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Title: Manures and the principles of manuring
Author: Aikman, Charles Morton
Language: English
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Historical Literature in Agriculture (CHLA), Cornell
University)



PRINCIPLES OF MANURING



MANURES

AND THE

PRINCIPLES OF MANURING


BY


C. M. AIKMAN, M.A., D.Sc., F.R.S.E., F.I.C.

FORMERLY PROFESSOR OF CHEMISTRY, GLASGOW VETERINARY COLLEGE, AND
EXAMINER IN CHEMISTRY, GLASGOW UNIVERSITY; AUTHOR OF 'FARMYARD MANURE,'
ETC.



THIRD IMPRESSION



WILLIAM BLACKWOOD AND SONS EDINBURGH AND LONDON MCMX


D. VAN NOSTRAND COMPANY NEW YORK


_All Rights reserved_



TO


SIR JOHN BENNET LAWES, BART., D.C.L., LL.D., F.R.S.,
OF ROTHAMSTED,

AND

SIR J. HENRY GILBERT, M.A., LL.D., F.R.S.,

FORMERLY SIBTHORPIAN PROFESSOR OF RURAL ECONOMY,
UNIVERSITY OF OXFORD,
WHOSE FAMOUS INVESTIGATIONS DURING THE LAST FIFTY YEARS
HAVE SO LARGELY CONTRIBUTED TO BUILD UP
THE SCIENCE OF MANURING,

THIS WORK,

EMBODYING MANY OF THE ROTHAMSTED RESULTS,

IS DEDICATED.



PREFACE.


When the present work was first undertaken there were but few works in
English dealing with its subject-matter, and hardly any which dealt with
the question of Manuring at any length. During the last few years,
however, owing to the greatly increased interest taken in agricultural
education, the demand for agricultural scientific literature has called
into existence quite a number of new works. Despite this fact, the
author ventures to believe that the gap which the present treatise was
originally designed to fill is still unfilled.

Of the importance of the subject all interested in agriculture are well
aware. It is no exaggeration to say that the introduction of the
practice of artificial manuring has revolutionised modern husbandry.
Indeed, without the aid of artificial manures, arable farming, as at
present carried out, would be impossible. Fifty years ago the practice
may be said to have been unknown; yet so widespread has it now become,
that at the present time the capital invested in the manure trade in
this country alone amounts to millions sterling. It need scarcely be
pointed out, therefore, that a practice in which such vast monetary
interests are involved is worthy of the most careful consideration by
all students of agricultural science, as well as, it may be added, by
political economists.

The aim of the present work is to supply in a concise and popular form
the chief results of recent agricultural research on the question of
soil fertility, and the nature and action of various manures. It makes
no pretence to be an exhaustive treatise on the subject, and only
contains those facts which seem to the author to have an important
bearing on agricultural practice. In the treatment of its subject it may
be said to stand midway between Professor Storer's recently published
elaborate and excellent treatise on 'Agriculture in some of its
Relations to Chemistry'--a work which is to be warmly recommended to all
students of agricultural science, and to which the author would take
this opportunity of acknowledging his indebtedness--and Dr J. M. H.
Munro's admirable little work on 'Soils and Manures.'

In order to render the work as intelligible to the ordinary agricultural
reader as possible, all tabular matter and matter of a more or less
technical nature have been relegated to the Appendices attached to each
chapter.

The author's somewhat wide experience as a University Extension
Lecturer, and as a Lecturer in connection with County Council schemes of
agricultural education, during the last few years, induces him to
believe that the work may be of especial value to those engaged in
teaching agricultural science.

He has to express the deep obligation he is under, in common with all
writers on Agricultural Chemistry, to the classic researches of Sir John
Bennet Lawes, Bart., and Sir J. Henry Gilbert, now in progress for more
than fifty years at Sir John Lawes' Experiment Station at Rothamsted.
His debt of gratitude to these distinguished investigators has been
still further increased by their kindness in permitting him to dedicate
the work to them, and for having been good enough to read portions of
the work in proof. In addition to the free use which has been made
throughout the book of the results of these experiments, the last
chapter contains, in a tabular form, a short epitome of some of the more
important Rothamsted researches on the action of different manures.

To the numerous German and French works on the subject, more especially
to Professor Heiden's encyclopædic 'Lehrbuch der Düngerlehre' and the
various writings of Dr Emil von Wolff, the author is further much
indebted.

Among English works he would especially mention the assistance he has
derived from the writings of Mr R. Warington, F.R.S., Professor S. W.
Johnson, Professor Armsby, the late Dr Augustus Voelcker, and others. He
would also tender his acknowledgments to the new edition of Stephens'
'Book of the Farm,' and he has to thank its editor, his friend Mr James
Macdonald, Secretary to the Highland and Agricultural Society of
Scotland, for having read parts of his proof-sheets.

It is also his pleasing duty to thank his friends Dr Bernard Dyer, Hon
Secretary of the Society of Public Analysts, Dr A. P. Aitken, Chemist to
the Highland and Agricultural Society of Scotland; Professor Douglas
Gilchrist of Bangor; Mr F. J. Cooke, late of Flitcham; Mr Hermann Voss
of London; and Professor Wright of Glasgow, for having assisted him in
the revision of proof-sheets.

ANALYTICAL LABORATORY,
128 WELLINGTON STREET, GLASGOW,
_January_ 1894.



CONTENTS.

                    PART I.--HISTORICAL INTRODUCTION.
                                                                     PAGE
 Beginning of agricultural chemistry                                    4
 Early theories regarding plant-growth                                  4
        Van Helmont                                                     4
        Digby                                                           6
        Duhamel and Stephen Hales                                       8
        Jethro Tull                                                     9
 Charles Bonnet's discovery of source of plants' carbon                11
 Researches of Priestley, Ingenhousz, Sénébier, on assimilation
   of carbon                                                        11-12
 Publication of first English treatise by Earl Dundonald               13
 Publication of Theodore de Saussure, 'Chemical Researches on
   Vegetation,' 1804                                                   14
 Theories on source of plant-nitrogen                                  15
        Early experiments on this subject                              16
 Sir Humphry Davy's lectures (1802-1812)                               17
 State of agricultural chemistry in 1812                               17
 Beginning of Boussingault's researches (1834)                         21
 Publication of Liebig's first report to the British Association       24
        Refutation of "humus" theory                                   26
        Liebig's mineral theory                                        26
        Liebig's theory of source of plants' nitrogen                  27
 Publication of Liebig's second report to British Association          30
        Liebig's services to agricultural chemistry                    31
 Development of agricultural research in Germany                       32
 The Rothamsted Experiment Station                                     33
 Sir J. B. Lawes and Sir J. H. Gilbert, the nature and value of
   their experiments                                                   33
 Review of the present state of our knowledge of plant-growth          36
        Proximate composition of the plant                             36
        Fixation of carbon by plants                                   37
        Action of light on plant-growth, Dr Siemens' experiments       38
        Source of oxygen and hydrogen in the plant                  39-40
        Source of nitrogen in the plant                                40
        Relation of the free nitrogen to leguminous plants          42-44
        Relation of nitrogen in organic forms, as ammonia salts,
         and nitrates to the plant                                  46-50
        Nitrification and its conditions                               51
        Ash constituents of the plant                                  53
        Methods of research for ascertaining essentialness of ash
          constituents of plants                                       53
        (_a_) Artificial soils, (_b_) water-culture                 53-55
        Method in which plants absorb their food-constituents          55
        Endosmosis                                                     55
        Retention by soils of plant-food                               57
        Causes of retention by soils of plant-food                     59
        Manuring                                                       60
        "Field" and "pot" experimentation                              60


                     PART II.--PRINCIPLES OF MANURING.

                     CHAPTER I.--FERTILITY OF THE SOIL.

 What constitutes fertility in a soil                                  65
     I. Physical properties of a soil                                  66
        Kinds of soils                                                 67
        Absorptive power for water of soils                            67
        Absorptive power for water of sand, clay, and humus            68
        Fineness of particles of a soil                                69
        Limit of fineness of soil-particles                            69
        Importance of retentive power                                  70
        Power of plants for absorbing water from a soil
          experiments by Sachs                                         73
        How to increase absorptive power of soils                      74
        Amount of water in a soil most favourable for plant growth     75
        Hygroscopic power of soils                                     75
        Capacity of soils for absorbing and retaining heat             76
        Explanation of dew                                             77
        Heat of soils                                                  78
        Heat in rotting farmyard manure                                78
        Causes of heat of fermentation                                 79
        Influence of colour on heat-retaining power                    80
        Power of soils for absorbing gases                             81
        Gases found in soils                                           81
        Variation in gas-absorbing power of soils                      82
        Absorption of nitrogen by soils                                82
        Requirements of plant-roots in a soil                          83
        Influence of tillage on number of plants in a certain area     86
        Comparison of English and American farming                     86
    II. Chemical composition of a soil                                 87
        Fertilising ingredients of a soil                              87
        Importance of _nitrogen, phosphoric acid_, and _potash_ in a
          soil                                                         88
        Chemical condition of fertilising ingredients in soils         89
        Amount of soluble fertilising ingredients in soils             90
        Value of chemical analysis of soils                            90
   III. Biological properties of a soil                                92
        Bacteria of the soil                                           92
 Recapitulation of Chapter I                                           96

                          APPENDIX TO CHAPTER I.
 NOTE
     I. Table of absorptive power of soil substances by Schübler       98
    II. Table of rate of evaporation of water in different soils
          by Schübler                                                  99
   III. Table of hygroscopic power of soils dried at 212° F.
          (Davy)                                                       99
    IV. Gases present in soil                                         100
     V. Amount of plant-food in soils                                 100
    VI. Chemical composition of the soil                              101
   VII. Forms in which plant-foods are present in the soil            107


 CHAPTER II.--FUNCTIONS PERFORMED BY MANURES.

 Etymological meaning of word manure                                  109
 Definition of manures                                                110
 Different classes of manures                                         111
 Action of different classes of manures                               113


 CHAPTER III.--POSITION OF NITROGEN IN AGRICULTURE.

 The Rothamsted experiments and the nitrogen question                 115
 Different forms in which nitrogen exists in nature                   116
        Relation of "free" nitrogen to the plant                      117
        Combined nitrogen in the air                                  118
        Amount of combined nitrogen falling in the rain               119
        Nitrogen in the soil                                          120
        Nitrogen in the subsoil                                       121
        Nitrogen of surface-soil                                      121
        Amount of nitrogen in the soil                                123
        Soils richest in nitrogen                                     123
        Nature of the nitrogen in the soil                            124
        Organic nitrogen in the soil                                  125
        Differences of surface and subsoil nitrogen                   126
        Nitrogen as ammonia in soils                                  127
        Amount of ammonia in soils                                    127
        Nitrogen present as nitrates in the soil                      128
        Position of nitric nitrogen in soil                           128
        Amount of nitrates in the soil                                129
        Amount of nitrates in fallow soils                            129
        Amount of nitrates in cropped soils                           130
        Amount of nitrates in manured wheat-soils                     131
        The sources of soil-nitrogen                                  131
        Accumulation of soil-nitrogen under natural conditions        133
        Accumulation of nitrogen in pastures                          134
        Gain of nitrogen with leguminous crops                        135
        The fixation of "free" nitrogen                               136
        Influence of manures in increasing soil-nitrogen              136
        Sources of loss of nitrogen                                   137
        Loss of nitrates by drainage                                  137
        Prevention of loss of nitrogen by permanent pasture and
          "catch-cropping"                                            138
        Other conditions diminishing loss of nitrates                 139
        Amount of loss of nitrogen by drainage                        140
        Loss of nitrogen in form of "free" nitrogen                   141
        Total amount of loss of nitrogen                              142
        Loss of nitrogen by retrogression                             142
        Artificial sources of loss of nitrogen                        144
        Amount of nitrogen removed in crops                           144
        Losses of nitrogen incurred on the farm                       146
        Loss in treatment of farmyard manure                          146
        Nitrogen removed in milk                                      147
        Economics of the nitrogen question                            147
        Loss of nitrogen-compounds in the arts                        148
        Loss due to use of gunpowder                                  148
        Loss due to sewage disposal                                   149
        Our artificial nitrogen supply                                150
        Nitrate of soda and sulphate of ammonia                       150
        Peruvian guano                                                151
        Bones                                                         151
        Other nitrogenous manures                                     152
        Oil-seeds and oilcakes                                        153
        Other imported sources of nitrogen                            153
        Conclusion                                                    153

                         APPENDIX TO CHAPTER III.
 NOTE
     I. Determination of the quantity of nitrogen supplied by
          rain, as ammonia and nitric acid, to an acre of land
          during one year                                             155
    II. Nitrogen in soils at various depths                           156
   III. Nitrogen as nitrates in cropped soils receiving no
          nitrogenous manures, in lb. per acre (Rothamsted soils)     157
    IV. Nitrogen as nitrates in Rothamsted soils                      157
     V. Examples of increase of nitrogen in Rothamsted soils
          laid down in pasture                                        158
    VI. Loss by drainage of nitrates                                  158
   VII. Examples of decrease of nitrogen in Rothamsted soils          159
  VIII. Amount of drainage and nitrogen as nitrates in
          drainage-water from unmanured bare soil, 20 and 60
          inches deep                                                 160


                        CHAPTER IV.--NITRIFICATION.

 Process of nitrification                                             161
 Occurrence of nitrates in the soil                                   162
 Nitre soils of India                                                 162
 Saltpetre plantations                                                163
 Cause of nitrification                                               165
        Ferments effecting nitrification                              167
        Appearance of nitrous organisms                               168
        Nitric organism                                               169
        Difficulty in isolating them                                  169
        Nitrifying organisms do not require organic matter            169
 Conditions favourable for nitrification--
        Presence of food-constituents                                 170
        Presence of a salifiable base                                 171
        Only takes place in slightly alkaline solutions               172
        Action of gypsum on nitrification                             173
        Presence of oxygen                                            173
        Temperature                                                   175
        Presence of a sufficient quantity of moisture                 176
        Absence of strong sunlight                                    176
        Nitrifying organisms destroyed by poisons                     176
 Denitrification                                                      177
        Denitrification also effected by bacteria                     178
        Conditions favourable for denitrification                     178
        Takes place in water-logged soils                             179
 Distribution of the nitrifying organisms in the soil                 179
        Depth down at which they occur                                180
        Action of plant-roots in promoting nitrification              181
        Nature of substances capable of nitrification                 181
 Rate at which nitrification takes place                              183
        Nitrification takes place chiefly during summer               183
        Process goes on most quickly in fallow fields                 184
        Laboratory experiments on rate of nitrification               185
        Certain portions of soil-nitrogen more easily
          nitrifiable than the rest                                   187
        Rate of nitrification deduced from field experiments          187
        Quantity of nitrates formed in the soils of fallow fields     188
        Position of nitrates depends on season                        188
        Nitrates in drainage-waters                                   188
        Amount produced at different times of year                    189
        Nitrification of manures                                      190
        Ammonia salts most easily nitrifiable                         191
        Sulphate of ammonia the most easily nitrifiable manure        191
        Rate of nitrification of other manures                        192
        Soils best suited for nitrification                           192
        Absence of nitrification in forest-soils                      193
        Important bearing of nitrification on agricultural
          practice                                                    193
        Desirable to have soil covered with vegetation                194
        Permanent pasture most economical condition of soil           194
        Nitrification and rotation of crops                           195

                           APPENDIX TO CHAPTER IV.
 NOTE
     I. Old theories of nitrification                                 196
    II. Nitrification takes place in solutions devoid of organic
          matter                                                      196
   III. Oxidising power of micro-organisms in soils                   197
    IV. Effect of urine on nitrification in soils                     197
     V. Solution used by Professor Frankland in cultivating
          nitrificative micro-organisms                               198
    VI. Experiments by Boussingault on rate of nitrification          198
   VII. Nitrogen as nitrates in Rothamsted soils after bare
          fallow in lb. per acre                                      198


          CHAPTER V.--POSITION OF PHOSPHORIC ACID IN AGRICULTURE.

 Occurrence of phosphoric acid in nature                              199
 Mineral sources of phosphoric acid                                   200
 Apatite and phosphorite                                              200
 Coprolites                                                           201
 Occurrence of phosphoric acid in guanos                              202
 Universal occurrence in common rocks                                 202
 Occurrence in the soil                                               203
 Condition in which phosphoric acid occurs in the soil                203
 Occurrence in plants                                                 204
 Occurrence in animals                                                205
 Sources of loss of phosphoric acid in agriculture                    205
 Loss of phosphoric acid by drainage                                  206
 Artificial sources of loss of phosphoric acid                        206
 Amount of phosphoric acid removed in milk                            207
 Loss of phosphoric acid in treatment of farmyard manure              208
 Loss of phosphoric acid in sewage                                    208
 Sources of artificial gain of phosphoric acid                        208

                          APPENDIX TO CHAPTER V.
 NOTE
     I. Composition of apatite (Voelcker)                             210
    II. Percentage of phosphoric acid in the commoner rocks           211


              CHAPTER VI.--POSITION OF POTASH IN AGRICULTURE.

 Potash of less importance than phosphoric acid                       212
 Occurrence of potash                                                 213
 Felspar and other potash minerals                                    213
 Stassfurt salts                                                      214
 Occurrence of saltpetre                                              215
 Occurrence of potash in the soil                                     215
 Potash chiefly in insoluble condition in soils                       216
 Percentage of potash in plants and plant-ash                         216
 Occurrence of potash in animal tissue                                217
 Sources of loss of potash                                            217
 Amount of potash removed in crops                                    218
 Amount of potash removed in milk                                     218
 Potash manures                                                       218

                          APPENDIX TO CHAPTER VI.
 NOTE
     I. Amount of potash in different minerals                        220
    II. Quantity of potash obtained from 1000 lb. of different
          kinds of vegetation in the manufacture of potashes          220


                            PART III.--MANURES.

                      CHAPTER VII.--FARMYARD MANURE.

 Variation in its composition                                         223
 Made up of three classes of constituents                             224
        _Solid excreta_--
            Its nature                                                224
            Difference in composition of the solid excreta of the
              different farm animals                                  224
            Causes of this difference                                 225
            Percentage of manurial ingredients in solid excreta
              of different animals                                    226
        _Urine_--
            Its nature                                                228
            Variation in its composition                              229
            Causes of this variation                                  229
            Manurial value of the urine of the different farm
              animals                                                 230
            Percentage of the _organic matter_, _nitrogen_, and
              _mineral substances_ in the food, voided in the
              solid excreta and urine                                 232
            Comparison of manurial value of total excrements of
              the different farm animals                              234
            Nature of changes undergone by food in process of
              digestion                                               235
        _Litter_--
            Its uses                                                  236
            _Straw_ as litter, and its qualifications                 237
            Composition of different kinds of straw                   238
            _Loam_ as litter                                          239
            _Peat_ as litter                                          240
            Comparison of properties of _peat-moss_ and _straw_       241
            The _bracken-fern_ as litter                              241
            _Dried leaves_ as litter                                  242
 Manures produced by the different animals--
        _Horse-manure_--
            Amount produced                                           243
            Its nature and composition                                243
            Amount of straw used for litter                           244
            Sources of loss on keeping                                245
            How to prevent loss                                       245
            Use of "fixers," and the nature of their action           245
        _Cow-manure_--
            Amount produced                                           248
            Its nature and composition                                248
            Amount of straw used as litter                            248
            Sources of loss on keeping                                249
            Advantages of _short dung_                                249
        _Pig-manure_--
            Amount produced                                           250
            Its nature and composition                                250
            Amount of straw used as litter                            251
        _Sheep-manure_--
            Amount produced                                           251
            Nature and composition                                    251
            Amount of straw used as litter                            252
            Methods of calculating amount of manure produced on
              the farm                                          252, note
 Fermentation of farmyard manure--
        Action of _micro-organic_ life in producing fermentation      255
        Two classes of _bacteria_ active in this work, _aerobies_
          and _anaerobies_                                            255
        Conditions influencing fermentation--
            _Temperature_                                             256
            _Openness to the air_                                     256
            _Dampness_                                                257
            _Composition of manure_                                   257
        Products of fermentation                                      257
 Analyses of farmyard manure--
        Dr Voelcker's experiments                                     259
        Variation in composition                                      259
        Amounts of _moisture_, _organic matter_
          (containing _nitrogen_), and _mineral matter_               260
        Its manurial value compared with _nitrate of soda_,
          _sulphate of ammonia_, and _superphosphate_                 260
 Comparison of fresh and rotten manure--
        The nature and amount of loss sustained in the process of
          _rotting_                                                   261
        Ought manure to be applied _fresh_ or _rotten_?               262
 Relative merits of _covered_ and _uncovered_ manure-heaps            263
 Methods of application of farmyard manure to the field--
        Merits and demerits of the different methods                  265
        Setting it out in _heaps_                                     265
        Spreading it _broadcast_, and letting it lie                  266
        Ploughing it in immediately                                   267
 Value and function of farmyard manure--
        As a supplier of the necessary elements of plant-food         268
        As a "universal" manure                                       269
        Proportion in which _nitrogen_, _phosphoric acid_, and
          _potash_ are required by crops                              269
        Proportion in which they are present in farmyard manure       270
        Farmyard manure _poor in nitrogen_                            270
        Lawes' and Gilbert's experiments                              271
        How it may be best reinforced by the use of "artificials"     271
        Indirect value of farmyard manure as a supplier of
          _humus_ to the soil                                         273
        Its influence on soil-texture                                 273
        Its influence in setting free inert fertilising matter
          in the soil                                                 274
 Rate at which farmyard manure ought to be applied                    275
 Lasting nature of farmyard manure                                    276
 Its economic value                                                   276

                         APPENDIX TO CHAPTER VII.
 NOTE
     I. Difference in amount of excreta voided for food consumed      279
    II. Solid excreta voided by sheep, oxen, and cows                 279
   III. Urine voided by sheep, oxen, and cows                         280
    IV. Percentage of food voided in the solid and liquid
          excrements                                                  281
     V. Pig excrements                                                281
    VI. Manurial constituents in 1000 parts of ordinary foods         282
   VII. Analyses of stable-manure, made respectively with
          peat-moss litter and wheat-straw                            283
  VIII. Analyses of bracken                                           283
    IX. Analyses of horse-manure                                      283
     X. The nature of the chemical reactions of ammonia "fixers"      284
    XI. Analyses of cow-manure                                        286
   XII. Composition of fresh and rotten farmyard manure               286
  XIII. Comparison of fresh and rotten manure                         288
   XIV. Lord Kinnaird's experiments                                   289
    XV. Drainings of manure-heaps                                     290
   XVI. Amounts of potash and phosphoric acid removed by
          rotation from a Prussian morgen (.631 acre)                 290
  XVII. Composition of farmyard manure (fresh)                        291
 XVIII. The urine (quantity voided)                                   291


                           CHAPTER VIII.--GUANO.

 Importance in agriculture                                            293
 Influence on British farming                                         294
 Influence of guano not wholly good                                   295
 Value of guano as a manure                                           296
 Origin and occurrence of guano                                       297
 Variation in composition of different guanos                         299
     I. Nitrogenous guano--
        (_a_) Peruvian guano                                          300
        Different deposits of Peruvian guano                          301
        Appearance, colour, and nature of Peruvian guano              303
        Composition of Peruvian guano                                 304
        (_b_) Other nitrogenous manures: Angamos, Ichaboe             306
    II. Phosphatic guanos--
        Occurrence of phosphatic guanos                               308
        Inequality in composition of phosphatic guanos                309
        "Dissolved" phosphatic guano                                  310
        "Equalised" or "rectified" guano                              311
        The action of phosphatic guanos as manures                    312
        Proportion of fertilising constituents in guano               314
        Mode of application of guanos                                 315
        Quantity of guano to be used                                  317
        Adulteration of guano                                         318
   So-called guanos--
        Fish-guano                                                    320
        Value of fish-guano                                           322
        Meat-meal guano                                               324
        Value of meat-meal guano                                      324
        Bat guano                                                     325
        Pigeon and fowl dung                                          325

                         APPENDIX TO CHAPTER VIII.
 NOTE
     I. Peruvian guano imported into United Kingdom, 1865-1893        327
    II. Guano deposits of the world                                   327
   III. Composition of concretionary nodules                          328
    IV. Table showing gradual deterioration of Peruvian guano,
          1867-1881                                                   329
     V. Composition of different guanos                               329
    VI. Liebig's theory as to the action of oxalic acid in guano      330
   VII. Analyses of dung of fowls, pigeons, ducks, and geese          331


                       CHAPTER IX.--NITRATE OF SODA.

 Amount of exports                                                    332
 Date of discovery of nitrate deposits                                333
 The origin of nitrate deposits                                       334
 Forbes and Darwin on the theory of their origin                      335
 Source of nitric acid in nitrate of soda                             337
 Guano theory of origin of nitrate of soda                            337
 Nitric acid in nitrate of soda probably derived from sea-weed        339
 Appearance of nitrate-fields                                         340
 The method of mining the nitrate of soda                             341
 Composition of _caliche_                                             342
 Extent of the nitrate deposits                                       342
 Composition and properties of nitrate of soda                        343
 Nitrate applied as a top-dressing                                    344
 Nitrate of soda encourages deep roots                                344
 Is nitrate of soda an exhausting manure?                             345
 Crops for which nitrate of soda is suited                            346
 Method of application of nitrate of soda                             347
 Importance of having a sufficiency of other fertilising
   constituents                                                       348
 Conclusions drawn                                                    349

                          APPENDIX TO CHAPTER IX.

 Total shipments from South America, 1830-1893                        351
 Total imports into Europe and United Kingdom, 1873-1892              351


                     CHAPTER X.--SULPHATE OF AMMONIA.

 Value of ammonia as a manure                                         352
 Sources of sulphate of ammonia                                       353
 Ammonia from gas-works                                               353
 Other sources                                                        354
 Composition, &c., of sulphate of ammonia                             355
 Application of sulphate of ammonia                                   356

                          APPENDIX TO CHAPTER X.

 Production of sulphate of ammonia in United Kingdom, 1870-1892       358


                            CHAPTER XI.--BONES.

 Early use of bones                                                   359
 Different forms in which bones are used                              360
 Composition of bones                                                 362
 The organic matter of bones                                          363
 The inorganic matter of bones                                        363
 Treatment of bones                                                   364
 Action of bones                                                      365
 Dissolved bones                                                      368
 Crops suited for bones                                               368
 Bone-ash                                                             369
 Bone-char or bone-black                                              369

                          APPENDIX TO CHAPTER XI.
 NOTE
     I. Analysis of bone-meal                                         371
    II. Analysis of dissolved bones                                   371
   III. Composition of bone-ash                                       372
    IV. Composition of bone-char                                      372


                     CHAPTER XII.--MINERAL PHOSPHATES.

 Coprolites                                                           373
 Canadian apatite or phosphorite                                      374
 Estremadura or Spanish phosphates                                    375
 Norwegian apatite                                                    376
 Charlestown or South Carolina phosphate                              376
 Belgian phosphate                                                    377
 Somme phosphate                                                      378
 Florida phosphate                                                    378
 Lahn phosphate                                                       379
 Bordeaux or French phosphate                                         379
 Algerian phosphate                                                   379
 Crust guanos                                                         379
 Value of mineral phosphates as manures                               380

                         APPENDIX TO CHAPTER XII.

 Imports of phosphates                                                381


                      CHAPTER XIII.--SUPERPHOSPHATES.

 Discovery of superphosphate by Liebig                                382
 Manufacture of superphosphate                                        383
 Nature of the reaction taking place                                  385
 Phosphates of lime                                                   385
 Reverted phosphate                                                   389
 Value of reverted phosphate                                          391
 Composition of superphosphates                                       391
 Action of superphosphates                                            392
 Action of superphosphate sometimes unfavourable                      395
 Application of superphosphate                                        395
 Value of insoluble phosphates                                        396
 Rate at which superphosphate is applied                              397

                         APPENDIX TO CHAPTER XIII.
 NOTE
     I. The formulæ, and molecular and percentage composition,
          of the different phosphates                                 398
    II. Reactions of sulphuric acid and phosphate of lime             398
   III. Table for conversion of soluble phosphate into insoluble
          phosphate                                                   399
    IV. Action of iron and alumina in causing reversion               399
     V. Relative trade values of phosphoric acid in different
          manures                                                     400


              CHAPTER XIV.--THOMAS-PHOSPHATE OR BASIC SLAG.

 Its manufacture                                                      401
 Not at first used                                                    403
 Discovery of its value as a manure                                   403
 Composition of basic slag                                            404
 Processes for preparing slag                                         406
 Solubility of basic slag                                             408
 Darmstadt experiments with basic slag                                410
 Results of other experiments                                         413
 Soils most suited for slag                                           414
 Rate of application                                                  414
 Method of application                                                416

                         APPENDIX TO CHAPTER XIV.

 Analysis of basic slag                                               417


                      CHAPTER XV.--POTASSIC MANURES.

 Relative importance                                                  418
 Scottish soils supplied with potash                                  419
 Sources of potassic manures                                          419
 Stassfurt potash salts                                               420
 Relative merits of sulphate and muriate of potash                    421
 Application of potash manures                                        422
 Soils and crops suited for potash manures                            423
 Rate of application                                                  423


                  CHAPTER XVI.--MINOR ARTIFICIAL MANURES.

 Scutch                                                               427
 Shoddy and wool-waste                                                427
 Soot                                                                 428


                    CHAPTER XVII.--SEWAGE AS A MANURE.

 Irrigation                                                           431
 Effects of continued application of sewage                           433
 Intermittent irrigation                                              434
 Crops suited for sewage                                              434
 Treatment of sewage by precipitation, &c.                            436
 Value of sewage sludge                                               439


 CHAPTER XVIII.--LIQUID MANURE                                        442


                          CHAPTER XIX.--COMPOSTS.

 Farmyard manure a typical compost                                    446
 Other composts                                                       447


                      CHAPTER XX.--INDIRECT MANURES.

 Lime                                                                 449
        Antiquity of lime as a manure                                 449
        Action of lime                                                449
        Lime a necessary plant-food                                   450
        Lime of abundant occurrence                                   452
        Lime returned to the soil in ordinary agricultural
          practice                                                    452
        Different forms of lime                                       453
        Caustic lime                                                  453
        Lime acts both mechanically and chemically                    455
     I. Mechanical functions of lime                                  455
            Action on soil's texture                                  455
            Lime renders light soils more cohesive                    457
    II. Chemical action of lime                                       457
   III. Biological action of lime                                     459
            Action of lime on nitrogenous organic matter              460
   Recapitulation                                                     461


            CHAPTER XXI.--INDIRECT MANURES--GYPSUM, SALT, ETC.

 Gypsum                                                               462
        Mode in which gypsum acts                                     462
 Salt                                                                 465
        Antiquity of the use of salt                                  465
        Nature of its action                                          465
        Salt not a necessary plant-food                               466
        Can soda replace potash?                                      466
        Salt of universal occurrence                                  467
        Special sources of salt                                       468
        The action of salt                                            468
        Mechanical action on soils                                    470
        Solvent action                                                470
        Best used in small quantities along with manures              472
        Affects quality of crop                                       472
        Rate of application                                           473


                CHAPTER XXII.--THE APPLICATION OF MANURES.

 Influence of manures in increasing soil-fertility                    474
 Influence of farmyard manure on the soil                             475
 Farmyard manure _v._ artificials                                     476
 Farmyard manure not favourable to certain crops                      477
 Conditions determining the application of artificial manures         477
        Nature of the manure                                          478
 Nitrogenous manures                                                  478
 Phosphatic manures                                                   480
 Potash manures                                                       480
 Nature of soil                                                       481
 Nature of previous manuring                                          482
 Nature of the crop                                                   483
 Amounts of fertilising ingredients removed from the soil by
   different crops                                                    484
 Capacity of crops for assimilating manures                           486
        Difference in root-systems of different crops                 488
        Period of growth                                              489
        Variation in composition of crops                             490
        Absorption of plant-food                                      490
        Fertilising ingredients lodge in the seed                     491
 Forms in which nitrogen exists in plants                             491
        Bearing of above on agricultural practice                     492
 Influence of excessive manuring of crops                             492


            CHAPTER XXIII.--MANURING OF THE COMMON FARM CROPS.

 Cereals                                                              493
        Especially benefited by nitrogenous manures                   494
        Power of absorbing silicates                                  494
        Barley                                                        495
            Period of growth                                          495
            Most suitable soil                                        496
            Farmyard manure not suitable                              497
            Importance of uniform manuring of barley                  497
        Norfolk experiments on barley                                 497
            Proportion of grain to straw                              498
        Wheat                                                         499
        Rothamsted experiments                                        500
            Continuous growth                                         500
        Flitcham experiments                                          500
        Oats                                                          501
            A very hardy crop                                         502
            Require mixed nitrogenous manuring                        502
        Arendt's experiments                                          503
            Avenine                                                   503
            Quantities of manures                                     504
 Grass                                                                504
        Effect of manures on herbage of pastures                      505
        Influence of farmyard manure                                  506
        Influence of soil and season on pastures                      507
        Manuring of meadow land                                       508
            Bangor experiments                                        508
            Norfolk experiments                                       509
        Manuring of permanent pastures                                509
 Roots                                                                510
        Influence of manure on composition                            512
        Nitrogenous manures increase sugar                            512
        Amount of nitrogen recovered in increase of crop              513
        Norfolk experiments                                           513
            Manure for swedes                                         514
        Highland Society's experiments                                515
            Manuring for rich crops of turnips                        516
        Experiments by the author on turnips                          516
        Potatoes                                                      517
        Highland Society's experiments                                518
        The Rothamsted experiments                                    519
            Effect of farmyard manure                                 520
            Manuring of potatoes in Jersey                            521
            The influence of manure on the composition                521
 Leguminous crops                                                     522
        Leguminous plants benefit by potash                           523
        Nitrogenous manures may be hurtful                            523
        Clover sickness                                               524
        Alternate wheat and bean rotation                             524
        Beans                                                         525
            Manure for beans                                          525
            Relative value of manurial ingredients                    526
            Gypsum as a bean manure                                   526
            Effect of manure on composition of crop                   527
        Peas                                                          527
        Hops                                                          528
        Cabbages                                                      528

                        APPENDIX TO CHAPTER XXIII.

 Experiments on bean-manuring                                         530


 CHAPTER XXIV.--ON THE METHOD OF APPLICATION, AND ON THE MIXING OF MANURES.

 Equal distribution of manures                                        531
 Mixing manures                                                       532
 Risks of loss in mixtures                                            533
        Loss of ammonia                                               533
        Effects of lime on ammonia                                    535
        Loss of nitric acid                                           536
        Reversion of phosphates                                       537
 Manurial ingredients should be applied separately                    538


         CHAPTER XXV.--ON THE VALUATION AND ANALYSIS OF MANURES.

 Value of chemical analysis                                           539
 Interpretation of chemical analysis                                  539
 Nitrogen                                                             540
 Phosphoric acid                                                      541
        Importance of mechanical condition of phosphate               542
 Potash                                                               542
 Other items in the chemical analysis of manures                      543
 Fertilisers and Feeding Stuffs Act                                   543
 Different methods of valuing manures                                 544
 Unit value of manurial ingredients                                   544
 Intrinsic value of manures                                           545
 Field experiments                                                    545
 Educational value of field experiments                               547
 Value of manures deduced from experiments                            548
 Value of unexhausted manures                                         549
 Potential fertility of a soil                                        549
 Tables of value of unexhausted manures.                              551

                         APPENDIX TO CHAPTER XXV.
 NOTE
     I. Factors for calculating compounds from manurial
          ingredients                                                 553
    II. Units for determining commercial value of manures and
          cash prices of manures                                 554, 555
   III. Manurial value of nitrogen and potash in different
          substances                                                  556
    IV. Comparative manurial value of different forms of
          nitrogen and potash                                         557
     V. Lawes' and Gilbert's tables for calculating unexhausted
          value of manures                                            559


                CHAPTER XXVI.--THE ROTHAMSTED EXPERIMENTS.

 Nature of experiments on crops and manures                           561
        Soil of Rothamsted                                            561
 Table I. List of Rothamsted field experiments                        562
 Wheat experiments--
        Unmanured plots                                               562
        Wheat grown continuously on same land (unmanured)             562
            Table  II. Results of first eight years                   562
            Table III. Results of subsequent forty years              562
        Table   IV. Wheat grown continuously with farmyard
          manure (14 tons per annum)                                  564

        Table    V. Wheat grown continuously with artificial
          manures                                                     565
 Table   VI. Experiments on the growth of barley, forty years,
   1852-91                                                            566
 Table  VII. Experiments on the growth of oats, 1869-78               567
 Table VIII. Experiments on root crops--Swedish turnips          568, 569
 Table   IX. Experiments on mangel-wurzel                        568, 569
 Table    X. Experiments with different manures on permanent
    meadow-land, thirty-six years, 1856-91                            570
 Table   XI. Experiments on the growth of potatoes--average for
   five seasons, 1876-80                                              571
 Table  XII. Experiments on growth of potatoes (continued)--
   average for twelve seasons, 1881-92                                572

        *       *       *       *       *

 INDEX                                                                573



PART I.

HISTORICAL INTRODUCTION



MANURES AND THE PRINCIPLES OF MANURING.


HISTORICAL INTRODUCTION.

Agricultural Chemistry, like most branches of natural science, may be
said to be entirely of modern growth. While it is true we have many old
speculations on the subject, they can scarcely be said to possess much
scientific value. The great questions which had first to be solved by
the agricultural chemist were,--What is the food of plants? and,--What
is the source of that food? The second of these two questions more
easily admitted of answer than the first. The source of plant-food could
only be the atmosphere or the soil. As the composition of the
atmosphere, however, was not discovered till the close of last century,
and the chemistry of the soil is a question which is still requiring
much work ere we shall be in possession of anything like a full
knowledge of it, it will be at once obvious that the very fundamental
conditions for a solution of the question were awanting. The beginning,
then, of a true scientific agricultural chemistry may be said to date
from the brilliant discoveries associated with the names of Priestley,
Scheele, Lavoisier, Cavendish, and Black--that is, towards the close of
last century.


_Early Theories on Source of Plant-food._

While this is so, and while we must regard the early attempts made
towards solving this question as being, for the most part, of little
scientific value, it is not without interest, from the historical point
of view, to glance briefly at some of these old interesting
speculations.

The Aristotelian doctrine, regarding the possibility of dividing matter
into the so-called four primary elements, _fire_, _air_, _earth_, and
_water_, which obtained in one form or another till the birth of modern
chemistry, had naturally an important influence on these early theories.


_Van Helmont's Theory._

Among the earliest and most important attempts made to solve the problem
of plant-growth was that by Jean Baptiste Van Helmont, one of the best
known of the alchemists, who flourished about the beginning of the
seventeenth century. Van Helmont believed that he had proved by a
conclusive experiment that all the products of vegetables were capable
of being generated from water. The details of this classical experiment
were as follows:--

"He took a given weight of dry soil--200 lb.--and into this soil he
planted a willow-tree that weighed 5 lb., and he watered this carefully
from time to time with pure rain-water, taking care to prevent any dust
or dirt falling on to the earth in which the plant grew. He allowed this
to go on growing for five years, and at the end of that period, thinking
his experiment had been conducted sufficiently long, he pulled up his
tree by the roots, shook all the earth off, dried the earth again,
weighed the earth and weighed the plant. He found that the plant now
weighed 169 lb. 3 ounces, whereas the weight of the soil remained very
nearly what it was--about 200 lb. It had only lost 2 ounces in
weight."[1]

The conclusion, therefore, come to by Van Helmont was that the source of
plant-food was _water_.[2]


_Digby's Theory._

Some fifty years later an extremely interesting book was published
bearing the following title: 'A Discourse concerning the Vegetation of
Plants, spoken by Sir Kenelm Digby, at Gresham College, on the 23d of
January 1660. (At a meeting of the Society for promoting Philosophical
Knowledge by Experiments. London: Printed for John Williams, in Little
Britain, over against St Botolph's Church, 1669.)' The author attributes
plant-growth to the influence of a _balsam_ which the air contains. This
book is especially interesting as containing the earliest recognition of
the value of saltpetre as a manure. The following is an extract from
this interesting old work:--

"The sickness, and at last the death of a plant, in its natural course,
proceeds from the want of that balsamick saline juice; which, I have
said, makes it swell, germinate, and augment itself. This want may
proceed either from a destitution of it in the place where the plant
grows, as when it is in a barren soil or bad air, or from a defect in
the plant itself, that hath not vigour sufficient to attract it, though
it be within the sphere of it; as when the root has become so hard,
obstructed and cold, as that it hath lost its vegetable functions. Now,
both these may be remedy'd, in a great measure, by one and the same
physick.... The watering of soils with cold hungray springs doth little
good; whereas muddy saline waters brought to overflow a piece of ground
enrich it much. But above all, well-digested dew makes all plants
luxuriate and prosper most. Now what may it be that endues these liquors
with such prolifick virtue? The meer water which is common to them all,
cannot be it; there must be something else enclosed within it, to which
the water serves but for a vehicle. Examine it by spagyric art, and you
will find that it is nothing else than a _nitrous salt_, which is
dilated in the water. It is this salt which gives foecundity to all
things: and from this salt (rightly understood) not only all vegetables,
but also all minerals draw their origine. By the help of plain
_salt-peter_, dilated in water and mingled with some other fit earthy
substance, that may familiarize it a little with the corn into which I
endeavoured to introduce it, I have made the barrenest ground far out-go
the richest, in giving a prodigiously plentiful harvest. I have seen
hemp-seed soaked in this liquor, that hath in due time made such plants
arise, as, for the tallness and hardness of them, seemed rather to be
coppice-wood of fourteen years' growth at least, than plain hemp. The
fathers of the Christian doctrine at Paris still keep by them for a
monument (and indeed it is an admirable one) a plant of barley
consisting of 249 stalks, springing from one root or grain of barley; in
which they counted above 18,000 grains or seeds of barley. But do you
think that it is barely the salt-peter, imbibed into the seed or root,
which causeth this fertility? no: that would be soon exhausted and could
not furnish matter to so vast a progeny. The salt-peter there is like a
magnet, which attracts a like salt which foecundates the air, and gave
cause to the Cosmopolite to say there is in the air a hidden food of
life."[3]


_Duhamel and Hales._

The names of the French writer, Duhamel, and of the English, Stephen
Hales, may be mentioned in passing as authors of works bearing on the
question of vegetable physiology. Both of these writers flourished about
the middle of the eighteenth century. The writings of the former
contained much valuable information on the effects of grafting, motion
of sap, and influence of light on vegetable growth, and also the results
of experiments which the author had carried out on the influence of
treating plants with certain substances. 'Statical Essays, containing
Vegetable Staticks; or an Account of some Statical Experiments on the
Sap of Vegetables, by Stephen Hales, D.D.' (2 vols.), was published in
London in 1738; and contained, as will be seen from its title, records
of experiments of very much the same nature as those of Duhamel.


_Jethro Tull's Theory._

Some reference may be made to a theory which created a considerable
amount of interest when it was first published--viz., that of Jethro
Tull. The chief value of Tull's contribution to the subject of
agricultural science was, that he emphasised the importance of tillage
operations by putting forward a theory to account for the fact,
universally recognised, that the more thoroughly a soil was tilled, the
more luxuriant the crops would be. As Tull's theory had a very
considerable influence in stirring up interest in many of the most
important problems in agricultural chemistry, and as it contained in
itself much, the value of which we have only of late years come to
understand, a brief statement of this theory may not be without
interest.

According to Tull the food of plants consists of the particles of the
soil. These particles, however, must be rendered very minute before they
become available for the plant, which absorbs them by means of its
rootlets. This pulverisation of the soil goes on in nature independently
of the farmer, but only very slowly, and the farmer has therefore to
hasten it on by means of tillage operations. The more efficiently these
operations are carried on, the more abundant will the supply of
plant-food be rendered in the soil. He consequently introduced and
advocated the system of horse-hoe husbandry. This theory, he informs us,
was suggested to him by the custom, which he had noticed on the
Continent, of growing vines in rows, and hoeing the intervals between
these rows from time to time. The excellent results which followed this
mode of cultivation induced him to adopt it in England for his farm
crops. He accordingly sowed his crops in rows or ridges, wide enough
apart to admit of thorough tillage of the intervals by ploughing as well
as by hand-hoeing. This he continued until the plant had reached
maturity. As to the exact width of the interval most suitable, he made a
large number of experiments. At first, in the cultivation of wheat, he
made this interval six feet wide; but latterly he adopted an interval of
lesser width, that finally arrived at being between four and five feet.
He likewise experimented on each separate ridge as to which was the best
number of rows of wheat to be sown, latterly adopting, as most
convenient, two rows at ten inches apart. The great success which he met
with in this system of cultivation induced him to publish the results of
his experiments in his famous work, 'Horse-Hoeing Husbandry.'

While Tull's theory was based on principles at heart thoroughly sound,
he was carried away by his personal success into drawing unwarrantable
deductions. Thus he came to the conclusion that rotation of crops was
unnecessary, provided that a thorough system of tillage was carried out.
Manures also, according to him, might be entirely dispensed with under
his system of cultivation, for the true function of all manures is to
aid in the pulverisation of the soil by fermentation.

The first really valuable scientific facts contributed to the science
were made by Priestley, Bonnet, Ingenhousz, and Sénébier.


_Discovery of the Source of Plants' Carbon._

To Charles Bonnet (1720-1793), a Swiss naturalist, is due the credit of
having made the first contribution to a discovery of very great
importance--viz., the true source of the _carbon_, which we now know
forms so large a portion of the plant-substance. Bonnet, who had devoted
himself to the question of the function of leaves, noticed that when
these were immersed in water bubbles were seen, after a time, to collect
on their surface. De la Hire, it ought to be pointed out, had noticed
this same fact about sixty years earlier. It was left to Priestley,
however, to identify these bubbles with the gas he had a short time
previously discovered--viz., oxygen. Priestley had observed, about this
time, the interesting fact that plants possessed the power of purifying
air vitiated by the presence of animal life.[4] The next step in this
highly interesting and important discovery was taken by John Ingenhousz
(1730-1799), an eminent physician and natural philosopher. In 1779,
Ingenhousz published a work in London entitled 'Experiments on
Vegetables.' In it he gives the results of some important experiments he
had made on the question already investigated by Bonnet and Priestley.
These experiments proved that plant-leaves only gave up their oxygen in
the presence of sunlight. In 1782 he published another work on 'The
Influence of the Vegetable Kingdom on the Animal Creation.'[5]

The source of the gas, which Bonnet had first noticed to be given off
from plant-leaves, Priestley had identified as oxygen, and Ingenhousz
had proved to be only given off under the influence of the sun's rays,
was finally shown by a Swiss naturalist, Jean Sénébier[6] (1742-1809),
to be the _carbonic acid gas_ in the air, which the plant absorbed and
decomposed, giving out the oxygen and assimilating the carbon.


_Publication of First English Treatise on Agricultural Chemistry._

In 1795, a book dealing with the relations between chemistry and
agriculture was published. This work was written by a Scottish nobleman,
the Earl of Dundonald, and possesses especial interest from the fact
that it is the first book in the English language on agricultural
chemistry. The full title is as follows: 'A Treatise showing the
Intimate Connection that subsists between Agriculture and Chemistry.'

In his introduction the author says: "The slow progress which
agriculture has hitherto made as a science is to be ascribed to a want
of education on the part of the cultivators of the soil, and to a want
of knowledge, in such authors as have written on agriculture, of the
intimate connection that subsists between the science and that of
chemistry. Indeed, there is no operation or process not merely
mechanical that does not depend on chemistry, which is defined to be a
knowledge of the properties of bodies, and of the effects resulting from
their different combinations."

In quoting this passage Professor S. W. Johnson remarks:[7] "Earl
Dundonald could not fail to see that chemistry was ere long to open a
splendid future for the ancient art that had always been and always will
be the prime supporter of the nations. But when he wrote, how feeble
was the light that chemistry could throw upon the fundamental questions
of agricultural science! The chemical nature of the atmosphere was then
a discovery of barely twenty years' standing. The composition of water
had been known but twelve years. The only account of the composition of
plants that Earl Dundonald could give was the following: 'Vegetables
consist of mucilaginous matter, resinous matter, matter analogous to
that of animals, and some proportion of oil.... Besides these,
vegetables contain earthy matters, formerly held in solution in the
newly-taken-in juices of the growing vegetables.' To be sure, he
explains by mentioning in subsequent pages that starch belongs to the
mucilaginous matter, and that on analysis by fire vegetables yield
soluble alkaline salts and insoluble phosphate of lime. But these salts,
he held, were formed in the process of burning, their lime excepted; and
the fact of their being taken from the soil and constituting the
indispensable food of plants, his lordship was unacquainted with. The
gist of agricultural chemistry with him was, that plants 'are composed
of gases with a small proportion of calcareous matter; for although this
discovery may appear to be of small moment to the practical farmer, yet
it is well deserving of his attention and notice.'"


_De Saussure._

The year 1804 witnessed the publication of by far the most important
contribution made to the science up till this time. This was
'Recherches Chimique sur la Végétation,' by Theodore de Saussure, one of
the most illustrious agricultural chemists of the century. De Saussure
was the first to draw attention to the mineral or ash constituents of
the plant; and thus anticipate, to a certain extent, the subsequent
famous "mineral" theory of the great Liebig. The French chemist
maintained that these ash ingredients were essential; and that without
them plant-life was impossible. He also adduced fresh experiments of his
own in support of the theory, based on the experiments of Bonnet,
Priestley, Ingenhousz, and Sénébier, that plants obtain their carbon
from the carbonic acid gas in the air, under the influence of the
sunlight. He was of opinion that the _hydrogen_ and _oxygen_ of the
plant were, probably, chiefly derived from water. He showed that by far
the largest portion of the plant's substance was derived from the air
and from water, and that the ash portion was alone derived from the
soil. To Saussure we owe the first definite statement on the different
sources of the plant's food. It may be said that the lapse of nearly a
century has shown his views to be, in the main, correct.


_Source of Plant-nitrogen._

There was one question, which, even at that remote period in the history
of the subject, engaged the attention of agricultural chemists--viz.,
the question of the source of the plant's _nitrogen_--a question which
may be fitly described at the present hour as still the burning
question of agricultural chemistry.[8]

As soon as it was discovered that nitrogen was a constituent of the
plant's substance; speculations as to its source were indulged in. The
fact that the air furnished an unlimited storehouse of this valuable
element, and the analogy of the absorption of carbon (from the same
source by plant-leaves), naturally suggested to the minds of early
inquirers that the free nitrogen of the air was the source of the
plant's nitrogen. As, however, no direct experiments could be adduced to
prove this theory, and as, moreover, nitrogen was found in the soil, and
seemed to be a necessary ingredient of all fertile soils, the opinion
that the soil was the only source gradually supplanted the older theory.
Little value, however, must be attached to these early theories, as they
can scarcely be said to have been based on experiments of serious value.
Indeed it may be safely affirmed, in the light of subsequent
experiments, that it was impossible for this question to be decided at
this early period, from the fact that analytical apparatus, of a
sufficiently delicate nature, was then wholly unknown. Indeed it is only
within the last few years that it has been possible to carry out
experiments which may be regarded as at all crucial. A short sketch of
the development of our knowledge of the relation of nitrogen to the
plant will be given further on.


_Sir Humphry Davy's Lectures._

A series of lectures on agricultural chemistry, delivered by Sir Humphry
Davy during the years 1802-1812, for the Board of Agriculture, and
subsequently published in book form in the year 1813,[9] affords us an
opportunity of gauging, pretty accurately, the state of knowledge on the
subject at the time.


_Position of Agricultural Chemistry at beginning of Century._

In his opening lecture Davy says: "Agricultural chemistry has not yet
received a regular and systematic form. It has been pursued by competent
experimenters for a short time only. The doctrines have not as yet been
collected into any elementary treatise, ... and," he adds, "I am sure
you will receive with indulgence the first attempt made in this country
to illustrate it by a series of experimental demonstrations."

He further on remarks: "It is evident that the study of agricultural
chemistry ought to be commenced by some general inquiries into the
composition and nature of material bodies, and the law of their changes.
The surface of the earth, the atmosphere, and the water deposited from
it, must either together, or separately, afford all the principles
concerned in vegetation, and it is only by examining the chemical
nature of these principles that we are capable of discovering what is
the food of plants, and the manner in which this food is supplied and
prepared for their nourishment."

Davy goes on further to say: "No general principles can be laid down
respecting the comparative merits of the different systems of
cultivation and the various systems of crops adopted in different
districts, unless the chemical nature of the soil, and the physical
circumstances to which it is exposed, are fully known."

He recognises the enormous importance of experiments. "Nothing is more
wanting in agriculture than experiments, in which all the circumstances
are minutely and scientifically detailed."

In dealing with the composition of plants he says: "It is evident that
the most essential vegetable substances consist of hydrogen, carbon, and
oxygen, in different proportions, generally alone; but in some few cases
combined as carbon and nitrogen. The acids, alkalies, earths, metallic
oxides, and saline compounds, though necessary in the vegetable economy,
must be considered as of less importance, particularly in their relation
to agriculture, than the other principles."

Further on: "It will be asked, Are the pure earths in the soil merely
active as mechanical or indirect chemical agents, or do they actually
afford food to the plant?"

This question he answers by saying that "water, and the decomposing
animal and vegetable matter existing in the soil, constitute the true
nourishment of plants; and as the earthy parts of the soil are useful in
retaining water, so as to supply it in the proper proportion to the
roots of the vegetables, so they are likewise efficacious in producing
the proper distribution of the animal or vegetable matter. When equally
mixed with it, they prevent it from decomposing too rapidly; and by
their means the soluble parts are supplied in proper proportions."


_Value of Davy's Lectures._

The chief value of these lectures is due to the fact that they form the
first attempt to connect in a systematic manner the various scattered
facts, up to that time ascertained, and to interpret their bearing on
agricultural practice. We have in them, it is true, a strange mixture of
facts belonging rather to botany and physiology than to agricultural
chemistry; still they undoubtedly furnished a great impetus to inquiry,
and at the same time they did much to popularise the science.

But not merely did Davy summarise and systematise the various results
arrived at by others, he also made many valuable contributions to the
science himself. The conclusions he drew from the results he obtained
were, no doubt, in many cases false, and in other cases exaggerated;
still the results possess a permanent interest. He may be said to have
worked out many of the most important _physical_ or _mechanical_
properties of a soil, although exaggerating the importance of the
influence of these properties on the question of fertility.[10]

These experiments had to do with the heat- and water-absorbing powers of
a soil. He experimented on a brown fertile soil, and a cold barren clay,
and found at what rate they lost heat. "Nothing," he says, "can be more
evident than that the genial heat of the soil, particularly in spring,
must be of the highest importance to the rising plant; ... so that the
temperature of the surface, when bare and exposed to the rays of the
sun, affords at least one indication of the degree of the fertility."

Again he says: "The power of soils to absorb water from air is much
connected with fertility.... I have compared the absorbent powers of
many soils, with respect to atmospheric moisture, and I have always
found it greatest in the most fertile soils; so that it affords one
method of judging of the productiveness of land."

Where he erred was in overestimating the functions of the mechanical
properties of a soil, and in considering fertility to be due to them
alone.

During the next thirty years or so, little progress seems to have been
made in the way of fresh experimentation.


_Boussingault._

In 1834, Boussingault,[11] the most distinguished French agricultural
chemist of the century, began that series of brilliant chemico-agricultural
experiments on his estate at Bechelbronn, in Alsace, the results of which
have added so much to agricultural science. It was the first instance of
the combination of "science with practice," of the institution of a
laboratory on a farm; a combination peculiarly fitted to promote the
interests of agricultural science, and an example which has been since
followed with such magnificent results in the case of Sir John Lawes's
famous Rothamsted Experiment Station, and other less known research
stations.

Boussingault's first paper appeared in 1836, and was entitled, "The
amount of nitrogen in different kinds of foods, and on the equal value
of foods founded on these data."

In the year following other papers were published on such subjects as
the amount of gluten in different kinds of wheat; on the meteorological
considerations of how far various agricultural operations--such as
extensive clearings of wood, the draining of large swamps,
&c.--influence of climate on a country; and on experiments on the
culture of the vine.

Boussingault was the first observer to study the scientific principles
underlying the system of _rotation of crops_. In 1838 he published
the results of some very elaborate experiments he had carried out on
this subject. He also was the first chemist to carry out elaborate
experiments with a view to deciding the question of the assimilation by
plants of free atmospheric nitrogen. His first contribution to the
subject was published in 1838, but can scarcely be regarded as
possessing much scientific value, except in so far as it stimulated
further research. Some thirteen years later he returned to this
question; and during the years 1851-1855 carried out most elaborate
experiments, the results of which, until quite recently, were generally
regarded as having, along with the experiments of Messrs Lawes, Gilbert,
and Pugh, definitely settled the question.[12]

In 1839 Boussingault was elected a member of the French Institute, an
honour paid to him in recognition of his great services to agricultural
chemistry.[13]

The foregoing is a brief epitome of the history of the development of
agricultural chemistry up to the year 1840, the year which witnessed the
publication of one of the most memorable works on the subject, which has
appeared during the present century--Liebig's first report to the
British Association, a work which may be described as constituting an
epoch in the history of the science. Liebig's position as an
agricultural chemist was so prominent, and his influence as a teacher so
potent, that a few biographical facts may not be out of place before
entering upon an estimate of his work.


_Liebig._

Liebig was born at Darmstadt in the year 1803. He was the son of a
drysalter, and early devoted himself to the study of chemistry in the
only way at first at his disposal--viz., in an apothecary's shop. Soon
finding, however, his opportunities of study limited, he left the
apothecary's shop for the University of Bonn. He did not remain long at
Bonn, but in a short time left that university for Erlangen, where he
studied for some years, taking his Ph.D. degree in 1822. His subsequent
studies were carried on at Paris under Gay-Lussac, Thénard, Dulong, and
other distinguished chemists. Through the influence of A. Humboldt, who
was at that time in Paris, and whose acquaintance he was fortunate
enough to make, he was received into Gay-Lussac's private laboratory.
In 1824--that is, when he was only twenty-one years of age--he was
appointed Professor _Extraordinarius_ of Chemistry at the University of
Giessen. Two years later he was appointed to the post of Professor
_Ordinarius_--an appointment which he held for twenty-five years. In
1845 he was created Baron, and in 1852 appointed Professor at Munich. He
died in 1873.


_His First Report to British Association._

The report above referred to was made by Liebig at the request of the
Chemical Section of the British Association. It was read to a meeting of
the Association held in Glasgow in 1840, and was subsequently published
in book form, under the title of 'Chemistry in its Application to
Agriculture and Physiology,' Liebig's position, past training and
experience were such as to peculiarly fit him for the part of pioneer in
the new science. As Sir J. H. Gilbert has remarked,[14] "In the
treatment of his subject he not only called to his aid the previously
existing knowledge directly bearing upon his subject, but he also turned
to good account the more recent triumphs of organic chemistry, many of
which had been won in his own laboratory."

In his dedication to the British Association at the beginning of the
book, Liebig says: "Perfect agriculture is the true foundation of all
trade and industry--it is the foundation of the riches of States. But a
rational system of agriculture cannot be formed without the application
of scientific principles; for such a system must be based on an exact
acquaintance with the means of nutrition of vegetables, and with the
influence of soils and actions of manure upon them. This knowledge we
must seek from chemistry, which teaches the mode of investigating the
composition and of studying the characters of the different substances
from which plants derive their nourishment."


_His criticism of the "Humus" Theory._

The first subject which Liebig discusses is the scientific basis of the
so-called "humus" theory. The humus theory seems to have been first
promulgated by Einhof and Thaer towards the close of last century. Thaer
held that humus was the source of plant-food. He stated in his published
writings that the fertility of a soil depended really upon its humus;
for this substance, with the exception of water, is the only source of
plant-food. De Saussure, however, by his experiments--the results of
which he had published in 1804--had shown the fallacy of this humus
theory; and his statements had been further developed and substantiated
by the investigations of the French chemist Braconnot and the German
chemist Sprengel. Despite, however, the experiments of Saussure,
Braconnot, and Sprengel, the belief that plants derived the
carbonaceous portion of their substance from humus still seemed to be
commonly held in 1840.

While Liebig, therefore, can scarcely be said to have been the first to
controvert the humus theory, he certainly dealt it its death-blow. He
reasserted de Saussure's conclusions, and by some simple calculations
showed very clearly that it was wholly untenable. One of the most
striking of the arguments he brought forward was the fact that the humus
of the soil itself consisted of the decayed vegetable matter of
preceding plants. This being so, how, he asked, could it be the original
source of the carbon of plants? To reason thus was simply to reason in a
circle. He pointed out, further, that the comparative insolubility of
humus in water, or even in alkaline solutions, told against its
acceptance as correct.


_His Mineral Theory._

Having thus controverted the humus theory, he then goes on to deal with
the question of the source of the various plant constituents. In
treating of the relation of the soil to the plant, he puts forward his
"mineral" theory. It cannot be doubted that, while the advance of
science since Liebig's time has induced us to considerably modify his
mineral theory, it contained the statement of one of the most important
facts in the chemistry of plant physiology. He was the first to fully
estimate the enormous importance of the mineral portion of the plant's
food, and point the way to one of the chief sources of a soil's
fertility. Up to this period the ash constituents had been generally
considered to be of minor importance. By emphasising the contrary
opinion, and insisting upon their essentialness to plant-life, he gave
to agricultural research a fresh impetus upon the right lines. His
statement of his mineral theory was in the main true, but was not the
whole truth.

De Saussure, as has already been pointed out, to a certain extent,
anticipated Liebig's mineral theory. He was of the opinion that whatever
might be the case with some of the mineral constituents of plants,
others were necessary, inasmuch as they were always found in the ash. Of
these he instanced the alkaline phosphates. "Their small quantity does
not indicate their inutility," he sagaciously remarks. Sir Humphry Davy,
as has already been pointed out, missed recognising the true importance
of the ash constituents. It was left to Liebig, then, to restate the
important doctrine of the essentialness of the mineral matter, already
implied to some extent by de Saussure.

Liebig says: "Carbonic acid, water, and ammonia are necessary for the
existence of plants, because they contain the elements from which their
organs are formed; but other substances are likewise necessary for the
formation of certain organs destined for special functions, peculiar to
each family of plants. Plants obtain these substances from inorganic
nature."

While insisting on the importance of the mineral constituents, he did
so in a more or less general way not sufficiently distinguishing one
mineral constituent from another.

As all plants contained certain organic acids, and as these organic
acids were nearly always found in a neutral state--_i.e._, in
combination with bases, such as potash, soda, lime, and magnesia--the
plant must be in a position to take up sufficient of these alkaline
bases to neutralise these acids. Hence the necessity of these mineral
constituents in the soil. According to him, however, the exact nature of
the bases was a point of not so much importance. He assumed, in short,
as has been pointed out by Sir J. H. Gilbert, a greater amount of mutual
replaceability amongst the bases than can be now admitted.

Passing on to a consideration of the difference of the mineral
composition of different soils, he attributes this to the difference in
the rocks forming the soils. "Weathering" is the great agent at work in
rendering available the otherwise locked-up stores of fertility. He
attributes the benefits of fallow exclusively to the increased supply of
these incombustible compounds which were thus rendered available to the
plant. Treating of this subject, he says: "From the preceding part of
this chapter" (in which he has been explaining weathering) "it will be
seen that fallow is that period of culture when the land is exposed to
progressive disintegration by the action of the weather, for the purpose
of liberating a certain quantity of alkalies and silica, to be absorbed
by future plants."


_His Theory of Manures._

Treating of manures, he showed how the most important constituents of
manures were _potash_ and _phosphates_. In the first edition of his work
he also insisted on the value of _nitrogen_ in manures, condemning the
want of precautions, in the treatment of animal manures, against loss of
nitrogen.

In the later editions of his work he seems to have receded from that
opinion, and considered that there was no necessity for supplying
nitrogen in manures, since the ammonia washed down in rain was a
sufficient source of all the nitrogen the plant required. It was here
that Liebig went astray, first in denying the importance of supplying
nitrogen as a manure; and secondly, in overestimating the amount of
ammonia washed down in rain, which has subsequently been shown to be
entirely inadequate to supply plants with the whole of their
nitrogen.[15]


_His Theory of Rotation of Crops._

In explaining the benefits of the rotation of crops, Liebig propounded a
very ingenious theory, but one which was largely of a speculative
nature, and which has since been shown to be unfounded on any scientific
basis. It was to the effect that one kind of crop excreted matters which
were especially favourable to another kind of crop. He did not say
whether he considered such excretion positively injurious to the crop
which excreted them; but he inferred that what was excreted by the crop
was what was not required, and what could, therefore, be of little
benefit to a crop of the same nature following it.

The second portion of Liebig's report dealt with the processes of
fermentation, decay, and putrefaction.


_Publication of Liebig's Second Report to British Association._

In 1842 Liebig contributed his second famous report to the British
Association, subsequently published under the title of 'Animal
Chemistry; or, Organic Chemistry in its Applications to Physiology and
Pathology.' The publication of this report created even greater interest
than the publication of his first work. In it he may be said to have
contributed as much to animal physiology, as, in his first, he did to
agricultural chemistry. His subsequent principal works on agricultural
chemistry were--'Principles of Agricultural Chemistry,' published in
1855, and 'On Theory and Practice in Agriculture,' 1856.


_Liebig's services to Agricultural Chemistry._

An attempt has been made to sketch in the very briefest manner some of
the main points in Liebig's teaching, as contained in his famous report
to the British Association in 1840. Agricultural chemistry up till that
year can scarcely be described as having a distinct existence as a
branch of chemistry. Much valuable work, it is true, had already been
done, especially by his two great predecessors, de Saussure and
Boussingault; but it was, down to the year 1840, a science made up of
isolated facts. Liebig's genius formed it into an important branch of
chemistry, supplied the necessary connection between the facts, and by a
series of brilliant generalisations formed the principles upon which all
subsequent advance has been built.

As has already been indicated, Liebig's chief claim to rank as the
greatest agricultural chemist of the century does not rest upon the
number or value of his actual researches, but on the formative power he
exercised in the evolution of the science. His master-mind surveyed the
whole field of agricultural chemistry, and saw laws and principles where
others saw simply a confusion of isolated, and, in many cases, seemingly
contradictory facts.

But great as the direct value of Liebig's work was, it may be questioned
whether its indirect value was not even greater. The publication of his
famous work had the effect of giving a general interest to questions
which up till then had possessed a special interest, and that for
comparatively few. Both on the Continent and in England a very large
amount of discussion took place regarding his various theories.


_Development of Agricultural Research in Germany._

It was especially in Germany, however, that Liebig's work bore its
greatest and most immediate fruit. Thanks to the great chemist, the
German Government recognised the importance of forwarding scientific
research by State aid. Agricultural Departments were added to some of
the universities, largely at State expense, while agricultural research
stations were, one after another, instituted in different parts of the
country.

The first of the agricultural research stations to be founded was the
now famous one of Möckern, near Leipzig. It was instituted in the year
1851. Others followed, until at the present day there are some seventy
to eighty of these _Versuchs-Stationen_ scattered throughout Germany,
all well equipped and doing excellent work. Some idea of the activity of
the German stations may be inferred when it is stated that up to the
year 1877 the total number of papers embodying the results of their
experiments published by them amount to over 2000.[16]

To trace the development of agricultural chemistry, subsequent to
Liebig's time, in the way it has been done prior to the year 1840, is no
longer possible. This is due to the enormous increase in the number of
workers in the field, as also to the overlapping nature of their work,
which renders a strict chronological record wellnigh an impossibility.
It will be better, therefore, to attempt to give a brief statement of
our present knowledge on the subject, naming the chief workers in the
various departments of the subject.


_The Rothamsted Experiments._

Before doing so, it is fitting that reference should be made to the work
and experiments of two living English chemists, who have done much to
contribute to our knowledge in every branch of the science--viz., Sir
John Lawes, Bart., and Sir J. H. Gilbert, F.R.S.

The fame of the Rothamsted experiments is now world-wide; and no single
experiment station has ever produced such an amount of important work as
the magnificently equipped research station at Rothamsted. The
Rothamsted station may be said to date from 1843, although Sir John
Lawes was engaged in carrying out field experiments for ten years
previous to that date.[17] In 1843 Sir John Lawes associated with
himself the distinguished chemist Sir J. H. Gilbert, and the numerous
papers since published have almost invariably borne the two names. The
expense of working the station has been borne entirely by Sir John Lawes
himself; who has further set aside a sum of £100,000, the Laboratory,
and certain areas of land, for the continuance of the investigations
after his death. The fields under experimentation amount to about fifty
acres. By a Trust-deed, which was signed on February 14, 1889, Sir John
Lawes has made over the Rothamsted Experimental Station to the English
nation, to be managed by trustees.

It is impossible to enter, in any detail, into the nature and scope of
the Rothamsted experiments.[18] It may be stated that, since the year
1847, some eighty papers have been published on field experiments, and
experiments on vegetation; while thirty papers have been published
recording experiments on the feeding of animals.[19]

What has all along characterised these valuable experiments has been
their practical nature. While their aim has been entirely scientific,
the scale of the experiments and the conditions under which they have
been carried out, have been such as to render them essentially
_technical_ experiments. For this reason their results possess, and will
always possess, a peculiar interest for every practical farmer.

The greatest services the Rothamsted experiments have rendered
agricultural chemistry have been the valuable contributions they have
made to our knowledge of the function of nitrogen in agriculture; its
relation in its different chemical forms to plant-life; and the sources
of the nitrogen found in plants. Researches of a most elaborate nature
have been carried out on what is still one of the most keenly debated
questions of the present hour--viz., the relation of the "free" nitrogen
in the atmosphere to the plant. Of the very highest value also have been
the elaborate researches of Mr R. Warington, F.R.S., on the important
question of _Nitrification_, which have been in course in the Rothamsted
Laboratory for the last fifteen years, and to which full reference will
be made in the chapter on Nitrification.

To the Rothamsted experiments also we owe the refutation of Liebig's
mineral theory. In fact it may safely be said that no experimenters in
the field of agricultural chemistry have made more numerous or valuable
contributions to the science than these illustrious investigators.


_Review of our present Knowledge of Agricultural Chemistry._

Some attempt may now be made to indicate briefly our present knowledge
of the more important facts regarding plant physiology, agronomy, and
manuring.


_Proximate Composition of the Plant._

The great advance made in the direction of the improvement of the
accuracy of old analytical processes and the discovery of numerous new
ones have furnished us with elaborate analyses of the composition of
plants. We now know that the plant-substance is made up of a large
number of complex organic substances, formed out of carbon, hydrogen,
oxygen, and nitrogen,[20] and that these substances form, on an average,
about 95 per cent of the dry vegetable matter; the other 5 per cent
being made up of mineral substances. As to the source of these different
substances, our knowledge is, on the whole, pretty complete. With regard
to the carbon of green-leaved plants, which amounts to from 40 to 50 per
cent, subsequent research has confirmed Sénébier and de Saussure's
conclusions, that its source is the carbonic acid gas of the air. The
decomposition of the carbonic acid gas is effected by the leaves under
the influence of sunlight. That a certain quantity of carbon may be
obtained from the carbonic acid absorbed by plant-roots, is indeed
probable. Especially during the early stages of plant-growth this source
of carbon may be of considerable importance. Generally speaking,
however, it may be said of all green-leaved plants, that the chief
source of their carbon is the carbonic acid gas in the atmosphere.


_Carbon Fixation by Plants._

The exact way in which this decomposition of carbonic acid gas is
effected by the leaves is not yet clear. It seems to be directly
dependent, in some way or other, on the chlorophyll, or green colouring
matter. This decomposition of carbonic acid, and the fixation of the
carbon by the plant with the formation of starch, takes place only under
the influence of sunlight. During the night a reflex action takes place,
which is commonly known as _respiration_, and which is exactly analogous
to animal respiration.[21] The rate at which the fixation of carbon
takes place depends on the strength of the sun's rays. It seems to take
place very rapidly under a strong tropical sun.[22] The action of
sunlight on the absorption of carbon has been studied by a number of
observers, among others by Sachs, Draper, Cloez, Gratiolet, Caillet,
Prillieux, Lommel, &c.


_Action of Light on Plant-growth._

Experiments made by several observers, more especially Pfeffer, have
shown that the yellow rays of the solar spectrum are the most potent in
inducing this decomposition.

Some interesting experiments have been carried out by different
observers on the possibility of growing plants under the influence of
artificial light. While it would seem that the light from oil-lamps or
gaslight is unable to promote growth, except in very exceptional cases,
the electric light, or other strong artificial light, seems to be
capable of taking the place of sunlight. Heinrich was the first to show
that sunlight could be replaced by the magnesium light.

Experiments with the electric light have been carried out by
Hervé-Mangon in France and Dr Siemens in England. The plants grown under
the influence of the electric light were observed to be of a lighter
green colour than those grown under normal conditions, thus indicating a
feebler growth; in fact, Siemens was of the opinion that the electric
light was about half as effective as daylight.[23]

These experiments are interesting from an industrial point of view; for
it is conceivable that at some distant time electricity might be called
to the aid of the agriculturist.


_Source of Plants' Oxygen._

With regard to the source of the oxygen, which, next to carbon, is the
element most largely present in the plant's substance--amounting to,
roughly speaking, about 40 per cent--all evidence seems to indicate that
it is chiefly derived from water, which is also the source of the
plant's hydrogen. In addition to water, carbonic acid and nitric acid
may also furnish small quantities. It has been pretty conclusively
proved that the atmospheric oxygen, while necessary to plant-growth, and
promoting the various chemical vital processes, is not a direct source
of the plant's oxygen. The important function played by atmospheric
oxygen in certain stages of the plant's growth has been long recognised.
Malpighi, nearly two hundred years ago, observed that for the process of
germination atmospheric air was necessary; and shortly after the
discovery of the composition of the air was made, oxygen was identified
as the important gas in promoting this process. Oxygen is also
especially necessary during the period of ripening.


_Source of Plants' Hydrogen._

Hydrogen, which amounts to about 6 per cent, is, as has already been
pointed out, chiefly derived from water. It is possible that ammonia
also may form a source.


_Source of Plants' Nitrogen._

When we come to treat of the source of the nitrogen, which is found in
the plant's substance to an extent varying from a fraction of a per cent
to about 4 per cent, we enter on a much more debated question.

What is the source, or, what are the sources, of plant-nitrogen? is a
question to the solution of which more time and more research have been
devoted than to the solution of any other question connected with
agricultural chemistry.

The most obvious source is the free nitrogen, which forms four-fifths of
the atmospheric air. Reference has already been made to this
question.[24] Priestley was the first of the long list of experimenters
on this interesting question.

As far back as 1771 he affirmed that certain plants had the power of
absorbing free nitrogen; and this opinion he supported by the results of
certain experiments he had made on the subject. Eight years
later,--viz., in 1779--Ingenhousz further supported this conclusion, and
stated that all plants could absorb, within the space of a few hours,
noticeable quantities of nitrogen gas. The first to oppose this theory
was de Saussure, who, in 1804, carried out experiments which showed that
plants were unable to utilise free nitrogen.

Subsequent experiments, carried out by Woodhouse and Sénébier, supported
de Saussure's conclusions. Mention has already been made of
Boussingault's elaborate researches on the subject.[25] His first
experiments were carried out in 1838. He concluded that plants did not
absorb free nitrogen. Georges Ville was the first to reassert the older
theory, put forward by Priestley and Ingenhousz. His opinion was founded
on experiments he had carried out during the years 1849-52. The subject
created so much interest at the time, that a committee of the French
Academy--consisting of Dumas, Regnault, Péligot, Chevreul, and
Decaisne--were appointed to investigate Ville's experiments. The result
of the investigation of the Commission was to confirm Ville's
experiments. It is a significant fact, however, that the plant
experimented with by the Commission was _cress--a non-leguminous
plant_. It has been commonly assumed that the results of recent
experiments have confirmed Ville's experiments. It is only proper to
point out that this is not a necessary inference. The assimilation of
free nitrogen by the _leguminosæ_, so far as modern research has
revealed, only takes place under the influence of micro-organic life.
Ville's experiments, however, were supposed to be conducted under
_sterilised_ conditions.

In the meantime the results of Boussingault's second series of
experiments, carried out between the years 1851 and 1855, were
published, and confirmed his earlier experiments.

The results of a large number of experiments subsequently carried out
were in support of Boussingault's conclusions. Among them may be
mentioned Mène, Harting, Gunning, Lawes, Gilbert and Pugh, Roy,
Petzholdt, and Bretschneider.

Such an amount of overwhelming evidence might naturally have been
regarded as conclusively proving that the free nitrogen of the air is
not an available source of nitrogen to the plant. The question, however,
was not decided. In 1876 Berthelot reopened it. From experiments he had
carried out, he concluded that free nitrogen was fixed by various
organic compounds, under the influence of silent electric discharges. In
1885 he carried out further experiments, from which he concluded that
argillaceous soils had the power of fixing the free nitrogen of the
atmosphere. This they effected, he was of opinion, through the agency of
micro-organisms. Schloesing has recently shown that this fixation of
free nitrogen by soils is extremely doubtful.[26] The gain of nitrogen
observed under such conditions can be explained by the absorption by the
soil of combined nitrogen--viz., ammonia--from the air.

Berthelot's early experiments in 1876 had the effect of stimulating a
number of other experiments, with the result that we now possess the
solution of this long-debated and most important problem.

The names of the better known investigators on this subject, in addition
to Berthelot's, are those of Hellriegel, Wilfarth, Dehérain, Joulie,
Dietzell, Frank, Emil von Wolff, Atwater, Woods, Nobbe, Ward, Breal,
Boussingault, Wagner, Schultz-Lupitz, Fleischer, Pagnoul, Schloesing,
Laurent, Petermann, Pradmowsky, Beyrenick, Lawes, and Gilbert.

It is impossible to enter into the details of these most important
experiments. An attempt may be made, instead, briefly to epitomise them.


_Recent Experiments on Nitrogen question._

In the first place, it may be asked, How is it possible that the
previous elaborate experiments, published prior to 1876, should now
prove unreliable? A satisfactory explanation may be found in the fact,
as Lawes and Gilbert have recently pointed out, that the fixation of the
free nitrogen by the plant, or within the soil, takes place, if at all,
through the agency of electricity or of micro-organisms, or of both.
The earlier experiments, however, were so arranged as to exclude the
influence of either of those agencies.

The question has further been limited in its scope. It is now supposed
that only plants of the _leguminous_ order have the power of drawing
upon the free atmospheric nitrogen. Of the experiments above referred
to, those of Hellriegel and Wilfarth are the most striking and
important. They found in their experiments, that while the legumes have
the power of obtaining their nitrogen from the air, cereals have not.
Similar experiments by Atwater in America, and others, support this
conclusion.

Their conclusions may be briefly epitomised as follows:--

(_a_) That the leguminous plants--such as peas, &c.--have the power of
drawing their nitrogen supplies from the free nitrogen of the air in a
way not possessed by other plants; and that they thus possess two
sources of nitrogen--the soil and the air.

(_b_) That this absorption of free nitrogen is not effected directly by
the plant, but is the result, so to speak, of the joint action of
certain micro-organisms present in certain soils and in the plant
itself, (_symbiosis_).

(_c_) That this fixation is connected with the formation of minute
tubercles on the roots of the plants of the leguminous class; and that
these tubercles may be the home of the fixing organism.

(_d_) That these fixing micro-organisms are not present in all
soils.[27]

While the relation of free nitrogen to the plant has long been, and
still is, a very obscure problem, it was early recognised that the
combined nitrogen present in soils and manures was an important source
of plant-food. Reference has already been made to the early theory of
Sir Kenelm Digby regarding the value of nitrates.[28] De Saussure, as we
have also already seen, was fully impressed with the importance of
applying nitrogen to the soil as a manure. Liebig's early attitude on
this question was to the effect, that to apply nitrogen in manures was
quite unnecessary, as the plant had a sufficient source in the ammonia
present in the air, which he erroneously supposed was sufficient in
quantity to supply all the needs of the crops. Despite this early
recognition of the value of combined nitrogen to the plant, it is only
of recent years that we have obtained any definite knowledge as to the
respective value of its different compounds as manures, or as to the
form in which it is assimilated by the plant. It exists in three
forms--(1) as organic nitrogen; (2) as ammonia salts; (3) as nitrates
and nitrites. Much experimental work has during late years been devoted
to studying the comparative action and merits of these three forms.


_Relation of Organic Nitrogen to the Plant._

First, as to the relation of organic nitrogen to the plant. There is a
large number of different organic compounds which contain nitrogen. That
the plant is able to assimilate certain of these organic compounds,
seems, from several experiments, to be extremely probable. From certain
researches, carried out as far back as the year 1857, Sir Charles
Cameron concluded that the plant could assimilate one of them--viz.,
_urea_. From what, however, we have subsequently learned regarding the
process of "nitrification," it is quite probable that the nitrogen in
these experiments was first converted into nitrates before being
assimilated. At any rate, as the plants were not tested for urea, the
experiments must be regarded as leaving the problem unsolved.

Other experiments were carried out of a similar nature by Professor S.
W. Johnson, the different kinds of nitrogen experimented with being
_uric acid_, _hippuric acid_, and _guanine_. But here, again, no
definite conclusion can be drawn, as no analyses were made of the
plants. More recently, however, Dr Hampe has carried out experiments
with _urea_, _uric acid_, _hippuric acid_, and _glycocoll_. These
experiments may be held as demonstrating the fact that at least one
organic compound of nitrogen is capable of being assimilated, as urea
was actually identified as being present in the plants experimented
with. From further experiments, carried out by Dr Paul Wagner and
Wolff, _glycin_, _tyrosin_, and _kreatin_ are able to be assimilated by
the plant.


_Plants able to absorb certain Forms of Organic Nitrogen._

We may conclude, then, from these interesting experiments, that plants
are able to absorb certain organic forms of nitrogen. That they do so in
nature to any extent is extremely improbable, such organic forms of
nitrogen being rarely present in the soil, or if present, being
converted into ammonia or nitrate salts before assimilation.


_Nature of Humus in the Soil._

While on the subject of organic nitrogen, reference may be briefly made
to that substance known as _humus_,--the name applied to the organic
portion of soils,--a substance which figures so largely in early
theories of plant-nutrition. The most elaborate investigation of the
composition of humus has been carried out by Mulder. According to
Mulder, it is composed of a number of organic bodies, and he has
identified the following substances--ulmin, humin, ulmic, humic, geic
acids, &c. These bodies are composed of carbon, hydrogen, and oxygen,
which are invariably associated with nitrogen. Detmer and Simon have
further investigated the subject. The true function of humus, it would
seem, in addition to its numerous mechanical properties, is to furnish,
by its decomposition, carbonic acid and nitrogen--in the form of
ammonia and nitric acid--to the soil; the former acting as a solvent of
the mineral food, the latter as the source of the plant's nitrogen. The
old theory, therefore, that the presence of humus in a soil is a
condition of fertility, is not so far removed from the truth. Where
there is an abundance of humus in the soil there is likely also to be an
abundance of nitrogen.


_Relation of Ammonia to the Plant._

It seems to be beyond doubt that nitrogen is directly absorbed by plants
in the form of ammonia. Liebig, as we have seen, concluded that this was
the great source of nitrogen for the plant, and that the ammonia
compounds present in the air were an all-sufficient supply. Subsequent
research, while confirming his belief so far as regards the capability
of plants to assimilate nitrogen in the form of ammonia, has proved that
the amount of ammonia present in the air is very minute, and utterly
inadequate to supply the plant with the whole of its nitrogen.
Investigations have been made on this subject by Graeger, Fresenius,
Pierre, Bineau, and Ville. According to Ville's researches, which are
among the most recent, the amount does not exceed 30 _parts per thousand
million parts of air_.[29] Some conception of the value of this source
of nitrogen may be gained by estimating the quantity falling, dissolved
in rain, on an acre of soil throughout the year. Various estimations of
the total amount of combined nitrogen, which is in this way brought to
the soil, have been made. A certain amount of discrepancy, it is true,
is to be found in these various estimations, no doubt largely due to the
difference in the circumstances under which the investigations were
carried out. Mr Warington has made several investigations at Rothamsted,
and, according to his most recently published figures, the total
quantity only amounts to 3.37 lb. per acre per annum--of which only 2.53
lb. is as ammonia itself.[30]

As already mentioned, there can be little doubt that plants can absorb
nitrogen in the form of ammonia. The question of how far plant-leaves
are able to absorb ammonia is a much debated one. It is probable that if
they can do so, it is only to a very small extent.[31] The question as
to whether the plant's roots can absorb ammonia or not, is also a very
keenly debated one. The point is a very difficult one to decide, and is
much complicated by the consideration that ammonia, when applied to the
the soil, is so speedily converted into nitric acid. Despite, however,
these difficulties, and the vast amount of controversy on the point, the
experiments of Ville, Hosäus and Lehmann, seem to indicate beyond doubt
that ammonia is a direct source of nitrogen. Lehmann's experiments would
seem, further, to indicate that there are certain periods of a plant's
growth when its preference for ammonia salts seems to be greater than at
other times. The point, however, it must be confessed, is still an
obscure one. The great difficulty in deciding it, as has just been said,
lies in the fact that ammonia salts, when applied to a soil, are, by the
process of nitrification, converted into nitrates. In experimenting,
therefore, with ammonia, and noting the results, it is wellnigh
impossible to say, except by subsequent analyses, whether the nitrogen
in the ammonia salts has not been converted into nitrates before
assimilation.


_Relation of Nitric Acid to the Plant._

Thirdly, as to nitrogen in the form of nitrates. While it is true that
plants can absorb nitrogen in certain organic forms and as ammonia
salts, it is now a well-known fact that the chief, and by far the most
important, source of nitrogen is nitric acid. Probably more than 90 per
cent of the nitrogen absorbed by green-leaved plants from the soil is
absorbed as nitrates. The tendency of all nitrogen compounds in the soil
is towards conversion into nitric acid. It is the final form of nitrogen
in the soil. The precise method in which this conversion takes place is
a discovery of only a few years' standing. The great economic importance
of this discovery, made by the French chemists Schloesing and Müntz, and
associated in this country with the names of Warington, Munro, and P. F.
Frankland, is only gradually being appreciated. It is without doubt one
of the most interesting made in the domain of agricultural chemistry of
late years.


_Nitrification._

It was in the year 1877 that the two French chemists above referred to
published the results of some experiments they had carried out, which
proved that nitrification--the name given to the process by which
ammonia or other nitrogen salts are converted in the soil into nitric
acid--was due to the action of micro-organic life.

The basis of the theory rests upon the fact that dilute solutions of
ammonia salts or urine, containing all the necessary constituents of
plant-food, if previously sterilised, may be kept for an indefinitely
long period of time, provided the air supplied be filtered through
cotton wool,--so as to prevent the entrance of micro-organisms--without
any formation of nitrates. Introduce, however, into such a solution a
little fresh soil, and nitrification will soon follow.

The conditions under which the nitrification ferment acts, as well as
the nature of the ferment, or rather ferments, have subsequently been
carefully studied by Schloesing and Müntz, Winogradsy, Dehérain,
Kellner, and other Continental observers, and especially by Warington,
Munro, and P. F. Frankland in this country. These conditions cannot be
gone into here. They will be fully discussed in the chapter on
Nitrification. Briefly stated, they are a certain range of temperature
(between slightly above freezing-point and 50° C., the maximum activity
taking place, according to Schloesing and Müntz, at about 30° C.); a
plentiful supply of atmosphere oxygen (hence the fact observed by
Warington, that nitrification is chiefly limited to the surface-soil); a
certain amount of moisture; and the presence of certain of the necessary
mineral plant constituents, and the presence of carbonate of lime.

The light which these discoveries throw upon the extremely complicated
question of the fertility of the soil is considerable, as it follows
that no soil can be regarded as really a fertile one in which the
process of nitrification does not freely take place. They furthermore
explain many facts, hitherto observed but not well understood, with
regard to the action of different nitrogenous manures.


_Ash Constituents of the Plant._

We now come to consider the present state of our knowledge on the
essentialness of the ash or mineral portion of the plant. While a
portion of the plant's substance which, up to Liebig's time, had
obtained little notice, it has, since the publication of his famous
"mineral" theory, obtained an ever-increasing amount of investigation.

Up till 1800 practically nothing was known of the function of the ash
constituents. In 1802 de Saussure wrote that it was unknown whether the
constituents of many plants were due to the soils on which they grew, or
whether they were the products of vegetable growth. Some two years
later, however, he was enabled to carry out a number of experiments
which really placed the subject on a firm scientific basis. The
essentialness of the ash constituents was only, however, placed beyond
all doubt by Wiegmann and Polstorff's researches, carried out in 1840.

Reference has already been made to the great stimulus given to research
by the promulgation of Liebig's mineral theory.


_Methods of Research._

In epitomising the vast amount of work carried on since 1840, with the
view of ascertaining the essentialness of the various substances found
in the ash of plants, two methods of experimentation have been
followed.


_Artificial Soils._

The first of these two methods was that adopted in the famous
experiments, carried out by Prince Salm-Horstmar, which have done so
much to further our knowledge on this question. It consisted in growing
plants on an artificial soil--formed out of sugar-charcoal, pulverised
quartz or purified sand--to which were added the different food
constituents.


_Water-culture._

While the results obtained by Prince Salm-Horstmar by this method were
of a most valuable nature, subsequent experimenters have abandoned his
method for the other method--viz., "water-culture." The medium used in
this process is pure water; and it is from experiments carried out in
water-culture that much of our present knowledge, in regard to the
relation of the ash constituents to the plant, is due.

The names of those who have worked in this department are very numerous.
Among them may be mentioned Knop, Sachs, Stohmann, Nobbe, Rautenberg,
Kühn, Lucanus, W. Wolff, Hampe, Beyer, E. Wolff, P. Wagner,
Bretschneider and Lehmann. The results obtained by these and other
experimenters have demonstrated the following facts.

The substances which have been found in the ash of plants are: _potash_,
_soda_, _lime_, _magnesia_, _oxide of iron_, _oxide of manganese_,
_phosphoric acid_, _sulphuric acid_, _silica_, _carbonic acid_,
_chlorine_, _lithia_, _rubidia_, _alumina_, _oxide of copper_,
_bromine_, _iodine_, and occasionally even other substances. Of these,
however, only six are probably absolutely necessary for
plant-growth--viz., _potash_, _lime_, _magnesia_, _oxide of iron_,
_phosphoric acid_, and _sulphuric acid_. Three other substances seem
also to be almost invariably present, and may possibly be essential--in
very minute quantities at any rate--viz., _chlorine_, _soda_, and
_silica_. With regard to _alumina_ and _oxide of copper_, these
constituents must be regarded as accidental; while _iodine_ and
_bromine_ only occur in the ash of marine plants.


_Method of Absorption of Plant-food._

A department of vegetable physiology which has had much work devoted to
it is the method in which plant-roots absorb their food. The plant's
nourishment is absorbed in solution by means of the roots. Its
absorption takes place, according to Fischer and Dutrochet, who have
investigated the subject at great length, by the process known as
_endosmosis_. It has also been established by numerous experiments, that
different plants require different constituents in different
proportions.


_Water as a Carrier of Plant-food._

The function performed by water, as the carrier of plant-food, and the
motion of the sap of the plant, are questions which have also received
much attention. The motion of the plant's sap seems to have attracted a
great deal of attention at a very early stage of the study of plant
physiology. As far back as 1679, Marriotte studied it. Among other old
experimenters were Hales, Guettard, Sénébier, Saint-Martin, de Candolle,
and Miguel. In more recent times, it has been investigated by Schübler,
Lawes and Gilbert, Knop, Sachs, Unger, and Hosäus. Some idea of the
enormous amount of water transpired by plant-leaves may be gained by the
statement that from 233 lb. to 912 lb. of water are transpired for every
pound of plant-tissue formed.[32]


_Agronomy._

When we come to deal with questions relating to the chemistry of the
soil, we find that so much investigation has been devoted to this one
branch of agricultural chemistry as to constitute it a special branch by
itself--known in France under the name of _agronomie_--and being taught
in the large agricultural colleges by special professors of the subject.
The value of studying the properties of soils was recognised at an early
period. This study was for long largely confined to their _physical_,
or, what are popularly known as their _mechanical_ properties. Thus Sir
Humphry Davy ascertained many important facts with regard to the heat
and water absorbing and retaining properties of soils.


_Retention by Soil of Plant-food._

It was not till a later period that the power soils possess of fixing
from their watery solutions various plant-foods, both organic and
inorganic, was discovered. The earliest recognition of this most
important property of soils was made by Gazzeri, who, in 1819, called
attention to the fact that the dark fluid portion of farmyard manure was
purified on passing through clay. He concluded that soils, more
especially clayey soils, possessed the property of being able to fix
from their watery solutions the necessary plant-food constituents, and
fix them beyond risk of loss, only affording a gradual supply to the
plant as required.

The first experiments carried out on this subject were those by Huxtable
and Thompson in 1850. The liquid portion of farmyard manure was filtered
through soil and subsequently examined, when it was found to have not
only lost its colour, but also to have lost its smell. Ammonia and
ammonia salts were also experimented with, and it was found that soils
possessed the power of fixing ammonia.

To Thomas Way, however, we are indebted for the most valuable
contribution on this important subject made by any one single
investigator. His experiments were not merely carried out with regard
to ammonia, but also with regard to other bases--such as potash, lime,
magnesia, soda, &c. Since Way's experiments much work has been done by
Liebig, Stohmann, Henneberg, and Heiden, as also by Voelcker, Eichhorn,
Knop, Rautenberg, Pochwissnew, Warington, Beyer, Bretschneider, Sestini,
Laskowsky, Strehl, Pillnitz, Peters, W. Wolff, Lehmann, and Biedermann.


_Bases and Acids fixed by Soil._

From these experiments it may be taken as proved beyond doubt that soils
have the power of fixing, to a greater or less extent, the following
bases: ammonia, potash, lime, magnesia and soda; as well as the two
acids, phosphoric and silicic. The order in which the different bases
are fixed is an important point. It would seem that the soil has a
greater affinity for the more valuable manurial substances, such as
ammonia, potash, and lime, and that these substances are first fixed.
That in fixing any one of the above-mentioned bases from its solution,
it can only do so at the expense of another base. Thus, in fixing
potash, either lime, magnesia, or soda must be given up. Further, when a
base in solution, as sulphate or chloride, is absorbed by a soil, the
base is alone fixed, while the sulphuric acid or chlorine is left in
solution. Lastly, the amount of base absorbed by a soil depends on the
concentration of its solution, on the nature of its combination, and the
temperature. Way found in his experiments that a clay soil has more
power than a peaty soil, and that a peaty soil has more power than a
sandy soil.


_Causes of this Fixation._

So much for the fact of soil absorption; as to the cause or causes of
this absorption, a great number of theories have been put forward. Those
may be divided into two classes--those accounting for it as due to
physical properties of the soil; and those, on the other hand,
explaining it as due to chemical action.

To the latter class Way's belonged. He explained it as due to the
formation in the soil of hydrated double silicates, consisting of a
silicate of alumina, along with a silicate of the base fixed. Brüstlein
and Peters, on the other hand, were of the opinion that it was purely
physical in its nature. A theory has been advanced that it is due to the
formation of insoluble ulmates and humates, formed by the union of ulmic
and humic acids, along with the bases fixed. Among others who devoted
investigation to this interesting question, may be mentioned Rautenberg
and Heiden.

On reviewing the evidence, it seems to be pretty well established that
it really is mainly a chemical act, due chiefly to the formation of
double silicates, and doubtless to a certain extent to the formation of
insoluble humates and ulmates. Heiden's experiments would seem to
indicate, however, that it is also partly of a physical nature.

With regard to the absorption of phosphoric acid, this has been shown to
be a chemical act, and depends on the formation of insoluble phosphates
of calcium, iron, aluminium, and magnesium, the percentage of iron
especially determining this.

Much analytical work has been accomplished of late years with a view of
ascertaining the amount of ash in different kinds of plants, and in the
different parts of the plant.


_Action of Manures._

The department of agricultural chemistry which has been most largely
developed of late years is that connected with the problems of
_manuring_. It is, from a practical point of view, of most value. It is
some considerable time since we have recognised that the only three
ingredients it is, as a rule, expedient to apply as artificial manures,
are _nitrogen_, _phosphoric acid_, and _potash_. The nature, mode of
action of the different compounds, and properties of these three
substances, and their comparative influence in fostering plant-growth,
together with the economic question of which form is, under various
circumstances, the most economical for the farmer to use, have together
given rise to a large number of "field" and "pot" experiments. As the
principles underlying this practice form the subject of the following
treatise, any further discussion of the question must be left to the
following chapters.

    _Note._--The reader interested in the historical development of
    agricultural chemistry is referred to Sir J. H. Gilbert's
    Presidential Address to the Chemical Section of the British
    Association, 1880.


FOOTNOTES:

[1] The History of the Chemical Elements. By Sir Henry E. Roscoe, F.R.S.
(Wm. Collins, Sons, & Co.)

[2] Van Helmont's science was, however, of an extremely rudimentary
nature, as may be evidenced by the belief he entertained that the smells
which arise from the bottom of morasses produce frogs, slugs, leeches,
and other things; as well as by the following recipe which he gave for
the production of a pot of mice: "Press a dirty shirt into the orifice
of a vessel containing a little corn, after about twenty-one days the
ferment proceeding from the dirty shirt, modified by the odour of the
corn, effects a transmutation of the wheat into mice." The crowning
point in this recipe, however, lay in the fact that he asserted that he
had himself witnessed the fact, and, as an interesting and corroborative
detail, he added that the mice were born full-grown. See 'Louis Pasteur:
His Life and Labours.' By his Son-in-law. Translated by Lady Claud
Hamilton. (Longmans, Green, & Co.) P. 89.

[3] He then goes on to relate a number of experiments by Cornelius
Drebel and Albertus Magnus, showing the refreshing power of this balsam,
and then those of Quercitan with roses and other flowers, and his own
with nettles.

[4] Priestley, however, did not realise that _carbonic acid gas_ was a
necessary plant-food; on the contrary, he considered it to have a
deleterious action on plant-growth. Percival was really the first to
point out that carbonic acid gas was a plant-food.

[5] It is recorded as an instance of the scientific enthusiasm of the
man, that he was wont to carry about with him bottles containing oxygen,
which he had obtained from cabbage-leaves, as also coils of iron wire,
with which he could illustrate the brilliant combustion which ensued on
burning the latter in oxygen gas.

[6] For a full account of Sénébier's researches, see 'Physiologie
végétale, contenant une description des organes des plantes, et une
exposition des phénomenes produits par leur organisation, par Jean
Sénébier.' (5 tomes. Genève, 1800.)

[7] How Crops Grow. By Professor S. W. Johnson. Macmillan & Co.
(Introduction, p. 4.)

[8] See p. 40 to 45.

[9] Elements of Agricultural Chemistry, in a course of Lectures for the
Board of Agriculture. By Sir Humphry Davy. (London, 1831.)

[10] This department of agricultural research was subsequently carried
on by Sprengel, Schübler, and others.

[11] Born in Paris, 1802; died 11th May 1887.

[12] See p. 40.

[13] While much of Boussingault's work was carried out previous to the
year 1840, he continued to enrich agricultural chemistry with numerous
valuable contributions up till the time of his death. It may be well
here to mention the names of his most important contributions to
agricultural science, made subsequent to 1840.

In 1843 he published, in a work entitled 'Economie Rurale,' the results
of his numerous experiments and researches. This work is well known to
English agriculturists from an English translation which appeared in
1845 (Boussingault's 'Rural Economy,' translated by G. Law. H. Ballière,
London).

In 1860 appeared the first volume of his last great work, 'Agronomie
Chimie Agricole et Physiologie' This work, which consisted of seven
volumes, was not finished till 1884. He died on the 11th of May 1887. It
may be added that the Royal Society of London awarded him the Copley
medal in 1887.

[14] See British Association Proceedings, 1880, p. 511.

[15] It may be pointed out that, while the amount of ammonia washed down
by the rain is small, Schloesing has found in some recent experiments
that a damp soil may absorb from the air in the course of a year 38 lb.
of combined nitrogen, chiefly ammonia, per acre. See p. 132.

[16] The example, set by Germany, has been followed by other countries
in which well-equipped research stations now exist. Perhaps the most
striking example of the rapid development of the means of agricultural
research is furnished by the United States of America. At present over
fifty agricultural experiment stations, more or less well equipped,
exist at present in that country, all liberally supplied by State aid.
The earliest to be founded, it may be added, was that at Middletown,
Connecticut, the date of its institution being 1875.

[17] It may thus claim to be the second oldest experimental station,
that instituted by Boussingault at Bechelbronn in Alsace being the
oldest.

[18] For an account of the Rothamsted experiments, and a short biography
of Sir John Lawes, the reader is referred to a pamphlet by the present
writer, entitled 'Sir J. B. Lawes, Bart., LL.D., F.R.S., and the
Rothamsted Experiments' ('Scottish Farmer' Office, 93 Hope Street,
Glasgow).

[19] Of these numerous elaborate experiments, perhaps those which have
attracted the most widespread interest amongst agriculturists have been
those carried out on the growth of wheat on the same land year after
year for a period of nearly fifty years. The important light which this
series of experiments has thrown upon the theory of the rotation of
crops, and the subject of the manuring of cereals, is very great.

[20] Associated in some cases with phosphorus and sulphur.

[21] It must be pointed out that plant-respiration does not take place
_only_ during the night-time. It probably goes on at all times, but it
is only during the night-time that its action is apparent, as the
reverse process of carbon assimilation, which goes on at an incomparably
greater rate, masks its action during the daytime.

[22] The length of the day has an important influence on plant-growth,
as is evidenced by the rapid growth of vegetation in Norway and Sweden.
In these countries there is a late spring, and a short and by no means
hot summer, but a very long period of daylight.

[23] A point of great interest which these experiments elucidated is
that nocturnal repose is not absolutely necessary for the growth and
development of all plants.

[24] See pp. 15 and 22.

[25] See p. 22.

[26] See Chapter III., pp. 120 and 131.

[27] Further reference is made to this subject in Chapter III., p. 136.

[28] See p. 6.

[29] See Phil. Trans., Part II., 1861, pp. 444-446. Lawes & Gilbert.
Schloesing has found in the air in the neighbourhood of Paris 1 lb. of
ammonia in 26,000,000 cubic yards; while Müntz found only about half
that amount in a similar quantity of air on the top of the Pic du Midi.

[30] See Chapter III., pp. 119, 120; Appendix, p. 155.

[31] Some recent experiments by Dyer and Smetham would seem to show that
comparatively small quantities of ammonia in the air prove actually
hurtful to plant-life. Thus they found that one volume of ammonia in
1000 volumes of air was fatal to hardy plants; while one volume in 3000
volumes killed tender ones.

[32] According to the experiments of Hellriegel and Wollny. The
quantity, it may be added, varies with the leaf-surface and the length
of the period of growth of the plant. It is greatest with clovers and
grasses, and least in the potatoes and roots.



PART II.

PRINCIPLES OF MANURING



CHAPTER I.

FERTILITY OF THE SOIL.


It is necessary to clearly understand to what the fertility of a soil is
due ere we can hope to master the theory of manuring.


_What constitutes Fertility in a Soil._

The question, What constitutes fertility in a soil? is by no means an
easy one to answer. If we say, The presence of a plentiful supply of the
constituents which form the plant's food, our answer will be incomplete.
Similarly, if we reply, A certain physical condition of the soil--here,
again, it will be found equally unsatisfactory; for fertility of a soil
depends both on its physical condition and on its chemical composition,
and indeed even on other circumstances. It may be well, then, before
proceeding to treat of the nature and action of the different manures,
to offer a brief statement of the conditions of fertility so far, at any
rate, as we at present know them. For it may be well to warn the reader
that, despite the great amount of work carried out on this subject by
experimenters, we still have much to learn before we shall be in a
position fully and clearly to understand the subject of soil-fertility
in all its bearings.

Apart altogether from the influence exerted by climate, latitude,
altitude, and exposure, the fertility of a soil may be said to depend on
the following properties. These we may divide into three groups or
classes:--

1. Physical or mechanical.
2. Chemical.
3. Biological.


=I. Physical Properties of a Soil.=--The physical properties of a soil
are generally admitted to have a very important bearing on its
fertility. This has been long practically recognised, and perhaps has in
the past been unduly exalted in importance, at the expense of the no
less important functions of the chemical.[33] The reason of this is
doubtless to be ascribed to the fact that it is much easier to study
the physical properties of a soil than it is to study the chemical; and
that, while we are in possession of a very large amount of useful
information with regard to the former, we are at present only on the
threshold of our knowledge of the latter.


_Variety of Soils._

It is a matter of common observation that soils differ widely in their
mechanical nature. The early recognition of this fact is evidenced by
the large number of technical terms which have been long in vogue among
farmers descriptive of these differences. Thus soils are in the habit of
being described as "heavy," "light," "stiff," "strong," "warm," "cold,"
"wet," "damp," "peaty," "clayey," "sandy," "loamy," &c., &c.


_Absorptive Power for Water._

One of the most important of the physical properties of a soil is its
power to absorb water.

Water to the plant economy is just as important and necessary as it is
to the animal economy. Consequently it is of primary importance to
examine into the conditions which regulate the absorption of this
important plant-food by the soil.

By the absorptive power of a soil is meant its capacity for drinking in
any water with which its particles may come in contact. This power
depends, first, on the predominance of its proximate constituents--viz.,
_sand_, _clay_, _carbonate of lime_, and _humus_; and secondly on the
fineness of the soil-particles.


_Absorptive Power of Sand, Clay, Humus._

First, then, with regard to the absorptive power of sand, clay, and
humus. Of these, sand possesses this power to the least extent, clay to
a greater extent, while humus possesses it most of all.[34]

The extent, therefore, of the absorptive power of a soil depends very
much on the proportions in which it possesses these three ingredients.
The more sandy a soil is, the less will its power be of absorbing water;
and this, there is little doubt, is one of the reasons why a sandy soil
is, as a rule, an unfertile soil. Of course there are other and even
more important reasons; but that this absorptive power has an important
bearing on the question is conclusively proved by the fact that sandy
soils are more fertile in a climate where rain is frequent than in one
where much dry weather prevails. The incapacity of a sandy soil to
absorb a large quantity of moisture is not fraught with such evil
effects to the crops in the former case, because it is counteracted by
the climatic conditions, which obviate the necessity, in a soil, of
possessing great absorptive powers.

The converse, of course, we may mention in passing, holds good of clayey
soils.


_Fineness of Soil-particles._

The second quality in a soil on which its absorptive power depends is
the fineness of its particles. The great benefit which a soil derives
from a good tilth, in this respect, was one of the reasons why Tull's
system of horse-hoeing husbandry was so successful in its results.[35]
The finer the soil-particles, it may be said generally, the greater is
the absorptive power of the soil.


_Limit to Fineness._

There is, however, a limit to the fineness to which the particles of a
soil ought to be reduced; for it has been found by experiment that when
a certain degree of fineness is reached, the absorptive power decreases
with any further pulverisation. A German experimenter found, for
example, that a garden loam, capable of absorbing 114 per cent of water
in its natural state, when pulverised very fine was able to absorb only
62 per cent of water. Here, clearly, the limit to which it is advisable
to pulverise a soil had been exceeded.


_Reason of the above._

It is not difficult to see why this should be so. The amount of water
that a soil can soak up is due to the number of pores, or air-spaces, it
contains of a certain size. If these pores are large and few in number,
the amount of water absorbed will be naturally less than when they are
numerous and smaller in size. Up to a certain extent, the more a soil is
broken the greater will be the number of pores created, of a size to
permit the water to soak in. Beyond that point the pores become too
minute, and the soil becomes too compact, each particle clinging
together too closely.


_Retentive Power of Soils for Water._

Now closely connected with this absorptive power of soils, which we have
just been considering, is the power soils possess of holding or
retaining the water they absorb. This power, it will be seen at a
glance, must have an important bearing on the fertility of a soil.


_Importance of Retentive Power._

As a considerable interval often elapses between the periods of
rainfall, soils, if they are to support vegetable growth, must be able
to store up their water-supply against periods of drought. This is all
the more necessary when we remember that, in the case of heavy crops,
the rainfall would often be inadequate to supply the water necessary for
their growth. In fact, it has been estimated that the average
evaporation from soils bare of any cultivation is equal to the rainfall.
That the evaporation from soils covered with vegetation is very much
greater, has been strikingly shown by a calculation made by the late
eminent American botanist, Professor Asa Gray, who calculated that a
certain elm-tree offered a leaf-surface, from which active transpiration
constantly went on, of some five acres in extent; while it has further
been calculated that a certain oak-tree, within a period of six months,
transpired during the daytime eight and a half times more water than
fell as rain on an area equal in circumference to the tree-top.[36] Just
as the state of the fineness of the soil-particles has an important
influence on the absorptive power of soils, so, too, it is found, it has
an important bearing on the rate at which evaporation takes place.
Evaporation goes on to the greatest extent in soils whose particles are
compacted together, capillary action in this case taking place more
freely, and effecting evaporation from a greater depth of soil. The
stirring of the surface portion of the soil, as for example by hoeing or
harrowing, has for this reason an important influence in lessening the
amount of evaporation, and minimising the risks of drought, by breaking
the capillary attraction. The amount of evaporation which takes place
from a soil covered with a crop, depends largely on the nature of the
crop; a deep-rooted crop, since it draws its moisture from a wider area
of soil, being more effective in drying a soil than a shallow-rooted
crop. The difference in the amounts evaporated from a cropped and a bare
fallow soil has been shown at Rothamsted to equal a rainfall of nine
inches, the crop being barley. The increase, of course, is due to the
water which the crop transpires.[37]

It may be generally said that the greater the absorptive power of a
soil, the greater is its retentive power; for soils that most largely
absorb water are the most reluctant to part with it.

While these properties are undoubtedly necessary for fertile soils, it
is needless to add that they may be possessed by a soil to too great an
extent. The soil that is unable to throw off any excess of water becomes
cold and damp, and does not admit of proper tillage. Its pores become
entirely choked up, and the circulation of air, which, as we shall see,
is of so much importance, is rendered impossible. Plants in such a soil
are apt to sicken and die, the water becomes stagnant, and certain
chemical actions are caused which give rise to poisonous gases, such as
sulphuretted hydrogen, &c. A stiff clayey soil offers a good example of
the disadvantage of over-retentiveness. Owing to the difficulty such
soils experience in throwing off their excessive water, they are
extremely difficult to till; and sowing operations are on that account
apt to be delayed.


_Power Plants have of absorbing Water from a Soil._

It is a strange fact, and one worth noticing in this connection, that
the power plant-roots have of drawing their moisture from a soil, seems
to depend on the retentive power of the soil. By this is meant that
plants have not the means of exhausting the water in a retentive soil to
such an extent as in a non-retentive soil.

In some extremely interesting experiments, carried out by the well-known
German botanist Sachs, it was found that plants wilted in a loamy soil,
whose water-holding capacity was 52 per cent, when its moisture reached
8 per cent; while in a sandy soil--water-holding capacity 21 per
cent--the same species of plant did not wilt until its moisture reached
1-1/2 per cent. Here, then, we see that on one kind of soil the plant
was able to live, and obtain sufficient water for its needs, while it
died of thirst in another soil, although that soil contained quite as
much moisture.

Speaking generally, we may say that Hellriegel's experiments have shown
that any soil can supply plants with all the water they need so long as
its moisture is not reduced below one-third of the whole amount it can
hold.[38]


_How to increase Absorptive Power of Soils._

The absence or presence, in excess, of the above properties, suggests a
word or two on how these natural defects may, to a certain extent, be
remedied artificially. It stands to reason, that if organic matter in a
soil renders its absorptive power greater, a simple method of improving
a soil defective in this property is by the addition of organic matter.
One of the benefits of ploughing-in green crops on sandy soils is
undoubtedly due to this fact; the addition of farmyard manure having
also a similar effect. The absence of a sufficient amount of
retentiveness, such as is found in sandy soils, in the same way
suggests, as a remedy, the addition of clay; and, _vice versâ_, where
the soil is too clayey, the natural method of improvement will be the
addition of sand.[39]


_Shrinkage of Soils._

In drying, soils shrink. Those which shrink least are sandy and chalky
soils. Humus soils, on the other hand, shrink most.


_Most favourable Amount of Water in a Soil._

The amount of water in a soil most favourable for plant-growth is a
question of considerable difficulty. Too great an amount of moisture
renders the land cold; air cannot obtain access to the soil-particles,
and the plants sicken and die. Hellriegel has found that as much as 80
per cent of what the soil can hold is hurtful to plants, and that from
50 to 60 per cent is the best amount.[40]


_Hygroscopic Power._

A property possessed by soils in relation to water, which is quite
distinct from absorptive power, is their hygroscopic power. By this is
meant their power of absorbing water from the air where it is present in
the gaseous form. This property is identical with the property which
will be adverted to immediately--viz., capacity for absorbing gases. The
extent to which soils possess this hygroscopic property seems to be
regulated very much by the same conditions as regulate their ordinary
absorptive power.[41] This property is considered to be of great
importance in the case of soils in hot climates, where their
agricultural value may be said to depend to a large extent upon it. The
amount of water, however, absorbed in this way is, comparatively
speaking, insignificant. Lastly, it may be observed that there are
certain methods of drying soils afflicted with too much moisture. These
consist in making open ditches, and thus relieving them of their
superabundance of water, or in planting certain kinds of trees, such as
willows and poplars. The amount of green surface presented by the large
number of leaves of trees, from which the constant evaporation of water
goes on, is very great. The consequence is that trees may be regarded as
pumping-engines. It is from this cause that foresters have noticed that
clay lands are apt to become wetter after the trees growing upon them
have been cut down.[42]


_Capacity for Heat in Soils._

A property which depends largely on those we have just been considering
is the capacity soils possess of absorbing and retaining heat.[43] The
temperature of a soil, of course, largely depends on the temperature of
the air; but this, we must not forget, depends also on the soil itself.
The heat given forth by the sun's rays strikes the soil, with the result
that, while so much of its heat is absorbed, a certain portion--and this
will vary according to the nature of the soil--of its heat is radiated
into the air.

The changes in the temperature of the soil naturally take place more
slowly than the changes in the temperature of the air; the depth of soil
thus affected by those changes varies also in different climes. It has
been calculated that in temperate climes the changes of temperature
occurring from day to night are not felt much below three feet down.


_The Explanation of Dew._

We have, it may be stated, generally two processes going on. During the
day the soil is engaged in absorbing its heat from the sun's rays; when
night comes, and the sun goes below the horizon, the air is chilled
below the temperature of the soil, which radiates out its stored-up heat
into the air. The result is that the temperature of the soil is soon
reduced below the temperature of the air, and the moisture, present in
the air in the form of vapour, coming in contact with the cold surface
of the earth, is condensed into dew, which is deposited, and is seen
best early in the morning before the sun has had time to evaporate it
again. Dew is most abundant in summer-time, for the reason that the
difference in temperature of the day and night is then greatest. In
winter-time it is seen as hoar-frost.


_Heat of Soils._

The temperature of a soil, however, is due to other sources than the
sun's rays. Whenever vegetable matter decays, there is always a certain
amount of heat generated. Soils, therefore, in which there is a large
amount of decaying vegetable matter, are certain to receive more heat
from this source than soils of more purely mineral nature.


_Heat in Farmyard Manure._

A good example of the amount of heat that accompanies fermentation, or
decay of vegetable matter, is seen in the case of rotting farmyard
manure. The danger of loss of the volatile ammonia from this cause is
often great, and care must be taken to prevent fermentation going on too
quickly, and the temperature from becoming too high.[44] The actual
increase in the temperature of a soil effected by the addition of
certain bulky organic manures, such as farmyard manure, may thus be
considerable. In some experiments carried out at Tokio, Japan, it was
found that the application of 20 tons of farmyard manure per acre
increased the temperature of the soil to a depth of five inches, for a
period of nearly a month, on an average, one and a half degrees
Fahrenheit. The amount of water present in a soil, it may be noticed in
passing, will have a considerable effect in regulating its temperature,
a damp soil being, as a rule, a cold soil.


_The Cause of the Heat of Fermentation._

It may be asked, How is the decay, or fermentation, of vegetable matter,
such as farmyard manure, caused? or rather, To what is it due? Decay of
any substance is just its slow combustion or burning. When a substance
unites with the active chemical element in air--the oxygen gas--it is
said to be oxidised. Now, this union of a substance with oxygen is the
explanation of burning, and the phenomena of burning and decay are
explained by the same chemical operation. When bodies decay, or when
they burn, they unite with oxygen: when this union of a body and oxygen
takes place very quickly, and the result is a flame and very great heat,
then we call it burning; when, however, it takes place slowly, it is not
called burning, but simply oxidation or decay. The ultimate products are
the same, however, whether the body burns or decays; and the process of
decay is always accompanied by heat, as well as the process of
burning.[45] It is not, of course, only the vegetable or organic matter
in a soil that decays, but also the mineral matter. The oxidation,
however, of the mineral matter in the soil takes place so slowly, and
the amount of heat generated by this oxidation is so slight, that the
temperature of the soil can scarcely be said to be much affected by it.


_Influence of Colour of a Soil._

There is still another quality of a soil on which its temperature
depends, and that is its colour. This may seem at first sight to be
scarcely worth taking into account, and yet it has been shown to have a
very striking influence on the temperature of a soil. This naturally is
best seen in climates where there is a good deal of sun. Dark-coloured
soils have a greater heat-absorbing capacity than light-coloured soils;
and experiments carried out for the purpose of determining the extent of
this influence have shown that under certain conditions the difference
between a soil covered with a black substance, and one covered with a
white substance, amounted to from 13° to 14° Fahr. Other things being
equal, a crop on a dark-coloured soil will be sooner ripened than one on
a light-coloured soil. A soil covered by a crop is cooler than one
without any crop.


_The Power Soils have for absorbing Gases._

We have just seen that one cause of the heat of soils is the oxidation
which is constantly going on in all soils, but more rapidly in soils
containing a large quantity of vegetable matter. This suggests a word or
two on the power soils have of absorbing gases.

The chief gases in the atmosphere are oxygen and nitrogen. Both these
gases are absorbed by soils, although not in similar proportions.[46]
With regard to the former, it is well known that a plentiful supply of
oxygen in the pores of the soil is a necessary condition of fertility.
This was long ago experimentally proved by de Saussure, who showed that
plants absorbed oxygen through their roots. At certain periods of their
growth this demand for oxygen on the part of the plant is greater than
at other times. For example, seeds in the process of germination require
to have free access to a plentiful supply of oxygen. This fact
emphasises the enormous importance of providing a good seed-bed, and of
seeing that the seed is not buried too deeply.


_Carbonic Acid and Ammonia._

In addition to oxygen and nitrogen, the air contains other gases which
are absorbed by the soil. Of these, carbonic acid is the most abundant.
By far the largest portion of the carbonic acid which the soil obtains
from the air, is washed down in solution in the rain.[47] Of the other
constituents of the atmosphere, the combined forms of nitrogen--viz.,
_ammonia_, _nitric_, and _nitrous acids_--are the most important. These
are all absorbed by the soil, but, like carbonic acid, they are chiefly
washed down by the rain. The amount of ammonia which may be absorbed by
a soil from the air, is very much greater than was formerly supposed.
Some recent experiments by Schloesing, referred to in a following
chapter,[48] show this. A damp soil may in the course of a year absorb
far more ammonia than that washed down in rain.


_Gas-absorbing Power of Soils varies._

The power of different soils to absorb these gases varies. This
variation depends not only on their physical properties, but also on
their chemical as well. Soils containing much organic matter have a
greater capacity for absorbing gases than the more purely mineral ones.


_Absorption of Nitrogen._

The absorption of nitrogen by the soil is a question of considerable
importance. It will be referred to later on under the heading of the
biological properties of soils, as it is fixed by the agency of
micro-organisms.[49]

To recapitulate, the chief physical or mechanical properties of a soil
are its absorptive and retentive powers for water; its capacity for
heat; and its power of absorbing gases. It will be easily seen how
tillage operations are calculated to influence these physical properties
of a soil. Thus, in the case of a stiff soil, tillage increases its
power for absorbing the atmospheric gases, chiefly oxygen, which are so
necessary for rendering its fertilising matters available. On the other
hand, in a light and too open soil it may exert quite a contrary effect.

It may be also well to refer here to the important influence these
physical properties exercise on the growth of the plant.


_Plant-roots require a certain Openness in the Soil._

One of the functions of the soil is to support the plant in an upright
position, and this is a function which requires in the soil a certain
amount of compactness or firmness. On the other hand, however, a soil
must not possess too great compactness, otherwise the plant-roots will
experience a difficulty in pushing their way downwards. This is
especially the case during the earlier periods of growth, when the
plant-roots are as yet extremely tender, and experience great difficulty
in overcoming much resistance. The importance of preparing a mellow
seed-bed will be thus at once seen to be based on sound scientific
principles; and this for a double reason. Not only does the young plant
require every facility for developing its roots, but also, as has just
been pointed out, an abundant supply of oxygen is of paramount
importance during the process of germination.


_Soil and Plant-roots._

The whole question of the influence of the mechanical condition of the
soil on the development of plant-roots is one of the highest importance
and interest, and is not so generally recognised as it ought to be.


_Natural tendency of Plant-roots to grow downwards._

It may be taken as certain that the tangled condition of plant-roots is
due to the resistance offered by the soil-particles, and that the
natural tendency of the plant-root is to grow downwards. The roots, in
short, would probably grow in as symmetrical a form as do the stalks or
branches, were it not that they are hindered from so doing by the
soil-particles. Where, then, the soil is such as to offer much
hindrance, the growth of the plant cannot but be retarded. Some
extremely interesting experiments have been performed by the eminent
German chemist Hellriegel on the influence which the closeness of the
soil-particles has on root-development. In these experiments peas and
beans were grown in moistened sawdust, more or less compactly
compressed. It was found that when the sawdust Was compressed to any
extent, plant-growth took place very slowly, or entirely ceased.

The importance of having plant-roots as widely developed in the soil as
possible, will be at once seen when we reflect that this means that the
area of soil from which the plant derives its soil-food is thereby
greatly increased. Another important consideration is, that the deeper
plant-roots can penetrate in a soil, the more able--other conditions
being equal--is the plant to withstand the action of drought, as it can
draw water for its needs from the deeper layers of the soil, long after
a plant, whose roots do not penetrate so deeply, has wilted.


_Plants require Room._

Another important bearing tillage has on plant-growth may here be
discussed. A problem of considerable difficulty is presented in the
question, How many individual plants will a certain piece of soil
support in a healthy way? For as plants require room, it is imperative
that they be not too closely crowded together.

The question resolves itself pretty much into one of quality against
quantity.

Experiments on this subject have shown that a certain area of soil is
only able to support the healthy growth of a certain number of plants.
If the limit be exceeded, the result is imperfect development.


_Number of Plants on certain Area increased by Tillage._

It is obvious, however, that the more thoroughly tilled a soil is, the
greater will be the number of plants it will be possible to grow on it.
The roots, instead of being forced to spread themselves along the
surface-soil, and thus take up a large amount of room, will find no
difficulty in striking downwards. Two or three plants may thus be
enabled to grow in a thoroughly tilled soil in the same space as only
one could before tillage.


_American and English Farming._

The above considerations throw considerable light on what seems to many
farmers a strange anomaly--viz., the fact that the return of farm
produce per acre on American farms is, as a rule, very much less than
that from our own impoverished soils in this country. To many, at first
sight, this seems to be in direct contradiction to our common belief,
and to point to the conclusion that the virgin soils of America are,
after all, actually inferior in fertility to the soils of Britain.

It is not, however, necessary to draw this conclusion, as the facts of
the case admit of another explanation. The inferior returns obtained
from American farms are due, not to the fact that the American soil is
less fertile than the British--for this is not true--but to the fact
that it is less _intensively_ cultivated.

In America land is cheap and labour is dear; it is consequently found to
be more economical to cultivate a large tract of land less thoroughly
than a small area more thoroughly. In Britain the reverse is the case,
labour being cheap and land being dear. It is thus necessary to make the
land go as far as possible, and produce as heavy a crop as it is
possible to produce. There can be little doubt, that were American
farming to be carried on as intensively as is British farming, the
present yield would be at least probably doubled.

We have now to consider the second class of properties which influence
the fertility of a soil. These are _chemical_.


=II. Chemical Composition of a Soil.=--Chemically considered, the soil
is a body of great complexity. It is made up of a great variety of
substances. The relations existing between these substances and the
plant are not all of equal importance; some--and these form by far the
largest proportion of the soil-substance--are concerned in acting simply
as a mechanical support for the plant, and in helping to maintain those
physical properties in the soil which, as we have just seen, exercise
such important functions in the plant's development.


_Fertilising Ingredients._

A small portion of the soil-substance, however, takes a very much more
active part in promoting plant-growth, by acting as direct food of the
plant. As we have already seen in the Introductory Chapter,[50] the
substances which have been found in the ash of plants are the following:
_potash_, _lime_, _magnesia_, _oxide of iron_, _phosphoric acid_,
_sulphuric acid_, soda, silica, chlorine, oxide of manganese, lithia,
rubidia, alumina, oxide of copper, bromine, and iodine. The general
presence of some of these substances is doubtful; the presence of
others, again, probably purely accidental; while some are only found in
plants of a special nature, as, for instance, iodine and bromine, which
are only found in the ash of marine plants.

Of these ash constituents, only the first six substances--those marked
in italics--are absolutely necessary to plant-growth. In addition to
these six ash constituents, the plant also derives its _nitrogen_, which
is a necessary plant-food, chiefly from the soil.[51]


_Importance of Nitrogen, Phosphoric Acid, and Potash._

But of these seven constituents of the soil which are necessary to
plant-growth, some have come to be regarded by the agriculturist with
very much greater interest than others. This is due to the fact that
they are normally present in the soil in very much smaller quantities
than is the case with the other equally necessary food ingredients;
that, in short, they are nearly invariably present in the soil, in a
readily available form, in lesser quantities than the plant is able to
avail itself of, and often, as in impoverished or barren soils, in
quantities too small for even normal growth. These ingredients are
_nitrogen_, _phosphoric acid_, and _potash_.[52]

The importance of seeing that all the necessary plant ingredients are
present in a soil in proper quantities will be at once properly
estimated when it is stated that the absence or insufficiency in amount
of one single ingredient is capable of preventing the growth of the
plant, although the other necessary ingredients may be even abundantly
present.

With lime, magnesia, iron, and sulphuric acid, most soils are abundantly
supplied. The substances with which the farmer has to concern himself,
then, are nitrogen, phosphates, and potash. It is these substances
therefore, that, as a rule, are alone added as manures.


_Chemical Condition of Fertilising Ingredients in Soil._

But in considering the chemical properties of a soil, a simple
consideration of the quantity of the different ingredients present is
not enough. A very important consideration is their chemical condition.
Ere any plant-food can be assimilated by the plant's roots, it must
first be rendered soluble. The quantity of soluble, or, as it is known,
_available_, plant-food in a soil is very small. It is, of course, being
steadily added to each day by the process of disintegration constantly
going on in soils.


_Amount of Soluble Fertilising Ingredients._

The exact nature and dissolving capacity of the soil-water, charged as
it is, to a greater or less extent, with different acids and salts, as
well as the dissolving power of the sap of the rootlets of the plant
itself, render the exact estimation of the available fertilising
constituents wellnigh impossible. An approximate estimate, however, may
be obtained by treating the soil with pure water and dilute acid
solutions. The treatment of the soil with dilute acid solutions is for
the purpose of simulating, as nearly as may be done, the conditions it
is submitted to in the soil. By treating a soil with water, we obtain a
certain amount of plant-food dissolved in the water. This can only be
regarded as indicating approximately the amount available at that moment
to the plant. But every day, thanks to the numberless complicated
reactions going on in the soil, this soluble plant-food is constantly
being added to. Considerations such as the above, together with our
ignorance as to the exact combinations in which the necessary minerals
enter the plant, will serve to indicate the great difficulty of this
part of the subject.[53]


_Value of Chemical Analysis of Soils._

It is largely for these reasons that a chemical analysis of a soil is
from one point of view of little value in giving evidence of its actual
fertility. What it demonstrates more satisfactorily is its potential
fertility. It is useful in revealing what there is present in it, not
necessarily, however, in an available condition. Under certain
circumstances it may be made of great value, as, for example, when we
are anxious to know what will be the result of certain kinds of
treatment, such as the application of lime, &c.

It is hardly advisable, therefore, to place before the reader a number
of soil analyses. That he may obtain an approximate idea of the
composition of a soil, one or two representative analyses will be found
in the Appendix,[54] along with a short account of the chief minerals
out of which soils are formed.

A point of considerable interest is the quantity per acre different
soils contain of nitrogen, phosphoric acid, and potash. Although the
amount of these ingredients when stated in percentage seems very
trifling, yet when calculated in lb. per acre, it is seen to be in large
excess of the amount removed by the different crops. This question will
be dealt with in succeeding chapters.

A point of further interest is the chemical form in which the necessary
plant constituents are present in the soil. For information on this
point the reader is referred to the Appendix.[55]

The third class of properties which affect the fertility of a soil are
those which have been termed the _biological_.


=III. Biological Properties of a Soil.=--The important functions which
modern discoveries have shown to be discharged by minute organic life in
the terrestrial economy are nowhere more strikingly exemplified than in
the important _rôle_ they perform in the soil.


_Bacteria of the Soil._

The soil of every cultivated field is teeming with bacteria whose
function is to aid in supplying plants with their necessary food. The
nature of, and the functions performed by, these organisms differ very
widely. Regarding many of them we know very little; every day, however,
our knowledge is being extended by the laborious researches of
investigators in all parts of the world, and it is to be anticipated
that ere long we shall be in possession of many facts regarding the
nature and the method of the development of these most interesting
agents in terrestrial economy. That they are present, however, in
enormous numbers in all soils we have every reason to believe, one class
of organism connected with the oxidation of carbonic acid gas being
estimated to be present to the extent of over half a million in one
gramme of soil[56] (Wollny and Adametz). One class--and their
importance is very great in agriculture--prepare the food of plants by
decomposing the organic matter in the soil into such simple substances
as are easily assimilated by the plant. The so-called "ripening" of
various organic fertilisers is effected, we now know, entirely through
the agency of bacteria of this class. Plant-life is unable to live upon
the complex nitrogenous compounds of the organic matter of the soil, and
were it not for bacteria these substances would remain unavailable.
Attention will be drawn in the Chapter on Farmyard Manure to this
question more in detail. Of these bacteria, among the most important are
those which are the active agents in the process known as
"nitrification"--_i.e._, the process whereby organic nitrogen and
ammonia salts are converted into nitrites and nitrates. The presence of
these organisms, it would appear, is indispensable to the fertility of
any soil. There are organisms, on the other hand, which have the power
of reversing the work of the nitrification bacteria by converting
nitrates into other forms of nitrogen. The reduction of nitrates in the
soil is often the source of much loss of valuable nitrogen, which
escapes in the free state, so that the action of bacteria is not
altogether of a beneficial nature.


_Three Classes of Organisms in the Soil._

So far as the subject has been at present studied, the micro-organisms
in the soil may be divided into three classes.[57]


_First Class of Organisms._

We have, first of all, those whose function it is to oxidise the soil
ingredients. Organisms of this class may act in different ways. They may
assimilate the organic matter of the soil and convert it into carbonic
acid gas and water; or, on the other hand, they may oxidise it by giving
off oxygen. Some of these organisms, whose action is of the first kind,
choose most remarkable materials for assimilation. One has been found to
require ferrous carbonate for its development, which it oxidises into
the oxide (Winogradsky); while another,[58] the so-called sulphur
organism, converts sulphur into sulphuretted hydrogen according to some,
and according to others into sulphates. To this class of organism the
nitrifying organisms belong. As will be seen more fully in a subsequent
chapter, two distinct organisms connected with this process have already
been isolated and studied--one of these effecting the formation of
nitrites from organic nitrogen or ammonia salts, and the other the
conversion of nitrites into nitrates. The second method in which these
oxidising organisms act is by giving off oxygen. There is much interest
attaching to this fact, as it was supposed till quite recently that all
evolution of oxygen in vegetable physiology was dependent on the
presence of light, and also intimately connected with chlorophyll, or
the green colouring matter of plants. It would seem, however, that among
the soil organisms these conditions are not necessary, and the evolution
of oxygen may be carried on in the case of colourless organisms as well
as in the case of light. With organisms of this kind every soil is
probably teeming. A typical example is the organism which is the active
agent in the oxidation of carbonic acid gas, and which has already been
referred to as existing in the soil in such numbers.[59]


_The Second Class of Organisms in the Soil._

The second class of organisms are those which reduce or destroy the soil
constituents. The most important of these, from the agricultural point
of view, are those which effect the liberation of nitrogen from its
compounds. In the putrefaction of organic matter the organisms chiefly
act, it is probable, in the entire absence of atmospheric oxygen; but
it would seem, however, that they may also act in the presence of
oxygen. It is through their agency that the soil may lose some of its
nitrogen in the "free" form. To this class belong the denitrifying
organisms already referred to which reduce the nitrates and nitrites in
the soil.[60]


_Third Class of Organisms._

The third class of organisms are those by whose agency the soil is
enriched. Of this class those fixing the free nitrogen from the air are
the most important. The nature of these organisms is still somewhat
obscure, but that leguminous plants have the power of drawing upon this
source of nitrogen is now a firmly established fact. Further reference
to these interesting organisms may be delayed to another chapter.

The important point to be emphasised is, that for the healthy
development of these organisms, which are so necessary in every fertile
soil, certain conditions must exist. These necessary conditions will be
treated more in detail later on. It is sufficient to notice that they
have to do with the physical properties as well as the chemical
composition of the soil. This furnishes a further reason for the
necessity of having the mechanical condition of a soil satisfactory.


_Recapitulation._

From what we have said, it will be seen that the question of
soil-fertility is a very complicated one, and depends on numerous and
varied conditions; that the properties which constitute fertility, while
seemingly very widely different in their nature, in reality influence
one another to a very great extent; that not merely is the presence in a
soil of the necessary plant constituents necessary to fertility, but
that the possession by the soil of certain physical or mechanical
properties is equally necessary; while, lastly, we have seen that the
presence of certain micro-organic life is bound up with the problem of
fertility in a very direct and practical manner.

The importance of the conditions, other than those of a purely chemical
nature, have been thus far somewhat prominently emphasised, for the
reason that in what follows attention will be almost exclusively devoted
to the purely chemical conditions of fertility. It is well, then, to
realise that, while the latter conditions are by far the most important,
so far as the farmer is practically concerned, inasmuch as they are most
under his control, they are not the only conditions, and are not by
themselves able to control fertility.


FOOTNOTES:

[33] This statement perhaps needs qualification. While the important
_rôle_ played by the physical qualities of the soil were in the early
years of the science recognised, of more recent years the chemical
composition of the soil has been engaging almost exclusive
investigation. Physical properties of the soil have recently acquired a
further importance in the eyes of the agricultural chemist, from the
important influence they exert on what we have here called the
biological properties of a soil--viz., the development of those
fermentative processes whereby plant-food is prepared to a large extent.

[34] A good example of the absorptive capacity of a soil containing a
large quantity of vegetable matter is furnished by peat-bogs, which,
sponge-like, can absorb enormous quantities of water. (See Appendix,
Note I., p. 98.)

[35] Jethro Tull, an early well-known agricultural writer, who lived
about the middle of last century, propounded the theory, that as the
food of plants consisted of the minute earthy particles of the soil, all
that was required by the skilful farmer was to see that his soil was
properly tilled. He accordingly published a work entitled 'Horse-hoeing
Husbandry,' in which he advocated a system of thorough tillage. (See
Historical Introduction, p. 10.)

[36] See Introduction, p. 55.

[37] See Introductory Chapter, p. 55.

[38] It is not exactly known why excess of water should prevent normal
growth in the plant. Probably it is on account of the fact that free
access of oxygen is hindered in such a case. The roots are thus not
freely enough exposed to this necessary gas, and fermentative processes
of the nature of nitrification are not promoted. It may be also due to
the fact that the solution of plant-food is too dilute when such excess
of water prevails.

[39] See Appendix, Note II., p. 98.

[40] Some experiments by E. Wollny show this. He found, when
experimenting with _summer rape_, that the best results were obtained
when the soil contained only 40 per cent of its total water-holding
power; when the amount was either lessened or increased the results
obtained fell off. The effect of either too little or too much water is
seen in the development of the different organs of the plant as well as
on its period of growth, much water seeming to retard the growth. The
quality of the plant seems also to be influenced by this condition.
Experiments on cereal grains by Wollny show that not merely is the
texture of the grain influenced, but that much moisture lessens the
percentage of nitrogen. Wollny is of the opinion that for crops
generally, the best amount is from 40 to 75 per cent of the total
water-holding capacity of the soil.

[41] See Appendix, Note III., p. 99.

[42] See p. 55.

[43] The effect of the temperature of the soil on the development of the
plant is most important. This is especially marked at the period of
germination, but is felt at subsequent periods of growth. Up to a
certain temperature the warmer the soil the more rapid the plant's
development. In this country the temperature most favourable to growth
is rarely exceeded, or indeed reached.

[44] See Chapter on Farmyard Manure.

[45] As will be seen further on, the fermentation of organic substances
is caused by the action of micro-organic life.

[46] See Appendix, Note IV., p. 100.

[47] Of course it must be remembered that a large amount of carbonic
acid in soils comes from the decay of vegetable matter. Soils are twenty
to one hundred times richer in carbonic acid than the air.

[48] See Chapter III., p. 119.

[49] See Introduction, p. 40.

[50] See Introductory Chapter, p. 54.

[51] See pp. 44 and 135.

[52] Occasionally also _lime_.

[53] See Appendix, Notes V. and VI., pp. 100, 101.

[54] Note VI., p. 101.

[55] Note VII., p. 107.

[56] Even larger estimates of the number of germs in a gramme of soil
have been made--from three-quarters to one million (Koch, Fülles, and
others).

[57] These organisms consist of molds, yeast, and bacteria, the
last-named being most abundant. In the surface-soil, among the bacteria,
bacilli are most abundant. Micrococei are not abundant.

[58] Investigated by Winogradsky, Olivier, De Rey Pailhade, and others.

[59] Organisms of this kind have been investigated among others by
Heraüs, Hueppe, and E. Wollny. According to the two first-mentioned
investigators, certain colourless bacteria effect the formation in the
absence of light from humus and carbonates a body resembling in its
nature cellulose.

[60] Investigated by Springer, Gayon and Dupetit, Dehérain, and
Marguenne.



APPENDIX TO CHAPTER I.


NOTE I. (p. 68).

The following determinations by Schübler show the absorptive power of
different kinds of soil-substances. These were obtained by soaking
weighed quantities of the soil in water, and allowing the excess of
liquid to drain away, and weighing the wet earth.

                        Per cent of water
                        absorbed by 100
                        parts of earth.

Siliceous sand                 25
Gypsum                         27
Calcareous sand                29
Sandy clay                     40
Strong clay                    50
Arable soil                    52
Fine calcareous                85
Garden-earth                   89
Humus                         190

It has been calculated that the absorptive power of a mixture of
different substances is not simply equal to the sum of their separate
ingredients.


NOTE II. (p. 74).

EVAPORATION.

The retentive property of a soil for water tends to retard evaporation.
The following table by Schübler shows the rate at which evaporation
proceeds in different soils. The experiment was conducted in the
following way. The soil experimented upon was saturated with water and
spread over a disc, and allowed to evaporate for four hours, when it was
weighed. The amount of time required for the evaporation of 90 per cent
of the water was also estimated. Of 100 parts of water in the wet soil
there evaporated, at 60° Fahr.--

                                            Time required to
                       In four hours--   evaporate 90 per cent.
From--                    per cent.         Hours.   Minutes.

Quartz                       88                4         4
Limestone                    76                4        44
Sandy clay                   52                5         1
Stiffish clay                46                6        55
Loamy clay                   46                7        52
Pure grey clay               32               11        17
Loam                         32               11        15
Fine calcium carbonate       28               12        51
Humus                        21               17        33
Magnesium carbonate          11               33        20


NOTE III. (p. 76).

HYGROSCOPIC POWER OF SOILS.

Davy found the hygroscopic power of soils to be as follows. He found
that 100 parts by weight of three samples of different sands absorbed 3,
8, and 11 parts of water, respectively, in one hour; while three loams
absorbed similarly 1.3, 1.6, and 1.8 parts.

The following samples of soil were dried at 212° Fahr., and exposed to
an atmosphere saturated with water and a temperature of 62° Fahr., when
it was found they absorbed the following amounts in twelve hours'
time:--

Quartz sand               0.0
Limestone sand            0.3
Lean clay                 2.1
Fat clay                  2.5
Clay soil.                3.0
Pure clay.                3.7
Garden-loam               3.5
Humus                     8.0


NOTE IV. (p. 81).

GASES PRESENT IN SOILS.

The air which we find enclosed in the pores of the soil is distinctly
_poorer_ in oxygen than ordinary air. Boussingault found the percentage
of oxygen in a sandy soil, freshly manured and wet with rain, to be as
low as 10.35 per cent; while the air in forest-soil contained 19.5 per
cent of oxygen, and .93 per cent of carbonic acid. The percentage of
oxygen in soils depends on the rate of decay of the organic portions.
The depth of the soil-layer also determines the quantity. This is owing
to the fact that diffusion takes place more slowly deep down than near
the surface.


NOTE V. (p. 90).

AMOUNT OF SOLUBLE PLANT-FOOD IN THE SOIL.

Two of the most reliable methods of ascertaining an approximation of the
quantity of soluble soil constituents are (1) by treating the soil with
distilled water, and (2) by analysing the drainage-water. With regard to
the former of these two methods, it has been found that even the amount
of fertilising matter dissolved out by pure distilled water varies. This
variation depends on the amount of distilled water used, as well as the
length of time the soil is left in contact with the solvent. By washing
the soil with different quantities of water, different amounts of
soluble soil ingredients will be found to have been washed out; for
although the first washings contain by far the greater portion of the
soluble matter, each subsequent washing will be found to contain further
quantities.

A number of experiments have shown that 1000 parts of distilled water
dissolved out from different soils from one half to one and a half parts
of soluble constituents; or from .05 to .15 per cent. Of this soluble
matter from 30 to 67 per cent is mineral in its nature, and from 33 to
70 per cent organic. Poor sandy soils yield the minimum quantity, while
peaty soils yield the maximum. The quantity of soluble matter in a
regular peaty soil may vary from .4 to 1.4 per cent; this consists
chiefly, however, of organic matter. (See Johnson's 'How Crops Feed,' p.
312.)

Perhaps a more satisfactory method is by analysing the drainage-water of
a soil. This has been found to vary very considerably in composition.
The average of a large number of analyses are .04 to .05 per cent of
dissolved matter. Of this dissolved matter the largest proportion is
made up of organic matter, nitric acid, lime, and soda salts. It must be
borne in mind, however, that even the drainage-water does not furnish an
exact indication of the amount of dissolved matter in a soil. Much,
perhaps the largest proportion of dissolved matter, never finds its way
into the drainage-water. That contained by the drainage-water really
represents the surplus quantity of dissolved matter which the soil is
unable to retain, and which is thus washed by the rain into the drains.
The composition of drainage-water is interesting, as it shows that,
practically speaking, all the necessary plant ingredients are in a state
of solution in the soil.


NOTE VI. (p. 90).

CHEMICAL COMPOSITION OF THE SOIL.

The most important substances present in soils are as follows: silica,
alumina, lime, magnesia, potash, soda, ferric oxide, manganese oxide,
sulphuric acid, phosphoric acid, and chlorine. Of these substances the
presence of alumina, silica, lime, and, in certain cases, magnesia,
along with the organic portion of the soil--the humus--has the chief
influence in determining the nature and the physical properties of a
soil.

In order to clearly understand to what it is soils owe the nature of
their chemical composition, it is necessary to consider the composition
of some of the chief minerals out of the disintegration of which soils
are formed.

While we know of some seventy elements present in the earth's crust, it
is practically made up of only some sixteen. These sixteen are--oxygen,
silicon, carbon, sulphur, hydrogen, chlorine, phosphorus, iron,
aluminium, calcium, magnesium, sodium, potassium, fluorine, manganese,
and barium.[61] Of these, oxygen is by far the largest constituent,
forming, roughly speaking, about 50 per cent.

The main mass of the rocks consists of silica, and this is generally
combined with alumina, as in clay, forming aluminium silicate, and with
the commoner alkalies and alkaline earths. Another extremely abundant
compound is carbonate of lime, which, as limestone, chalk, and marl,
forms one-sixth of the earth's total rocks.

The word "mineral" means a definite chemical compound of natural
occurrence. The number of minerals is very great, and it is impossible
to go into the subject here. Reference can only be made to a few of the
more prominent ones, which are chiefly concerned in the formation of
soils.

Those formed out of silicates are, from the agricultural point of view,
the most important, as they form a very large group; and it is by their
disintegration that soils are chiefly formed. They consist of silica and
alumina, along with various other substances, chiefly alkalies and
alkaline earths. It is important to note one peculiarity about the
solubility of silicates. We have two classes of silicates: the one,
which is called "acid," and contains an excess of silica; the other,
"basic," and which contains an excess of base. Now, while the former of
these is more or less insoluble, the second is soluble. This fact has an
important signification in the process of the disintegration of the
silicate minerals we are about to consider.

The first and most important class are the _Felspars_. Felspar is not
really a definite mineral, with a definite chemical composition, but
rather the name of a class of minerals of which there are several
different kinds. The felspars are composed of silica and alumina, along
with potash, soda, and lime, with traces of iron and magnesia. Their
principal constituents, however, are silica and alumina, along with
either potash, soda, or lime. According as the base potash, soda, or
lime predominates, the felspar is known as Orthoclase, Albite, and
Oligoclase, respectively.

The following are the analyses of the three minerals (by the late Dr
Anderson):--

--------------------+-----------------+-----------------+-----------------
                    |   Orthoclase.   |     Albite.     |    Oligoclase.
                    +--------+--------+--------+--------+--------+--------
                    |   1.   |   2.   |   1.   |   2.   |   1.   |   2.
--------------------+--------+--------+--------+--------+--------+--------
 Silica             |  65.72 |  65.00 |  67.99 |  68.23 |  62.70 |  63.51
 Alumina            |  18.57 |  18.64 |  19.61 |  18.30 |  23.80 |  23.09
 Peroxide of iron   | traces |   0.83 |   0.70 |   1.01 |   0.62 |   none
 Oxide of manganese | traces |   0.13 |   none |   none |   none |   none
 Lime               |   0.34 |   1.23 |   0.66 |   1.26 |   4.60 |   2.44
 Magnesia           |   0.10 |   1.03 |   none |   0.51 |   0.02 |   0.77
 Potash             |  14.02 |   9.12 |   none |   2.53 |   1.05 |   2.19
 Soda.              |   1.25 |   3.49 |  11.12 |   7.99 |   8.00 |   9.37
--------------------+--------+--------+--------+--------+--------+--------
                    | 100.00 |  99.47 | 100.08 |  99.83 | 100.79 | 101.37
--------------------+--------+--------+--------+--------+--------+--------

According as these various felspars are present in a soil, so will the
quality of the soil be. It stands to reason that as the presence of
potash in a soil is one of the distinguishing features of its fertility,
much will depend on the extent to which the orthoclase felspar is
present; and also, not only on the extent, but on the state and degree
of its disintegration. It is important to note the method of this
disintegration. It is effected by the absorption of water. This water is
not merely absorbed mechanically, but actually enters into the
composition of the mineral. It is not present as moisture merely,
capable of being expelled at ordinary boiling temperature, but it forms
what is known as water of composition. In this process of hydration, the
mineral loses its lustre and crystalline appearance, crumbles away into
a more or less--according to its state of disintegration--powdery mass.
A very great change is also effected in its chemical composition; it
loses nearly all its base. This is effected in the following way. As
water enters into the mineral's composition, it sets free a certain
portion of the base; there is thus formed a basic silicate, which, being
soluble in water, is washed away in solution. This change may be
illustrated by quoting the analysis of a kaolin clay formed by the
disintegration of orthoclase felspar.

_Kaolin Clay formed by disintegration of Orthoclase._

             Silica                    46.80
             Alumina                   36.83
             Peroxide of iron           3.11
             Carbonate of lime          0.55
             Potash                     0.27
             Water                     12.44
                                      ------
                                      100.00
                                      ------

The chief difference here is the almost total loss of potash and a
portion of the silica, and the gain of water. The other constituents
practically remain insoluble.

Another important mineral is _Mica_. Its composition is not unlike
felspar. It contains silica, alumina, and iron, in considerable
quantities, also magnesia and potash. There are two kinds of mica--that
containing potash, and that containing magnesia, in excess. The analyses
of these two kinds are as follows (by the late Dr Anderson):--

                                 MICAS.
                      (_a_) Potash.  (_b_) Magnesia.
Silica                    46.36           42.65
Alumina                   36.80           12.96
Peroxide of iron           4.53            none
Protoxide of iron          none            7.11
Oxide of manganese         0.02            1.06
Magnesia                   none           25.75
Potash                     9.22            6.03
Hydrofluoric acid          0.70            0.62
Water                      1.84            3.17
                          -----           -----
                          99.47           99.35
                          -----           -----

The decomposition of mica is very slow, however, as it is a peculiarly
hard mineral.

Other important minerals are _Hornblende_ and _Augite_. These are
composed of silica, alumina, iron oxide, manganese oxide, lime and
magnesia. These are the chief minerals out of which soils are formed. It
is scarcely necessary to say that few soils are made up out of any of
these three minerals alone. Nearly all rocks are formed out of a mixture
of these minerals. Where, however, any one mineral predominates over the
rest, the nature of the soil will be thereby affected. In order to
illustrate this, it may be well to mention the composition of one or two
of the commoner rocks.

1. _Granite_, which is so abundant in certain parts of the north of
Scotland, and which gives rise to the soils in the neighbourhood of
Aberdeen, is made up of a mixture of quartz, felspar, and mica. It
depends on the felspar present--_i.e._, whether it is orthoclase,
oligoclase, or albite--whether the soil will be rich in potash or not.
Granite containing orthoclase felspar produces a fairly fertile soil. An
important consideration, which is apt to complicate this question, is
the situation of such soils. They are generally so high above sea-level,
that their fertility is seriously impaired on these grounds.

2. _Gneiss_, another common rock, is similar in composition, only that
it contains very little felspar, and a correspondingly greater amount of
mica.

3. _Syenite_ contains quartz, felspar, and hornblende.

The rocks of which greenstone and trap are types, are found very largely
scattered over the country. They are of two kinds, diorite and dolorite.

4. _Limestone_ is of two great classes. We have (1) Common, (2)
Magnesian. The following are the analyses of these two classes by Dr
Anderson:--

----------------------+---------------------------+------------------------
                      |          Common.          |         Magnesian.
                      +-------------+-------------+-------------+----------
                      | Mid-Lothian | Sutherland. | Sutherland. | Dumfries.
----------------------+-------------+-------------+-------------+----------
Silica                |    2.00     |     7.43    |     6.00    |    2.31
Iron oxide and alumina|    0.45     |     0.76    |     1.57    |    2.00
Carbonate of lime     |   93.61     |    84.11    |    50.21    |   58.81
Carbonate of magnesia |    1.62     |     7.45    |    41.22    |   36.41
Phosphate of lime     |    0.56     |      -      |      -      |     -
Sulphate of lime      |    0.92     |      -      |      -      |     -
Organic matter        |    0.20     |      -      |      -      |     -
Water                 |    0.50     |      -      |      -      |     -
                      |   -----     |    -----    |    -----    |   -----
                      |   99.86     |    99.75    |    99.00    |   99.53
----------------------+-------------+-------------+-------------+----------

Clays are formed by the disintegration of any of the crystalline rocks;
the purest clays being formed from felspar. A pure clay consists simply
of silica and alumina, all the other constituents having been washed
out. Disintegration, however, seldom reaches such an extent; otherwise
clay soils would be completely barren, which they are notably not. The
impurities present in clay, which consist of alkalies, especially potash
and other mineral ingredients of the plant, are what confer on clay
soils their fertility. Clays differ, however, very considerably in
their composition. The following is an analysis of a clay soil by Dr
Anderson:--

Silica                        60.03
Alumina                       14.91
Peroxide of iron               8.94
Lime                           2.08
Magnesia                       4.22
Potash                         3.87
Soda                           0.06
Water and carbonic acid        5.67
                              -----
                              99.72
                              -----


NOTE VII. (p. 91).

FORMS IN WHICH PLANT-FOODS ARE PRESENT IN SOIL.

The forms in which the bases necessary for plant-food are present in the
soil, are chiefly as _hydrated silicates_, and in combination with
organic acids, forming humates, &c., as well as in the form of sulphates
and chlorides.

Phosphoric acid is present in combination with iron, alumina, or lime,
or possibly also as magnesium-ammonium-phosphate. Sulphuric acid is
generally present in a more or less insoluble condition, in combination
with iron and lime; whereas chlorine is combined with the alkali bases
in an easily soluble form. An important point is as to the form in which
the plant absorbs these food constituents. In this connection reference
may be made to a theory put forward by a very distinguished French
agricultural chemist, Professor Grandeau. His theory is that the
necessary ingredients of plant-food are absorbed into the plant as
humates, or, at any rate, that the medium of this transference is humic
acid, and organic acids of a similar nature. This theory, however, while
ingenious, has not yet been supported by sufficient evidence to make its
acceptance advisable. It is probable that it is only in the form of
soluble salts that the plant can absorb its food. It is quite probable,
however, at the same time, that the exact form in which the different
food substances enter the plant may be largely determined by
circumstances. According to Nobbe, chloride of potassium is the most
suitable form of potassium salts, although the plant may absorb its
potassium as sulphate, phosphate, or even silicate.


FOOTNOTES:

[61] Composition of the earth's solid crust in 100 parts by weight:--

Oxygen       44.0  to  48.7  |  Calcium      6.6  to  0.9
Silicon      22.8  to  36.2  |  Magnesium    2.7  to  0.1
Aluminium     9.9  to   6.1  |  Sodium       2.4  to  2.5
Iron          9.9  to   2.4  |  Potassium    1.7  to  3.1

(Roscoe's 'Lessons in Elementary Chemistry,' p. 8.)



CHAPTER II.

FUNCTIONS PERFORMED BY MANURES.


Having now considered the general conditions on which fertility of soil
depends, we are in a position to deal with the nature and function of
manures.

Manures may be classified in several different ways, and a considerable
amount of confusion is sometimes caused by the variety of classification
adopted by different writers on this subject.


_Etymological meaning of the word Manure._

Let us, in the first place, clearly understand what we mean by a manure.
The word manure comes from the French word _manoeuvrer_, which simply
means "to work with the hand," hence "to till," and this etymological
meaning of the word illustrates the old belief in the function of
manures. We have already seen in the historical introduction that,
according to Tull, the true and only function of manures was to aid in
the pulverisation of the soil by fermentation. In advancing his system
of _thorough tillage_, he claimed that since tillage effected the
pulverisation of the soil, where it was practised, manures could be
dispensed with.


_Definition of Manures._

We no longer, of course, attach this old meaning to the word. The word
manure is now applied to any substance which by its application
contributes to the fertility of a soil. As has been shown in the
previous chapter, the substances necessary for plant-growth which are
apt to be lacking in a soil, are only generally three in number--viz.,
_nitrogen_, _phosphoric acid_, and _potash_. A manure, therefore, is
understood to be any substance containing these ingredients, either
singly or together, and its commercial value is determined by the amount
it contains of these substances. But while this is so, it must not be
forgotten that if we define a manure to be a substance which contributes
in any way to the fertility of the soil, substances other than these
above mentioned may be fairly regarded as manures. The fertility of a
soil, we have seen, depends not merely on the presence of certain
constituents, but also on their chemical condition--_i.e._, whether they
are easily soluble or not. It further depends, as we have also seen, on
the possession by the soil of certain mechanical and biological
properties. Thus there are substances which act upon the soil's inert
fertilising matter, and by their action convert it into a more speedily
available form. There are other substances which by their application
exert a considerable effect on the texture of the soil, and thereby
influence its physical and biological properties. All such substances,
according to the above definition of a manure, must be included under
the term. It will thus be seen that since fertility in a soil can be
promoted in a variety of ways, and the functions performed by manures
are of different kinds, we can divide them into different classes,
according to their respective action.


_Different Classes of Manures._

In the first place, we can divide manures into two great classes,--(1)
those supplying to the soil necessary plant-food constituents, and thus
contributing directly to fertility; and (2) those influencing
soil-fertility in an indirect manner. The first class we may call
_direct_ manures, and the second _indirect_. Those two classes admit
further of being subdivided into other smaller classes. Among the direct
manures we have a number of subdivisions in use. They may be divided
into _general_ manures and _special_ manures, according as they contain
all the elements necessary for plant-growth, or only some of them; or
they may be divided according to their source into _natural_ and
_artificial_, _mineral_ and _vegetable_. Similarly we have a number of
subdivisions among the second class, depending on the special nature of
the action they exert. Some manures act in both capacities--both
directly and indirectly--and in order that their value be fully
appreciated must be studied under both heads. The most striking example
of such a manure is farmyard manure. There are other manures which may
in certain circumstances act in two different ways. Such a substance is
lime. There are soils which are actually lacking in a sufficiency of
lime for the needs of crops. On such soils an application of lime would
act both as a direct and also as an indirect manure. There may also be
cases of an exceptional nature, in which magnesia salts or even iron
salts may act as direct manures. Many manures commonly regarded as
purely direct manures would exert an indirect influence were the
quantities in which they were applied sufficiently large. This is the
case, indeed, with many artificial manures, such as guano, bones,
nitrate of soda, and basic slag. It has been claimed for nitrate of soda
that it not merely promotes fertility by supplying nitrogen in its most
available form to the soil, but that the soda it contains exerts a
valuable indirect influence in consolidating the soil and increasing its
absorptive powers. When we reflect, however, on the small quantity of
this manure which is applied per acre, its mechanical influence must be
insignificant. The same applies to basic slag, which contains a
considerable quantity of free lime in its composition. As this manure,
however, is sometimes applied in considerable quantities, it is
reasonable to suppose that its indirect value may not be altogether
insignificant. Indeed we have proof of this in the fact that its most
favourable action has been found to be on soils rich in organic
matter.[62] The action of bones and guano, and indeed of all other
manures containing a large percentage of decomposable organic matter, is
likewise of a double nature, inasmuch as their decomposition or
putrefaction in the soil gives rise to the formation of carbonic and
organic acids, which are capable of exerting a chemical action on the
soil ingredients. There is one point in connection with the action of
these manures which is worthy of notice, and it is that, however slight
their indirect value may be, their action as a direct manure is very
much accelerated by the way in which their organic matter putrefies. In
short, they may be described as providing, to a certain extent, the
solvents which render them available for the requirements of the plant.
It may be here convenient to classify the manures which we intend
subsequently to deal with.

I. Manures, action of which is both direct and indirect--_e.g._, _green
manures_, _farmyard manure_, _composts_, and _sewage_.

II. Manures which may be regarded as having only a direct
action--_e.g._, _guano_ of all kinds, _bones_ in all forms, _nitrate of
soda_, _sulphate of ammonia_, _dried blood_, _superphosphates_, _mineral
phosphates_ of all kinds, _horns_ and _hoofs_, _shoddy_, _wool-waste_,
_fish-guano_, _muriate of potash_, _sulphate of potash_, and _kainit_.

III. Manures which may be regarded as having only an indirect
value--_e.g._, _lime_, _mild_ and _caustic_, _marl_, _gypsum_, _salt_,
&c.

We shall now proceed to discuss the nature and action of these different
manures, starting with those exercising both a _direct_ and _indirect_
influence. Before doing so it may be well to consider the occurrence and
natural sources of the three important soil constituents, nitrogen,
phosphoric acid, and potash, with a view of seeing to what extent these
are being removed from our soils by the various natural processes
constantly going on, as well as by the crops, and how far their natural
sources are capable of making good this loss--in short, to clearly
understand the economic reasons for the application of artificial
manures.


FOOTNOTES:

[62] See Chapter on Basic Slag.



CHAPTER III.

THE POSITION OF NITROGEN IN AGRICULTURE.


Of manurial ingredients, nitrogen is by far the most important, and on
the presence and character of the nitrogen it contains, the fertility of
a soil may be said to be most largely dependent. Most soils, as a rule,
are better supplied with available ash ingredients than with available
nitrogen compounds. The expensive nature of most artificial nitrogenous
manures also gives to nitrogen the first position from an economic point
of view. A thorough study, therefore, of the different forms in which it
exists in nature, of the numerous and complicated changes it undergoes
in the soil, by which it is prepared for the plant's needs, of the
relation of its different forms to plant-life, and of the natural
sources of its loss and gain, is of the highest importance if we are to
hope to understand the difficult question of soil-fertility.


_The Rothamsted Experiments and the Nitrogen question._

The position of nitrogen in agriculture is a question of great
difficulty and complexity. It has engaged much attention, and has had
devoted to its elucidation much elaborate and painstaking research. To
the Rothamsted experiments we owe most of the information we possess on
the subject, and the facts contained in this chapter are almost entirely
derived from the results of these famous experiments, as embodied in the
memoirs and writings of Messrs Lawes, Gilbert, and Warington.


_Different forms in which Nitrogen exists in Nature._

We have already referred to the nitrogen question in the historical
introduction. In order, however, to have a comprehensive view of the
subject, it may be well to recapitulate some of the facts there
mentioned.

Nitrogen, as we have already seen, exists in the "free" or elementary
condition, as nitrates and nitrites, as ammonia, and in a large number
of different organic forms.


_Nitrogen in the Air._

It occurs in greatest abundance (amounting to about 80 per cent) in the
first of these forms in the air. That this free nitrogen, which is
practically unlimited in quantity,[63] has originally been the source of
all its other forms, is of course obvious. But this conversion of free
nitrogen into the various compound forms in which it occurs throughout
the mineral, vegetable, and animal kingdoms, has been a process effected
by a variety of indirect methods, and only at the expense of a vast
amount of time. For practical purposes, the free nitrogen of the air may
be regarded chiefly as a non-available source for most bodies containing
it. It may be described as of all forms of nitrogen the least active, as
far as plant-life is concerned.


_Relation of "free" Nitrogen to the Plant._

The relation of the "free" nitrogen to the plant has formed the subject
of much research, more especially during the last few years, and a brief
epitome of the main results arrived at has already been given in the
Introductory Chapter.[64]

That this source of nitrogen is not so inaccessible to the plant as was
formerly believed, has now been abundantly proved. As the considerations
which have led to this conclusion, and have suggested the very recent
elaborate experiments on the fixation of free nitrogen by the plant--the
results of which bid fair, it would seem, to largely revolutionise our
agricultural practice--have been due to the study of the relation of the
soil-nitrogen to the plant, it will be best to defer further discussion
of this question till we have dealt with the other sources of nitrogen.


_Combined Nitrogen in the Air._

In addition to nitrogen in the free state, air contains very small
quantities of this element in combined forms. We have it in minute
traces as nitrates and nitrites, as ammonia,[65] and also in still
smaller traces as organic nitrogen in the minute dust-particles which
modern researches have revealed as being present in such enormous
numbers in our atmosphere. What the sources of these nitrates and
nitrites (which exist in quantities so minute that accurate
determination of their amount is rendered extremely difficult) are is a
disputed point. That nitrogen and oxygen unite together to form nitric
and nitrous oxides under the influence of intense heat, such as the
electric spark, has been proved beyond doubt. One source, therefore, is
probably the electrical discharges which are taking place more or less
frequently on different parts of the earth's surface. Nitrates may also
be formed in the combustion of nitrogenous bodies.[66] In the burning of
coal-gas, for example, it is probable that small quantities of nitrates
may be produced. Similarly the slow combustion or decay of nitrogenous
organic matter, which constantly takes place all over the earth's
surface, may be regarded as another source of this form of combined
nitrogen. Ammonia may be similarly formed by the combustion, either
quick or slow, of nitrogenous organic matter. It exists in the air as
nitrate or nitrite of ammonia, and also as carbonate of ammonia.[67]


_Amount of combined Nitrogen falling in the Rain._

The importance of the combined nitrogen in the air as a source of
soil-nitrogen is best gauged by the amount falling annually on the soil
dissolved in rain. This has been found to vary considerably. In the rain
falling in the vicinity of large towns the amount is greater than in
rain falling in the country. Thus at Rothamsted, in England, the average
amount for several years was only 3.37 lb. nitrogen per annum per acre,
of which 2.53 lb. were as ammonia,.84 being as nitric acid. At Lincoln,
in New Zealand, 1.74 lb. fell annually per acre--as ammonia,.74, as
nitric acid, 1.00; while at Barbadoes the amount was 3.77 lb., of which
.93 was as ammonia, and 2.84 as nitric acid.[68] That the combined
nitrogen derived from the air by the soil may be considerably in excess
of this is highly probable. Soils, especially when damp, may absorb much
larger quantities from the air of the combined nitrogen it contains. We
must remember that the air in contact with the soil-surface is
constantly being changed, and that there is thus a constant renewal of
the air passed over the ground. The result is that the amount of air
from which combined nitrogen may be removed is very great.[69]


_Nitrogen in the Soil._

It has been remarked as a fact worthy of notice that nitrogen is
essentially a superficial element. By this is meant that it is only
found, as a rule, on the earth's immediate surface. This statement can
only be admitted to be true within certain limits. The chief source of
nitrogen, in addition to the atmosphere, is, of course, vegetable and
animal tissue.[70] As vegetable and animal tissue are only found to any
extent on the earth's surface, nitrogen is therefore chiefly found
there. The natural deposits of nitrogen salts, such as the
nitrate-fields of Chili and the saltpetre soils of India, &c., also
only occur superficially. Notwithstanding these facts, however, the
amount of nitrogen which exists at probably considerable depths from the
surface must be very great. There are few sedimentary rocks which do not
contain it. At Rothamsted a sample of calcareous clay, taken from a
depth of 500 feet, contained .04 per cent--that is, as much as is found,
on an average, in the Rothamsted clay subsoils.


_Nitrogen in the Subsoil._

On the whole, however, as we have said, nitrogen is chiefly found in the
surface-soil. The amount found in the subsoil at Rothamsted seems to
vary very slightly at different depths, the percentage amounting to from
.06 to .03.[71] Unlike the nitrogen of the surface-soil, that in the
subsoil seems to be of very ancient origin, being probably derived from
the remains of animal and vegetable life in the mud deposited at the
bottom of the ocean. It is more abundant in the case of a clay subsoil
than in a sandy subsoil.


_Nitrogen of Surface-Soil._

Nitrogen has a tendency to collect on the top layers of the
surface-soil, the first 9 inches or foot containing by far the largest
proportion of it. In the table given in the Appendix,[72] the rate at
which it decreases in amount the further down we go is clearly shown.
Determinations of the respective amounts of nitrogen in every 3 inches
of the soil, taken to a depth of one foot of the experimental
wheat-field at Rothamsted, showed that the percentage between the first
3 inches and the second 3 inches varied very slightly. A more marked
difference, however, was shown to exist between the nitrogen in the
second and third 3 inches; while the fourth 3 inches were distinctly
poorer--differing very little in their percentage of nitrogen from the
subsoil. This was the case in unmanured soil. In the case of heavily
manured soil, the increase in the soil's percentage, due to manure, was
shown to be felt to the depth of a foot, but not much below it.[73]

A careful perusal of the tables in the Appendix will show that the
quantity of nitrogen in the case of both arable and pasture soils
steadily decreases for the first 3 feet, but that below this depth
little decrease is seen, the percentage evidently becoming fairly
constant.


_The amount of Nitrogen in the Soil._

Very considerable difference exists in the amount of nitrogen present in
different soils. The majority of analyses refer only to the amount found
in the surface-soil--generally in the first 9 or 12 inches. As the soil,
further, is not a body exactly homogeneous in its character, very
considerable difficulty exists in obtaining reliable results. A great
deal depends, therefore, on the method of sampling and the basis of
calculation adopted; and it may be that this may occasionally explain,
to some extent at least, the great discrepancies in the estimation of
the quantities of nitrogen present in different soils as found by
different investigators.


_Peat-soils richest in Nitrogen._

Of all soils, peat-soils are richest in nitrogen. Professor S. W.
Johnson found the nitrogen in fifty separate samples of peat to range
from .4 per cent to 2.9 per cent, the average being 1.5 per cent. On the
other hand, marls and sandy soils are poorest, the analyses of a number
of these soils showing only from .004 to .083 per cent for the former,
and .025 to .074 for the latter. As a general rule most arable soils
contain over one-tenth per cent of nitrogen, or, say, over 3500 lb. per
acre. A good pasture-soil, taken to a depth of 9 inches at Rothamsted,
was found to contain about a quarter per cent. In ten samples of soil,
taken to a depth of 9 inches, from different parts of Great Britain and
Ireland, Munro found from .128 to .695 per cent of nitrogen, the average
being .3278 per cent. The Rothamsted soils, it may be pointed out, are
probably poor in nitrogen compared with most soils. A. Müller's
investigations showed that in some of the soils he has analysed, the
nitrogen amounted to little short of one per cent, while for the others
the average was over half a per cent; even the poorer soils he examined
contained about one quarter per cent on an average. Anderson's analyses
of Scottish wheat-soils showed a variation of from .074 to .22 in the
surface-soil, while he found in their subsoil from .15 to .92 per cent.
Boussingault's results are also very much higher. The amount of nitrogen
in a number of loams coming from widely different localities he examined
contained from 6000 to 30,000 lb. per acre--the soil taken to a depth of
17 inches.[74]


_Nature of the Nitrogen in the Soil._

When we compare the amount of nitrogen removed by different crops
(which, even in the case of those most exhaustive of nitrogen, does not
often amount to more than 150 lb. per acre), with the amount contained
in the soil, the former amount seems very insignificant when compared to
the latter. Such being the case, it would seem at first sight that the
addition of nitrogen in the form of manures is quite superfluous. We
must remember, however, that while the _total_ amount of nitrogen is
relatively large when compared to that removed by crops, only a very
small proportion is in a condition _available_ to the plant. This leads
us to consider the different forms in which nitrogen is present in the
soil, and their respective quantities.


_Organic Nitrogen in the Soil._

Nitrogen occurs in the soil as organic nitrogen, nitric acid, nitrous
acid, and ammonia. By far the largest proportion is present in the first
of these forms. This is a wise provision, for otherwise the soil would
be apt to become very speedily impoverished in nitrogen; for that
present as nitrates it has scarcely any power to retain, while that
present as ammonia is soon converted into nitrates by the process of
_nitrification_.

The organic nitrogen of the soil, although we are apt to think of it as
such, is by no means of a homogeneous character, or of equal value as a
source of plant-food. Some of it, it would seem from recent
investigations, is in a condition more susceptible of being converted
into an available form than the rest. Thus in the process of
nitrification, a process which we shall consider at length immediately,
there seems to be generally a certain small proportion more ready to
undergo this change than the rest; so that when this small amount is
used up nitrification proceeds more slowly. In short, although we as yet
know very little of the nature of the organic nitrogen of soils, we
cannot doubt but that there is a constant series of changes in its
composition taking place, resulting in the gradual elaboration of more
available forms, until ultimately these are converted into ammonia and
nitrates.

The great bulk of the organic nitrogen, however, in the soil must be
regarded as in an _inert_ condition, and by no means available for the
crop. What the exact chemical form of this nitrogen is it is extremely
difficult to say. Mulder was of the opinion that a considerable
proportion was in the form of humate of ammonia. This opinion, as we
shall have occasion to see immediately, was based on false grounds. It
is highly probable that it may be in some form approximating to amide
nitrogen. Its inert character is against the belief that it long remains
as albuminoid nitrogen.


_Different Character of Surface and Subsoil Nitrogen._

A point of very considerable importance to notice is, that the
nitrogenous organic matter of the surface-soil is very different from
that found in the subsoil. This difference is shown by the variation in
the ratio of nitrogen to carbon, which points to the fact that, just as
we should naturally suppose, the origin of the latter is very much more
ancient than the origin of the former. Thus in the first 9 inches of old
pasture-soil at Rothamsted, the ratio was 1:13; while in the subsoil, 3
feet from the surface, it was only 1:6. In the surface-soil it thus
approaches more nearly in composition ordinary vegetable matter.


_Nitrogen as Ammonia in Soils._

The second form in which nitrogen is present in soil is as ammonia. A
very considerable misapprehension has existed in the past as to the
amount of nitrogen in this form in soils. This mistake was due to the
method adopted in estimating it, which consisted in treating the soil
with boiling caustic alkalies and counting as ammonia what was given off
as such. It is now known that certain forms of organic nitrogen--as, for
example, amides--if treated in this way are slowly converted into
ammonia. Statements, therefore, which are found in the older text-books,
representing the amount of ammonia in soils as at over a tenth per cent,
must be regarded as utterly unreliable. Indeed it is highly probable
that ammonia only occurs in most soils in very minute traces. From what
we know of the process of nitrification, we see how it is wellnigh
impossible that ammonia should exist to any extent in the soil except
under very exceptional circumstances.


_Amount of Ammonia present in the Soil._

In ordinary soils it probably does not amount to more than from .0002
per cent to .0008 per cent, or an average of .0006 per cent.[75] In
rich soils, or in garden-soils, the amount may be considerably more.
Thus Boussingault found in a garden-soil .002 per cent. In peat and in
peat-mould even a higher percentage has been found--viz.,.018 for the
former and .05 for the latter.


_Nitrogen present as Nitrates in the Soil._

The third form of nitrogen in the soil is nitric acid. It is more
abundant in this form than as ammonia; but still, compared with the
organic nitrogen, its amount is trifling. Probably not more than 5 per
cent of the total nitrogen of a soil is ever present as nitrates. The
reason of this is twofold. First, as we have already remarked, the soil
has very little power to retain nitrogen in this form; and secondly,
where the soil is covered with growing vegetation the nitrates are
quickly assimilated by the plant as they are formed. It is for this
reason that we find the quantity of nitrogen as nitrates very much
greater in fallow soils than in those covered with a crop.


_Position of Nitric Nitrogen in Soil._

As we shall have occasion to see more fully in the following chapter on
Nitrification, the formation of nitrates is chiefly limited to the
surface-soil, the largest proportion being formed within the first 9 or
12 inches. For this reason we find the largest quantity of nitrates in
the surface-soil. But inasmuch as they are easily washed into the lower
layers of the soil after formation, we often find a considerable
proportion beyond the first 9 inches. The position of nitrates in the
soil thus depends very considerably on the season of the year and the
weather. In dry weather, where the evaporation of the soil-water takes
place at a considerable rate, the tendency will be to concentrate the
nitrates in the superficial portion of the soil. In wet weather, on the
other hand, the tendency will be to wash the nitrates into the lower
layers.


_Amount of Nitrates in the Soil._

The determination of the amount of nitrates in a soil is not of very
great economic importance; as this varies so much, and depends on such a
number of different conditions, such as the season, the condition of the
land, and prevailing weather. A point of very much greater economic
importance is the total amount formed in the year, and the rate at which
nitrification takes place. These questions will be discussed elsewhere,
and therefore need not here be referred to. Some interesting analyses
made at Rothamsted, however, of the amount of nitrates in soils at
different depths, merit careful consideration.


_Nitrates in Fallow Soils._

In the Appendix to the chapter on Nitrification,[76] will be found a
table containing the amounts of nitrates found in the first 27 inches of
fallow soils. The amounts vary from 33.7 lb. to 59.9 lb. per acre. The
analyses were made in September or October. In four out of the six
analyses, it will be found that by far the largest proportion is found
in the first 9 inches. In these cases the preceding summer had been dry,
and thus the nitrates had not been washed down to any depth. In the
other two cases the largest amount is found in the second 9 inches of
soil, and a considerable amount is also found in the third 9 inches.


_Nitrates in Cropped Soils._

In the case of cropped soils we find the amount of nitrates very much
less. A table containing an elaborate series of determinations of
nitrates in cropped soils, receiving, however, no manure, and taken to a
depth of 9 feet, will be found in the Appendix.[77] The first 27 inches
only contain some 5 to 14 lb. per acre, and the most of that is found in
the first 9 inches. This shows how speedily nitrates are assimilated by
the growing crop. An interesting point shown by these analyses is that
nitrates almost entirely cease in cropped soils a certain depth down,
but that at a still lower depth they again occur in small quantities.


_Nitrates in manured Wheat-soils._

Lastly, we give in the Appendix[78] the amount of nitrates found in
wheat and barley soils, differently manured, at Rothamsted. From a
perusal of these tables, it will be seen that the amount (under various
conditions of manuring) of nitrates in the first 27 inches varies from
21.2 lb. per acre to 52.2 lb. for the wheat-soils, and 20.1 to 44.1 lb.
per acre for the barley-soils.


THE SOURCES OF SOIL-NITROGEN.

We shall now consider the sources of soil-nitrogen, the conditions which
determine its increase, and the amount of that increase, as well as the
sources of loss, and the conditions which determine this loss.


_That dissolved in Rain._

The natural sources of the soil-nitrogen are several. We have first of
all the atmospheric nitrogen. Of this let us first consider that present
as combined nitrogen. This, as we have already seen, consists chiefly of
nitrates, nitrites, and ammonia, and reaches the soil dissolved in rain
or in other meteoric forms of water, such as snow, hail, fog,
hoar-frost, &c.


_That absorbed by the Soil from the Air._

It is also absorbed by the soil from the air, especially when the soil
is in a damp condition, as has been proved by Schloesing's experiments,
already referred to. The total amount which falls dissolved in the rain,
per acre per annum, varies very considerably in different parts of the
world, but in any case only amounts yearly to a few pounds per acre.[79]
That absorbed by the soil from the air may be probably very much more
considerable. Schloesing in his experiments found that this latter might
amount to 38 lb. per acre per annum. These results, however, were
obtained under circumstances most favourable for absorption--viz., with
a damp soil and in the vicinity of Paris, where the air is presumably
richer in combined nitrogen than it is in the country. The nitrogen
absorbed, it may be mentioned, was almost entirely in the form of
ammonia. It is to be noted that the nitrogen the soil obtains in this
way from the combined nitrogen of the air is not all pure gain. With
regard to the nitrates and nitrites, no doubt most of these are formed
by electrical discharge, although a small portion of them may be formed
by the oxidation of ammonia by means of ozone and peroxide of hydrogen.
With regard to the ammonia and the combined nitrogen present in the
organic particles in the air, a not inconsiderable proportion is
probably derived from the soil. Schloesing considers the chief source of
the ammonia present in the air to be the tropical ocean; but we must
remember that the source of much of the nitrogen in the tropical ocean
is, after all, the soil.

Leaving aside for a moment the question of the availability of the free
nitrogen of the air, let us consider the other sources of soil-nitrogen.


_Accumulation of Soil-nitrogen under Natural Conditions._

The chief source is of course the remains of vegetable and animal
tissue.[80] Plants are the great conservers of soil-nitrogen. By
assimilating such available forms of it as nitrates, and converting them
into organic nitrogen, they prevent the loss of this most valuable of
all soil constituents that would otherwise take place.

They also serve to collect the nitrogen from the lower soil-layers and
concentrate it in the surface portion. In a state of nature, where the
soil is constantly covered with vegetation, the process going on,
therefore, will be one of steady accumulation of nitrogen in the
surface-soil. To what extent this accumulation goes on, and how far it
is limited by the conditions of loss, will be considered immediately.
That it may go on to a very great extent is amply proved by the
existence of the so-called _virgin_ soils of countries like America and
Australia. There are cases, also, where the accumulation of nitrogen is
practically unlimited, although the result in such cases is not
necessarily a fertile soil. Such cases are peat-bogs. But let us pass on
to the accumulation of soil-nitrogen under the ordinary conditions of
husbandry.


_Accumulation of Nitrogen in Pastures._

The case which, under the conditions of ordinary farming, most resembles
a state of nature, is that of permanent pasture. It will be best,
therefore, to study first the conditions under which gain of nitrogen
takes place in this case.


_Increase of Nitrogen in the soil of Pasture-land._

That there is a steady increase of nitrogen in the soil of land under
pasture is a fact of universal experience. The older a pasture is the
richer is its soil in nitrogen. The comparison of the analyses of the
soil of arable land with the soil of pastures of different ages shows
this in a striking way.[81] Thus at Rothamsted it was found that while
the amount of nitrogen in an ordinary arable soil was .140 per cent,
that in pastures eight, eighteen, twenty-one, and thirty years old was
respectively .151, .174, .204, and .241 per cent. In the last two
analyses we have a record of the actual gain in nitrogen made by the
same pasture, this being .04 per cent in nine years' time. From these
statistics it may be inferred that the surface-soil of a pasture may
increase at the rate of 50 lb. per acre per annum. A point of great
interest in connection with this subject is the fact that there seems to
be a limit to the accumulation of nitrogen in pastures; for it would
seem that pastures centuries old are not any richer in nitrogen than
those thirty to forty years old.


_Gain of Nitrogen with Leguminous Crops._

Another case where the gain of nitrogen to the surface-soil is very
striking is in that of leguminous crops, such as clover, beans, peas,
&c. This fact has been long recognised--especially with regard to
clover--by farmers, and has been largely instrumental in leading to the
investigation of the "free" nitrogen question. That a soil bearing a
leguminous crop increases in nitrogen at a very striking rate is a
problem that requires to be solved. A partial explanation of the
phenomenon is found in the extraordinary capacity such a crop as clover
has, by means of its multitudinous and ramifying roots, for collecting
nitrogen from the subsoil. This, however, would only account for the
increase in nitrogen to a certain extent. There must be some other
source, and the only other source is the air. That the free nitrogen of
the air is, after all, available for the plant's needs, is a supposition
which has long seemed extremely probable, and which, within the last
few years, has been proved beyond doubt to be a fact in the case of
leguminous plants.


_The Fixation of "Free" Nitrogen._

The method in which these plants are able to make use of the free
nitrogen is still a point requiring much research. So far as the
question is at present investigated, it would seem that the fixation is
effected by means of micro-organisms present in tubercles or root
excrescences found on the roots of leguminous plants.[82] Not merely has
this been placed beyond doubt, but attempts have been made to isolate
and study the bacteria effecting this fixation. From Nobbe's exceedingly
interesting experiments, recently carried out, it would seem that the
different kinds of leguminous plants have different bacteria. Thus the
bacteria in the tubercle on the pea seems to be of a different order
from the bacteria in the tubercles of the lupin, and so on. This
discovery is of great importance, it need scarcely be pointed out, as it
throws much light on the principles of the rotation of crops.


_Influence of Manures in increasing Soil-nitrogen._

It may be doubted, however, if under any other conditions there is a
positive gain of soil-nitrogen. In other cases the amount in the soil is
only _maintained_ under liberal manuring. In connection with this point
a very striking fact has been observed with regard to the effect of
continuous large applications of farmyard manure. It has been found at
Rothamsted that in such a case, after a while, the manure does not seem
to increase the soil-nitrogen, although where the nitrogen goes to
remains a mystery. In the case of the application of artificial manures,
there does not seem to be almost any appreciable gain to the
soil-nitrogen. The soil-nitrogen is only increased by means of the
residue of crops. In this way, of course, by increasing the amount of
this crop-residue, artificial manures may be said indirectly to increase
the soil-nitrogen.[83]


SOURCES OF LOSS OF NITROGEN.

We now come to consider the sources of loss. The chief source, of
course, is that by drainage. Land under cultivation will suffer very
much more from this source of loss than in a state of nature. Our modern
system of husbandry, involving as it does thorough drainage, can
scarcely fail to very considerably increase this source of loss.


_Loss of Nitrates by Drainage._

The form in which nitrogen is lost in this way is as nitrates. It is a
somewhat striking fact, and one worthy of note, that of the three
important manurial ingredients--nitrogen, phosphoric acid, and potash,
the first of these, in its final and most valuable form, is alone
incapable of being fixed by the soil, and thus retained from loss by
drainage.

As nitrates are constantly being formed in the soil, the loss to its
total nitrogen must be considerable. It is due to the fact of the great
solubility of nitrates, as well as to the fact, as already mentioned, of
the incapacity of the soil-particles to fix them. To this one exception
must be made. According to Knop, small quantities of nitric acid are
held in the _insoluble_ condition in soils in the form of highly _basic
nitrates of iron and alumina_. The quantity, however, of these insoluble
compounds probably amounts to a very minute trace indeed.


_Permanent Pasture and "Catch-cropping" prevents Loss._

The amount of loss varies, and will depend on a number of different
circumstances--thus the nature of the soil, climate, and season of the
year will all influence its quantity. The way in which the soil is
cultivated is also another important factor. Where it is constantly
covered with vegetation, as in the case of permanent pasture, the loss
will be at a minimum. Under such conditions, plant-roots are always
there ready to fix, in the insoluble organic form, the soluble nitrates
as they are formed. A consideration of this fact forms one of the
strongest arguments in favour of the practice of what is known as
"catch-cropping." The practice consists in sowing some quickly-growing
green crop--_e.g._, _mustard_, _vetches_, &c.--so as to occupy the soil
immediately after harvest, and subsequently to plough it in. The
nitrates, which it is known are most abundantly formed towards the end
of summer,[84] and which are allowed to accumulate in the soil from the
period at which the active growth of, and consequently assimilation of
nitrates by, the cereal crop have ceased, are thus fixed in the organic
matter of the plant, and removed from danger of loss by drainage
incidental to autumn rains.


_Other Conditions diminishing Loss of Nitrates._

The nature of the soil is another important condition regulating this
loss. Some soils are very much opener and more porous than others; in
such soils, of course, the loss by drainage will be greatest. We are apt
at first sight, however, knowing the great solubility of nitrates, to
overrate this source of loss. We have to remember that while nitrates
are constantly being washed down to the lower layers of the soil, there
is likewise an upward compensating movement of the soil-water constantly
taking place. This is due to the evaporation of water from the surface
of the soil, which induces an upward capillary movement of water from
its lower to its higher layers.[85] This upward movement of water is
very much increased, in the case of soil covered with vegetation, by the
transpiration of the plants. The climate and the season of the year will
affect the extent of this upward movement. Where there is a heavy
rainfall it will be very much less than in dry climates. After a long
period of drought the nitrates will be found to be concentrated in the
top few inches of the soil; and in hot climates this sometimes takes
place to such an extent that the surface of the soil has been actually
covered with a saline crust, caused by the rapid evaporation of
soil-water under the influence of a burning tropical sun. From this
point of view it will be seen how very much less powerful a single
shower of rain is--even although at the time it is heavy--in causing
loss of nitrates by drainage, than a continuance of wet weather. In the
former case, where the showers are separated by an interval of dry
weather, the nitrates washed down into the lower layers of the soil are
slowly brought up again by the capillary action caused by evaporation.


_Amount of Loss by Drainage._

What the actual amount of loss is which takes place in this way it is
wellnigh impossible to say. What it amounts to under certain definite
circumstances has been discovered by actual experiment at Rothamsted.
Taking the circumstances most favourable to extreme loss--viz.,
unmanured fallow land--the highest amount registered at Rothamsted for a
year is 54.2 lb. per acre from soil 20 inches deep, while the smallest
amount is 20.9 lb. In the former case, the drainage-water was equivalent
to 21.66 inches, while in the latter, to 8.96 inches. The average for
thirteen years on unmanured fallow soil has been 37.3 lb. (for 20
inches), 32.6 lb. (for 40 inches), 35.6 lb. (for 60 inches). The point
of especial interest in this connection is that an annual loss of
nitrogen, equal to over 2 cwt. of nitrate of soda, may take place from a
comparatively poor arable soil lying fallow.

The loss on cropped soils is of course very much less--in short, should
amount to very little--especially in permanent pasture, where it is
reduced to a minimum. Taking an average, Mr Warington is of opinion that
the loss in England may be put at 8 lb. per annum per acre.[86]


_Loss in Form of Free Nitrogen._

The other chief natural source of loss of nitrogen is due to its escape
from the soil in its "free" state. This source of loss is very much less
important than that by drainage, and probably amounts to very little.
That, however, it takes place is beyond a doubt; and that it may--as we
shall see by-and-by--under certain circumstances amount to something
very considerable is also proved. Where large quantities of nitrogenous
organic matter decay, and where, consequently, the supply of atmospheric
oxygen is insufficient to effect complete oxidation, "free" nitrogen may
be evolved in considerable quantities. Similarly, it may be evolved in
the case of vegetable matter decaying under water. In soils rich in
organic matter the reduction of even nitrates may take place,
accompanied with the evolution of free nitrogen, which is thus lost.


_Total Amount of Loss of Nitrogen._

What the rate of total loss of nitrogen is from these different sources
does not admit of easy calculation. Sir John Lawes, in dealing with the
question of soil-fertility, estimated some years ago, by comparing the
soil of old pasture at Rothamsted with that which had been under arable
culture for 250 years, that during that period some 3000 lb. of nitrogen
per acre had disappeared from the arable land. Examples of decrease of
nitrogen in Rothamsted soils, under various conditions of culture, will
be found in the Appendix.[87]


_Loss of Nitrogen by Retrogression._

A source of loss of nitrogen may be here mentioned which has to do with
diminution of amount of available nitrogen, rather than absolute loss of
nitrogen to the soil, and which we may term _loss by retrogression_.
Nitrogen in an available form, such as nitrates, has been found to be
converted into a less available form. This retrogression may be
effected, as in the case of nitrates, by reduction--_i.e._, by removal
of the oxygen in combination with the nitrogen, which in many cases may
be set free, and thus partially although not necessarily entirely lost.
Such reduction is due to the action of bacteria of the denitrifying
order.[88] Or, on the other hand, nitrogen may be converted into some
kind of insoluble form which seems to resist decomposition and lies in
an inert condition in the soil utterly unavailable for the plants'
needs. A striking example of this retrogression of nitrogen seems to be
afforded in the case of farmyard manure. It has been found in the
Rothamsted experiments, as has been pointed out in the preceding pages,
that when farmyard manure is applied, year after year, to the same land
in large quantities, a very considerable percentage of its nitrogen does
not (_i.e._, within a reasonable number of years) become available for
the crop's uses. What, indeed, becomes of the nitrogen is a mystery; but
it is highly probable that some such kind of retrogression as that above
referred to, whereby the nitrogen is converted into some inert organic
form, takes place.


_Artificial Sources of Loss of Nitrogen._

So far, the sources of loss of nitrogen considered have been what we may
term _natural_ sources. By this is meant that the loss of nitrogen from
the above sources takes place in a state of nature, and not merely under
conditions of cultivation. No doubt the loss due to drainage is very
much greater under arable farming than would be the case where
artificial drainage does not obtain; still, under any conditions, this
loss must be reckoned with. On the other hand, by _artificial_ sources
of loss are meant those entirely dependent on our modern system of
agriculture and our modern system of sewage disposal, whereby the
nitrogen contained in that portion of the produce of the farm which goes
to supply our food is not returned to the soil, but is totally lost.


_Amount of Nitrogen removed in Crops._

The modern tendency towards centralisation in large towns has rendered
this loss--despite all that has been said to the contrary--a necessity.
It is extremely difficult, however, to form any estimate of its amount.
We know, of course, the amount of nitrogen removed from the soil by
different crops. We cannot, however, estimate how much of this may find
its way back again to the soil. The amount of nitrogen contained in the
different crops will be fully dealt with in the chapter on the manuring
of different crops. It may be, however, not without interest to give
here some approximate indication of the amount of this loss, in order to
render the view of the subject as comprehensive as possible.

Recent agricultural returns for Great Britain give the total produce of
_wheat_ at over 76 million bushels, that of _barley_ at over 69 million,
and that of _oats_ at over 150 million. Calculating the amount of
nitrogen, these quantities of wheat, barley, and oats respectively and
collectively contain, and calculating also how much _sulphate of
ammonia_ and _nitrate of soda_ these amounts of nitrogen represent, the
following are the results:--

                         Nitrogen.   Sulphate of   Nitrate of
                                      Ammonia.       Soda.
           Bushels.        Tons.        Tons.        Tons.
Wheat      76,224,940      37,432      176,465      227,266
Barley     69,948,266      27,324      128,813      165,896
Oats      150,789,416      56,835      267,936      345,068
          -----------     -------      -------      -------
Total     296,962,622     121,591      573,214      738,230
          ===========     =======      =======      =======

Of course these figures, so far as the amounts of nitrogen are
concerned, can only be regarded as approximate, as it is only possible
in such calculations to obtain approximate results. Accepting these
calculations as merely approximate, they are, nevertheless, of the
highest interest and importance. It is of great importance to understand
that in the annual produce of our three common cereal crops--supposing
them to be all consumed off the farm--there is removed from the soil a
quantity of nitrogen equal to that contained in over _half a million
tons of sulphate of ammonia, and three quarters of a million tons of
nitrate of soda._

As has already been remarked, it is impossible to estimate exactly what
proportion of this total nitrogen finds its way back to the soil. In the
case of wheat, it may be pointed out that the portion which is used as a
feeding-stuff--viz., _bran_--is very much richer in nitrogen than the
flour. While, then, we are unable to estimate with any exactitude this
source of loss of nitrogen, it cannot for a moment be doubted that it is
enormous, from what has been already stated. We must remember that the
portion of the crop richest in nitrogen is that which is generally
removed--the straw which is grown in producing a bushel of wheat,
barley, or oats, containing less than half the amount of nitrogen
contained by a bushel of the grain itself.


_Losses of Nitrogen incurred on the Farm._

In addition to the loss due to removal of crops from the farm, there are
one or two other sources of loss which it may be well to briefly refer
to.


_Loss in Treatment of Farmyard Manure._

There can be little doubt that in the past a very considerable source of
loss was the improper treatment of farmyard manure. The way in which
this loss may take place will be fully considered in the chapter on
farmyard manure. Suffice it to say here, that this may take place by
volatilisation of the nitrogen as carbonate of ammonia, caused by
carelessness in allowing the temperature of the manure-heap to rise too
high; or by drainage of the soluble nitrogen compounds, caused by
allowing the rich black liquor of the manure-heap to be washed away, and
not properly conserved.


_Nitrogen removed in Milk._

Another source of loss which is apt to be overlooked is the amount of
nitrogen removed in milk. Professor Storer has calculated that in the
case of a cow giving 2000 quarts, or 4300 lb., of milk in a year, and
the milk being all sold as such, there would be carried away from the
farm 22 lb. of nitrogen.[89]


_Economics of the Nitrogen question._

And here, before concluding our survey of the different sources of loss
of nitrogen, it may be well to regard for a moment the subject from a
somewhat wider standpoint than that from which we have been considering
it. The total supply of nitrogen in a combined form is limited. As we
have pointed out, it may be regarded as the element on which, more than
any other, life, animal as well as vegetable, depends. To animal life it
is alone available in combined form; to vegetable life it is chiefly
also only available in combined form. In the air we have an unlimited
quantity of nitrogen, but it is almost entirely in an _uncombined_ form,
and therefore largely unavailable. The conversion of nitrogen from the
free state to a combined form is a process which takes place only very
slowly. Any source which diminishes the sum-total of our already all too
limited supply of combined nitrogen must be regarded as worthy of most
serious consideration. The question, therefore, of the artificial waste
of nitrogen daily taking place around us, is one which ought to possess
for economists a very great interest indeed. This waste has, of late
years, enormously increased, and would seem to threaten us at no very
distant date with a nitrogen famine. It is incidental to the use of
certain nitrogenous substances in the manufacture of various articles,
and to our present system of sewage disposal.


_Loss of Nitrogen-compounds in the Arts._

The articles referred to are such as explosives, starch, textile
substances, malt liquors, &c. The question is strikingly dealt with in
an able paper on "The Economy of Nitrogen" in the 'Quarterly Journal of
Science.'[90]


_Loss due to Use of Gunpowder._

The explosives--more particularly gunpowder--are the most important of
these articles. Gunpowder contains 75 per cent of saltpetre, which in
its turn contains about 10 per cent of nitrogen. When gunpowder
explodes, practically the whole of this nitrogen is converted into
"free" nitrogen. The loss is thus in a sense irreparable. In the paper
above, referred to, our total annual exports of this substance are
estimated at 19,000,000 lb.; while the total annual production of the
world is estimated at not less than 100,000,000 lb. The annual loss of
nitrogen due to this source alone would amount to about 10,000,000
lb.[91] Similarly, loss of nitrogen, although to a less extent, is
caused by the use of other explosives, as well as in the manufacture of
the other articles above mentioned.


_Loss due to Sewage Disposal._

The loss due to our present system of sewage disposal has been already
taken into account in dealing with the loss due to removal of crops. It
may be well, however, to treat it from the sewage aspect. Taking the
amount of nitrogen in the excreta of every individual as, on an average,
half an ounce, the annual amount voided in the excreta of the total
population of the British Isles would amount to 365,000,000 lb.[92]--of
this, the amount in the London sewage alone being 91,000,000 lb.[93] By
the water system, which is almost universally adopted in this country,
the above quantity of nitrogen is entirely lost to the soil. A small
portion of it, it may be argued, is eventually recovered in sea weed and
fish, which may be used for manure. This, however, is to argue too much
_sub specie æternitatis_. Not all the nitrogen originally present in the
excreta finds its way into the sea; for it is highly probable that a
considerable quantity escapes in the process of the decomposition of the
sewage as "free" nitrogen.

From the above statement of the sources of loss and gain of nitrogen
taking place in the soil, it may be pretty safe to conclude that while
in a state of nature the gain balances the loss, if indeed it does not
do more, under conditions of arable farming such is very far from being
the case; and that if fertility of the land is to be maintained,
recourse to nitrogenous manures must be had,--in short, that the
application of artificial nitrogenous manures is a necessary condition
of modern husbandry.


_Our Artificial Nitrogen Supply._

Before concluding this chapter, it may be interesting to enumerate very
briefly the chief sources of our artificial nitrogen supply.


_Nitrate of Soda and Sulphate of Ammonia._

The most important artificial nitrogenous manures in use at present are
nitrate of soda and sulphate of ammonia. Of the former, the annual
exportation from Chili is close on one million tons, of which quantity
about 120,000 tons is imported into the United Kingdom. Of sulphate of
ammonia, on the other hand, the total production in this country is
about 130,000 tons per annum,[94] the greater proportion of which is
exported, leaving only from 30,000 to 40,000 tons for consumption.
Nitrate of soda, it must be remembered, is not entirely used for
manurial purposes, a small proportion of the above imports being used
for chemical manufacturing purposes.


_Peruvian Guano._

Peruvian guano is another important nitrogenous manure very much less
abundant now than formerly, as the different guano-beds have become
nearly exhausted. While the imports of this important manure into the
United Kingdom amounted in 1870 to nearly 250,000 tons, at present not
more than 11,000 tons are being imported.


_Bones._

A further source of nitrogen is bones, which, of course, are chiefly
valuable as a phosphatic manure, but which contain also some 3 to 4 per
cent of nitrogen. Of this valuable manure we import at present about
30,000 tons, while about 60,000 tons are collected in this country,
bringing up our total consumption to 100,000 tons.


_Other Nitrogenous Manures._

The above mentioned are the most important of nitrogenous manures; there
are, however, a number of other nitrogenous manures used in this country
in very much smaller quantities. As most of these substances are made in
this country, it is very difficult to estimate the amount of their
annual production with exactness. These substances are as follows:
fish-guano, meat-meal guano, dried blood, shoddy, scutch, horns and
hoofs, hair, bristles, feathers, leather-scrap, &c. Of fish-guano, the
total consumption per annum may be put down at about 8000 tons, of which
a fourth is imported into this country, the remaining 6000 tons being
manufactured at home. Of meat-meal guano, dried blood, hoof-guano, &c.,
about 2500 tons are annually imported, the home production bringing up
the total amount to some 10,000 tons. Of shoddy, some 12,000 tons are
manufactured in this country; while scutch--the name given to a manure
manufactured from the waste products incidental to the manufacture of
glue and the dressing of skins--is produced only to the extent of a few
thousand tons annually.

It is a fact worthy of notice, that while the use of phosphatic manures
has increased very considerably of late years, the same cannot be said
of nitrogen. According to Mr Hermann Voss, some 34,000[95] tons of
nitrogen were used in the form of artificial manures in 1873, while now
only about 28,000 tons are used--_i.e._, some 6000 tons less.


_Oil-seeds and Oilcakes._

There still remains a very important source of nitrogen which has not
yet been mentioned, in the shape of oil-seeds and oilcakes, used for
feeding purposes. Oilcakes are both manufactured in this country and
imported in large quantities. Recent Agricultural Returns show the total
imports of oilcakes at 256,296 tons; that of linseed at 370,000 tons;
that of rape-seed at 80,000 tons; and that of cotton-seed at 289,413
tons.


_Other imported Sources of Nitrogen._

We have further, in considering this question, to take into account the
large amount of maize, peas, beans, wheat, and oats which are imported
into this country, a certain quantity of which is used as cattle-food,
and will therefore go to enrich their manure. Also the imported straw
used for purposes of litter must not be forgotten. In 1887 this amounted
to 52,393 tons.


_Conclusion._

In conclusion, it may be asked how far are the artificial sources of
nitrogen able to make good the loss? In the opinion of such a reliable
authority as Sir John Lawes, they do not. There are some soils which
depend almost entirely upon imported fertility, and could not be
cultivated without it. Upon some of them it is possible that the imports
of nitrogen are in excess of the exports. Taking the agricultural
acreage as a whole, however, he is of opinion that there is a decided
loss of nitrogen, which he estimates at _from 15 lb. to 20 lb. per acre
per annum_.[96]


FOOTNOTES:

[63] The total amount of nitrogen in the air has been estimated
approximately at four million billion tons.

[64] See Introductory Chapter, pp. 40 to 45.

[65] Although ammonia is more abundant than nitrates and nitrites, it
only amounts to a few parts per million of air. According to Müntz, the
air at great heights contains more ammonia than in its lower strata. The
opposite, however, is the case with regard to nitrates, which are only
found in air near the surface of the earth. See p. 49.

[66] Nitric acid may also be formed by the oxidation of ammonia by
ozone, or peroxide of hydrogen.

[67] According to Schloesing, the chief source of the ammonia present in
the air is the tropical ocean, which yields gradually to the atmosphere,
under the action of the powerful evaporation constantly going on, a
large amount of nitrogen in this form. The sources of the nitrogen of
the ocean are the nitrates which it receives from the drainage of land,
animal and vegetable matter, sewage, &c.

[68] See Appendix, Note I., p. 155.

[69] To illustrate this point, it may be mentioned that on the least
windy of days, when the wind is only moving at the rate of two miles an
hour--and this, it may he added, is so slow as to be scarcely
noticeable--the air in a space of 20 feet is changed over five hundred
times in an hour. The combined nitrogen thus absorbed is probably
entirely in the form of ammonia. It would seem so at any rate, from some
experiments by Schloesing. See p. 132.

[70] No vegetable or animal cell exists which does not contain nitrogen.

[71] This is less on the whole than what has been found in subsoils by
Continental investigators. Thus, for example, A. Müller found the
average of a number of analyses of subsoils to be .15 per cent., and the
late Dr Anderson found the nitrogen in the subsoil of different Scottish
wheat-soils to run from .15 per cent to .97 per cent.

[72] See Appendix, Note II., p. 156.

[73] "Under prolonged kitchen-garden culture the subsoil becomes
enriched with nitrogenous matter to a far more considerable depth; this
has been shown by the analyses of the soil of the old kitchen-garden at
Rothamsted. This is doubtless due to the practice of deep trenching
employed by gardeners."--R. Warington, 'Lectures on Rothamsted
Experiments.' U.S.A. Bulletin, p. 24.

[74] The comparatively insignificant effect the addition of various
nitrogenous manures have in increasing the total soil-nitrogen is
strikingly illustrated in the tables given in the Appendix, Note IV., p.
157.

[75] See Storer's Agric. Chem., vol. i. p. 357.

[76] See Chapter IV., Appendix, Note VII., p. 198.

[77] See Appendix, Note III., p. 157.

[78] See Appendix, Note IV., p. 157.

[79] See Appendix, Note I., p. 155.

[80] The original source of the nitrogen in the soil must have been the
nitrogen in the air. When plants first begin to grow on a purely mineral
soil, they must obtain nitrogen from some source. The small traces
washed down in the rain will supply sufficient nitrogen to enable a
scanty growth of the lower forms of vegetable life; whereas these by
their decay furnish their successors with a more abundant source, which
rapidly increases, until we have a fair percentage of humus accumulated.

[81] See Appendix, Note V., p. 158.

[82] See Historical Introduction, pp. 40-45.

[83] The evidence demonstrating this is to be found in the fact that the
amount of carbon found in different soils rises or falls in proportion
to the nitrogen. See p. 126.

[84] See Chapter IV. on Nitrification.

[85] Diffusion as well as capillary attraction is a means of bringing
nitrates again to the surface-soil after rain.

[86] See Appendix, Note VI., p. 158, and Note VIII., p. 160; also p.
154.

[87] See Appendix, Note VII., p. 159.

[88] See following Chapter on Nitrification, p. 178.

[89] According to the Agricultural Returns for 1888, the number of cows
in milk in Great Britain amounted to 2,450,444. If we multiply this
number by 22 the result is 54,000,000 lb., or in tons 24,107. This
quantity represents 154,067 tons of ordinary commercial nitrate of soda.

[90] For 1878 (p. 146 _et seq._) The reader interested in the subject is
referred to the paper itself.

[91] In tons 4464, and represents 28,530 tons of nitrate of soda.

[92] This in tons 162,946, which represents 1,041,384 tons of nitrate of
soda.

[93] This in tons 40,625, which represents 259,633 tons of nitrate of
soda. See paper in 'Journal of Science' already referred to.

[94] Europe's total production may be stated at 200,000 tons.

[95] 10,500 tons of which were as guano.

[96] Mr Warington estimates this at about 8 lb. See p. 141.



APPENDIX TO CHAPTER III.


NOTE I. (p. 119).

DETERMINATIONS OF THE QUANTITY OF NITROGEN SUPPLIED BY RAIN, AS
AMMONIA AND NITRIC ACID, TO AN ACRE OF LAND, DURING ONE YEAR.

(_From Dr Fream's 'Soils and their Properties_,' p. 62.)

----------------------+----------------+---------+----------------+--------
                      |                |         |  Nitrogen per  |
                      |                |         |  million, as   | Total
                      |                |         +--------+-------+Nitrogen
                      |     Year.      |Rainfall.|        |       |  per
                      |                |         |Ammonia.| Nitric| acre.
                      |                |         |        | Acid. |
----------------------+----------------+---------+--------+-------+--------
                      |                |         |        |       |   lb.
Kuschen               |    1864-65     |  11.85  |  0.54  | 0.16  |  1.86
   "                  |    1865-66     |  17.70  |  0.44  | 0.16  |  2.50
Insterburg            |    1864-65     |  27.55  |  0.55  | 0.30  |  5.49
   "                  |    1865-66     |  23.79  |  0.76  | 0.49  |  6.81
Dahme                 |    1865        |  17.09  |  1.42  | 0.30  |  6.66
Regenwalde            |    1864-65     |  23.48  |  2.03  | 0.80  | 15.09
   "                  |    1865-66     |  19.31  |  1.88  | 0.48  | 10.38
   "                  |    1866-67     |  25.37  |  2.28  | 0.56  | 16.44
Ida-Marienhütte, mean |                |         |        |       |
  of six years        |    1865-70     |  22.65  |        |       |  9.92
Proskau               |    1864-65     |  17.81  |  3.21  | 1.73  | 20.91
Florence              |    1870        |  36.55  |  1.17  | 0.44  | 13.36
   "                  |    1871        |  42.48  |  0.81  | 0.22  |  9.89
   "                  |    1872        |  50.82  |  0.82  | 0.26  | 12.51
Vallombrosa           |    1872        |  79.83  |  0.42  | 0.15  | 10.38
Montsouris, Paris     |    1877-78     |  23.62  |  1.91  | 0.24  | 11.54
   "                  |    1878-79     |  25.79  |  1.20  | 0.70  | 11.16
   "                  |    1879-80     |  15.70  |  1.36  | 1.60  | 10.52
                      |                |         |        |       |
                      |                +---------+--------+-------+--------
                      |                |         |        |       |
                      |Mean of 22 years|  27.63  |        |       | 10.23
                      |                |         |        |       |
----------------------+----------------+---------+--------+-------+--------


NOTE II. (p. 122).

NITROGEN IN SOILS AT VARIOUS DEPTHS.

(1) _Rothamsted Soils._

------------------+---------------------------+---------------------------
      Depth.      |        Arable soil.       |     Old pasture soil.
------------------+-----------+---------------+------------+--------------
                  | per cent. | lb. per acre. |  per cent. | lb. per acre.
 1st 9 inches     |   0.120   |     3,015     |    0.245   |     5,351
 2d  9    "       |   0.068   |     1,629     |    0.082   |     2,313
 3d  9    "       |   0.059   |     1,461     |    0.053   |     1,580
 4th 9    "       |   0.051   |     1,228     |    0.046   |     1,412
 5th 9    "       |   0.045   |     1,090     |    0.042   |     1,301
 6th 9    "       |   0.044   |     1,131     |    0.039   |     1,186
                  |   -----   |     -----     |    -----   |     -----
 Total, 54 inches |     -     |     9,554     |      -     |    13,143
                  |-----------+---------------+------------+--------------
 7th 9 inches     |   0.042   |     1,049     |      -     |       -
 8th 9    "       |   0.041   |     1,095     |      -     |       -
 9th 9    "       |   0.044   |     1,173     |      -     |       -
10th 9    "       |   0.043   |     1,076     |      -     |       -
11th 9    "       |   0.043   |     1,112     |      -     |       -
12th 9    "       |   0.045   |     1,198     |      -     |       -
                  |   -----   |    ------     |    -----   |     -----
 Total, 9 feet    |           |    16,257     |            |
------------------+-----------+---------------+------------+--------------

(2) _Manitoba Soils._

----------+----------|-------------+-----------+----------
  Depth.  | Brandon. | Niverville. | Winnipeg. | Selkirk.
----------+----------+-------------+-----------+----------
          | per cent.|  per cent.  | per cent. | per cent.
1st foot  |   0.187  |    0.261    |   0.428   |   0.618
2d    "   |   0.109  |    0.169    |   0.327   |   0.264
3d    "   |   0.072  |    0.069    |   0.158   |   0.076
4th   "   |   0.019  |    0.038    |   0.107   |   0.042
----------+----------+-------------+-----------+----------


NOTE III. (p. 130).

NITROGEN AS NITRATES IN CROPPED SOILS RECEIVING NO NITROGENOUS
MANURE, IN LB. PER ACRE (_Rothamsted Soils_).

-------------+----------------+---------+---------+---------+--------
             |     Wheat.     |         |         |         |
             +-------+--------+ Bokhara |         |         |  White
             | After | After  | clover, | Vetches,| Lucern, | clover,
   Depth.    |fallow,|clover, |  1882.  |  1883.  |  1885.  |  1885.
             | 1883. | 1883.  |         |         |         |
-------------+-------+--------+---------+---------+---------+--------
             |  lb.  |   lb.  |   lb.   |   lb.   |   lb.   |   lb.
 1st 9 inches|  3.4  |   6.1  |   3.4   |  10.2   |   8.9   |  11.5
 2d  9   "   |  3.1  |   4.4  |   1.0   |   2.7   |   1.1   |   1.4
 3d  9   "   |  0.8  |   1.6  |   0.6   |   1.1   |   0.8   |   0.9
 4th 9   "   |  1.0  |   1.3  |   1.0   |   1.5   |   0.8   |   1.9
 5th 9   "   |  0.8  |   1.5  |   0.8   |   2.5   |   1.0   |   7.1
 6th 9   "   |  0.6  |   0.8  |   1.7   |   4.4   |   0.9   |  11.3
 7th 9   "   |  0.8  |   2.2  |         |   4.5   |   0.6   |  13.1
 8th 9   "   |  0.9  |   1.7  |         |   4.9   |   0.8   |  12.6
 9th 9   "   |  0.7  |   2.4  |         |   4.8   |   0.7   |  11.2
10th 9   "   |  2.0  |   2.1  |         |   5.1   |   0.6   |  10.7
11th 9   "   |  1.5  |   2.1  |         |   6.4   |   0.4   |  11.1
12th 9   "   |  3.8  |   2.8  |         |   6.5   |   0.4   |  10.0
-------------+-------+--------+---------+---------+---------+--------


NOTE IV. (p. 124 and p. 131).

NITROGEN AS NITRATES IN WHEAT-SOILS VARIOUSLY MANURED, OCTOBER 1881,
IN LB. PER ACRE (_Rothamsted Soils_).

----+----------------------------------+------+------+------+--------+-------
    |                                  |      |      |      |        |Excess
    |                                  |      |      |      |        |over
Plot|            Manuring.             |1st 9 |2nd 9 |3rd 9 |Total 27|plots
    |                                  |inches|inches|inches|inches  |3 and 4
----+----------------------------------+------+------+------+--------+-------
    |                                  |  lb. |  lb. |  lb. |   lb.  |   lb.
 3  |No manure, 38 years               |  9.7 |  5.3 |  2.8 |  17.8  |
 4  |    "      30   "                 |  9.2 |  4.0 |  1.8 |  15.0  |
16a |    "      17   "                 | 10.6 |  5.0 |  2.3 |  17.9  |   1.5
 5a |Ash constituents, 30 years        | 12.6 |  7.1 |  4.6 |  24.3  |   7.9
17a | "       "         1 year         | 10.3 |  7.5 |  3.4 |  21.2  |   4.8
 6a | " and ammonium salts,  200 lb.   | 16.5 |  7.5 |  4.7 |  28.7  |  12.3
 7a | "        "             400  "    | 22.8 | 11.3 |  5.7 |  39.8  |  23.4
 8a | "        "             600  "    | 21.1 | 13.9 |  7.8 |  42.8  |  26.4
 9a |Ash and sodium nitrate, 550  "    | 19.7 | 10.0 |  8.2 |  37.9  |  21.5
 9b |Sodium nitrate,          "   "    | 16.3 | 20.1 | 17.7 |  54.1  |  37.7
10a |Ammonium salts,         400  "    | 14.2 | 11.9 |  7.3 |  33.4  |  17.0
11a |Superphosphate and ammonium salts,|      |      |      |        |
    |  400 lb.                         | 17.9 |  9.3 |  3.6 |  30.8  |  14.4
19  |Rape-cake, 1700 lb.               | 14.1 | 13.0 |  7.1 |  34.2  |  17.8
 2  |Farmyard manure, 14 tons--38 years| 30.0 | 15.4 |  6.8 |  52.2  |  35.8
----+----------------------------------+------+------+------+--------+-------

NITROGEN AS NITRATES IN BARLEY-SOILS VARIOUSLY MANURED, MARCH 1892,
IN LB. PER ACRE (_Rothamsted Soils_).

-------+------------------------+-------+-------+-------+-------+--------
       |                        |       |       |       | Total | Excess
       |                        | 1st 9 | 2d 9  | 3d 9  |  27   |  over
 Plot. |        Manuring.       |inches.|inches.|inches.|inches.|plot 10.
-------+------------------------+-------+-------+-------+-------+--------
       |                        |  lb.  |  lb.  |  lb.  |  lb.  |  lb.
10     |No manure               |  5.9  |  4.7  |  5.1  | 15.7  |   -
20-40  |Ash constituents (mean) |  6.7  |  7.0  |  6.4  | 20.1  |  4.4
1 A    |Ammonium salts, 200 lb. |  6.1  |  8.3  |  7.0  | 21.4  |  5.7
2A-4A  |Ammonium and ash        |       |       |       |       |
       |  constituents (mean)   |  7.7  |  7.8  |  7.6  | 23.1  |  7.4
1 AA   |Sodium nitrate, 275 lb. |  9.7  |  6.8  |  9.0  | 25.5  |  9.8
2AA-4AA|Sodium nitrate and ash  |       |       |       |       |
       |  constituents (mean)   |  8.3  |  7.4  |  7.5  | 23.2  |  7.5
1C     |Rape-cake, 1000 lb.     | 10.6  | 13.7  |  7.9  | 32.2  | 16.5
2C-4C  |Rape-cake and ash       |       |       |       |       |
       |  constituents (mean)   |  8.8  | 11.9  |  8.7  | 29.4  | 13.7
7-1    |No manure, 10 years--   |       |       |       |       |
       |  formerly dung         | 14.8  | 11.8  | 10.9  | 37.5  | 21.8
7-2    |Farmyard manure, 14 tons| 18.6  | 14.6  | 10.9  | 44.1  | 28.4
-------+------------------------+-------+-------+-------+-------+-------


NOTE V. (p. 134).

EXAMPLES OF INCREASE OF NITROGEN IN ROTHAMSTED SOILS LAID DOWN IN
PASTURE.

--------------------+----------+---------------
                    |  Age of  |  Nitrogen in
                    | pasture. | 1st 9 inches.
--------------------+----------+---------------
                    |  Years.  |   Per cent.
Arable land         |     -    |     0.140
Barn-field pasture  |     8    |     0.151
Apple-tree pasture  |    18    |     0.174
Dr Gilbert's meadow |    21    |     0.204
Dr Gilbert's meadow |    30    |     0.241
--------------------+----------+---------------


NOTE VI. (p. 141).

In connection with the loss by drainage of nitrogen in the form of
nitrates, it may be mentioned that the water of many of the famous
rivers contains large quantities of nitrates. Thus the water of the
Seine has been found to contain fifteen parts of nitrates per million of
water, and the Rhine eight parts per million. Some idea of what this
amounts to per annum may be obtained by the statement that "the Rhine
discharges daily 220 tons of saltpetre into the ocean, the river Seine
270, and the Nile 1100 tons."--(Storer's Agric. Chem., vol. i. p. 318.)


NOTE VII. (p. 142).

EXAMPLES OF DECREASE OF NITROGEN IN ROTHAMSTED SOILS.

--------------------------------------+----------------
                                      |  Nitrogen in
                                      | 1st 9 inches.
--------------------------------------+----------------
                                      |   Per cent.
Old pasture                           |     0.250
Arable land in ordinary culture       |     0.140
Wheat unmanured, 38 years             |     0.105
Wheat and fallow unmanured, 31 years  |     0.096
Barley unmanured, 30 years            |     0.093
Turnips unmanured, 25 years           |     0.085
--------------------------------------+----------------

MANURING, PRODUCE OF WHEAT, AND ALTERATION IN THE COMPOSITION OF THE
SOIL IN BROADBALK FIELD, ROTHAMSTED, FROM 1865 TO 1881.

-----+----------------------------------+----------------+----------------------
     |                                  |    Average     | Nitrogen per acre
     |                                  |    produce     |  in 1st 9 inches
Plot.|         Manures per acre,        |   per acre.    |     of soil.
     |         annually applied,        +-------+--------+------+-----+---------
     |         16 years, 1865-81.       |       |        |      |     | Gain or
     |                                  |Dressed| Total  |1865. |1881.| loss in
     |                                  |grain. |produce.|      |     |16 years.
-----+----------------------------------+-------+--------+------+-----+---------
     |                                  | bush. |  lb.   | lb.  | lb. |   lb.
 3   |Unmanured                         |11-7/8 |  1715  | 2507 |2404 |  -103
 5_a_|Mixed mineral manure              |12-3/4 |  1963  | 2574 |2328 |  -246
10_a_|Ammonium salts, 400 lb.           |17-7/8 |  2881  | 2548 |2471 |   -77
11_1_|Ammonium salts, with              |       |        |      |     |
     |                  superphosphate  |23-1/4 |  3856  | 2693 |2676 |   -17
 7_a_|Ammonium salts, with mixed mineral|       |        |      |     |
     |                  manure          |  28   |  4993  | 2829 |2908 |   +79
 9_a_|Nitrate of soda, 550 lb., and     |       |        |      |     |
     |  mixed mineral manure            |  36   |  6949  | 2834 |2883 |   +49
16_a_|Unmanured*                        |13-1/2 |  2194  | 2907 |2557 |  -350
 2   |Farmyard manure, 14 tons          |31-1/2 |  5356  | 4329 |4502 |  +173
-----+----------------------------------+-------+--------+------+-----+---------
* During 1852-64 received annually ammonium salts, 800 lb., with mixed mineral
manure, and yielded an average product of 39-1/2 bushels of grain and 46-5/8
cwt. of straw.


NOTE VIII. (p. 141).

$1$2

--------------+--------+-----------------+----------------------------------
              |        |                 |       Nitrogen as nitrates.
              |        |                 +-----------------+----------------
              |        |    Amount of    | Per million of  |    Per acre.
              |Rainfall|    drainage.    |     water.      |
              |        +--------+--------+--------+--------+--------+-------
              |        |20-inch |60-inch |20-inch |60-inch |20-inch |60-inch
              |        |gauge.  |gauge.  |gauge.  |gauge.  |gauge.  |gauge.
--------------+--------+--------+--------+--------+--------+--------+-------
              |Inches. |Inches. |Inches. |        |        |  lb.   |  lb.
March         |  1.70  |  0.85  |  0.94  |  7.3   |  8.9   |  1.41  |  1.89
April         |  2.25  |  0.72  |  0.79  |  8.3   |  9.0   |  1.35  |  1.61
May           |  2.48  |  0.80  |  0.79  |  8.4   |  9.1   |  1.53  |  1.63
June          |  2.59  |  0.78  |  0.78  |  9.2   |  9.1   |  1.62  |  1.60
July          |  2.85  |  0.68  |  0.62  | 13.5   | 11.8   |  2.08  |  1.66
August        |  2.69  |  0.84  |  0.76  | 15.1   | 13.3   |  2.87  |  2.28
September     |  2.70  |  0.97  |  0.82  | 17.7   | 13.4   |  3.86  |  2.50
October       |  3.12  |  1.86  |  1.68  | 13.8   | 11.9   |  5.83  |  4.53
November      |  3.20  |  2.44  |  2.32  | 11.8   | 11.4   |  6.50  |  5.98
December      |  2.34  |  1.88  |  1.88  |  9.5   | 10.6   |  4.06  |  4.51
January       |  2.13  |  1.79  |  1.93  |  7.4   |  8.9   |  2.99  |  3.88
February      |  2.16  |  1.84  |  1.74  |  7.7   |  9.1   |  3.19  |  3.57
              +--------+--------+--------+--------+--------+--------+-------
March-June    |  9.02  |  3.15  |  3.30  |   8.3  |  9.0   |  5.91  |  6.73
July-September|  8.24  |  2.49  |  2.20  |  15.6  | 13.0   |  8.81  |  6.44
October-Feb.  | 12.95  |  9.81  |  9.55  |  10.2  | 10.4   | 22.57  | 22.47
              +--------+--------+--------+--------+--------+--------+-------
Whole year    | 30.21  | 15.45  | 15.05  |  10.7  | 10.5   | 37.29  | 35.64
--------------+--------+--------+--------+--------+--------+--------+-------



CHAPTER IV.

NITRIFICATION.


The most important compound of nitrogen for the plant is _nitric acid_.
It is as nitrates that most plants absorb the nitrogen they require to
build up their tissue. In nature the nitrogen, present in the soil as
ammonia and different organic forms, is constantly being converted into
nitric acid. This conversion of nitrogen into nitrates, known as
_nitrification_, is a process of very great importance, and, as has been
already pointed out in the Introductory Chapter, is effected through the
agency of micro-organisms (ferments).[97] The process of nitrification,
as well as the nature of the other changes taking place in the soil
between the various compounds of nitrogen, are as yet but most
imperfectly understood, but much light has been thrown on this most
interesting department of agricultural research during the last few
years; and it cannot be doubted that the increased attention which it is
receiving from different investigators, both on the Continent and in
this country, will be fraught with most important results for practical
agriculture.


_Occurrence of Nitrates in the Soil._

The occurrence of nitre,[98] or potassium nitrate, in soils has been
long known, although it is only within the last few years that we have
obtained any precise knowledge with regard to the mode of its
production. While its amount in most soils, especially in this
country,[99] is very minute, there are certain parts of the world where
nitrates are found in large quantities. The nitrate fields of Chili and
Peru are the chief natural sources of nitrates, and they are referred to
in the chapter on Nitrate of Soda. We have other parts of the world,
however (in China and India), where soils rich in nitre occur, and which
in the past have formed a source of the commercial article.[100]


_Nitre Soils of India._

The most important of these nitre soils are those found in the
North-west of India, in the province of Bengal. In these districts the
soil is of a light porous texture, rich in lime, and situated at a
considerable height above water-level. They are the sites of old
villages, and the nitre is found in the form of an efflorescence on the
surface of different parts of the soil. The occurrence of nitre under
such conditions is due, partly to the natural richness of the soil in
nitrogen, and partly to its artificial enrichment through receiving the
nitrogenous excrements of the inhabitants of the villages and their
cattle. The constant process of evaporation going on in such a warm
climate has the effect of inducing an upward tendency of the soil-water,
the result being a concentration of all the nitre the soil contains in
its surface layer. This goes on until a regular incrustation is formed,
and the soil is covered by a white deposit of nitre. Whenever this
becomes apparent, the surface portion of the soil is scraped off by the
_sorawallah_, or native manufacturer, and collected and treated for the
purpose of recovering, in a pure state, the saltpetre.


_Saltpetre Plantations._

The large demand for saltpetre, larger than could be supplied by these
nitre soils, soon gave rise to the semi-artificial method of production,
formerly so largely practised in Switzerland, France, Germany, Sweden,
and in many other parts of the Continent, by means of the so-called
"nitre beds," "nitraries," or "saltpetre plantations." Previous to the
introduction of this method of manufacture, the demand for saltpetre for
gunpowder had become so great, that every source of nitre was eagerly
sought for. Thus, when it was discovered that the earth from the floors
of byres, stables, and farmyards were particularly rich in nitre, and
when mixed with wood-ashes formed an important source of it, the right
to remove these in France was vested in the Government under the
Saltpetre Laws, which obtained till the French Revolution. This great
scarcity soon led, however, to a careful investigation being made into
the conditions under which potassium nitrate was formed in nitre
soils.[101] These conditions, which included the presence of rich
nitrogenous matter, warmth, free aeration of the soil, and a certain
proportion of moisture, became, in the course of years, more and more
thoroughly understood, and the result was the institution of numerous
"saltpetre plantations." These generally consisted of heaps of mould,
rich in nitrogen, mixed with decomposing animal matter, rubbish of
various kinds, manurial substances, ashes, road-scrapings, and lime
salts.[102] The heap was interlaid with brushwood, and was watered from
time to time with liquid manure from stables, consisting chiefly of
dilute urine. In forming the heap care was taken to keep the mass
porous, so as to admit of the free access of air. The heap was further
protected from the rain by covering it with a roof. In course of time
considerable quantities of nitrates were developed, and the nitre was
occasionally collected by scraping it from the surface, where it became
concentrated just as in the nitre soils. In all cases, however, the
heaps, when considered rich enough in nitre, were treated from time to
time with water which, by subsequent evaporation, yielded the nitre in a
more or less pure condition.[103]

This mode of obtaining nitre is no longer practised to any extent, since
it is now more conveniently obtained from the treatment of nitrate of
soda with potassium chloride.


_Cause of Nitrification._

We have adverted to these nitre plantations as showing how the
conditions most favourable for the development of nitrification were
recognised long before anything was known as to the true nature of the
process. It was only in 1877 that the formation of nitrates in the soil
was proved to be due to the action of micro-organic life,[104] by the
two French chemists, Schloesing and Müntz, who discovered the fact when
carrying out experiments to see if the presence of humic matter was
essential to the purification of sewage by soil. In these experiments
sewage was made to filter slowly through a certain depth of soil (the
time occupied in this filtration being eight days). It was found that
nitrification of the sewage took place. By treating the soil with
chloroform[105] it was found that it no longer possessed the power of
inducing the nitrification of the sewage. When, however, a small portion
of a nitrifying soil was added, the power was regained. From this it was
naturally inferred that nitrification was effected by some kind of
ferment. This conclusion was soon confirmed by subsequent experiments by
Warington at Rothamsted, who showed that the power of nitrification
could be communicated to media, which did not nitrify, by simply
seeding them with a nitrifying substance, and that light was
unfavourable to the process. Since then the question has formed the
subject of a number of researches by Mr Warington at Rothamsted, as well
as by Schloesing and Müntz, Munro, Dehérain, P. F. Frankland,
Winogradsky, Gayon and Dupetit, Kellner, Plath, Pichard, Landolt, Leone,
and others. From these researches we have obtained the following
information with regard to the nature of the organisms concerned in this
process, and the conditions most favourable for their development.


_Ferments effecting Nitrification._

The importance of isolating and studying them microscopically was
recognised at an early period in these researches. Messrs Schloesing and
Müntz were the first to attempt this. They reported that they had
successfully accomplished this, and described the organism as consisting
of very small, round, or slightly elongated corpuscles, occurring either
singly or two together. According, however, to the most recent
researches of Warington, Winogradsky, and P. F. Frankland, nitrification
is not effected by a _single_ micro-organism, but by _two_, both of
which have been successfully isolated and studied.[106] The first of
these to be discovered and isolated was the _nitrous_ organism, which
effects the conversion of ammonia into nitrous acid; the second, which
has only been lately isolated by Warington and Winogradsky, effects the
conversion of nitrous acid into nitric acid. Each of these ferments thus
has its distinctive function to perform in this most important process,
the nitric ferment being unable to act on ammonia, as the nitrous
ferment is unable to convert nitrites into nitrates. Both ferments occur
in enormous quantities in the soil, and seem to be influenced, so far as
is at present known, by the same conditions. Their action will thus
proceed together. Nearly all we know as yet on the subject of their
nature is with regard to the nitrous ferment.


_Appearance of Nitrous Organism._

Mr Warington[107] thus describes the appearance of the nitrous organism:
"As found in suspension in a freshly nitrified solution, it consists
largely of nearly spherical corpuscles, varying extremely in size. The
largest of these corpuscles barely reaches a diameter of 1/1000th of a
millimeter; and some are so minute as to be hardly discernible in
photographs, although shown there with a surface one million times
greater than their own. The larger ones are frequently not strictly
circular. These forms are universally present in nitrifying cultures.
The larger organisms are sometimes seen in the act of dividing."


_Nitric Organism._

So far as at present known, the nitric organism is very similar in
appearance to the nitrous organism, so much so that it is difficult to
distinguish the one from the other. As the same conditions influence
their development, the process may be regarded as a whole.


_Difficulty in isolating them._

A great difficulty has been experienced in the attempt to isolate these
micro-organisms for the purpose of studying their nature. This arises
from the fact that they refuse to grow on the ordinary solid cultivating
media used by bacteriologists. Winogradsky, however, has recently
succeeded in cultivating them in _a purely mineral_ medium--viz.,
_silica-jelly_.[108]


_Nitrifying Organisms do not require Organic Matter._

The fact that they can develop in media destitute of organic matter, is
one of very great interest and importance to Vegetable Physiology. It
implies that they can derive their carbon from carbonic acid--a power
which it was believed was possessed by green plants alone among living
structures. For organisms destitute of chlorophyll, the source of their
protoplasmic carbon, it has been hitherto commonly believed, must be
_organic matter_ of some sort. While it would appear that the nitrifying
organisms can, when opportunity affords, feed upon organic matter, yet
it has been proved beyond doubt that they can also freely develop in
media entirely devoid of it, and are capable, under such circumstances,
of deriving their carbon from a purely mineral source.[109] This fact,
which is subversive of what was believed to be a fundamental law of
Vegetable Physiology, is one of the most important of the many important
and interesting facts which these nitrification researches have
elicited.[110]


CONDITIONS FAVOURABLE FOR NITRIFICATION.

We may now proceed to discuss the conditions favourable for
nitrification.


_Presence of Food-constituents._

Among these conditions the first is the presence of certain
food-constituents. To both animal and vegetable life alike a certain
amount of mineral food is absolutely necessary. Among these phosphoric
acid is one of the most important, and in the experiments on
nitrification it has been found that the nitrifying organisms will not
develop in any medium destitute of it. That other mineral
food-constituents are necessary is highly probable, although the
influence of their absence on the development of the process has not
been similarly studied. Probably potash, magnesia, and lime salts are
necessary. In the cultivating solutions used in the experiments on the
subject, the mineral food-constituents added consisted of lime,
magnesia, and potash salts and phosphoric acid.[111]

As we have seen above, the presence of organic matter is not necessary
for the process. In this respect these organisms are differentiated from
all other ferments hitherto discovered.


_Presence of a Salifiable Base._

The presence of a sufficient quantity of a base in the soil with which
the nitric acid may combine, when it is formed, is another necessary
condition.[112] The process only goes on in a slightly alkaline
solution. The substance which acts as this salifiable base is _lime_.
The presence of a sufficient quantity of carbonate of lime in the soil
will thus be seen to be of first-rate importance. This furnishes an
explanation of one of the many benefits conferred by lime on soils. The
activity of nitrification in many soils may be hindered by the absence
of a sufficiency of lime salts, and in such cases most striking results
may follow the application of moderate dressings of chalk. The absence
of the nitrifying organisms in certain soils, such as peaty and forest
soils, may be thus accounted for. In such soils humic acids are present
and the requisite alkalinity is thus awanting.


_Only takes place in slightly Alkaline Solutions._

But while a certain slight amount of alkalinity is necessary, this must
not exceed a certain strength, otherwise the process is retarded. This
is the reason why strong urine solutions do not nitrify. The amount of
carbonate of ammonia generated in them by putrefaction renders the
development of nitrification impossible by rendering the alkalinity of
the solution too great.[113] The practical importance of this fact is
considerable, as it shows the importance of diluting urine very
considerably before applying it as a manure. Similarly, when large
quantities of lime, especially burnt lime, are applied to soils, the
result will be to arrest the action of nitrification for the time. The
presence of alkaline carbonates in the soil, unless in minute
quantities, is apt, therefore, to seriously interfere with the
process.[114]


_Action of Gypsum on Nitrification._

It has been found by Pichard that the action of certain mineral
sulphates is extremely favourable to the process, and among these
_gypsum_. Warington has carried out some experiments on the action of
gypsum in promoting nitrification. The reason of its favourable action
is probably because it neutralises the alkalinity of nitrifying
solutions. It thus permits the process to go on in unfavourable
conditions. Where, therefore, too great alkalinity exists for the
maximum development of nitrification, the best specific will be found to
be gypsum.[115] The practical value of gypsum as an adjunct to certain
manurial substances, where nitrification is desired to be promoted as
rapidly as possible, such as sewage and farmyard manure, will thus at
once become apparent. So far as there is a proper degree of alkalinity
maintained, the presence of large quantities of saline matter does not
seem to interfere with the process.


_Presence of Oxygen._

The nitrification bacteria belong, it would seem, to the aerobic[116]
class of ferment--_i.e._, they cannot develop without a free supply of
oxygen. Exclusion of the air is sufficient to kill them, and in those
portions of the soil where access of air is not freely permitted,
nitrification will be found to be correspondingly feeble. Thus it has
been found in experiments with different portions of soils, that but
little signs of nitrification occur in the lower soil layers. According
to experiments by Schloesing on a moist soil, in atmospheres
respectively containing no oxygen and varying quantities of it, the
action of oxygen in promoting nitrification was strikingly demonstrated.
In an atmosphere of pure nitrogen, entirely devoid of oxygen, the
process no longer took place, but the nitrates already present in the
soil were reduced and free nitrogen was evolved. In an atmosphere, on
the other hand, containing 1.5 per cent of oxygen, a considerable amount
of nitrification took place; while in the presence of 6 per cent,
nitrification took place to double the extent. An addition of 10 to 15
per cent again doubled the quantity. When the amount of moisture added
was increased, the effect of larger percentages of oxygen was found to
be less marked. The reason of this is that the oxygen probably acts as
dissolved oxygen; the addition of water meaning at the same time an
addition of available oxygen. This condition exemplifies the value of
tillage operations. The more thoroughly a soil is tilled the more
thoroughly will the aeration of its particles take place, and
consequently the more favourable will this necessary condition of
nitrification be rendered. The benefits conferred on clayey soils by
tillage will in this respect be especially great.


_Temperature._

Another of the conditions determining the rate at which nitrification
takes place, and one which is most important, is _Temperature_.
According to Schloesing and Müntz the temperature at which maximum
development takes place is 37° C.[117] (99° F.), at which temperature it
is ten times as active as at 14° C. (57° F.) Below 5° C. (40° F.) the
action is extremely feeble. It is clearly appreciable at 12° C. (54°
F.), and from there up to 37° C. (99° F.) it rapidly increases. From 37°
C. (99° F.) to 55° C. (131° F.), at which temperature no nitrification
takes place, its activity decreases; at 45° C. (113° F.) it is less
active than at 15° C. (59° F.), and at 50° C. (122° F.) it is very
slight. These results by Schloesing and Müntz have not been exactly
confirmed by Warington. He has found that a considerable amount of
nitrification goes on at a temperature between 3° and 4° C. (37° and 39°
F.), while the highest temperature at which he has found it to take
place is considerably lower than 55° C. (131° F.) Thus he was unable to
start nitrification in a solution maintained at 40° C. (104° F.) It
would thus seem that the nitrifying ferments are able to develop at
lower temperatures than most organisms; and although nitrification
entirely ceases during frost, yet in a climate such as our own there
must be a considerable proportion of the winter during which
nitrification is moderately active.


_Presence of a sufficient quantity of Moisture._

The presence of moisture in a soil is another of the necessary
conditions of nitrification. It has been shown that it is at once
arrested, and indeed destroyed, by desiccation. Other conditions being
equal, and up to a certain extent, the more moisture a soil contains the
more rapid is the process. Too much water, however, is unfavourable, as
it is apt to exclude the free access of air, which, as we have just
shown, is so necessary, as well as to lower the temperature. During a
period of drought the rate at which nitrification takes place will,
therefore, be apt to be seriously diminished.


_Absence of strong Sunlight._

It has been found that the process goes on much more actively in
darkness; indeed Warington has found in his experiments that
nitrification could be arrested by simply exposing the vessel in which
it was going on to the action of sunshine.


_Nitrifying Organisms destroyed by Poisons._

It has already been pointed out that nitrification is arrested by the
action of antiseptics, such as chloroform, bisulphide of carbon, and
carbolic acid. Another substance which has been found to have an
injurious action is ferrous sulphate or "copperas," a substance which is
apt to be present in badly drained soils, or soils in which there is
much actively putrefying organic matter. Maercker has found that in moor
soils containing ferrous sulphate, no nitrates, or mere traces of
nitrates, could be found. A substance such as gas-lime, unless submitted
to the action of the atmosphere for some time, would also have a bad
effect in checking nitrification, owing to the poisonous sulphur
compounds it contains. Common salt, it would seem, also arrests the
process; and this antiseptic property which salt exercises on
nitrification throws a certain amount of light on the nature of its
action when applied, as it is often done, along with artificial
nitrogenous manures.


_Denitrification._

In connection with the process of nitrification, it is of interest to
notice that a process of an opposite nature may also take place in
soils--viz., _denitrification_--a process which consists in reducing the
nitrates to nitrites, nitrous oxide, or free nitrogen. That a reduction
of nitrates takes place in the decomposition of sewage with the
evolution of free nitrogen, was a fact first observed by the late Dr
Angus Smith in 1867; and the reduction of nitrates to nitrites, and
nitric and nitrous oxides in putrefactive changes has been subsequently
noticed by different experimenters, who have further observed that such
reduction takes place in the case of putrefaction going on in the
presence of large quantities of water or where there is much organic
matter.


_Denitrification also effected by Bacteria._

This change was supposed to be of a purely chemical nature, and it has
only been recently discovered that it is effected, like nitrification,
by means of bacteria. It has been surmised by some that the action of
denitrification may be effected by the same organisms that effect
nitrification, and that it depends on merely external conditions which
process goes on. There is no reason, however, to suppose that this is
so, and several of the denitrifying organisms have been identified.


_Conditions favourable for Denitrification._

That it is a process that goes on to any extent in properly cultivated
soils is not to be supposed. The conditions which favour denitrification
are exactly the opposite of those which favour nitrification. It is only
when oxygen is excluded, or, which practically means the same thing,
when large quantities of organic matter are in active putrefaction, and
the supply of oxygen is therefore deficient, that denitrification takes
place. Schloesing, as we have already seen, found that in the case of a
moist soil, kept in an atmosphere devoid of oxygen, a reduction of its
nitrates to free nitrogen took place.


_Takes place in water-logged Soils._

The exclusion of oxygen from a soil may be effected by saturating the
soil with water; and Warington has found in experiments carried out in
an arable soil, by no means rich in organic matter, that complete
reduction of nitrates may be effected in this way. It would thus seem
that the process of denitrification will take place in water-logged
soils, or in the putrefaction of sewage matter in the presence of large
quantities of water. Whether this reduction will result in the
production of nitrites, nitrous oxide, or free nitrogen, depends on
different conditions. This process is one of great importance from an
economic point of view, as it reveals to us a source of loss which may
take place in the fermentation of manures. In the rotting of our
farmyard manure it is possible that the denitrifying organisms may be
more active than we have hitherto suspected, and that a considerable
loss of nitrogen may in this way be effected.


_Distribution of the Nitrifying Organisms in the Soil._

The nitrifying organisms are probably chiefly confined to the soil, and
do not usually occur in rain or in the atmosphere. That, however, they
are found in spots which we might be inclined to think extremely
unlikely, is shown by some recent interesting researches carried out by
Müntz, who discovered that the bare surfaces of felspathic, calcareous,
schistose, and other rocks at the summit of mountains in the Pyrenees,
Alps, and Vosges, yielded large numbers of them, and that they occurred
to a considerable depth in the cracks and fissures of the rocks. The
nitrifying organisms are also found in river-water, in sewage, and
well-waters.


_Depth down at which they occur._

In Warington's earlier experiments, the conclusion he arrived at was
that the occurrence of the nitrifying organisms was almost entirely
limited to the superficial layers of the soil, and that they were seldom
to be met with much below a depth of 18 inches. His subsequent
experiments, however, considerably modified this conclusion, and showed
that nitrification may take place to a depth of at least 6 feet.[118]
But although it may take place at this depth, it probably, as a general
rule, is limited to the surface-soil, as it is only there the conditions
for obtaining circulation of air are sufficiently favourable. A great
deal, of course, will depend on the nature of the soil--_i.e._, as to
its texture. In a clayey subsoil the principal hindrance to
nitrification will be the difficulty of obtaining sufficient aeration.
In clay soils it is probable, therefore, that nearly all the
nitrification goes on in the surface layer; in sandy soils it may take
place to a greater depth.[119]


_Action of Plant-roots in promoting Nitrification._

In this connection the action of plant-roots in permitting a more
abundant access of air to the lower layers of the soil, and thus
promoting nitrification, is worth noticing. This has been observed in
the case of different crops. Thus the action of nitrification has been
found to be more marked in the lower layers of a soil on which a
leguminous crop was growing than on that on which a gramineous. "The
conditions which would favour nitrification in the subsoil are such as
would enable air to penetrate it, as artificial drainage, a dry season,
the growth of a luxuriant crop causing much evaporation of the water in
the soil. Such conditions, by removing the water that fills the pores of
the subsoil, will cause the air to penetrate more or less deeply and
render nitrification possible. Subsoil nitrification will thus be most
active in the drier periods of the year" (Warington).


_Nature of Substances capable of Nitrification._

What kinds of nitrogenous substances are capable of undergoing this
process of nitrification are not yet well known. The question is, of
course, one of great importance, as the rapidity with which a
nitrogenous body nitrifies will be an important factor in determining
its value as a manure. Unfortunately, on this subject we know, as yet,
very little. We are well aware that the nitrogen present in the humic
matter of the soil is readily nitrifiable. In the experiments on
nitrification the nitrogenous bodies used have been chiefly ammonia
salts, so that it is difficult to say whether, in the case of other
nitrogenous substances, micro-organic life of a different sort has not
also been active and has converted the nitrogen into ammonia, and
thereby prepared the way for the process of nitrification.

That various manures, such as bones, horn, wool, and rape-cake are
readily nitrifiable, has been shown by experiment. Laboratory
experiments have also been carried out on such different nitrogenous
substances as ethylamine, thiocyanates, gelatin, urea, asparagin, and
albuminoids of milk. But in all these experiments, how far these bodies
have been directly acted upon by the nitrifying organisms, or how far
they have first undergone a preparatory change in which their nitrogen
has been first converted into ammonia, is impossible to say. It is at
least quite probable that all the organic forms of nitrogen have first
to be converted into ammonia ere they are nitrified.


_Rate at which Nitrification takes place_.

A question which is practically of no little importance is the rate at
which nitrification takes place. From what has been already said as to
the nature of the conditions favourable for the process, it will be at
once seen that this will depend on how far these conditions are present
in the soil. In point of fact the rate at which nitrification takes
place will vary very much in different soils. A greater difference,
however, in the rate at which it takes place, will be found even in the
same soils at different periods of the year. In this country, where the
most favourable temperature for its development is seldom reached, it
never goes on at the same rate as in tropical climates. One of the
causes of the greater fertility of tropical soils is due, doubtless, to
the very much longer duration of the period of nitrification, as well as
to its greater intensity. As, however, temperature is not the only
condition, and the presence of moisture is quite as necessary, it may be
that its development is seriously retarded in many tropical climates by
the extreme dryness of the soil during long periods.


_Takes place chiefly during the Summer Months_.

Although in this climate, as has already been pointed out, nitrification
probably goes on during most of the winter months, owing to the fact
that the temperature of our soils is only occasionally below the
minimum temperature at which the process takes place, yet there can be
little doubt that the great bulk of the soil-nitrates are produced
during a few months in summer. A fair conception of this amount is
afforded by the interesting experiments on the composition of
drainage-waters made at Rothamsted, which we shall have occasion to
refer to immediately. It may be pointed out, however, that it is not
always safe to take the amount of nitrates found in drainage-waters as
an infallible indication of this rate, for this amount will depend to a
certain extent on the amount of rainfall, and would be misleading in the
case of a long period of drought. On the whole, however, it furnishes us
with extremely useful data for the elucidation of this important
problem.


_Process goes on most quickly in Fallow Fields._

It has been shown in the Rothamsted experiments that the process goes on
best in fields lying in bare fallow; and in this fact lies the
explanation of one of the many reasons why the practice of leaving
fields in bare fallow, so common in past times, and still practised in
the case of clay soils in some parts of the country, was so beneficial
to the land thus treated. But despite this fact, the practice of leaving
soils in bare fallow can scarcely be justified from this point of view,
as the loss of nitrates through the action of rain is very great in our
moist climate.


_Laboratory Experiments on Rate of Nitrification._

Several interesting experiments have been carried out with the object of
affording data for estimating the rate at which the process may go on in
our soils under certain conditions. An old experiment, carried out by
Boussingault, illustrates, in a general way, how rapid the process is
under favourable circumstances. A small portion of rich soil was placed
on a slab protected by a glass roof, and was moistened from time to time
with water. The amount of nitrate of potash formed under these
circumstances was estimated from time to time during a period of two
months. During the first month (August) the percentage was increased
from .01 to .18 (equal to about 5 cwt. of nitrate of potash per acre).
The increase during the second month (September) was very much
less,--indeed only about a seventh of the amount.[120] The soil
experimented with was an extremely rich garden soil, and all the
conditions for nitrification were most favourable.

Of recent experiments on the rate of nitrification, the most striking,
perhaps, are those by Schloesing. He mixed sulphate of ammonia with a
quantity of soil fairly rich in organic matter, and containing 19 per
cent of water. During the twelve days of active nitrification no less
than 56 parts of nitrogen per million of soil were nitrified per day.
Taking the soil to a depth of 9 inches, this would be equal to more
than 1 cwt. per acre--an amount of nitrogen equal to that contained in
6 cwt. of commercial nitrate of soda. These experiments are interesting
as showing what is probably the maximum rate of nitrification under the
most favourable circumstances, and where there is an abundant supply of
easily nitrifiable nitrogen. That nitrification ever takes place in our
soils to this extent is not to be for a moment supposed.

Warington, in his Rothamsted experiments, has found that the greatest
rate, working with ordinary arable soil (first 9 inches) from the
Rothamsted farm, was .588 parts per million of air-dried soil per
day--_i.e._, 1.3 lb. per acre (equal to about 8 lb. of nitrate of soda).
Similar soil, when supplied with ammonia salts, showed nearly double
this quantity. Higher results were obtained by Lawes and Gilbert with
rich Manitoba soils, the average rate being .7 parts per million per
day.

The last of these interesting laboratory experiments on the rate of
nitrification we shall refer to, are those by Dehérain. He experimented
with soils containing different amounts of nitrogen and moisture. With a
soil containing .16 per cent of nitrogen he obtained, during a period of
90 days, rates of nitrification varying from .71 to 1.09 per million
parts of soil. The maximum quantity was formed when the soil contained
25 per cent of moisture. On a soil considerably richer--viz.,.261 per
cent of nitrogen--a higher rate of nitrification took place--1.48 parts
per million. The highest rate obtained in these experiments showed,
when calculated to pounds per acre, about 5-1/2, taking the soil to a
depth of 9 inches. When the soil was alternately dried and moistened the
process was most rapid.


_Portion of Soil-nitrogen more easily Nitrifiable than the rest._

Lastly, it may be noticed that in the above-cited experiments, and
others of a similar kind, the process goes on most rapidly at first, and
steadily diminishes thereafter. This is due to the fact, that there is
generally a certain quantity of nitrogen in most soils in a more easily
nitrifiable condition than the rest, so that when this becomes oxidised
nitrification proceeds more slowly. It would further seem that the
nitrogen of the subsoil is less easily nitrified than that of the
surface-soil.


_Rate of Nitrification deduced from Field Experiments._

While the above experiments throw much light on the question of the rate
at which nitrification may go on under different circumstances, the
results furnished by actual analyses of soils and their drainage-waters
are of still more practical value; and the Rothamsted experiments
fortunately furnish us with a number of these valuable results.


_Quantity of Nitrates formed in the soils of Fallow Fields._

These researches had to be carried out on soil taken from fields lying
in bare fallow; for no true estimate of the amount of nitrates formed
could have been obtained from _cropped_ fields. In the first 27 inches
of soil of six separate fields, nitrate-nitrogen was found to vary from
36.3 lb. to 59.9 lb. per acre. In four of these fields the largest
proportion was found in the first 9 inches of soil; in the remaining
two, in the second 9 inches; while the third 9 inches in two fields
showed almost as large a proportion as the first 9 inches.[121]


_Position of Nitrates depends on Season._

The position of nitrates in the soil depends largely on the season; for,
as has been already pointed out, their production is almost entirely
limited to the surface-soil, and it is only by being washed down in rain
that they find their way to the lower layers. A wet season, therefore,
has the effect of increasing their percentage in the lower soil-layers.


_Nitrates in Drainage-waters._

As there is a certain proportion of nitrates that finds its way even
below the first 27 inches of soil, the above results do not show their
total production. To accurately estimate this amount we must ascertain
the quantity escaping in drainage-water. Here, again, the Rothamsted
experiments furnish us with valuable data. The amount found in
drainage-waters of course naturally varies very much, and depends
largely on the rainfall; but taking an average of twelve years, this has
been found to amount to between 30 and 40 lb. per acre--an amount not so
very far short of that found in the first 27 inches of the soil itself.
This was from comparatively poor soil, it must be remembered, and a much
larger quantity would undoubtedly be produced in the case of richer
soils. Adding then the results together, we find that in soils like
those at Rothamsted, when in bare fallow, between 80 and 90 lb. of
nitrogen are converted into nitrates in some fourteen months' time--an
amount equal to about 5 cwt. of nitrate of soda. It is a fact of no
little practical significance that nearly one-half of this large
quantity is found in the drainage-water.


_Amount produced at Different Times of the Year._

Some indication of the rate at which nitrification takes place during
the different months of the year is obtained from a study of the results
of the analyses of drainage-waters which we have just referred to. This,
however, it must be remembered, only furnishes us with a very
approximate indication. The month showing the greatest amount of
nitrates in the drainage-water must not necessarily be regarded as that
during which nitrification has been most active, for the amount chiefly
depends on the rainfall. In illustration of this it will be found that
the drainage-water during the autumn and early winter months contains
most nitrates, not because nitrification is most active then, but
because the rainfall is greatest, and a large proportion of the nitrates
formed during the drier summer months is being only then washed from the
soil. The amount of nitrates in drainage-waters steadily diminishes from
autumn through the winter months, and is least in spring. The total
amount of nitrates found in the drainage-water is, therefore, not a safe
guide. What, however, does furnish us with a more reliable indication is
the _percentage_ of nitrates in the drainage-water. Regarding the
results of the analyses of drainage-water (see Appendix) from this point
of view, it will be seen that this is greatest during the month of
September, and least during April.[122]


_Nitrification of Manures._

A subject which has not yet been specially referred to, but which is of
great practical importance, is the nitrification of manurial substances.
It is unfortunate that the amount of research hitherto devoted to this
important question has been slight, and that the knowledge we possess is
therefore very limited.


_Ammonia Salts most easily Nitrifiable._

One fact, however, about which there can be little doubt, is that
nitrogen in the form of ammonia salts is, of all compounds of nitrogen,
the most easily nitrifiable. Indeed, as we have already indicated, it is
highly probable that the conversion of the different forms of organic
nitrogen into ammonia is an intermediate stage in the nitrification of
these bodies. At any rate it seems to be invariably the case that when a
mixture of nitrogen compounds, including ammonia salts, are allowed to
nitrify, the nitrogen in the form of ammonia is the first to become
nitrified.


_Sulphate of Ammonia most easily Nitrifiable Manure._

It follows from this that sulphate of ammonia, the most common of
ammoniacal manures, is one of the most speedily nitrified when applied
to the soil. The rate at which the nitrification of this manure takes
place naturally varies according to the quantity applied, and other
circumstances, such as the nature of the soil and the weather, &c. That,
under favourable circumstances, the conversion of ammonia into nitrates
is very rapid, has been shown by a number of experiments. Dehérain has
found that when sulphate of ammonia was mixed with soil at the rate of 2
cwt. per acre, nitrification took place at the rate of 1/100th of its
nitrogen per day.

_Rate of Nitrification of other Manures._

Of other nitrogenous manures, guano, it would seem, comes next to
sulphate of ammonia in the rate at which it becomes nitrified in the
soil; while next to guano stand green manures, dried blood, meat-meal,
&c. As we should expect, such a manure as shoddy is very slowly
nitrified. The rate at which the nitrogen compounds in farmyard manure
become nitrified, when incorporated with the soil, vary very much
according to circumstances. It goes on probably at a greater rate than
the ordinary nitrification of soil-nitrogen. It is a somewhat striking
fact that the effect of adding nitrate of soda to the soil may be at
first to check nitrification. That the addition of common salt, even in
small quantities, has this result, is at any rate certain. The presence
of salt to the extent of one-thousandth of the weight of the soil, has a
prejudicial effect.

_Soils best suited for Nitrification._

To recapitulate, then, nitrification is effected through the agency of
micro-organisms, which are present to a greater or less extent in all
soils. It requires for its favourable development air, warmth, moisture,
absence of strong light, presence of a salifiable base--viz., carbonate
of lime--the presence of certain mineral food-constituents, such as
phosphates, and a certain amount of alkalinity. It consequently takes
place to the least extent in barren sandy soils. Soils rich, light, well
ventilated, uniformly moist, warm, and chalky, are best suited for its
development. Other things being equal, it develops better in a
fine-grained soil than in a coarse-grained soil, because, in the case of
the former, aeration and uniform moistening of the soil are best
secured.


_Absence of Nitrification in Forest-soils._

A point of considerable interest is the practical absence of the process
in forest-soils. The absence, or occurrence in the most minute traces,
of nitrates in forest-soils has been accounted for by the lowness of the
normal temperature of such soils and their extreme dryness. This latter
condition is accounted for by the enormous transpiration of water which
takes place through the trees, especially in summer-time, which is such
as to render the soil almost air-dry. Lastly, it may be accounted for by
the want of mineral food ingredients.


_Important Bearing of Nitrification on Agricultural Practice._

Before concluding this chapter, it may be well to draw attention to the
important bearing which nitrification has on agricultural practice. The
light which our present knowledge--imperfect as it is--of this most
interesting process throws on the theory of the rotation of crops is
very striking, for it shows how the adoption of a skilful rotation may
be made to prevent the loss of enormous quantities of the most valuable
of all our soil-constituents,--the one on the presence of which
fertility may be said most to depend--viz., nitrogen.


_Desirable to have Soil covered with Vegetation._

The constant production of nitrates going on in the soil, the inability
of the soil to retain them, and the consequent risk of their being
removed in drainage, furnish a strong argument in favour of keeping our
soils as constantly covered with vegetation as possible.


_Permanent Pasture most Economical Condition of Soil._

From the point of view of conservation of soil-nitrates, permanent
pasture may be said to be the most economical condition for the soil to
be in. In such a case the nitrates are assimilated as they are formed,
and, by being converted in the plant into organic nitrogen, they are at
once removed from all risk of loss. A consideration, therefore, of the
process of nitrification furnishes many arguments in favour of laying
down land in permanent pasture--a practice which of late years has been
increasingly followed in many parts of the country. As, however, it is
not possible or desirable to carry out this practice beyond certain
limits, the rotation which most nearly conforms to the condition of
keeping the soil covered with vegetation, and most approximates in this
respect to permanent pasture, is most to be recommended.


_Nitrification and Rotation of Crops._

The chief risk of loss of nitrates is in connection with a cereal crop
such as wheat. Where turnips follow wheat, there is a period during
which the soil is left uncovered, and during which most serious loss of
nitrates is apt to ensue. The risk of loss is enhanced by the fact that
the assimilation of nitrates by cereals ceases before the season of
their maximum production in the soil. The soil is then left bare of
vegetation during the autumn, which is the most critical period of all,
and the result must be serious loss. In order to minimise this loss, the
practice of growing catch-crops has been had recourse to. As, however,
this practice will be dealt with elsewhere, nothing further need here be
said.


FOOTNOTES:

[97] As the formation of nitrites is a stage in the process, the term
_nitrification_ includes the formation of nitrites as well as nitrates.

[98] Nitre seems to have been known as early as the thirteenth century.

[99] Lawes and Gilbert, for example, have shown that in the Rothamsted
soils it only amounts to a few parts per million of soil.

[100] See Appendix, Note I., p. 196.

[101] The artificial production of nitre seems to have been first
effected by Glauber in the seventeenth century.

[102] The lime-rubbish from old buildings, especially those parts which
have come in contact with the earth, or plastering from the walls of
damp cellars, barns, stables, &c., have been found to be rich in nitrate
of lime, and, as has been long well known, constitute by themselves a
valuable manure. The formation of the nitrate of lime can be accounted
for by the contact of the lime with nitrogenous matter of different
kinds.

[103] As much of the nitric acid in this solution was present as nitrate
of lime, it was usually treated with a solution of potassium carbonate,
the result being the precipitation of the lime as carbonate, pure
saltpetre being left in solution, according to the following equation--

K_{2}CO_{3} + Ca(NO_{3})_{2} = 2 KNO_{3} + CaCO_{3}.

Under the French mode of manufacture, the process was considered to have
developed satisfactorily when 1000 lb. of earth, at the expiration of
two years, yielded 5 lb. of nitre.

[104] Pasteur had already in 1862 expressed the opinion that
nitrification might probably be in some way connected with ferments. A.
Müller (see 'Journal of Chemical Society,' 1879, p. 249) was the first
to advance the opinion that nitrification was due to the action of a
ferment. This conclusion he was led to by the observation that while the
ammonia in sewage was converted into nitric acid, no change took place
in solutions of ammonia or urine prepared in the laboratory.

[105] Bisulphide of carbon and phenol (carbolic acid) have also been
experimented with in connection with their antiseptic action on
nitrification. In these experiments the former had a similar effect to
chloroform; the phenol, however, while hindering it did not entirely
suspend it, due probably to the difficulty of bringing the phenol vapour
into thorough contact with the soil-particles.

[106] Winogradsky has named the nitrous organism _nitrosomonas_, and the
nitric organism _nitrobaeter_.

[107] From a series of Lectures delivered by him in connection with
Lawes Agricultural Trust, in the United States.

[108] This silica-jelly consists of dialysed silicic acid, ammonium
sulphate, potassium phosphate, magnesium sulphate, calcium chloride, and
magnesium carbonate.

[109] This fact is all the more striking when we remember that this
decomposition of carbonic acid is best effected in the dark, since light
is prejudicial to nitrification.

[110] See Appendix, Note II., p. 196, and Note III., p. 197.

[111] See Appendix, Note V., p. 198.

[112] This is shown by the fact that nitrification will only continue in
a solution of carbonate of ammonia till one-half the ammonia is
nitrified. It then stops. The base, with which the nitrous acid combines
as it is formed, being at that stage entirely used up, nitrification is
no longer possible. With regard to urine solutions the same is the case.
Nitrification thus will only take place where there is a sufficiency of
base.

[113] See Appendix, Note IV., p. 197.

[114] It would seem that an alkalinity much exceeding four parts of
nitrogen per million is prejudicial to the process.

[115] According to Warington, solutions containing 50 per cent of urine
become nitrifiable when sufficient gypsum is added. The gypsum
neutralises the alkalinity of nitrifying solutions by converting the
alkaline ammonium carbonate into neutral ammonium sulphate, the calcium
carbonate being precipitated.

[116] See Chapter on Farmyard Manure.

[117] As practically illustrating this fact, a solution kept at 10° C.
required ten days, while a solution kept at 30° C. required only eight
days for nitrification.

[118] In sixty-nine trials no failure to produce nitrification by
seeding with soil from a depth, of 2 feet was experienced. Similarly in
eleven trials only one failure took place with soil from a depth of 3
feet. With clay soil from a depth of 6 feet success took place to the
extent of 50 per cent. No nitrification was obtained with clay from a
depth of 8 feet. Entire failure was experienced with chalk subsoil. The
process thus diminishes in activity the lower down we go.

[119] Koch has found that in soils he has examined few organisms were
found at a depth below 3 feet.

[120] See Appendix, Note VI., p. 198.

[121] For full analytical results see Appendix, Note VII., p. 198.

[122] We find the least amount in the month of April. In the water, from
a 20-and 60-inch gauge respectively, the amounts were 1.35 lb. and 1.61
lb. per acre (rainfall 2.25 inches). From then on to November the amount
steadily increases. In the latter month it reaches its maximum--viz.,
6.50 lb. (20-inch gauge) and 5.98 lb. (60-inch gauge) per acre (rainfall
2.30 inches). See Appendix to Chapter III., Note VIII, p. 160.



APPENDIX TO CHAPTER IV.


NOTE I. (p. 162).

OLD THEORIES OF NITRIFICATION.

According to the old theories, nitrification was regarded as a simple
case of the oxidation of nitrogen by the oxygen of the air, or by ozone.
The union of nitrogen and oxygen, however, probably takes place only at
very high temperatures, such as are formed during electric discharges.
It is needless to point out that the union of nitrogen and oxygen in
this way is not likely to occur in soils. According to other theories,
nitrification was effected by means of the oxidation of ammonia.
Ammonia, however, can only be oxidised to nitric acid by means of
certain powerful oxidising agents, such as ozone or hydrogen peroxide.
As, however, these substances are not found in the soil, it is much to
be doubted whether nitric acid is ever formed in the soil in this way.
It is possible, however, as held by some, that ferric oxide is capable
of inducing this conversion. On the whole, however, most evidence points
to the conclusion that all nitric acid produced in the soil is formed
through the agency of micro-organic life.


NOTE II. (p. 170).

The important fact that nitrification can take place in solutions
practically devoid of organic matter, was first shown by Dr J. H. M.
Munro ('Chemical Society Journal,' August 1886, p. 561). It was further
corroborated by Warington and P. F. Frankland. Winogradsky, however,
has carried out the most conclusive experiments on the subject. "He
prepared vessels and solutions, carefully purified from organic matter,
and these solutions he sowed with the nitrifying organism. Finding that
under these conditions the nitrifying organism increased enormously and
displayed its full vigour, he proceeded further to determine the amount
of carbonaceous organic matter formed in solutions after the
introduction of the organism. By making the nitrification intensive, he
was able to obtain considerable quantities of carbon from the nitrified
solutions by the process of wet combustion. In his third memoir he
publishes figures which apparently show a close relation between the
amount of nitrogen oxidised, and the amount of carbon assimilated; the
ratio is about 35:1."--See Bulletin of U.S. Department of Agriculture,
No. 8, containing Lectures on Rothamsted Experiments by R. Warington,
F.R.S., p. 50.


NOTE III. (p. 170).

The oxidising power of the micro-organisms of soil is not confined to
the oxidation of ammonia or of organic matter. Müntz has shown that soil
is capable of oxidising iodides to hypo-iodides and iodates, and
bromides to hypo-bromides and bromates. This is a very important result,
and seems to indicate that nitrification is part of a general oxidising
action, and that we must not assume that nitrites or nitrates are
produced because they are in themselves of advantage to the organism.


NOTE IV. (p. 172).

"When urine in different degrees of dilution was treated with soil, 1
gram of soil being added to 100 c.c. of diluted urine, nitrification
commenced in the 1-per-cent solution in 11 days, in the 5-per-cent
solution in 20 days, in the 10-per-cent solution in 62 days, in the
12-per-cent solution in 90 days. The alkalinity of the last-named
solution when nitrification commenced was equal to 447 mgs. of ammonia
per litre. A solution with an alkalinity of 500 mgs. of ammonia per
litre is apparently unnitrifiable."--American Department of Agriculture
Bulletin, Warington's Lectures on Rothamsted Experiments, p. 51.


NOTE V. (p. 171).

Professor P. F. Frankland in his experiments used the following
solutions:--

             grms.
NH_{4}Cl      .5   }
H_{3}PO_{4}   .1   }
MgSO_{4}      .02  >  In 1000 c.c. of distilled water.
CaCl_{2}      .01  }
CaCO_{3}     5.00  }


NOTE VI. (p. 185).

Experiment by Boussingault on Rate of Nitrification.

              Percentage of Nitrate
   1857.           of Potash.        = lb. per acre.
August 5              .01                   34
August 17             .06                  222
September 2           .18                  634
September 17          .22                  760
October 2             .21                  728


NOTE VII. (p. 188).

NITROGEN AS NITRATES IN ROTHAMSTED SOILS AFTER BARE FALLOW IN LB.
PER ACRE.

------------+-----------+---------------------------+-----------+----------
            | Alternate |   Four-course rotation.   |           |
            |  Wheat    +-----------+---------------+           |
 Depth of   |   and     |  Super-   |               | Claycroft | Foster's
  Soil.     |  Fallow.  | phosphate | Mixed Manure. |   Field.  |  Field.
            |           |  only.    |               |           |
            +-----------+-----------+-------+-------+-----------+----------
            |   1878.   |   1878.   | 1878. | 1882. |   1881.   |   1881.
------------+-----------+-----------+-------+-------+-----------+----------
            |    lb.    |    lb.    |  lb.  |  lb.  |    lb.    |    lb.
 1st 9 ins. |   28.5    |   22.3    | 30.0  | 40.1  |   16.4    |   14.6
 2d 9 ins.  |    5.2    |   14.0    | 18.8  | 14.3  |   26.5    |   24.6
 3d 9 ins.  |    -      |    -      |  -    |  5.5  |   15.9    |   17.3
------------+-----------+-----------+-------+-------+-----------+----------
  Total     |   33.7    |   36.3    | 48.8  | 59.9  |   58.8    |   56.5
------------+-----------+-----------+-------+-------+-----------+----------



CHAPTER V.

THE POSITION OF PHOSPHORIC ACID.


We now come to consider the position of phosphoric acid in agriculture.
The question is, however, very much simpler in its nature than that of
nitrogen, and may be consequently discussed in a much shorter space.

Most soils, as we have already had occasion to point out, are better
supplied with available ash-plant ingredients than available nitrogen
compounds. The quantity of phosphoric acid absorbed by the plant is also
less than that of nitrogen; and lastly, the different chemical compounds
of phosphoric acid occurring in the soil are not nearly so numerous as
those of nitrogen. Phosphoric acid, however, must be regarded as ranking
next to nitrogen in its importance as a soil-constituent.


_Occurrence of Phosphoric Acid in Nature._

That phosphoric acid is of universal occurrence may be assumed from the
fact of the almost universal occurrence of vegetable life on the
earth's surface; for plants are unable to grow without it. While thus of
practically universal occurrence, its amount in most soils is very
trifling. As the only source of it in the soil is from the
disintegration of the different rocks, a short description of its
occurrence in the mineral kingdom may first be given.


_Mineral Sources of Phosphoric Acid._

It was first discovered in the mineral kingdom towards the close of last
century; but we have only of late years ascertained any exact knowledge
of its percentage in the different rocks out of which soils are formed.
This has been shown in many cases to be very trifling. It most
abundantly occurs as _apatite_, a mineral consisting of calcium
phosphate, with small quantities of calcium fluoride or calcium
chloride. This apatite, or phosphorite, is found in certain parts of the
world in large masses; but as a rule, it only occurs in small quantities
in most rocks. It may be stated that the older rocks are, as a general
rule, richer in it than those of more recent formation; and Daubeny has
drawn attention to this fact as furnishing a useful guide in estimating
the probable richness of a soil in phosphoric acid. The older,
therefore, a rock is, the richer it is likely to be in phosphoric acid.


_Apatite and Phosphorite._

Of apatite there are a variety of kinds, which differ in their
appearance as well as in their composition. It occurs chiefly in a
crystalline form, and is found sometimes in regular crystals, but it
also occurs in the amorphous form. In colour it may be white, yellow,
brown, red, green, grey, or blue. Two classes of apatite are found. The
first consists of calcium phosphate along with calcium fluoride; and in
other kinds of apatite the calcium fluoride is replaced by calcium
chloride. Phosphorite is another name for apatite, but is chiefly
applied to impure amorphous apatite. The percentage of phosphate of lime
in different kinds of apatite may be stated at from 70 to 90 per cent.
It occurs in very large quantities in Canada, the Canadian apatite being
very rich in phosphate of lime--80 to 90 per cent. In many parts of the
world it forms portions of mountain-masses, and is quarried, crushed,
and used for artificial manurial purposes. Further details of its
occurrence and chemical composition will be found in the Appendix.[123]


_Coprolites._

In many parts of the world round nodules, largely consisting of
phosphate of lime, have been found, to which the name "coprolites" has
been given, on the assumption that they consisted of fossilised animal
excrements. These coprolites, or osteolites as they have also been
called, vary in the percentage of phosphate of lime they contain.
Sometimes this amounts to 80 per cent, but as a rule it is very much
less. They also in the past have formed an important source of manure,
and will be referred to subsequently.


_Guano._

We have, lastly, phosphoric acid occurring in large quantities in
guano-deposits, chiefly found on the west coast of South America. These
deposits, which have been of enormous importance as a source of
artificial manure, are of animal origin, and will be discussed at
considerable length in a chapter specially devoted to the subject; so
that we need do no more than mention them here.

Phosphoric acid is also found in the form of phosphate of lime in
certain rocks as "layers" and "pockets."


_Universal Occurrence in Common Rocks._

But while it is thus found in considerable quantities in various parts
of the world, and while no anxiety need thus be felt as to its abundance
for artificial manurial purposes, its occurrence in the common rocks,
which, as we have already pointed out, is practically universal, is in
many cases very minute.

Fownes first identified it in the felspathic rocks in 1844; and since
then its percentage in granite, lava, trachyte, basalt, porphyry,
dolomite, gneiss, syenite, dolerite, diorite, and a number of other
rocks, has been determined by numerous investigators. For analyses of
these rocks the reader is referred to the Appendix.[124]


_Occurrence in the Soil._

That no soil is actually without phosphoric acid is highly probable, but
in many soils it is present in the merest traces, and even in fertile
soils it is rarely present in quantities over two-tenths of a per cent;
while half that amount may be taken as an average for most fairly
fertile soils. This would be about 3500 lb. per acre, calculating the
soil to a depth of 9 inches. In exceptional cases it has been found to
the extent of .3 per cent; and in the famous Russian _black earth_ it
has been found to amount to .6 per cent.[125] Like nitrogen, it is found
in greatest amount in the surface portion of the soil, but its amount at
different depths does not vary to the same extent as we have found to be
the case with nitrogen.


_Condition in which Phosphoric Acid is present in the Soil._

Unlike nitrogen, phosphoric acid occurs in the soil almost entirely in
an _insoluble_ form; and when applied to the soil in a soluble form, is
speedily converted into an insoluble condition. Its most commonly
occurring forms are as phosphates of lime, iron, and alumina. These
facts are of importance to remember, as they explain why phosphoric acid
is not found in drainage-water in any quantity. It also shows how little
the risk of loss from drainage is in the application of artificial
phosphatic manure to the soil.


_Occurrence in Plants._

The percentage of phosphoric acid in plants, like other
ash-constituents, is subject to considerable variation, and depends on a
variety of conditions, such as the state of the plant's development,
nature of soil, climate, season, treatment with manures, &c. All these
conditions have a certain influence. The different parts of the plant
have been found to contain it in different quantities. The tendency of
phosphoric acid is to travel up to the higher portions of the plant with
the progress of growth, and to finally accumulate in the seed. As
illustrating this, it may be mentioned that the inner portion of the
stalk of a ripe oat-plant has been found to contain only a seventeenth
of the amount of phosphoric acid found in the same portion of the stalk
of a young oat-plant. Similarly it may be mentioned that, while the ash
of the grain of rye and wheat contains nearly half their weight of
phosphoric acid, the percentage present in the ash of other parts of the
plant amounts only to from 5 to 16 per cent. The percentage of
phosphorus is greater in young plants than in mature plants; it is
greater also in quickly developed plants than in slowly developed
plants.

In the plant, phosphorus is present chiefly in the albuminoids; and its
absorption from the soil takes place in greatest quantity during the
period of maximum growth. In beans and peas an oil containing phosphorus
has been found.


_Occurrence in Animals._

That phosphorus in different forms exists in animal tissue is well
known. It is found both in the brain and in the nerves, as well as in
nearly all the fluids of the animal body. It is, however, in the bones
that it is most abundant, the mineral portion of which is almost
entirely made of phosphate of lime,--a fact which renders bones such a
valuable artificial manure. Altogether, phosphoric acid occurs in the
animal body to the extent of 2.3 per cent. There is a point which we
shall have occasion to draw the student's attention to further on in
discussing the nature of farmyard manure--and that is, that the urine of
the common farm animals is practically devoid of phosphoric acid.


_Sources of Loss of Phosphoric Acid in Agriculture._

As we have already done in the case of nitrogen, we may now attempt to
form some conception of the sources of loss and gain of phosphoric acid
in the soil. The sources of loss may be divided into natural and
artificial. Of natural sources of loss we have only one, and that is
loss by drainage.


_Loss of Phosphoric Acid by Drainage._

We have already seen that the condition in which phosphoric acid is
present in the soil is as insoluble phosphate. In drainage-water it
occurs in mere traces. Minute though the amount seems when stated as
percentage, and small as it appears beside the loss (from the same
source) of nitrogen, it is yet, if considered for large areas,
sufficiently striking. Thus it has been estimated that in the river Elbe
there is carried off by drainage from the fields of Bohemia 2-3/4
million pounds (1200 tons) of phosphoric acid annually. This, it is
true, is a very trifling amount compared with the annual loss of
nitrogen from an equal area; but then it must be remembered, on the
other hand, the sources of gain to the soil of this ingredient are not
so numerous as are those of nitrogen, the only sources of phosphoric
acid being in the manure applied to the soil, and that coming from the
gradual disintegration of phosphatic minerals.


_Artificial Sources of Loss._

The other sources of loss may be classed under the term artificial, and
are connected with agricultural practice. Just as we have seen that in
the case of nitrogen enormous quantities of that substance are
constantly being removed from the soil in those crops which are
consumed off the farm, so, too, enormous quantities of phosphoric acid
are being removed in the same way. As illustrating this fact, it may be
mentioned that Professor Grandeau has recently estimated that in the
entire crops grown in France in one year there are about 298,200 tons of
phosphoric acid; while the amount returned in the dung of farm animals
is only 157,200, or only about one-half of what is removed in the crops,
leaving a deficit of 147,000 tons to be made good by the addition of
artificial phosphatic manures, if the fertility of the soil is to be
maintained. The same authority has calculated that in the bones of the
entire farm animals in France there is no less a quantity than 76,820
tons of phosphoric acid.

As an example of how, in many cases, the amount of phosphoric acid
removed from the farm is very often much greater than that restored, a
case quoted by Crusius may be cited. This was a farm of 670 acres
(Saxon) which had received only farmyard manure, and from which, during
sixteen years, 985.67 cwt. of phosphoric acid had been sold off in the
crops; while only 408.33 cwt. had been restored in the manure, leaving a
loss of 577.34 cwt.


_Phosphoric Acid removed in Milk._

A further source of loss is the phosphoric acid removed in milk. In the
total annual yield of milk from one cow there may be from 11 to 12 lb.
of phosphoric acid.


_Loss in Treatment of Farmyard Manure._

The risks of loss of phosphoric acid in the treatment of farmyard manure
are not so great as in the case of nitrogen. There is, however, a
considerable risk, through want of proper precautions, of the soluble
phosphates being washed away by rain.


_Loss in Sewage._

The loss of phosphoric acid incurred by the present method of sewage
disposal is not so large as the loss of nitrogen, inasmuch as the
quantity of phosphoric acid contained in human excreta is very much
less. Roughly speaking, it may be said to amount to a little less than
one-third of the nitrogen lost in this way.


_Sources of Artificial Gain of Phosphoric Acid._

To balance these losses, we have a practically unlimited supply of
mineral phosphates for application as artificial manure, as well as
large quantities of other manures, many of them already mentioned in
connection with nitrogen, such as bones and guanos of all kinds. Quite
recently, also, a large source of phosphoric acid has been opened up in
the basic slag, a rich phosphatic bye-product obtained in considerable
quantity in steel-works from the basic process of steel manufacture. We
have also large quantities of phosphoric acid in the imported
feeding-stuffs, for statistics regarding which we would refer our
readers to a previous chapter. The question of the actual amount
contained in these sources is not of the same interest as in the case of
nitrogen, and need not therefore detain us. We have sufficiently
indicated the importance of phosphoric acid in agriculture by the
statements above given. All further consideration of phosphoric acid
must therefore be deferred to future chapters.


FOOTNOTES:

[123] See Appendix, Note I., p. 210.

[124] See Appendix, Note II., p. 211.

[125] These results, as indeed all soil percentages, are calculated on
the soil in a dry condition.



APPENDIX TO CHAPTER V


NOTE I. (p. 201).

COMPOSITION OF APATITE (Voelcker).

   (_Krageröe, Norway._)

Lime                   52.16
Phosphoric acid        41.25
Chlorine                4.10
Fluorine                1.23
Oxide of iron           0.29
Alumina                 0.38
Potash and soda         0.17
Water                   0.42
                      ------
                      100.00
                      ======

Apatite is found in considerable quantities in America, Germany, France,
Spain, Hungary, Norway, and Great Britain. According to Rose, apatite is
made up of three molecules of tribasic calcium phosphate (Ca(PO_4)_2),
combined with one molecule of calcium fluoride (Ca F_2) or one molecule
of calcium chloride (CaCl_2) respectively.

The composition of the pure mineral should be--

      _Chlorapatite._
                      Per cent.
Calcium phosphate      89.38
Calcium chloride       10.62

      _Fluorapatite._

Calcium phosphate      92.31
Calcium fluoride        7.69


NOTE II. (p. 203).

The following is a list of the commoner rocks in which the percentage of
phosphoric acid has been determined. The results are taken from analyses
by Nesbit, Schramm, Bergemann, Rose, Dehérain, Handtke, Petersen,
Nessler, Muth, Fleischmann, Storer, and others:--

                               Per cent.
Felspar                 1.7
Granite                 0.09   0.25   0.58  0.68
Lava                    1.21   1.8
Trachyte                0.30   0.66
Basalt                  0.50   1.11
Porphyry                0.26
Marl                    1.45   2.31   3.8
Calcareous stones       0.064  0.176
Dolomite                1.24
Lias chalk              1.39
Gneiss                  0.18   0.78   1.51
Syenite                 0.10
Dolerite                0.3    1.1    1.2
Diorite                 0.5    0.69



CHAPTER VI.

THE POSITION OF POTASH IN AGRICULTURE.


We may, lastly, consider the position of _potash_ in agriculture, the
only ash ingredient of the plant, in addition to phosphoric acid, which
it is as a rule necessary to add as a manure.


_Potash of less Importance than Phosphoric Acid._

It is of far less importance than phosphoric acid, from the fact of its
much more abundant occurrence in the soil, as well as from the fact that
under the ordinary conditions of agriculture, although removed from the
soil in considerable quantities by crops, it finds its way back again in
the farmyard manure; for it has not the same tendency to accumulate in
large quantities in the grain or seed as we have seen to be the case
with phosphoric acid. On this account straw contains a much greater
proportion of potash than phosphoric acid, and hence farmyard manure may
be regarded as fairly rich in potash.


_Occurrence of Potash._

Of all sources of potash the ocean must be regarded as the chief.
Millions and millions of tons are present in a state of solution in the
salt water of the ocean.[126] Like phosphoric acid, its occurrence in
the rocks forming the earth's crust may be said to be practically
universal. Many of the commonly occurring rocks and minerals are
extremely rich in it, and by their disintegration furnish large
quantities to the soil. Some of these rocks contain it in such abundance
that they have been tried as potash manures; and were other more
valuable sources less available than they actually are, such a practice
might well be recommended. A volcanic rock known as _palagonite_, and
that most commonly occurring of all potash minerals--viz., felspar--have
both been experimented with in this way with considerable success.


_Felspar and other Potash Minerals._

That felspar should prove, when finally ground, a valuable source of
potash, is not to be wondered at when we remember that some varieties of
it contain over 16 per cent. It has been calculated that a single cubic
foot of this mineral is sufficient to supply an oak-wood, covering a
surface of 26,910 square feet, with potash for a period of no less than
five years.[127] Some idea of the enormous _potential_ fertility of a
soil containing felspar, so far as potash is concerned, may be obtained
from this statement. It must be remembered, however, that it is only the
orthoclase or potash felspars which contain large quantities of
potash--other felspathic rocks, such as oligoclase and labradorite,
being comparatively poor in it. Another commonly occurring mineral which
is rich in potash is mica, which has been found to contain from 5 to 13
per cent. From this it follows that rocks which have large amounts of
these minerals in their composition--such as granite, for example, which
often contains 5 or 6 per cent of potash--form by their disintegration
soils rich in this ingredient.


_Stassfurt Salts._

But in addition to the sources of potash already mentioned, it exists in
other forms in the earth's surface. Till within recent years it was
obtained for commercial purposes from the ashes of plants, which, as we
shall immediately see, are extremely rich in this ingredient; from salt
water--this source giving rise to the so-called "salt gardens" on the
coast of France; and from nitre soils in different parts of India,
referred to already at considerable length. Large mineral deposits,
however, have been recently discovered in the neighbourhood of Stassfurt
in Germany, and have since their discovery supplied all the potash
required for manurial and other purposes. In these deposits (similar
ones have also been found at Kalusz in the Carpathian Mountains) there
are no less than five different minerals which contain potash. The form
in which it is present is as sulphate or chloride, so that it is readily
available for plants, and is of altogether very much greater value than
the form in which it occurs in the minerals already mentioned--viz., as
an insoluble silicate. Of the Stassfurt potash salts, the best known as
a manure is _kainit_, which contains about 32 per cent of sulphate of
potash. A list of the other potash minerals, with the particulars of
their composition and the percentage of potash they contain, will be
found in the Appendix.[128]


_Occurrence of Saltpetre._

We have already had occasion, in Chapter IV., when discussing the
question of nitrification, to refer to the occurrence of nitrate of
potash in certain soils in India, which have formed a large source of
saltpetre used in commerce in the past.


_Occurrence of Potash in the Soil._

From what has been said regarding the richness in potash of certain
commonly occurring minerals, such as felspar, it is only natural to
infer that most soils must contain large quantities of this substance;
and this is so. The wonder is that potash, when applied as an artificial
manure, should have such a marked effect in increasing the fertility of
the soil, as is often the case. We must remember, however, that although
a soil may contain large quantities of potash, there may be a very small
percentage of the whole in an available form for the plant's needs.


_Potash chiefly in insoluble Condition in Soils._

Potash occurs almost entirely in soils in a very insoluble form--viz.,
combined with silica as a silicate of potash. It is only by the slow
disintegration of potash rocks that the potash they contain is set free
for plant uses. When it is applied as an artificial manure, on the other
hand, it is in a soluble form. In most soils the amount soluble in water
probably lies between .001 and .009 per cent; that soluble in dilute
acid solutions from .1 to .5 per cent; and that insoluble from .2 to 3.5
per cent of the soil. It is highly probable that a certain quantity of
potash in the soil may exist in combination with humic and ulmic acids,
forming insoluble potassium humates and ulmates.


_Potash in Plants._

Of all the ash ingredients of plants, potash is the most abundant, as it
forms on an average about 50 per cent of the total ash of plants--about
90 per cent of the alkalies. The ash of plants, indeed, was for long
the chief source of potash. Certain plants remove very large quantities
from the soil. Of these roots, potatoes, the vine, the tobacco-plant,
and hops may be mentioned as examples. It is present in large quantities
in the grain of cereals, although, as we have already pointed out, not
to the same proportional extent as phosphoric acid. It is found in the
plant's extremities, such as twigs and new leaves, in greatest
abundance.[129]


_Potash in the Animal Tissue._

It is also found in all parts of the animal body. Especially rich in
potash salts are the blood corpuscles, which contain about ten times the
amount contained in the serum. It is found in especial abundance in the
fleece of sheep, which may contain more potash than that in the whole
body of the sheep. Animal urine also contains potash in considerable
quantities.


_Sources of Loss of Potash._

The capacity of the soil to retain soluble potash compounds, while not
equal to its capacity for retaining phosphoric acid, is yet very much in
excess of its capacity for retaining nitrates. The result is, that
potash is only found in comparatively minute traces in drainage
water.[130] Taking the same example as we already cited in illustration
of the loss of phosphoric acid, we find that the amount carried away in
the course of a year in the waters of the Elbe from Bohemia is
97,000,000 lb. (43,300 tons).


_Potash removed in Crops._

The amount of potash removed by the different crops from the soil will
be considered in a subsequent chapter. We need only say here that the
class of crops which remove the largest quantity are the root crops,
especially mangels. The loss is least in the case of the cereals. The
amount of potash contained in the straw of cereals is about three times
the amount of that removed in the grain.


_Potash removed in Milk._

Lastly, we may refer to the potash removed in milk, which, on an
average, may be taken at 10 lb. per annum for each cow.


_Potash Manures._

Of potash manures the chief are the sulphate and the chloride, or, as it
is commercially known, the "muriate." The chief source of potash manures
are the Stassfurt deposits already referred to. Wood-ashes have also
been used in large quantities in the past (chiefly as a potash manure),
and in some parts of the world are still used. A considerable source of
artificial potassic manures is the refuse manufacture of sugar-beet,
such a large industry in Germany. Potash occurs as a constituent of
certain other manures, more valuable for nitrogen and phosphoric acid,
such as guano and dried blood.


FOOTNOTES:

[126] According to Boguslawski and Dittmar, the total amount of potash
calculated as sulphate of potash in salt water equals 1141 × 10^{12}
tons.

[127] See Storer's 'Agricultural Chemistry,' vol. ii. p. 291.

[128] See Appendix, Note I., p. 220.

[129] See Appendix, Note II., p. 220.

[130] According to Way, different samples of drainage waters were found
only to contain from .00003 to .00031 per cent.



APPENDIX TO CHAPTER VI.


NOTE I. (p. 215).

AMOUNT OF POTASH IN DIFFERENT MINERALS.

Felspars--
                                  Percentage of potash.
  (_a_) Orthoclase        {  9.11  10.28  11.07  12.12  12.47
                          { 13.49  14.35  15.21  16.7
  (_b_) Oligoclase           0.50
  (_c_) Labradorite          0.33
Mica                      {  5.61   6.20   7.23   8.26   8.95
                          {  9.00  10.25  12.40  13.15
Amphibole                    0.25   2.96
Pyroxene                     0.34   2.48
Leucite                     13.60  18.61
Zeolites                     0.30   9.35   0.98   4.93

Stassfurt potash salts--                                          Per cent.
  (_a_) Polyhallite, _potassium sulphate_                            28
  (_b_) Karnallite (KCl.MgCl_{2}6H_{20}), _potassium chloride_    24 to 27
  (_c_) Sylvin, pure _potassium chloride_.
  (_d_) Kainit (K_{2}SO_{4}MgSO_{4}MgCl_{2}6H_{2}O), _potassium
          sulphate_                                                  32
  (_e_) Schoenite (K_{2}SO_{4}, MgSO_{4}, 6H_{2}O), pure
          _potassium magnesium sulphate_.


NOTE II. (p. 217).

The quantity of potash obtainable from various plants in the manufacture
of potashes on a large scale is illustrated by the following statements.
1000 lb. of the following vegetative products yield the following
quantities of potashes:--

                           lb.
Old spruce-wood              1/2
Old poplar-wood              3/4
Old oak-wood               1-1/2
Corn-stalks               17-1/2
Bean-stalks               20
Grape-vine twigs          40

(Storer, 'Agricultural Chemistry,' vol. ii. p. 108.)



PART III.

MANURES



CHAPTER VII.

FARMYARD MANURE


Farmyard manure is the oldest, and is still undoubtedly the most
popular, of all manures. It has stood the test of long experience, and
has proved its position as one of the most important of all our
fertilisers. It is highly desirable, therefore, to make a somewhat
detailed examination of its composition, and to see on what the
variation in this depends; and, finally, to examine into the mode of its
action as a manure.

That it should prove a valuable manure is scarcely to be wondered at, as
it is originally formed from vegetable substance, and as it therefore
contains all the elements present in the plant itself.

Its composition is very variable, and probably no two samples would
yield exactly similar analyses. In this fact lies one of the chief
difficulties of the treatment of the subject, and all statements made
in the following pages as to its chemical composition must be taken as
only _approximate_.

We may divide its constituents into three classes.

1. That portion due to _solid excreta_.

2. The liquid portion, largely made up of dilute _urine_.

3. The _straw_, or other material, which is used as litter.

The composition of the manure will vary according to the proportion in
which these three substances are present, as well as according to the
composition of the substances themselves. It will consequently tend to a
clearer apprehension of the subject if we first examine briefly the
chemical composition of the solid excreta and urine of the farm animals.


1. _Solid Excreta._

The manurial value of the solid excreta of animals--_i.e._, the
proportion they contain of _nitrogen_, _phosphoric acid_, and
_potash_--depends on a variety of conditions.

The solid excreta of horses, sheep, cows, and pigs, are well known to
possess different properties, as well as to vary in their composition.

What, however, has a still greater influence is the nature of the food.
This is owing to the fact that the solid excreta are made up of
undigested food. We can scarcely expect the same quality of solid
excreta from an animal fed on poor diet as from an animal fed on very
much richer diet. Again, the percentage of the food voided in the solid
excreta varies in the case of different animals.[131]

Another consideration which enters into the question is the age, as well
as the treatment, of the animal. A young animal, during the period of
its growth, absorbs from its food into its system a larger quantity of
the three fertilising substances, nitrogen, phosphoric acid, and potash,
than is the case with an adult animal whose weight is neither increasing
nor diminishing. A working horse, similarly, will return more of the
nitrogen, phosphates, and potash in its dung than one not at work and
which is permitted to gain in weight. The nature of the composition of
the solid excreta, therefore, will depend on the nature of the _food_,
_age_, _breed_, _condition_, and _treatment_ of the animal.

Let us now investigate shortly the influence of the above
considerations. The solid excrements of the common farm animals are
generally distinguished from one another according to the rate at which
they decompose or ferment on keeping. Thus horse-dung is generally known
as a "hot" dung; while cow-dung, on the other hand, is known as "cool."
Why this should be so is not absolutely clear. Probably it is owing to
the fact that the former contains less water, as well as to the fact
(and this probably has more to do with it) that it contains a larger
percentage of fertilising matter, especially nitrogen, thus affording
conditions more favourable for rapid fermentation than in the case of
the more moist and less rich cow-dung.

The composition of the solid excreta of various animals, as we have just
said, varies with the nature of their food; so that it is impossible to
take any analyses as absolutely representing its composition. It may be
interesting, however, to compare the analyses of samples of horse-dung
with those of some other of the commoner farm animals, with a view to
obtaining an _approximate_ idea of this difference.

Stoeckhardt has found that in 1000 lb. of the fresh solid excreta of the
animals below mentioned, there were the following amounts of _nitrogen_,
_phosphoric acid_, and _alkalies_:--

-----------------------+----------+-------------+-------------+-----------
                       |          |             | PHOSPHORIC  |
                       |          |  NITROGEN.  |   ACID.     | ALKALIES.
                       |  WATER.  |-----+-------+-----+-------+---+-------
                       |          |     |Reduced|     |Reduced|   |Reduced
                       |          |     |  to   |     |  to   |   |  to
-----------------------+----+-----+-----+-------+-----+-------+---+-------
                       |lb. | per | lb. |  per  | lb. |  per  |lb.|  per
                       |    |cent.|     | cent. |     | cent. |   | cent.
Horses (winter food)   |760 |76   |5    |  .50  |3-1/2|  .35  | 3 |  .30
Cows (winter food)     |840 |84   |3    |  .30  |2-1/2|  .25  | 1 |  .10
Swine (winter food)    |800 |80   |6    |  .60  |4-1/2|  .45  | 5 |  .50
Sheep (2 lb. hay per   |    |     |     |       |     |       |   |
  diem)                |580 |58   |7-1/2|  .75  |6    |  .6   | 3 |  .30
-----------------------+----+-----+-----+-------+-----+-------+---+-------

From the above table it will be seen that the sheep's dung contains the
least percentage of _water_, and is richer in _nitrogen_ and _phosphoric
acid_ than any of the other three. The percentage of alkalies, of which
the most important is potash, is, however, not so large. This may be
accounted for by the interesting and well-known fact that a large
percentage of potash is to be found in the wool of sheep.[132]

The solid excrement of the sheep is, therefore, weight for weight, the
most valuable as a manure, as it contains more nitrogen and phosphates
than the others, and at the same time is much drier.

If, however, we compare the composition of the solid excreta in a dry
state, we shall find that the following are the results (basing our
calculation on Stoeckhardt's analyses):--

          Nitrogen,   Phosphoric acid,   Alkalies,
          per cent.      per cent.       per cent.

Horse       2.08           1.45            1.25
Cow         1.87           1.56            0.62
Pig         3.00           2.25            2.50
Sheep       1.78           1.42            0.71

It will be seen from the above that the dry substance of the solid
excreta of the pig is richest in fertilising substances. Too much
stress, however, as has already been pointed out, must not be put on any
single analysis, as so much depends on various conditions, especially
the food.[133] The most reliable method of studying this question,
therefore, is to study it in its relation to the food consumed. Wolff
has calculated from numerous investigations that, with regard to the
amount of solid excreta produced by the food, the following percentage
of _organic matter_, _nitrogen_, and _mineral substances_, originally
present in the dry matter of the food, is voided in the dung:--

                     Cow.    Ox.   Sheep.   Horse.  Average.

Organic matter       39.5   42.5    44.0     44.1     42.5
Nitrogen             47.5   33.9    46.7     32.4     40.1
Mineral substances   53.9   64.6    57.9     62.5     59.7

There is one fact to be borne in mind in estimating the manurial value
of the dung of different animals--viz., that the quantity of dung voided
by one animal is much greater than that voided by another. Thus the
amount voided by the cow, for example, is much greater than that voided
by the horse; so that, in this way, the inferior quality of the former
is, to some extent, compensated for by its greater quantity.


2. _Urine._

The solid excreta possess, however, very much less manurial value than
the urine. The former, as already stated, are undigested
food-substances: any fertilising matters which they contain are such as
have failed to be digested or absorbed into the animal system. The
urine, on the other hand, contains those fertilising substances which
have been digested.

The amount of nitrogen and mineral matter, however, in the urine, does
not represent necessarily the total amount of these substances. Thus, in
the case of a growing or fattening animal, there is always a certain
amount of these substances being absorbed to build up the animal tissue
and put on flesh.

In this respect it will be seen that the composition of urine will vary
in the same way as that of the dung. In the case of the urine, however,
there is a compensating influence to be taken into account. Urine is a
waste product, and there is more waste in a young than in an adult
animal.

Another very important condition which determines the composition of
urine is the nature of the food, especially the quantity of water drunk.
This, of course, is obvious: the more water drunk, the poorer must the
composition of the urine be. But here again, as in the case of the dung,
this is largely compensated for by the total quantity voided--the more
dilute the urine, the larger will its quantity be; so that the inferior
quality is in this way made up for by its increased quantity.

Keeping in mind, then, the fact we have just stated--viz., that the
composition of urine will vary according to different conditions--we may
obtain an approximate idea of what its composition is from the following
results of analyses by Stoeckhardt. In 1000 parts the following
quantities of _water_, _nitrogen_, _phosphoric acid_, and _alkalies_
were found to be present.

From the following table it will be seen that the urine of swine
(containing 97 per cent of water) is much poorer in nitrogen and
alkalies than is the case with the urine of the sheep, horse, or
cow.[134] While this is the case, the amount of phosphoric acid it
contains is greater than that contained in the sheep's urine.

-------------------+------------+-------------------------+------------
                   |            |            | Phosphoric |
                   |   Water.   |  Nitrogen. |    Acid.   |  Alkalies.
                   +------+-----+------+-----+------+-----+------+-----
                   | Per  | Per | Per  | Per | Per  | Per | Per  | Per
                   | 1000 |cent.| 1000 |cent.| 1000 |cent.| 1000 |cent.
                   |parts.|     |parts.|     |parts.|     |parts.|
-------------------+------+-----+------+-----+------+-----+------+-----
Sheep (2 lb. hay  }| 865  | 86.5|  14  | 1.4 |  .5  | .050|  20  | 2.0
  per diem)       }|      |     |      |     |      |     |      |
                   |      |     |      |     |      |     |      |
Swine (winter food)| 975  | 97.5|   3  |  .3 | 1.25 | .125|   2  |  .2
                   |      |     |      |     |      |     |      |
Horses (hay and   }| 890  | 89.0|  12  | 1.2 |   -  |  -  |  15  | 1.5
  oats)           }|      |     |      |     |      |     |      |
                   |      |     |      |     |      |     |      |
Cows (hay and     }| 920  | 92.0|   8  |  .8 |   -  |  -  |  14  | 1.4
  potatoes)       }|      |     |      |     |      |     |      |
-------------------+------+-----+------+-----+------+-----+------+-----

Phosphoric acid is present in the urine of the farm animals in the most
minute traces: practically, it may be considered to be wanting in the
urine of the horse and the cow, and is present only in small quantities
in sheep's urine. The pig's urine, indeed, contains it in larger
quantities; but the percentage is still so small as to justify the
statement that the urine of the common farm animals is not a complete
manure, and must be supplemented by phosphates, if it is to be used
alone. The incomplete nature of urine as a manure constitutes a strong
argument in favour of its being applied along with the solid excreta,
which contain, as we have seen, considerable quantities of phosphoric
acid. It is on this account that the drainings of rotten manure-heaps
are more valuable, from a manurial point of view, than urine itself,
since these contain the soluble portion of the phosphates in the solid
excreta.[135] The urine of all animals, however, is not equally poor in
phosphates. In the case of flesh-eating animals, such as the dog, the
urine is found to contain them in considerable quantities.

The above tables show that the most valuable urine, weight for weight,
is that of the sheep, as it contains the largest amount of alkalies
(including potash) and nitrogen; that the urine of the horse comes next;
then that of the cow; while, as has already been pointed out, that of
the pig is the poorest.

In order to make our survey of the composition of urine uniform with
that of the dung, let us see how the urine of the common farm animals
compares in the matter of the composition of its dry substance. The
following results (basing our calculations on Stoeckhardt's figures,
previously given) show this:--

          Nitrogen,    Phosphoric acid,    Alkalies,
          per cent.        per cent.       per cent.

Pig         12.0               5              8
Horse       10.9             trace           13.6
Sheep       10.4              3.7            14.9
Cow         10.0             trace           17.5

From these figures we see that the dry substance of the urine of the pig
is richest in nitrogen and phosphoric acid, but poorest in alkalies, of
the four common farm animals; that of the horse comes next in the amount
of nitrogen it contains, but that, on the whole, there is very little
difference between the horse, cow, and sheep in this respect.[136]

As in the case of the dung, this subject is best studied in relation to
the food consumed. We are again indebted to Wolff's investigations for
valuable information on this point. He has found that the following
percentages of _organic matter_, _nitrogen_, and _mineral substances_,
originally present in the dry matter of the food, are voided in the
urine:--

                      Cow.     Ox.   Sheep.  Horse.  Average

Organic matter         4.0     4.4     2.0     3.3     3.4
Nitrogen              31.0    54.8    42.3    60.7    47.2
Mineral substances    43.1    34.3    41.0    37.5    39.0[137]

We have now considered briefly the composition of the solid excrements
and urine of the common farm animals, and have also enumerated some of
the principal causes of the variation in their composition.

The solid excreta consist, as we have seen, of _undigested_ food, while
the urine contains the manurial ingredients of the food which have been
_digested_ by the animal system.[138] The latter is, weight for weight,
as a rule, very much more valuable as a manure than the former. From
the table given in the Appendix[139] it will be seen that the
proportions of the nitrogen and ash-constituents originally present in
the food consumed, which are voided in the excrements, vary with
different circumstances. Wolff, in summarising his results, points out
that, as a rule, the solid and liquid excrements will contain about 46
per cent of the organic matter, 87.3 of the nitrogen, and 98.7 of
mineral matter; while the experiments of Lawes and Gilbert at Rothamsted
show that, with fattening oxen and sheep and with horses, more than 95
per cent of the nitrogen and 96 per cent or more of the ash-constituents
are voided in the manure. The pig retains a larger proportion of the
nitrogen--about 85 per cent appearing in the manure--while in the
milking-cow only about 75 per cent is returned in the excrements.
Generally speaking, we may say that the nitrogen originally present in
the food suffers very little loss in passing through the animal system,
and that, practically speaking, the ash-constituents suffer no loss
whatever.

As to the distribution of the manurial ingredients, much will depend on
the nature of the food. Almost invariably more than a _half_ of the
total nitrogen excreted will be found in the urine, in many cases very
much more.[140] Of the mineral constituents, about a third on the
average may be said to be excreted in the urine. Of this mineral matter
it may be noted that nearly all the alkalies (potash and soda), or about
98 per cent, are found in the urine. Of phosphoric acid and lime, on the
other hand, there are the merest traces in the urine. Horse-urine,
however, is an exception with regard to lime, as it contains about 60
per cent of the lime consumed in the food. For information on the
subject of pig-manure the reader is referred to Appendix, Note V.[141]

Before passing from this part of the subject, it may be desirable to
place before our readers the composition of the dung and urine taken
together, so that we may be able to form some idea of their relative
value, weight for weight. As the nitrogen constitutes by far the most
valuable portion of the manurial ingredients, it will be sufficient if
we compare them as to their percentage of this ingredient.

           Water,    Nitrogen,   Calculated on
         per cent.   per cent.   dry substance,   Analyses by
                                  per cent.
Sheep       67          .91           2.7         Jürgensen.
Horse       76          .65           2.7         Boussingault.
Pig         82          .61           3.4         Boussingault.
Cow         86          .36           2.6         Boussingault.

From these figures we see that, in their natural condition, the excreta
of the sheep are the most valuable; those of the horse and pig coming
next; while those of the cow are the poorest, containing one-third as
much nitrogen as those of the sheep, and one-half as much as those of
the horse and pig. This difference, however, is due almost entirely to
the different percentage of water the excreta of the various animals
contain in their natural state; for in the dry state they are seen to
contain, with the single exception of the pig, practically the same
amount.

In conclusion, then, the important points to be noticed are--

1. That in the passage of the food through the system of the common farm
animals, only a very small percentage of the fertilising substances,
nitrogen, phosphoric acid, and potash, is assimilated and retained in
the animal body; and that, therefore, theoretically at least, the
excreta should contain nearly the same amount of fertilising matter as
the food originally did.

2. That even in the case of a fattening animal, the loss of fertilising
matter sustained by the food in passing through the system is not great.

3. That with regard to the total amount of solid excreta and urine
voided, the latter contains, as a rule, more nitrogen than the former;
the nitrogen in the urine, further, being more valuable, as it is in a
soluble condition.

4. That as regards the distribution of the ash-constituents, _lime_,
_phosphoric acid_, and _magnesia_ are almost entirely found in the solid
excrements; while the urine contains nearly all the _potash_.

5. That the best results can be expected only when the liquid and solid
excreta are used together as a manure.

As the composition of the manure depends so largely on the nature of the
food, a table will be found in the Appendix, Note VI.,[142] containing
the manurial composition of some of the commoner feeding-stuffs.


3. _Litter._

We have now to consider the third constituent of farmyard manure--viz.,
the _litter_, which generally consists of straw.

The uses of the litter, in addition to providing a dry and comfortable
bed for the animal, may be briefly summed up as follows:--

1. To absorb and retain the liquid portion of the excreta.

2. To increase the quantity of the manure, and thus secure its more
equal distribution when applied to the field than could otherwise be
done.

3. To add to its value as a manure, both physically and chemically.

4. To retard and regulate the decomposition of the excreta.

Of course litter also performs a very useful function sanitarily,
inasmuch as it serves to keep the stall or byre fresher and cleaner, and
more free from noxious gases, which it absorbs, than would otherwise be
the case.

_Straw_ is almost universally used for this purpose. Besides being one
of the bye-products of the farm, it is admirably suited in many ways,
both owing to its peculiar shape--its tubular structure being
excellently adapted for this purpose--as well as on account of its
composition, being largely composed of cellulose, a very absorptive
substance. Straw thus possesses considerable absorptive power. In
manurial ingredients it is not very rich; for, of the various parts of
the ripened plant, straw contains the least percentage of nitrogen and
phosphates. This is due to the fact that, as the straw ripens, a
considerable proportion of these ingredients passes up from the stalk to
the seeds, where they are retained.

Generally speaking, straw may be said to contain not more than _a half
per cent_ of nitrogen--_i.e._, 11.2 lb. per ton. Its percentage of
nitrogen varies, of course; the recorded analyses[143] for wheat-straw
ranging from .22 to .81 per cent, or furnishing an average of .48 per
cent--_i.e._, 10.75 lb. per ton. Barley-straw is somewhat richer in
nitrogen, the recorded analyses ranging from .41 to .85 per cent, or
giving an average of .57 per cent--_i.e._, 12.76 lb. per ton; while
oat-straw is the richest of the commoner straws, ranging from .32 to
1.12 per cent, an average of .72 per cent--_i.e._, 16.12 lb per ton.

Of mineral matter, however, straw contains a very much larger
percentage, proportionally, than of nitrogen; for, with the exception of
phosphates, there is a considerable quantity of inorganic fertilising
matter, in the shape of potash, lime, &c., present in

               Composition of Straw.[144]
-----------------+---------------+------------------------+--------
                 |     Ash.      |  Composition of Ash.   |
                 +------+--------+------------------------+
                 |      |        |       Lb. per ton.     |
                 |      |  Lb.   |-------+----------+-----| Number
                 |  Per |  per   |Potash.|Phosphoric|Lime.|   of
                 | cent.|  ton.  |       |   Acid   |     |Analyses
-----------------+------+--------+-------+----------+-----+--------
Wheat (winter)   | 5.54 | 124.09 | 18.61 |   5.05   | 7.18|   8
Wheat (summer)   | 5.14 | 115.13 | 25.76 |   6.47   | 7.12|   6
Rye (winter)     | 5.33 | 119.39 | 20.61 |   5.89   | 9.73|   8
Rye (summer)     | 6.14 | 137.53 | 42.41 |   6.73   |10.53|   1
Barley           | 4.90 | 109.76 | 26.83 |   5.75   | 8.73|   8
Oats             | 5.09 | 114.01 | 26.22 |   4.17   | 9.12|   4
-----------------+------+--------+-------+----------+-----+--------

it. Of total ash ingredients, on an average, there are generally about 5
per cent--or 112 lb. per ton. The largest percentage of the fertilising
matter in this 5 per cent is potash, which varies in the ashes of the
straws of the commoner crops from 30 to 15 per cent. The above table
will show the variation in composition of the straws of some of the
commoner farm crops, and may be valuable for purposes of reference. The
crops are wheat (winter and summer), barley, oats, and rye (winter and
summer), and the amount is also calculated in lb. per ton. The results
represent the average of a number of analyses.[145] From the table it
will be seen that the percentage of phosphates is, as has already been
noticed, very small.

But while straw is well adapted for the purposes for which litter is
used, it is not the only substance. Its almost exclusive use as litter
is largely owing to the fact that it is a bye-product of the farm.

_Loam as Litter._--Generally speaking, any substance which has great
absorptive as well as retentive powers for nitrogen and the soluble
fertilising matters present in farmyard manure, and whose price is
nominal, is well suited for acting as litter. Ordinary loamy soil
possesses the above qualifications, and is, besides, a substance to be
had for nothing, and, under certain circumstances and in certain
countries, is actually used for this purpose, often along with straw. A
great objection against loam, however, is that it forms a dirty litter.
Moreover, it possesses a very small percentage of fertilising matter.
The tendency, consequently, in using ordinary loam, would be to dilute
the manure too much, besides retarding fermentation to an undesirable
extent. Except, therefore, under very exceptional circumstances, loam is
not to be regarded as a good litter.

_Peat as Litter._--Some kinds of soil, however, are well suited for this
purpose. Of these, the best are those rich in organic matter, the
so-called peaty soils. Peat, when dried and freed from any earthy
matter, forms an excellent absorbent of the liquid portion of the
manure, surpassing in this respect straw itself. It is, further,
generally very much richer in nitrogen--some peats having been found to
contain between 4 and 5 per cent of nitrogen. In some thirty samples of
peat analysed by Professor S. W. Johnson, the percentage of nitrogen
varied from .4 to 2.9, giving an average of 1.5 per cent.

While it has a very great capacity for absorbing liquids, it possesses
in an unequalled degree the power of retaining the soluble nitrogen
compounds. This is undoubtedly one of the most important properties
which recommend peat for the purposes of litter.[146]

Some interesting experiments on the value of peat-moss as a litter have
been recently carried out by Dr Bernard Dyer.[147] From these
experiments Mr Dyer has found that both its liquid-absorbing and
liquid-retaining powers are very much greater than those of straw. While
straw was only able to absorb three times its weight of water, peat-moss
was found to absorb nearly ten times its weight. With regard to its
water-retaining power, this was also found to be in excess of that of
straw. Both these properties are, it need scarcely be pointed out, of
very great value in a litter. Another point of interest in these
experiments was the respective amounts of nitrogen absorbed and retained
by the peat-moss and the straw. It was found that, in this respect, the
peat-moss had again an advantage over the straw. Lastly, the manure
produced by the peat-moss was shown to be richer in fertilising matter
than that produced by the use of straw.[148] These experiments are
interesting as demonstrating the fact that in peat-moss we have a
substance which is capable of acting as an excellent substitute for the
more costly straw, and which might increasingly be used as a fodder with
great benefit to the farmer.

Another substance which has been suggested as an excellent litter is the
common _bracken-fern_. According to some analyses made by Mr John
Hughes, the bracken, especially if cut in a young state, is a substance
of considerable manurial value. When dried, it is very much richer in
nitrogen, potash, and lime than straw. Its absorbent properties,
however, are probably not so great. Where it can easily and cheaply be
had, as in many parts of Scotland and Ireland, it might well be used for
littering purposes.[149]

_Dried leaves_ have also been used as a litter. Autumn leaves, however,
contain a very small percentage of fertilising matter. This is due to
the fact that the most of their potash, phosphoric acid, and nitrogen
pass into the body of the trees at the approach of winter. According to
Professor Storer, dried leaves only contain from .1 to .5 per cent
potash,.006 to .3 per cent phosphoric acid, and about .75 per cent of
nitrogen. Leaves, however, besides being poor in manurial ingredients,
make a bad litter, as they ferment but slowly. There is in this
fermentation a large quantity of cold sour humic acid formed, which
seriously impairs the value of the manure.[150]

Having now considered the composition of the three separate ingredients
of farmyard manure--viz., the _dung_ or _solid excreta_, the _urine_,
and the _litter_--we are in a position to consider the composition of
farmyard manure. In this connection it will be well to consider
separately the manures produced by the different farm animals.


1. _Horse-manure._

The composition of horse-manure is perhaps the most uniform of all the
manures produced by the different farm animals. This is due to the fact
that the food of the horse is generally of the same kind, consisting of
oats, hay, and straw.

The total excrements voided by a horse in a day have been calculated,
according to the average of experiments by Boussingault and Hofmeister,
at 28.11 lb., of which only 6.37 lb. consisted of dry matter.[151] These
28.11 lb. contained .18 lb. of nitrogen and .92 lb. of mineral matter.
The amount of straw necessary to absorb this amount of excrement may be
stated at from 4 to 6 lb. The amounts of nitrogen and mineral matter in
4 lb. of straw are .01 and .23 lb. respectively. The total amount of
nitrogen and ash, therefore, in the farmyard manure produced by a horse
in one day, would be .19 lb. nitrogen and 1.15 lb. mineral matter; or,
if we take the larger quantity of straw, somewhat more.

Taking these figures, we find that the amount of manure produced by a
horse in a year will be from 11,720 to 12,450 lb. (_i.e._, from 5-1/4
to 5-1/2 tons),[152] containing from 69 to 73 lb. nitrogen, and from 420
to 460 lb. mineral matter.[153]

A word or two may be of value regarding the treatment in the stable of
horse-manure. The great object to be aimed at is the prevention of loss
of valuable fertilising constituents. This loss may be due to two
causes. It may be, in the first place, caused by drainage of the soluble
matter of the manure; or secondly, it may be due to volatilisation of
the volatile constituents.

The first of these two sources of loss depends on the precautions taken
in the way of providing a proper impervious flooring to the stable. This
source of loss is extremely difficult to prevent, inasmuch as nearly all
materials used for flooring absorb a certain percentage of urine. The
judicious use of litter, however, will minimise this loss to within a
trifling extent.

Dr Heiden states that the amount of straw used as litter for the horse
in Germany is from 4 to 6 lb. per day. The quantity should be regulated
according to the percentage of water the excreta contain; the more
watery excreta requiring naturally a larger quantity of litter. The most
eminent authorities on this subject recommend that the amount of litter
should equal one-fourth of the food in its natural state, or about
one-third of its dry substance.

The second source of loss, which is due to volatilisation of the
volatile ingredients, may be largely prevented by the use of certain
preservatives.

Horse-dung being, comparatively speaking, of a dry nature, it is
extremely difficult to effect its thorough mixture with the litter. For
this reason the manure formed from horse excreta is particularly liable
to rapid fermentation.[154] In the process of fermentation, as will be
seen more in detail further on, the nitrogen is converted into carbonate
of ammonia. As nitrogen in this form is of an extremely volatile nature,
the risks of loss from this source are considerable. As illustrating
this fact, it may be mentioned that Boussingault has found by experiment
that the total percentage of nitrogen contained by fresh horse-manure
might be reduced in the process of fermentation to one-half of its
original amount by loss from this source.

The preservatives used to prevent this volatilisation are technically
known as "fixers." This they do by chemically combining with the
volatile ammonia and forming non-volatile compounds with it.

Of the acid fixers, hydrochloric and sulphuric acids have been
recommended. The former, however, is not well suited for this purpose.
It is a strongly fuming acid, and when brought into contact with
ammonia forms dense white fumes. The use of sulphuric acid is not open
to this objection. Sulphate of ammonia, the salt formed in this case, is
one of the most stable (or least volatile) of the compounds of ammonia.
If used, it should be largely diluted with water, and the whole mixed
with sand. Such a mixture, when sprinkled over the stable-floor in even
very small quantities, has been found to effectually prevent any loss of
the volatile carbonate of ammonia.

It is not, however, on the whole advisable to use an acid substance as a
fixer, since such a substance may act deleteriously on the horses'
hoofs.

Such substances as _gypsum_, _copperas_, and _sulphate of magnesia_,
while equally efficient, are not open to this objection. The
above-mentioned substances owe their efficacy to the fact that they are
compounds of sulphuric acid, which, by combining with the volatile
ammonia and forming sulphate of ammonia, prevent its escape.

Gypsum, or sulphate of lime, although, comparatively speaking, an
insoluble substance, when brought in contact with carbonate of ammonia
has been proved to effect the conversion of the ammonia into sulphate of
ammonia. It is also believed to retard the decomposition of the
manure.[155] Copperas, or ferrous sulphate, while a soluble salt, and
while thus acting in a more speedy manner in fixing the ammonia, is not
so well suited, owing to the hurtful influence it is well known to
possess on plant-life. It is only right to remember that there may be
circumstances in which copperas may, in small quantities, act even
beneficially as a manure, as Griffiths' experiments would seem to
indicate. The above objection, however, cannot be urged against sulphate
of magnesia. In addition to fixing the ammonia, sulphate of magnesia may
very probably fix the soluble phosphoric acid. Kainit, which consists of
a mixture of sulphates and chlorides of potassium and magnesium, has
also been suggested for this purpose. By using such a fixer, the value
of the resulting manure would be much enhanced. In conclusion, it must
be remembered that all the above-named fixers act very much in the same
way--viz., by converting the volatile carbonate of ammonia into sulphate
of ammonia.[156]


2. _Cow-manure._

The composition of the manure formed from the excrementitious matter of
the cow is very much less constant than is the case in the horse-manure.
An average statement of that composition is therefore very much more
difficult to obtain. The number of analyses available for the purpose of
forming this average is, however, very large. The manure produced by
cows contains a large percentage of water. This is due to the large
quantity of water they drink. It has been estimated that milch-cows
drink along with their winter food, for every pound of dry substance, 4
lb. of water, and in summer about 6 lb.

According to some experiments by Boussingault, the excrements of a cow
in a day amounted to 73.23 lb., of which only 9.92 lb. were dry
matter.[157] These excrements contained .256 lb. of nitrogen and 1.725
lb. of mineral matter. The amount of straw necessary to use as litter
for this amount of excrements may be taken at 6 to 10 lb. The manure,
therefore, formed by a cow per day, would contain from .274 to .286 lb.
of nitrogen, and from 2.046 to 2.278 lb. of mineral matter. In a year
this would amount to from 100 to 104.4 lb. of nitrogen, and from 746.8
lb. to 831.5 lb. of mineral matter; or from 6 cwt. 75 lb. to 7 cwt. 47
lb.

Cow-dung is, owing to its more watery nature and poorer quality, very
much slower in its fermentation than horse-dung. When applied alone,
cow-manure is very slow in its action, and makes its influence felt for
at least three or four years. It is difficult to spread it evenly over
the soil, owing to the fact that, when somewhat dried, it has a tendency
to form hard masses, which, when buried in the soil, may resist
decomposition for a very long period. The cause of this is due to the
presence of a considerable amount of mucilaginous and resinous matter in
the solid excreta, which prevents the entrance of moisture and air into
the centre of the mass. This tendency of cow-manure to resist
decomposition will be greatly lessened in the case of the excrements of
a cow richly fed.

The risks of loss of volatile ammonia are, therefore, in its case not so
great as we have seen them to be in the case of the "hot" horse-dung.
Notwithstanding this fact, much of what has been said on the use of
preservatives for horse-manure may be also applied to the cow-dung. This
is owing to the fact that the dung is allowed to accumulate in the court
for some time. The amount of straw it is advisable to use as litter
varies, as has been said, from 6 to 10 lb. per day. The best method of
calculating this amount, according to Dr Heiden, is by taking one-third
of the total weight of the dry substance of the food. The above
authority also recommends that the straw is best applied in blocks of
about one foot in length; and this for the following reasons:--

1. The strewing of it is more convenient.

2. The absorption of the fluid portion is more complete.

3. The cleaning out of the manure from the byre is easier.

4. The manure is more easily distributed when applied to the field.

Among the advantages incidental to allowing the manure to accumulate in
the court may be mentioned the following:--

1. The more thorough absorption of the urine by the straw, and,
consequently, the more uniform mixture which will be thus effected of
the more valuable urine with the less valuable solid excreta.

2. A certain retardation of decomposition effected by the treading under
foot of the manure.

3. The protection of the manure from rain and wind, and the securing of
a uniform temperature.

Against those advantages must be placed the risk of seriously affecting
the health of the animal. Although this is a point of very great
importance, it scarcely falls within the scope of this work. It may be
pointed out, however, that the judicious use of some of the chemical
fixers previously referred to may do much to keep the air of the byre or
court free of noxious gases.[158]


3. _Pig-manure._

The food of the pig is so very variable in its character that it is
wellnigh impossible to obtain anything like an average analysis of its
excrements. When the food of the pig is rich, then the manure may be
quite equal in quality to the other manures. According to Boussingault,
the total amount of excrements, on an average, voided by a pig in
twenty-four hours is about 8.32 lb., of which 1.5 lb. is dry
matter.[159] The amount of nitrogen these excrements contain is only .05
lb., and of mineral ingredients .313 lb. If we take the amount of straw
most suitable for absorbing this quantity of excrementitious matter at
from 4 to 8 lb., then we shall find that the manure produced by a pig
will contain from .06 to .074 lb. nitrogen and .545 to .772 lb. mineral
matter. These quantities, calculated for a year, give from 22 to 27 lb.
of nitrogen, and from 1 cwt. 87 lb. to 2 cwt. 57 lb. of mineral matter.
That is about as much nitrogen as would be contained in 1-1/4 to 1-1/2
cwt. of nitrate of soda (95 per cent purity); or from slightly less than
1 cwt. to slightly over 1 cwt. of sulphate of ammonia (97 per cent
purity).

As has already been pointed out, the excrements of the pig are, as a
rule, very poor in nitrogen. This accounts for the fact that pig-manure
is a "cold" manure, slow in fermenting.[160]


4. _Sheep-manure._

The dung and the urine of the sheep, as we have already seen, are,
weight for weight, the most valuable of any of the common farm animals.
The total weight of the excrements voided by a sheep in a day may be
taken, on an average,[161] at 3.78 lb., of which .97 lb. is dry matter.
These excrements contain .038 lb. of nitrogen and .223 lb. mineral
matter. Taking the amount of straw most suitable for absorbing this
quantity of excrementitious matter at three-fifths of a pound, then the
manure produced by a sheep in a day will contain .0429 lb. nitrogen and
.264 lb. mineral matter. That is, in a year the quantities of nitrogen
and mineral matter in the manure produced by a sheep would be 15.66 lb.
of nitrogen and 96.36 lb. of mineral matter.

From its richness in nitrogen, and from its dry condition, sheep-dung is
peculiarly liable to ferment. While richer in fertilising substances
than horse-manure, it is not so rapid in its fermentation. This is due
to the harder and more compact physical character of the solid excreta.
The risks of loss of volatile ammonia are, in its case, exceptionally
great. The use of artificial "fixers" is therefore to be strongly
recommended.[162]


_Fermentation of Farmyard Manure._

Having now considered the nature of the different manures produced by
the four common farm animals separately, it is of importance to consider
the exact nature of the fermentation, decomposition, or putrefaction
which takes place in the manure-heap.

It is now more than thirty years since Pasteur showed that the
fermentation which ensued on keeping a sample of urine was due to the
action of a minute organism, for the propagation of which a certain
amount of warmth, air, and moisture, as well as the presence of certain
food-constituents, especially nitrogenous bodies, were necessary.

Subsequent researches by Pasteur and others have conclusively
demonstrated that the micro-organic life instrumental in effecting the
putrefaction or decay of organic matter of any kind, may be divided into
two great classes:--

1. Those which require a plentiful supply of oxygen for their
development, and which, when bereft of oxygen, die--known as _aerobies_.

2. Those which, on the contrary, develop in the complete absence of
oxygen, and which, when exposed to oxygen, die--known as _anaerobies_.

In the fermentation of the manure-heap, therefore, we must conceive of
the two classes of organisms as the active agents. In the interior
portion of the manure-heap, where the supply of oxygen is necessarily
limited, the fermentation going on there is effected by means of the
anaerobic organism--_i.e._, the organism which does not require oxygen;
while on the surface portion, which is exposed to the air, the aerobic
(or oxygen-requiring) organism is similarly active. Gradually, as decay
progresses, the aerobic organisms increase in number. It is through
their instrumentality that the final products of decomposition are
largely produced. The functions of the anaerobic organisms may be, on
the contrary, regarded as largely preparatory in their nature. By
breaking up the complex organic substances in the manure into new and
simpler forms, they advance the process of putrefaction through the
initial stages; and when this is accomplished, they die and give place
to the aerobic, which, as we have just seen, effect the final
transformation of the organic matter into such simple substances as
_water_ and _carbonic acid gas_.

The conditions influencing the fermentation of farmyard manure may be
summed up as follows:[163]--

1. _Temperature._--The higher the temperature the more rapidly will the
manure decay.

2. _Openness to the Air._--Of course it will be seen that the effect of
exposing the manure to the action of the air is to induce the
development of the aerobic type of organism, and thus to promote more
rapid fermentation. If, on the other hand, the manure be impacted, the
slower but more regular fermentation, due to the anaerobic type of
organism, will be chiefly promoted. It must be remembered that in the
proper rotting of farmyard manure both kinds of fermentation should be
fostered. It is, in fact, on the careful regulation of the two classes
of fermentation that the successful rotting of the manure depends. It
must further be remembered that, even with a certain amount of openness
in a manure-heap, anaerobic fermentation may take place. This is due to
the fact that the evolution of carbonic acid gas, in such a case, is so
great as to exclude the access of the atmospheric oxygen into the pores
of the heap.

3. The _dampness_ of the manure-heap is another important influence.
This, of course, will act in two ways. First, by lowering the
temperature. Where the manure-heap is found to be suffering from
"fire-fang," the common method in practice is to lower the temperature
by moistening the heap with water. Secondly, it acts as a retarder of
fermentation by limiting the supply of atmospheric oxygen, and thus
preventing, as we have just seen, aerobic fermentation.

4. The fourth chief influence in regulating fermentation of the
manure-heap is its _composition_, and more especially the amount of
nitrogen it contains in a soluble form. The rate at which fermentation
takes place in any organic substance may be said chiefly to depend on
the percentage of soluble nitrogenous matter it contains: the greater
this is in amount, the more quickly does fermentation go on. There are
always a number of soluble nitrogenous bodies in farmyard manure. These
are chiefly found in the urine, such as _urea_, _uric_ and _hippuric
acids_, and _ammonia_ salts.


_Products of Decomposition of Farmyard Manure._

The most important of the changes which take place in the rotting of
farmyard manure may be briefly enumerated as follows:--

1. The gradual conversion into gases of a large portion of the organic
elements in the manure. Of these gaseous products the most abundant is
_carbonic acid gas_ (CO_2). It is in this form that the carbonaceous
matter which constitutes the chief portion of the manure escapes into
the air. Carbon also escapes into the air, combined with hydrogen, in
the form of _carburetted hydrogen_ or _marsh-gas_ (CH_4), a product of
the decomposition of organic matter in the presence of a large quantity
of water. This gas is consequently found bubbling up through stagnant
water. Next to carbonic acid gas, _water_ (H_2O) is the most abundant
gaseous product of decomposition. The nitrogen present in the manure, in
different forms, is converted by the process of decomposition chiefly
into _ammonia_, which, combining with the carbonic acid, forms carbonate
of ammonia, a very volatile salt. It is to this fact that one of the
great sources of loss in the decomposition of farmyard manure is due. If
the temperature of the manure-heap be permitted to rise too high, the
carbonate of ammonia volatilises. It is probable, also, that a not
inconsiderable portion of the nitrogen escapes into the air in the free
state. The last of the most important gaseous products of decomposition
are _sulphuretted_ and _phosphoretted hydrogen_. It is to these gases
that much of the smell of rotting farmyard manure is due.

2. The second class of substances formed are _soluble organic acids_,
such as _humic_ and _ulmic acids_. The function performed by these acids
is a very important one. They unite with the ammonia and the alkali
substances in the mineral portion of the manure, forming humates and
ulmates of ammonia, potash, &c. It is these ulmates that form the black
liquor which oozes out from the manure-heap.

In very rotten farmyard manure traces of _nitric acid_ may be found; but
it must be remembered that the formation of nitrates is practically
impossible under the ordinary conditions of active fermentation of
farmyard manure, except perhaps in its very last stages.

3. The third class of changes taking place have to do with the mineral
portion of the manure. The result of the formation of so much carbonic
and other organic acids is to increase the amount of _soluble_ mineral
matter very considerably.


_Analyses of Farmyard Manure._

It is chiefly to the valuable researches of the late Dr Augustus
Voelcker that we owe our knowledge of the composition of old and fresh
farmyard manure. All interested in this important question should peruse
the original papers on this subject contributed to the 'Journal of the
Royal Agricultural Society' by Dr Voelcker. Typical analyses
illustrating the variation in the composition of farmyard manure at
different stages of decomposition will be found in the Appendix.[164]
From what has been already said, it is obvious chat the composition of
farmyard manure is of a very variable nature.

The quantity of moisture naturally varies most, and this variation will
depend on the age of the manure, and the conditions under which it is
permitted to decay. It may be taken at from a minimum of 65 per cent in
fresh to 80 per cent in well-rotted manure. The total organic matter may
be taken at from 13 to 14 per cent, containing nitrogen .4 to .65 per
cent. The total mineral matter will range from about 4 to 6.5 per cent,
containing of potash from .4 to .7 per cent, and of phosphoric acid from
.2 to .4 per cent.[165]

As Mr Warington[166] has pointed out, one ton of farmyard manure would
thus contain 9 to 15 lb. of nitrogen, about the same quantity of potash,
and 4 to 9 lb. of phosphoric acid. These quantities of nitrogen and
phosphoric acid, calculated to (95 per cent) nitrate of soda, and (97
per cent) sulphate of ammonia, and (25 per cent) superphosphate, give
respectively 57.25 to 96 lb. nitrate of soda, 45 to 75 lb. sulphate of
ammonia, and 35 to 79 lb. superphosphate. That is, in order to apply as
much nitrogen to the soil as is contained in one ton of nitrate of soda,
we should require to use from 23 to 41 tons of farmyard manure:
similarly one ton of sulphate of ammonia contains as much nitrogen as 30
to 50 tons farmyard manure. In the same way one ton of superphosphate
of lime contains as much phosphoric acid as 28 to 64 tons farmyard
manure.

The value of rotten manure is, weight for weight, greater than that of
fresh manure. This is due to the fact that, while the water increases in
amount, the loss of organic matter of a non-nitrogenous nature more than
counterbalances the increase in water. The manure, therefore, becomes
more concentrated in quality. The loss on the total weight, according to
Wolff, in the rotting of farmyard manure, should not exceed in two or
three months' time 16 to 20 per cent--viz., a sixth to a fifth of its
entire weight. Not only, however, does the manure become richer in
manurial ingredients, but the forms in which the manurial ingredients
are present in rotten manure are more valuable, as they are more
soluble. These statements must not be taken as proving that it is more
economical to apply farmyard manure in a rotten condition than in a
fresh one. The distinction must not be lost sight of which exists
between relative increase--increase in the percentage of valuable
constituents--and absolute increase. The increase in the value of the
manure by the changes of the manurial ingredients from the insoluble to
the soluble condition may be effected at the expense of a considerable
amount of absolute loss of these valuable ingredients. This is a point
which is probably too often left out of account in discussing the
relative merits of fresh and rotten farmyard manure; and it is
important that it should be clearly understood. In the words of the late
Dr Voelcker: "Direct experiments have shown that 100 cwt. of fresh
farmyard manure are reduced to 80 cwt. if allowed to lie till the straw
is half rotten; 100 cwt. of fresh farmyard manure are reduced to 60 cwt.
if allowed to ferment till it becomes 'fat or cheesy'; 100 cwt. of fresh
farmyard manure are reduced to 40-50 cwt. if completely decomposed. This
loss not only affects the water and other less valuable constituents of
farmyard manure, but also its most fertilising ingredients. Chemical
analysis has shown that 100 cwt. of common farmyard manure contain about
40 lb. of nitrogen, and that during fermentation in the first period 5
lb. of nitrogen are dissipated in the form of volatile ammonia; in the
second, 10 lb.; in the third, 20 lb. Completely decomposed common manure
has thus lost about one-half of its most valuable constituent."[167]
While, of course, a very great amount of absolute loss of the valuable
constituents--the nitrogen and ash-constituents--of farmyard manure may
take place through volatilisation and drainage, by taking requisite
precautions this loss may be very much minimised. As regards the total
loss, this, in two or three months' time, should only amount to 16 to 20
per cent--or one-sixth to one-fifth of the weight.[168] The use of
fixers, to which reference has already been made, will greatly minimise
this loss. The application of fixers is best made to the manure when
still in the stall or byre. The health of the animal benefits by so
doing, while the manure is at once guarded against loss from this
source.

As to the relative merits of covered and uncovered manure-heaps, much
difference of opinion exists. It is one of those questions which does
not admit of final decision one way or another, as it depends so largely
on the individual circumstances of each case. That manure produced under
cover is more valuable than manure made in the open is readily granted.
The question, however, is as to whether the increase in its value is
sufficiently great to warrant the extra expense involved in building
covered courts. This depends on the individual circumstances of each
case, and cannot be decided in a general way. For experiments on the
relative value of manure made under cover and in the open, see
Appendix.[169]

The method of applying farmyard manure to the field is a question which
belongs more to the practical farmer than, to the scientist, and must be
largely decided by economic considerations. There is an aspect, however,
of the question which may well be treated here. The first point in the
production of good manure is in connection with its even distribution.
It is of great importance that the excrements of the different farm
animals be thoroughly mixed together. By the intimate incorporation of
the "hot" horse-dung with the "cold" cow and pig dung, uniform
fermentation is secured. Fire-fang--or too rapid fermentation--may occur
from this not being properly done, and from the manure becoming too dry.
It is important, also, as we shall see immediately, to have the manure
uniform in quality when applied to the field. The manure ought to be
firmly trodden down, to moderate the rate of fermentation. Where the
manure-heap is exposed to rain, the quantity of water it will naturally
receive will probably be quite sufficient, if indeed not too much, to
ensure a proper rate of fermentation--except, perhaps, in very warm
weather. The great point to be aimed at is to ensure regular
fermentation. What has to be especially avoided is any sudden exposure
of the manure to large quantities of water. The result of such a
washing-out of the soluble nitrogen is to retard fermentation, besides
incurring the risk of great actual loss by drainage.[170]


_Application of Farmyard Manure to the Field._

In applying the manure to the field, and before ploughing it in, two
methods of procedure may be pursued. First, the manure may be set out in
heaps, larger or smaller, over the field, and be allowed to remain in
these heaps some time before being spread; and secondly, it may be
directly spread broadcast over the field, and thus allowed to lie for
some time. Lastly, the manure may be ploughed in immediately; and it may
be stated that such a method is, where circumstances permit, the safest
and most economical method.[171]

In discussing the merits and demerits of these two methods, Dr Heiden
points out, first, with regard to the distribution of the manure in
small heaps over the field, that this is not to be recommended, on the
following grounds:--

1. Because the chances of loss by volatilisation are thereby increased.
The manure is distributed several times instead of only once or twice.

2. It is apt to ensure unequal distribution. The separate heaps run the
risk of losing their soluble nitrogenous matter, which soaks into the
ground beneath the heaps. The other portions of the field not covered by
the manure-heaps are thus manured with washed-out farmyard manure,
bereft of its most valuable constituents. The result is, that while
certain portions of the field are too strongly manured, other portions
are too weakly manured.

3. The proper fermentation of the manure is apt to be interfered with by
the loss of that which is its most important agent--viz., the soluble
nitrogenous matter--and also by the drying action of the wind.

The same objections hold good to a large extent with regard to the
setting out in the fields of the manure in large heaps. The risks of
loss, in one respect, may be said to be less, owing to the smaller
surface presented. On the other hand, they may be greater, owing to
fermentation taking place more quickly. Agricultural practice, however,
often renders this custom necessary; and if precautions are taken not to
let the heap lie too long, and to cover it over with earth, the risk of
serious loss may be rendered inconsiderable.

With regard to the second method of procedure--viz., the spreading of
the manure broadcast over the field, and allowing it thus to lie--Dr
Heiden is of opinion that this should only be done when the field is
level. In the case of uneven ground the risks are, of course, obvious.
It has been affirmed that, by allowing farmyard manure thus to lie
exposed for some time, an important loss of volatile ammonia--carbonate
of ammonia--is apt to take place. This could only take place where the
former treatment of the farmyard manure had been bad. Hellriegel has
shown that in the case of properly prepared farmyard manure there is no
danger of loss in this way. The absorptive power of the soil for
ammonia, it must be remembered, is very great, and the amount of
volatile ammonia in farmyard manure is relatively so small that it is
scarcely possible that any could escape in this way. Hellriegel's
experiments have demonstrated this in a very striking way. He has found
that in the case of a chalky soil, and during the summer and autumn
months, practically no loss of ammonia takes place. The following
considerations may be further urged in support of this method of
application, as against immediately ploughing in the manure, viz.:--

1. That fermentation takes place more quickly.

2. That it results in a more equable distribution of the manurial
constituents in the dung, by gradually and thoroughly incorporating the
liquid portion of the manure with the soil-particles.

Against, however, these undoubted advantages, one serious disadvantage
may be urged--viz., that the manure, before being ploughed in, becomes
robbed to a large extent of its soluble nitrogenous compounds, which, as
we have repeatedly observed, are so necessary for fermentation; and
that, therefore, when it is ploughed in, it does not so readily ferment.
This being so, it is highly advisable, in the case of light or sandy
soils, not to follow such a practice, but to plough the manure directly
in.

As to the depth to which it is advisable to plough the manure in, it may
be here noticed that it should not be too deep, so as to permit of the
access of sufficient moisture to ensure proper fermentation, and to
prevent rapid washing down of nitrates to the drains. Lastly, it need
scarcely be pointed out that it is highly important to have the manure
evenly and thoroughly incorporated with the soil-particles. Where the
manure is permitted to cake together in lumps, it may successfully
resist the action of fermentation for several years.


_Value and Function of Farmyard Manure._

Practical experience has long demonstrated the fact that farmyard manure
is, taking it all round, the most valuable, and admits of the most
universal application, of all manures; and science has done much to
explain the reason of this. The influence of farmyard manure is so
many-sided that it is difficult even to enumerate its different
functions. As has already been pointed out, its indirect value as a
manure is probably as great as, if indeed even not greater than, its
direct value. In concluding our study of farmyard manure, we shall
endeavour to summarise, in as brief a manner as possible, its chief
properties.

First, as to its value as a supplier of the necessary elements of
plant-food. This, there can be little doubt, has been, and still is,
grossly exaggerated by the ordinary farmer. Much has been claimed for it
as a "general" manure. How far it merits pre-eminence on this score
among other manures will be seen in the sequel. It is true that, since
it is composed of vegetable matter, it contains all the necessary plant
ingredients.[172] As has been shown in the Introduction, there is
practically in the case of most soils no necessity to add to a manure
any more than the three ingredients, _nitrogen_, _phosphoric acid_, and
_potash_. Its value, then, as a direct manure, must depend on the
quantity and proportion in which these three ingredients are present.
These substances, as we have already seen, it contains only in very
small quantities. It is, judged from this point of view, a comparatively
poor manure. Furthermore, only a certain percentage of these substances
is in a soluble or immediately available condition,--in this respect the
rotten manure being very much more valuable than the fresh manure.

Again, a point of great importance in a universal manure is the
proportion in which the necessary plant-foods are present. If it be
asked, Are the nitrogen, phosphoric acid, and potash in farmyard manure
present in the proportion in which crops require these constituents? the
answer must be in the negative. Heiden[173] has very strikingly
illustrated this point, in so far as the relations between the two ash
ingredients are concerned, by some computations as to the amount which
would be removed from the soil in the course of different
rotations.[174] In the case of five different rotations it was found
that the ratio between the potash and phosphoric acid removed was as
follows:[175] (1) 2.96 to 1; (2) 2.76 to 1; (3) 2.95 to 1; (4) 4.13 to
1; (5) 3.78 to 1. This would give a mean of 3.32 to 1. This is not the
ratio in which these ingredients are generally present in farmyard
manure. Farmyard manure may be said to be much richer in the mineral
constituents of plants than in nitrogen. Professor Heiden found that in
the case of a farm at Waldau, the crops in the course of ten years
removed from a _morgen_ (.631 of an acre) the following quantities:--

                       lb.
Nitrogen               329
Potash                 263
Phosphoric acid        121

In order to supply these amounts the following quantities of manure
would require to be supplied:--

1. For the nitrogen, 26 or 27 tons (manure containing .606 per cent
nitrogen).

2. For the potash, 20 to 25 tons (manure containing .672 per cent
potash).

3. For the phosphoric acid, 13 to 19 tons (manure containing .315 per
cent phosphoric acid).

From the above it will be seen that farmyard manure contains too little
nitrogen in proportion to its ash ingredients.

It is not merely the amount of fertilising ingredients removed by the
crop we have to take into account in estimating the value of certain
manurial ingredients for the different crops. Two other considerations
have to be remembered--viz., the amount of the constituents already
present in the soil, and the ability of the different crops to obtain
the ingredients from the soil. If we take into account these two
considerations in estimating the value of farmyard manure as a general
manure, we shall find that they accentuate the inadequacy of the ratio
existing between the nitrogen and the mineral ingredients. Messrs Lawes
and Gilbert have found in the Rothamsted experiments with farmyard
manure, that while it restored the mineral ingredients, it was
inadequate as a sufficient source of nitrogen. Nitrogen is, of all
manurial ingredients, in least abundance in soils. It is consequently
found that the ingredient in which farmyard manure requires to be
reinforced is nitrogen. With regard to phosphoric acid and potash, it
has already been shown that the ratio between them is probably greater
than that in a good average manure. We should, arguing from this alone,
be inclined to think that farmyard manure would be best reinforced with
potash. The reverse is the case, however, as every farmer knows. This is
due, first, to the fact that the potash, unlike the phosphoric acid, is
entirely of a soluble nature, and therefore immediately available for
the plant's needs; and secondly, to the fact that the necessity for the
application of potash as a manure is generally not nearly so great as in
the case of phosphoric acid. The result is, that farmyard manure will
be, as a rule, more valuably supplemented by phosphoric acid than by
potash.

Another point of great importance, in estimating the value of farmyard
manure as a chemical manure, is the inferior value possessed by much of
the nitrogen it contains, as compared with the nitrogen in such
artificial manures as nitrate of soda and sulphate of ammonia. According
to the Rothamsted experiments, weight for weight, the nitrogen in
farmyard manure is not half so valuable as it is in sulphate of ammonia.
Much of the nitrogen becomes only very slowly available; not a little of
it perhaps actually takes years to be converted into nitrates.[176]

Thus, with regard to the direct value of farmyard manure as a manure, we
have seen--

1. That it contains a very small quantity of the three fertilising
ingredients.

2. That the proportion in which these three ingredients are present is
not the best proportion for the requirements of crops.

3. That the form in which a portion of these ingredients--nitrogen and
phosphoric acid--is present is not of the most valuable kind.

It is consequently not as a direct chemical manure that farmyard manure
is pre-eminently valuable. We must seek for perhaps its most valuable
properties in its indirect influence.

It adds to the soil a large quantity of organic matter. Most soils are
improved by the addition of _humus_. The water-absorbing and retaining
powers of a soil are increased by this addition of _humus_, while it
enables the soil to attract an increased amount of moisture from the
air. This is often of great importance, as in the period of germination
of seed.[177] The influence it exerts on the texture of the soil in the
process of fermentation is also very great. This is especially so in
soils whose texture is too close, such as heavy clayey soils. It opens
up their pores to the air, and renders them more friable. Where such an
influence is most required, as in clayey soils, the manure ought to be
applied in a fresh condition, so that the maximum influence exerted by
the manure in this direction may be experienced. On light soils, on the
contrary, whose friability and openness are already too great, and which
do not require to be increased, the manure will be best applied in a
rotten condition. It adds, further, greatly to the heat of the soils by
its decomposition. Thus on cold damp soils it effects one very marked
benefit. The influence it exerts in its decomposition upon the
fertilising ingredients present in the soil is also by no means
inconsiderable. In the process of its fermentation large quantities of
carbonic acid gas are generated. This carbonic acid probably acts in a
double capacity. It will, in the first place, greatly increase the
solvent power of the soil-water, and thus enable it to set free an
increased amount of mineral plant-food; and secondly, it will help to
conserve a certain quantity of the soil-nitrogen, by preventing its
conversion into nitrates.

As its indirect and mechanical properties are greatest when in its fresh
condition, it will be better to apply it in that condition to soils most
lacking in these mechanical properties. We may therefore say that
farmyard manure is best applied in a rotted condition to light sandy
soils, and to soils in a high state of cultivation, where its mechanical
properties are not so much required.

An important point still remains to be discussed--viz., the rate at
which the farmyard manure should be applied. This, of course, should
naturally depend on a variety of circumstances--the amount of artificial
manures used as supplementary to the farmyard manure, the frequency of
its application, and the nature of the soil.

These considerations naturally vary so much, that the quantities of
farmyard manure it is advisable to apply in different cases are widely
different. There is a strong probability that the rate at which farmyard
manure has been applied in the past has been grossly in excess of what
could be profitably employed. Opinion is gaining ground among practical
farmers, that smaller and more frequent applications of farmyard manure
to the soil would be fraught with better results than the older custom
of applying a large dressing at a time. This is an opinion in the
support of which science can urge strong arguments. It is only of late
years that we have come to recognise sufficiently the various risks
which all fertilisers are subject to in the soil, and the importance,
therefore, of minimising these risks as much as possible by putting into
the soil at one time only as much manure as it is safely able to retain.

"The famous old German writer Thaer regarded 17 or 18 tons as an
abundant dressing; 14 tons he called good, and 8 or 9 tons light. Other
German authorities speak of 7 to 10 tons as light, 12 to 18 tons as
usual, 20 or more tons as heavy, and 30 tons as a very heavy
application."[178]

In the new edition of Stephens' 'Book of the Farm,'[179] from 8 to 12
tons per acre for roots, and from 15 to 20 tons for potatoes, along with
artificials, which may cost from 25s. to 60s. per acre additional, are
quoted as general dressings.

The majority of recent experiments with farmyard manure would seem to
indicate that, even in the case of what are considered small dressings,
the extra return in crop the first year after application is not such as
to cover the expense of the manure. Of course, as is commonly pointed
out, the effect of farmyard manure is of a lasting nature, and is
probably felt throughout the whole rotation, or even longer. This, to a
certain extent, is no doubt true; still it may be strongly doubted
whether farmyard manure is, after all, an economical manure, as compared
with artificial manures. The desirability of manuring the soil and not
the crop is, in this age of keen competition, no longer believed in; and
the Rothamsted experiments have shown that it is highly doubtful whether
even the soil benefits to anything like a commensurate extent by the
application of large quantities of farmyard manure. This is of course
assuming for farmyard manure the value that it would fetch when sold,
or, to put it rather differently, the price it would cost if the farmer
had to purchase it. Farmyard manure is a necessary bye-product of the
farm, and can scarcely be regarded, therefore, in the same light as the
artificial manures which the farmer buys.[180]


FOOTNOTES:

[131] See Appendix, Note I., p. 279.

[132] "The large amount of potash in unwashed wool is very remarkable: a
fleece must sometimes contain more potash than the whole body of the
shorn sheep."--Warington's 'Chemistry of the Farm,' p. 78.

[133] See Appendix, Note II., p. 279.

[134] The urine of the pig, from the nature of its food, is, as a
general rule, a very poor nitrogenous manure.

[135] See Appendix, Note XV., p. 290.

[136] See Appendix, Note III., p. 280.

[137] See Appendix, Note XVIII., p. 291.

[138] The nitrogen present in the urine, it may be well to point out, is
derived from the waste of nitrogenous tissue as well as from nitrogenous
matter of the food digested.

[139] Note IV., p. 281.

[140] Warington puts this matter admirably in the following words: "If
the food is nitrogenous and easily digested, the nitrogen in the urine
will greatly preponderate. If, on the other hand, the food is one
imperfectly digested, the nitrogen in the solid excrement may form the
larger quantity. When poor hay is given to horses, the nitrogen in the
solid excrement will exceed that contained in the urine. On the other
hand, corn, cake, and roots yield a large excess of nitrogen in the
urine." ('Chemistry of the Farm,' p. 137).

[141] See p. 281.

[142] See p. 282.

[143] See Heiden's 'Düngerlehre,' vol. ii. p. 58.

[144] Heiden's 'Düngerlehre,' vol. i. p. 404.

[145] The following quantities of nitrogen are found in rye, pea, and
bean straw:--

                 Ranging from     Average       Lb.
                   per cent.     per cent.   per ton.

Rye-straw         .30 to  .73       .57       12.76
Pea-straw         .76 to 1.61      1.21       27.10
Bean-straw       1.15 to 2.62      1.92       43.00

[146] Dr J. M. H. Munro recommends the sprinkling of a little finely
sifted peat-powder in addition to straw, as an excellent means of
preventing loss of volatile ammonia in the fermentation of manure.

[147] See 'Mark Lane Express,' October 7, 1889, p. 475.

[148] See Appendix, Note VII., p. 283.

[149] For analyses see Appendix, Note VIII., p. 283.

[150] According to Storer, in a ton of autumn leaves of the best quality
there would be 6 lb. of potash, less than 3 lb. of phosphoric acid, and
10 or 15 lb. of nitrogen. Another substance that may be used as a litter
is sawdust. This substance is a good absorbent, but is of little value
as a manurial substance.

[151] Heiden's 'Düngerlehre,' vol. ii. pp. 34, 66. In Boussingault's
experiments the food consisted of 15 lb. _hay_, 4.54 lb. _oats_, and 32
lb. water; the total excrements amounting to 31.16 lb., containing 7.42
lb. dry matter. In Hofmeister's experiments the food consisted of 5.23
lb. _hay_, 6.18 lb. _oats_, 1 lb. _chopped straw_, and 25.57 lb. water;
the excrements amounting to 25.07 lb., containing 5.32 lb. dry matter.

[152] This is taking no account of the amount of water which the manure
will absorb, and which will probably double the quantity.

[153] See Appendix, Note IX., p. 283.

[154] The rapid fermentation of horse-manure is due to its mechanical as
well as its chemical nature. The horse does not reduce its food to such
small pieces, and its urine is rich in nitrogen.

[155] Schulze recommends one-third of a pound per day of sulphate of
lime for each horse.

[156] See Appendix, Note X., p. 284.

[157] The food consisted of 30 lb. _potatoes_, 15 lb. _hay_, and 120 lb.
_water_.

[158] For further analyses of cow-manure, see Appendix, Note XI., p.
286.

[159] This is for a pig of six to eight months old, and fed on potatoes.

[160] It has been asserted that the use of pig-manure, when applied
alone, is apt to give an unpleasant taste to the produce grown.

[161] Taken from a very large number of analyses by a number of
experimenters. See Heiden's 'Düngerlehre,' vol. i. p. 99.

[162] See Storer, 'Agricultural Chemistry,' vol. ii. p. 96.

A question of great importance is as to the amount of farmyard manure
produced on a farm in a year, and its value. This is a question which is
extremely difficult to satisfactorily deal with. Various methods of
calculating this amount have been resorted to. It may be well to state
these pretty fully. Some practical authorities estimate the amount by
calculating that every ton of straw should produce 4 tons of manure.
Another method consists in estimating the amount from the size of the
farm. Sir John Lawes has calculated the composition of farmyard manure
which should be produced in the case of a farm of 400 acres, farmed on
the four-course system. He assumes that half of the roots and 100 tons
of hay are consumed at the homestead; that the whole of the straw of the
corn crops is retained at home as food and litter; that twelve horses
have corn equal to 10 lb. of oats per head per day; and that about ten
shillings per acre are expended in the purchase of cake for feeding
stock. Under these conditions the amount of farmyard manure should be
855 tons (or an average of 8-1/2 tons for each of the 100 acres of
root-crop) of _fresh undecomposed dung_. (For composition, see Appendix,
Note XVII., p. 291.) Another method is by taking, as the data of
calculation, the number of cattle, horses, sheep, &c., producing the
manure. Lloyd considers that a fattening animal requires 3 tons of straw
in the year, and makes about 12 tons of manure. A farmer, therefore,
should make 8 tons of manure for every acre of that part of his land
which, in the four-course rotation, is put down to turnips.

The last method consists in taking as the data the amount of food
consumed and litter used in the production of the manure. Of these
methods Heiden considers the last as alone satisfactory and trustworthy.
Applying this method to the horse, he shows, from experiments, that a
little over 47 per cent of the dry matter of its food has been proved to
be voided in the solid and liquid excreta. Taking the average percentage
of water in the excreta as about 77.5, the percentage of dry matter in
the excreta will be 22.5. That is, every pound of dry matter in the food
eaten by the horse yields a little over 2 lb. of excrementitious matter.
To this of course must be added the amount of straw used as litter,
which may be taken at 6.5 lb.

From these data we may calculate the amount of manure produced in a year
by a horse, making certain assumptions as to the amount of work
performed. This Heiden does by assuming that a horse works 260 days, of
twelve hours each, in the course of a year, or 130 whole days, spending
235 days in the stall. Calculating from the above data, he estimates
that a well-fed working horse will produce about 50 lb. of manure in a
day, or 6.5 tons in a year. Of course this does not necessarily
represent all the manure actually produced by the horse, but how much of
the remaining portion of the manure actually finds its way to the farm
it is impossible to say. According to the 'Book of the Farm,' Division
III. p. 98, a farm-horse makes about 12 tons of manure in a year.

It has been calculated that cows void about 48 per cent of the dry
matter of their food in the solid and liquid excreta, which contain of
water, on an average, 87.5 per cent. That is, every pound of dry matter
will furnish 3.84 lb. of total excreta. By adding the necessary amount
of straw for litter (which may be taken at one-third the weight of the
dry matter of the fodder), Heiden calculates that an ox weighing 1000
lb. should produce 113 lb. of manure in a day, or 20 tons in a year. The
'Book of the Farm,' Division III. p. 98, gives the annual amount at from
10 to 14 tons. According to Wolff, one may assume that on an average the
fresh excrements (both liquid and solid) of the common farm animals
(with the exception of the pig) contain of every 100 lb. of dry matter
in the food consumed about 50 lb., or a half. Estimating the dry matter
in the litter used at equal to about 1/4 of the dry matter of the food,
this would mean that for every 100 lb. of dry matter consumed in food
there would be 75 lb. of dry manure (viz., 50 lb. dry excrements + 25
lb. dry litter), which would yield 300 lb. of farmyard manure in the wet
state--_i.e._, with 75 per cent water. The amount of food daily required
per every 1000 lb. of live-weight of the common farm animals may be
taken, roughly speaking, at 24 lb. dry food material and 6 lb. of straw
as litter. The daily production of manure for 1000 lb. of live-weight
would amount, therefore, to 18 lb. of dry, or 72 lb. wet manure. (See
Appendix, Note XVII., p. 291.) According to J. C. Morton and Evershed,
oxen feeding in boxes require 20 lb. of straw per head per day as
litter. An ox, therefore, will make 8 tons of fresh dung in six months,
using 32 cwt. of litter. This means that each ton of litter gives 5 tons
of fresh dung. It is calculated that nearly twice as much litter must be
used in open yards.

[163] It has been calculated that under ordinary circumstances
sheep-dung, when allowed to ferment by itself, should do so in about
four months, horse-dung in six months, and cow-dung in eight months.

[164] See Appendix, Note XII., p. 286.

[165] See Heiden's 'Düngerlehre,' vol. ii. p. 156.

[166] Warington, 'Chemistry of the Farm,' p. 33.

[167] Recent experiments by Müntz and Girard in France have shown that
the loss in sheep excreta from volatilisation of the carbonate of
ammonia amounted to over 50 per cent. By the use of straw litter this
was reduced to about a half less, and with earth litter one quarter
less.

[168] See Appendix, Note XIII., p. 288.

[169] See Appendix, Note XIV., p. 289.

[170] See Appendix, Note XV., p. 290.

[171] For spring application rotten farmyard manure is generally used,
because in this condition its fertilising matter is more quickly
available. On light land it is best to apply it in the rotten condition
shortly before it is likely to be used. (See p. 261.)

[172] The total amount of plant-food in a ton of farmyard manure is
together less than 1/20th of its total weight.

[173] See Heiden's 'Düngerlehre,' vol. ii. p. 171.

[174] For full details see Appendix, Note XVI., p. 290.

[175] Storer reproduces these results in his 'Agricultural Chemistry,'
vol. ii. p. 21.

[176] This aspect of farmyard manure has been ably stated by Mr F. J.
Cooke, a well-known Norfolk farmer. In commenting on the results of the
Rothamsted experiments, he says: "It is clear enough that the faith of
the farmer in the soil-enriching character of his home-made manure is
amply justified; the only question being, indeed, if this quality be not
too highly appreciated. It is not, after all, so much by the fattening
of our land as by the bounty of the crop grown upon it that we reap the
fruit of our exertions. The man of scientific mind keeps his purpose
fixed on the _production of good crops_ mainly, and the cheapest way to
grow them. The experiments under consideration show that richness of
land may be purchased much too dearly, and that richness of crop by no
means bears the necessary relation to richness of soil which has
sometimes been imagined. We may boast of the 'lasting qualities' of our
dung, but the answer of science by these experiments is, that so great
is the last that the life of one man may not be long enough to exhaust
it. In the extravagant use of dung, therefore, such considerations,
amongst many others, as length of purse, as well as length and character
of tenure, must clearly be taken into account."

[177] See paper on "Manurial Experiments with Turnips" by author, in
'Transactions of the Highland and Agricultural Society of Scotland;'
1891.

[178] Storer's 'Agricultural Chemistry,' vol. i. p. 498.

[179] Division III. p. 130.

[180] Mr F. J. Cooke, who has already been quoted, has kindly furnished
the author with his views on the peculiar functions of farmyard manure
as a manure. He says: "I look upon it, broadly speaking, as chiefly of
value in restoring to good land, after cropping, those particular
advantages which good land alone can give, and in helping better than
any other manure, when applied to poor land, to bring it up to the level
of good land in those particular merits which belong alone to fine
soils. I speak now of an inherent value in good soils, beyond that
attaching to them as mere reservoirs of abundant plant-food. For
instance, one may supply a poor soil by artificial manure with much more
food--and in a highly soluble condition--than is needed by the crop to
be grown upon it, and yet not get so good a crop as upon a naturally
richer but otherwise similar soil less abundantly filled with
immediately available food. This may arise from a more perfect
distribution of the plant-food in the rich soil, or from the steady way
in which it becomes available to the crop, as well as for other reasons.
But whatever the cause, there, I think, is the broad fact of the power
of farmyard manure to enrich poor soils, so to speak, more
naturally--that is, in a way which makes them more nearly correspond to
better soils than artificial manures can."

Hence the indirect benefit to the farmer from farmyard manure is
probably greater than its direct value as a mere manure. And the usual
provision and use of it amongst all straw-growing farmers is
sufficiently justified. The extent, however, to which that course may be
beneficially carried, is one of the most important of the many difficult
economic and scientific problems which the farmer has to face.

On the economic side must of course be considered the cost of
manufacture in individual instances, as ruled by the market value of the
straw, and the different circumstances and conditions under which the
various farm animals are kept and fed (I have the figures by me of one
well-known farmer, which show the cost to him of every ton of home-made
manure to be 20s. or more); the price the resultant crops may be
expected to command; the cost at the moment of artificial manures, &c.,
&c. Whilst on the scientific side must be considered the nature of the
soil, the particular rotation of crops, &c.

It was, amongst others, just these scientific and yet very definite and
practical problems we have tried to throw light on in the series of
field experiments conducted for several years by the Norfolk Chamber of
Agriculture. (See reprint of summary of same in last year's Report of
the Board of Agriculture.)



APPENDIX TO CHAPTER VII.


NOTE I. (p. 225).

DIFFERENCE IN AMOUNT OF EXCRETA VOIDED FOR FOOD CONSUMED.

With regard to the difference in the composition of the solid excreta
voided by different fattening animals fed on the same amount of food,
see Warington's 'Chemistry of the Farm,' p. 125, where it is shown that
for equal amount of live-weight, the sheep produces on the same weight
of dry food very much more manure than the pig, while the ox produces
even more than the sheep. Of course this does not refer to the total
amount of manure produced by the different animals, but only to the
amount of manure produced from the consumption of equal quantities of
food. This would seem to be owing to the greater capacity the pig has
for assimilating its food.


NOTE II. (p. 227).

SOLID EXCRETA VOIDED BY SHEEP, OXEN, AND COWS.

To contrast with the analyses given by Stoeckhardt, it may be well to
cite those based on Lawes and Gilbert's experiments, and quoted by
Warington ('Chemistry of the Farm,' p. 138):--

I.--SHEEP (fed on _meadow-hay_).

                       SOLID EXCREMENT.
                       Fresh.      Dry.
Water                   66.2        -
Organic matter          30.3       89.6
Ash                      3.5       10.4
                        ----       ----
Nitrogen                  .7        2.0

II.--OXEN (fed on _clover-hay_ and _oat-straw_, with 8 lb.
_beans_ per day).

                       Fresh.      Dry.
Water                   86.3        -
Organic matter          12.3       89.7
Ash                      1.4       10.3
                        ----       ----
Nitrogen                  .3        1.9

III.--COWS (fed on _mangels_ and _lucerne hay_).

                      Mangels.  Lucerne hay.
Water                  83.00       79.70
Nitrogen                 .33         .34
Phosphoric acid          .24         .16
Potash                   .14         .23


NOTE III. (p. 232).

URINE VOIDED BY SHEEP, OXEN, AND COWS.

The following are the results for urine, the animals being fed as in
Note II.:--

                      Sheep.            Oxen.
                   Fresh.  Dry.      Fresh.  Dry.
Water               85.7    -        94.1    -
Organic matter      8.7    61.0        3.7   63.0
Ash                 5.6    39.0        2.2   37.0
                    ----   ----       ----   ----
Nitrogen            1.4     9.6        1.2   20.6

                              Cows.
                      Mangels.  Lucerne hay.
Water                  95.94       88.25
Nitrogen                 .12        1.54
Phosphoric acid          .01        .006
Potash                   .59        1.69


NOTE IV. (p. 233).

PERCENTAGE OF FOOD VOIDED IN THE SOLID AND LIQUID EXCREMENTS.

According to Wolff, the following table shows the percentage of the dry
substance of the food which is voided in the solid and liquid excrements
of the cow, ox, sheep, and horse:--

                 Cow.    Ox.   Sheep.   Horse.   Average.
Solid excreta    38.0   44.0    42.6     46.7      42.8
Urine             5.8    6.3     6.8      5.7       6.2
                 ----   ----    ----     ----      ----
Total            43.8   50.3    49.4     52.4      49.0


NOTE V. (p. 234).

PIG EXCREMENTS.

The excrements voided by pigs are poor in manurial constituents, because
the food on which they are fed is generally of a very poor nature. In
their case the urine is always very much richer in manurial ingredients
than the solid excreta. The relative composition of the solid excreta
and the urine will be best illustrated by quoting some experiments
carried out by Wolff on this subject. The experiments were carried out
with two pigs nine and a half months old, and each 121.9 kilogrammes (a
kilogramme is equal to about 2-1/4 lb.) in weight. The first consumed
daily 1000 grammes of barley, 5000 grammes of potatoes, and 2572 grammes
of sour-milk. The second one consumed the same quantities of potatoes
and sour-milk as the first, and 1000 grammes of peas. The following
table gives the results of excreta and urine daily voided, in grammes:--

                 Dry      Nitrogen. Ash. Potash. Lime. Magnesia. Phosphoric
              substance                                            acid.
Solid   {  I.   217.7       8.7     28.6   7.3    4.4     3.0      10.3
excreta { II.   161.1       9.1     31.1   5.9    4.9     2.8      11.1

Urine   {  I.   112.8      19.3     56.2  33.0    0.4     0.9       6.7
        { II.   137.7      30.6     62.2  37.1    0.2     1.1       7.1


NOTE VI.(p. 236).

MANURIAL CONSTITUENTS IN 1000 PARTS OF ORDINARY FOODS.

Based on Lawes and Gilbert's Analyses.

(Warington's 'Chemistry of the Farm,' p. 139.)

-----------------------------+---------+-----------+---------+------------
                             |  Dry    | Nitrogen. | Potash. | Phosphoric
                             | matter. |           |         |   acid.
-----------------------------+---------+-----------+---------+------------
 Cotton-cake, decorticated   |   918   |   70.4    |  15.8   |    30.5
 Rape-cake                   |   887   |   50.5    |  13.0   |    20.0
 Linseed-cake                |   883   |   43.2    |  12.5   |    16.2
 Cotton-cake, undecorticated |   878   |   33.3    |  20.0   |    22.7
 Linseed                     |   882   |   32.8    |  10.0   |    13.5
 Palm-kernel meal, English   |   930   |   25.0    |   5.5   |    12.2
 Beans                       |   855   |   40.8    |  12.9   |    12.1
 Peas                        |   857   |   35.8    |  10.1   |     8.4
 Malt-dust                   |   905   |   37.9    |  20.8   |    18.2
 Bran                        |   860   |   23.2    |  15.3   |    26.9
 Oats                        |   870   |   20.6    |   4.8   |     6.8
 Rice-meal                   |   900   |   19.1    |   6.1   |    23.8
 Wheat                       |   877   |   18.7    |   5.2   |     7.9
 Rye                         |   857   |   17.6    |   5.8   |     8.5
 Barley                      |   860   |   17.0    |   4.7   |     7.8
 Maize                       |   890   |   16.6    |   3.7   |     5.7
 Brewers' grains             |   234   |   7.8     |   0.4   |     3.9
 Clover-hay                  |   840   |   19.7    |  18.6   |     5.6
 Meadow-hay                  |   857   |   15.5    |  16.0   |     4.3
 Bean-straw                  |   840   |   13.0    |  19.4   |     2.9
 Oat-straw                   |   857   |   6.4     |  16.3   |     2.8
 Barley-straw                |   857   |   5.6     |  10.7   |     1.9
 Wheat-straw                 |   857   |   4.8     |   6.3   |     2.2
 Potatoes                    |   250   |   3.4     |   5.8   |     1.6
 Swedes                      |   107   |   2.2     |   2.0   |     0.6
 Carrots                     |   140   |   2.1     |   3.0   |     1.1
 Mangels                     |   120   |   1.8     |   4.6   |     0.7
 Turnips                     |    80   |   1.6     |   2.9   |     0.8
-----------------------------+---------+-----------+---------+------------


NOTE VII. (p. 241).

ANALYSES OF STABLE-MANURE, MADE RESPECTIVELY WITH PEAT-MOSS LITTER
AND WHEAT-STRAW (by BERNARD DYER, B.Sc.)

                               Peat-moss litter.   Wheat-straw.
                                   Per cent.         Per cent.
Total nitrogen                       0.88              0.61
Equal to ammonia                     1.07              0.74
Phosphoric acid                      0.37              0.43
Equal to Tribasic phosphate of
  lime (or Tricalcic phosphate)      0.80              0.94
Potash                               1.02              0.59


NOTE VIII. (p. 242).

ANALYSES OF BRACKEN (by J. HUGHES, F.C.S.)

                               No. 1         No. 2
                            Young fern.    Old fern.
                              Per cent.    Per cent.

      Water                     11.66       14.90
    * Organic matter            83.38       80.54
    + Mineral matter             4.96        4.56
                               ------      ------
                               100.00      100.00
                               ------      ------
Containing--
    * Nitrogen                   2.42        0.90
    + Silica                     1.60        2.81
      Potash                     1.15        0.10
      Soda                       0.64        0.26
      Lime                       0.44        0.62
      Magnesia                   0.13        0.47
      Phosphoric acid            0.60        0.30


NOTE IX. (p. 244).

ANALYSES OF HORSE-MANURE.

For a fuller discussion of this question, the reader is referred to
Heiden's 'Düngerlehre,' vol. ii. p. 185, and also to Storer's
'Agricultural Chemistry,' vol. i. p. 575. The statements in the
different text-books as to the quantity of manure produced by the horse
are such as naturally to perplex the student. This discrepancy is due,
however, to the different methods adopted by different writers of
calculating this amount. The subject is further discussed in the
footnote to p. 252. The following analyses of horse-manure may be
valuable for reference. They are taken from Storer's 'Agricultural
Chemistry,' vol. i. p. 496:--

----------------+-------+-------+-------+-------+-------+---------
                |   1.  |   2.  |   3.  |   4.  |   5.  | Average.
----------------+-------+-------+-------+-------+-------+---------
Water           | 75.76 | 69.30 | 67.23 | 72.13 | 71.30 |  71.15
Dry matter      | 24.24 | 24.82 | 32.72 | 27.87 | 28.70 |  27.67
Ash ingredients |  5.07 |  5.05 |  6.49 |  3.37 |  3.30 |   4.65
Potash          |  0.51 |  0.63 |  0.22 |  0.59 |  0.53 |   0.49
Lime            |  0.30 |  0.74 |  0.17 |  0.41 |  0.21 |   0.36
Magnesia        |  0.19 |  0.29 |  0.20 |  0.17 |  0.14 |   0.20
Phosphoric acid |  0.41 |  0.67 |  0.35 |  0.12 |  0.28 |   0.36
Ammonia         |  0.26 |  0.12 |  0.15 |  0.44 |   -   |   0.24
Total nitrogen  |  0.53 |  0.69 |  0.47 |  0.67 |  0.58 |   0.59
----------------+-------+-------+-------+-------+-------+---------


NOTE X. (p. 247).

THE NATURE OF THE CHEMICAL REACTIONS OF AMMONIA "FIXERS."

For the student, the exact nature of the chemical reactions taking place
may be of interest.

In the first place, it must be distinctly understood that the form in
which ammonia escapes from the manure-heap is not, as is so commonly
erroneously stated in agricultural text-books, as "free" ammonia.
Whenever ammonia is brought into contact with carbonic acid, carbonate
of ammonia is formed. When it is remembered that carbonic acid is by far
the most abundant of the gaseous products of the decomposition of
organic matter, it will be at once seen that free ammonia could not
exist under such circumstances.

1. In the case of _hydrochloric acid_, the following chemical equation
will represent the nature of the reaction--

2HCl          + (NH_{4})_{2}CO_{3}      = 2NH_{4}Cl      + H_{2}O+CO_{2}
(Hydrochloric   (carbonate of ammonia,)   (sal-ammoniac,)  (carbonic acid.)
 acid,)

2. In the case of _sulphuric acid_, the equation will be--

H_{2}SO_{4} + (NH_{4})_{2}CO_{3} = (NH_{4})_{2}SO_{4}    + H_{2}O+CO_{2}
(Sulphuric    (carbonate of        (sulphate of ammonia,)  (carbonic acid.)
 acid,)        ammonia,)

3. With _gypsum_ (CaSO_{4})--

CaSO_{4}  + (NH_{4})_{2}CO_{3} = CaCO_{3}     +  (NH_{4})_{2}SO_{4}
(Gypsum,)   (carbonate of        (calcium        (sulphate of ammonia.)
             ammonia,)            carbonate,)

4. With _copperas_ (FeSO_{4})--

FeSO_{4}     + (NH_{4})_{2}CO_{3} = FeCO_{3}    + (NH_{4})_{2}SO_{4}
(Sulphate of   (carbonate of        (ferrous      (sulphate of ammonia.)
iron,)          ammonia,)            carbonate,)

5. With _sulphate of magnesia_ (MgSO_{4})--

MgSO_{4}     + (NH_{4})_{2}CO_{3} = MgCO_{3}      + (NH_{4})_{2}SO_{4}
(Sulphate of   (carbonate of        (carbonate of   (sulphate of
 magnesia,)     ammonia,)            magnesia,)      ammonia.)

Reference has been made to the fact that magnesium sulphate may probably
not only fix the ammonia, but the phosphoric acid. When magnesium
sulphate, soluble phosphoric acid, and ammonia are brought in contact
with one another, the double insoluble phosphate of ammonium and
magnesium (MgNH_{4}PO_{4}6Aq) is formed. While such a reaction is
possible, it is highly improbable that it takes place to any extent. The
double phosphate is a crystalline salt which only separates after a
considerable time, and in the presence of a large excess of ammonia.


NOTE XI. (p. 250).

ANALYSES OF COW-MANURE.[181]

----------------+-------+-------+-------+-------+-------+-------+--------
                |   1.  |   2.  |   3.  |   4.  |   5.  |   6.  |Average.
                +-------+-------+-------+-------+-------+-------+--------
Water           | 85.30 | 77.71 | 74.02 | 72.87 | 75.00 | 77.50 |  77.06
Dry matter      | 14.70 | 22.30 | 25.98 | 27.13 | 25.00 | 22.50 |  22.93
Ash ingredients |  2.04 |  4.71 |  3.94 |  6.70 |  6.22 |  2.20 |   4.30
Potash          |  0.36 |  0.46 |  0.56 |  1.69 |  0.39 |  0.40 |   0.64
Lime            |  0.29 |  0.37 |  0.58 |  0.41 |  0.24 |  0.31 |   0.48
Magnesia        |  0.19 |  0.11 |  0.13 |    -  |  0.18 |  0.11 |     -
Phosphoric acid |  0.16 |  0.13 |  0.07 |  0.20 |  0.14 |  0.16 |   0.14
Ammonia         |  0.06 |  0.16 |  0.07 |    -  |  0.27 |    -  |   0.14
Total nitrogen  |  0.38 |  0.54 |  0.41 |  0.79 |  0.46 |  0.34 |   0.48
----------------+-------+-------+-------+-------+-------+-------+--------


NOTE XII. (p. 259).

COMPOSITION OF FRESH AND ROTTEN FARMYARD
MANURE (VOELCKER).

Composition of fresh manure, composed of horse, cow,
and pig dung, about fourteen days old:--

   Water                                               66.17
 * Soluble organic matter                               2.48
   Soluble inorganic matter                             1.54
 + Insoluble organic matter                            25.76
   Insoluble inorganic matter                           4.05
                                                      ------
                                                      100.00
                                                      ------
 * Containing nitrogen                                   .149
   Equal to ammonia                                      .181
 + Containing nitrogen                                   .494
   Equal to ammonia                                      .599
     Total percentage of nitrogen                        .643
     Equal to ammonia                                    .780
     Ammonia in a volatile state                         .034
     Ammonia in form of salts                            .088

Composition of the whole ash:--

  Soluble in water, 27.55 per cent:--
    Soluble silica                                      4.25
    Phosphate of lime                                   5.35
    Lime                                                1.10
    Magnesia                                            0.20
    Potash                                             10.26
    Soda                                                0.92
    Chloride of sodium                                  0.54
    Sulphuric acid                                      0.22
    Carbonic acid and loss                              4.71

  Insoluble in water, 72.45 per cent:--
    Soluble silica                                     17.34
    Insoluble silicious matter                         10.04
    Oxide of iron and alumina with phosphates           8.47
      (Containing phosphoric acid, 3.18 per cent.)
      (Equal to bone-earth, 6.88 per cent.)
    Lime                                               20.21
    Magnesia                                            2.56
    Potash                                              1.78
    Soda                                                0.38
    Sulphuric acid                                      1.27
    Carbonic acid and loss                             10.40
                                                      ------
                                                      100.00
                                                     -------

Composition of rotten dung, six months old, is as
follows:--

    Water                                              75.42
  * Soluble organic matter                              3.71
    Soluble inorganic matter                            1.47
  + Insoluble organic matter                           12.82
    Insoluble inorganic matter                          6.58
                                                      ------
                                                      100.00
                                                      ------

  * Containing nitrogen                                  .297
    Equal to ammonia                                     .360
  + Containing nitrogen                                  .309
    Equal to ammonia                                     .375
      Total amount of nitrogen                           .606
      Equal to ammonia                                   .735
      Ammonia in a volatile state                        .046
      Ammonia in form of salts                           .057
Composition of the whole ash:--

Soluble in water, 18.27 per cent:--

Soluble silica                                          3.16
Phosphate of lime                                       4.75
Lime                                                    1.44
Magnesia                                                0.59
Potash                                                  5.58
Soda                                                    0.29
Chloride of sodium                                      0.46
Sulphuric acid                                          0.72
Carbonic acid and loss                                  1.28

Insoluble in water, 81.73 per cent:--
Soluble silica                                         17.69
Insoluble silica                                       12.54
Phosphate of lime                                        -
Oxides of iron alumina with phosphates                 11.76
  (Containing phosphoric acid, 3.40 per cent.)
  (Equal to bone-earth, 7.36 per cent.)
Lime                                                   20.70
Magnesia                                               1.17
Potash                                                 0.56
Soda                                                   0.47
Chloride of sodium                                      -
Sulphuric acid                                         0.79
Carbonic acid and loss                                16.05
                                                     ------
                                                     100.00
                                                     ------


NOTE XIII. (p. 263).

COMPARISON OF FRESH AND ROTTEN MANURE (WOLFF).

                         Fresh.         Moderately rotten
                        (Taking the quantity of dry matter
                                  as the same.)
Dry matter                25.00               25.00
Ash                        3.81                4.76
Nitrogen                   0.39                0.49
Potash                     0.45                0.56
Lime                       0.49                0.61
Magnesia                   0.12                0.15
Phosphoric acid            0.18                0.23
Sulphuric acid             0.10                0.13
Silica                     0.86                1.08


NOTE XIV. (p. 263).

LORD KINNAIRD'S EXPERIMENTS.[182]

"Lord Kinnaird has given the particulars of a very careful experiment.
He tried to test the comparative value of manure kept in an open court
with that kept under cover. He selected the same kind of cattle, gave
them the same kind and quantity of food, and bedded them with the same
kind of straw. A field of 20 acres of uniform land was selected. This
having been equally divided, 2 acres out of each 10 gave the following
results:--

_Potatoes grown with Uncovered Manure._

                                      Tons.   cwt.   lb.
First measurement--1 acre produced.     7      6      8
Second     do.       do.     do.        7     18     99

_Potatoes grown with Covered Manure._

First measurement--1 acre produced.    11     17     56
Second     do.       do.     do.       11     12     26

This shows an increase of about 4 tons of potatoes per acre with the
covered manure.

"The next year the weather was wet, grain soft and not in very good
order, but the following was the amount of produce:--

_Wheat grown with Uncovered Manure._

                             Weight per
         Produce in grain.     bushel.     Produce in straw.
 Acre.    bushels.    lb.        lb.        stones.    lb.
First       41        19       61-1/2         152  of  22
Second      42        38       61-1/2         160  of  22

_Wheat grown with Covered Manure._

First       53         5         61           220  of  22
Second      53        47         61           210  of  22"


NOTE XV. (pp. 231, 264).

DRAININGS OF MANURE-HEAPS.

The importance of not separating the liquid portion from the solid
portion has already been pointed out in dealing with the composition of
the solid excreta and the urine. These two constituents of the manure
are complementary to one another, and the value of farmyard manure as a
general manure is very much impaired if the liquid portion is not
applied along with the solid. In one important respect do the drainings
of manure-heaps differ from urine--that is, in the percentage of
phosphates they contain, the latter being practically devoid of
phosphoric acid.

The following is an analysis of drainings from a manure-heap (Wolff):--

Dry substance           18.0  |  Magnesia                 0.4
Ash                     10.7  |  _Phosphoric acid_        0.1
Nitrogen                 1.5  |  Sulphuric acid           0.7
Potash                   4.9  |  Silica                   0.2
Lime                     0.3  |


NOTE XVI. (p. 270).

AMOUNTS OF POTASH AND PHOSPHORIC ACID REMOVED BY THE FOLLOWING
ROTATIONS FROM A PRUSSIAN MORGEN (.631 ACRE).

               Potash.   Phosphoric acid.
                 lb.           lb.
1. Wheat        16.40         10.67
   Oats         10.47          4.59
   Potatoes     66.41         18.33
   Hay          39.54         11.32
               ------         -----
               132.82         44.91
               ------         -----
The ratio of potash to phosphoric acid is 2.96 to 1.

2. Wheat        16.90         10.67
   Barley       17.44         10.65
   Potatoes     66.41         18.33
   Hay          39.54         11.32
               ------         -----
               140.29         50.97
               ------         -----
The ratio of potash to phosphoric acid is 2.76 to 1.

3. Rye          20.03         12.15
   Oats         10.97          4.59
   Potatoes     66.41         18.33
   Hay          39.54         11.32
               ------         -----
               136.95         46.39
               ------         -----
The ratio of potash to phosphoric acid is 2.95 to 1.

4. Wheat        16.90         10.67
   Oats         10.97          4.59
   Mangels     148.54         25.62
   Hay          39.54         11.32
               ------         -----
               215.95         52.20
               ------         -----
The ratio of potash to phosphoric acid is 4.13 to 1.

5. Rye          20.03         12.15
   Barley       17.44         10.65
   Mangels     148.54         25.62
   Hay          39.54         11.32
               ------         -----
               225.55         59.74
The ratio of potash to phosphoric acid is 3.78 to 1.


NOTE XVII. (pp. 253, 254).

COMPOSITION OF FARMYARD MANURE (FRESH), (calculated by SIR
JOHN LAWES).

                                          Phosphoric acid
                Total dry  Total mineral  calculated as   Potash.  Nitrogen.
                 matter.      matter.     phosphate of
                                              lime.
Percent           30.0          2.77            .50          .53       .64
Per ton (in lb.)  67.2         62.0           11.1         12.0      14.3


NOTE XVIII. (p. 232).

THE URINE.

An important consideration we have omitted to take note of in the text
is the quantity of the urine voided. It is this consideration that
renders the urine so much more valuable than the solid excreta. In the
case of a man it has been estimated that the urine voided is fifteen
times as much, is twelve times as rich in nitrogen, three times in
potash, and two in phosphoric acid, as the solid excreta (Munro). The
relation of solid matter in the case of the farm animals is not exactly
similar. The urine of the ox is about twice the weight of its solid
excreta. Both the horse and the sheep, however, void as a rule more
solid excreta than urine. Munro, in his work on 'Soils and Manures,'
contrasts the composition of the urine and solid excreta of the
different farm animals by the following statement:--

         1 ton of urine contains  1 ton of solid excreta
                 in lb.:             contains in lb.:

            Nitrogen.  Potash.           Nitrogen.
Cow            30        20                  9
Horse          36        22                 12
Sheep          38        30                 16


FOOTNOTES:

[181] Storer's 'Agricultural Chemistry,' vol. I. p. 496.

[182] Scott's 'Manures and Manuring,' p. 19.



CHAPTER VIII.

GUANO.


_Importance in Agriculture._

In the consideration of _artificial_ manures, guano deserves the first
place. This it does mainly on historical grounds, as it is now largely a
manure of the past. Not merely has it been used in agriculture to an
extent to which no other artificial manure has as yet ever approximated,
but its influence on agricultural practice has been enormous. Introduced
into this country about the middle of the present century, it was the
first of artificial manures to be used in large quantities.[183] It may
be thus described as having introduced the modern system of _intensive_
cultivation, and given rise to the now almost universal practice of
artificial manuring.


_Influence on British Farming._

It is, indeed, difficult to over-estimate the important influence which
the introduction of this most valuable fertiliser has exercised on
British as well as, to a large extent, on European husbandry. Before its
introduction the farmer was almost completely dependent on his farmyard
manure. He was tied down to a great extent, by the exigencies of the
then prevailing agricultural customs, to certain rotations of crops. He
could do little in the way of enriching barren soils or of ensuring a
heavy yield of crop. By the use of this very potent fertiliser, he
quickly discovered that the most wonderful results ensued--results which
must have seemed to him at first little short of miraculous. He found
that by the application of a few hundredweights per acre, poor soils
could be made to yield large returns, and that barren patches in a field
could be brought up to the average of the surrounding portions by
sprinkling merely a few handfuls of it; that by its means a good start
could be ensured to every crop, and one slow of coming away could be
hastened on. In short, in this wonderful brown powder, with such a
characteristic odour, the astonished farmer discovered a manure which,
for the speed of its action, and for the increase of crop it gave,
completely threw into the shade both farmyard manure and bones. What
wonder, then, that its fame as a manure should have become so quickly
known and its use so extensive! It thus gave a most powerful impetus to
intelligent farming by bringing home to the minds of those who used it
the important position nitrogen and phosphates occupied as constituents
of the soil, and the influence they exercised on plant-growth. It
furnished, in fact, on an enormously large scale, a practical
demonstration of the principles of manuring. The educational value which
the use of guano thus exercised may be said to have been very great. It
also led the way to the use of the various artificial manures so much
used during the last fifty years. Impressed by the value of guano,
farmers were favourably disposed towards the use of other fertilisers;
and, largely owing to its widespread popularity, the new practice
speedily gained ground.


_Influence not wholly for Good._

But its influence, it must be admitted, was not wholly for good. In its
very popularity lay the danger of its abuse. Had its value and the
method of its action been more widely understood, and had the principles
upon which the practice of artificial manuring depends been better
realised, agriculturists would have been spared much of the needless
pecuniary losses they sustained by being imposed upon by unscrupulous
manure-dealers. Among the farming community the word guano soon became a
name to conjure with, and under this title many spurious and worthless
manures were attempted to be palmed off on the unwary farmer. Even the
genuine article, there can be little doubt, was at one time largely
adulterated; and as the farmer was almost invariably content to purchase
the article not on any guaranteed chemical analysis, but simply on the
ground of its appearance, colour, and more especially smell, every
facility was given for the successful perpetration of such fraudulent
imposition. Guano, it was very soon found, varied in its composition,
but this variation in quality the farmer did not recognise. In the early
days of its use all guano was in his eyes of the same value. Too often,
as we have just pointed out, provided it had a good colour and a strong
odour, it was all right. Under such conditions, it can scarcely be
wondered at that its introduction should have proved not an unmixed
blessing to agriculture.


_Its Value as a Manure._

Guano derives its value as a manure from the nitrogen, phosphates, and
the small amount of potash it contains. This at any rate is true of the
great bulk of guano which has been used in the past. There are, as we
shall immediately see, certain kinds of guano, known as phosphatic
guanos, which only contain phosphates. The amount of such purely
phosphatic guano directly used as a manure in this country is, however,
inconsiderable, and guano may truly be described as owing its value
chiefly to its nitrogen. Not a little of its value and popularity as a
manure may be said to be due to the fact that it contains all of the
three important manurial constituents, and that in this respect it may
be regarded in a sense as a _general_ manure, thus resembling most
nearly, of all artificial manures, farmyard manure. Although its sources
are now, to a very large extent, exhausted, and its total annual imports
into this country are at present considerably less than what they were
thirty or forty years ago,[184] it may be well, on account of its
historical importance, to give a somewhat detailed account of its
origin, occurrence, and value as a manure.


_Origin and Occurrence._

Guano (which means _dung_)--or huano, as it is spelt in the Spanish
language--was first used in Peru. It seems to have been used there long
before that country was discovered by the Spaniards--probably as early
as the twelfth century. Regarding its origin there can be little doubt.
It is almost entirely derived from the excrements of sea-birds, such as
pelicans, penguins, and gulls, as well as from the remains of the birds
themselves, and of seals, walruses, and various other animals.[185]
Under the influence of a tropical sun, and in a region in which rain
scarcely ever falls, these excrements are soon dried, and remain little
changed in their composition through centuries. Many of the Peruvian
deposits must be extremely old, as they are covered up with sand and
other _débris_, and are of considerable depth. Especially is this the
case with deposits occurring on the mainland, such as those at Pabellon
de Pica, where the layer of sand or conglomerate covering up the deposit
varies in depth from a few feet to over a hundred. The effect of this
superficial covering has been to protect the guano, to a certain extent,
from loss of nitrogen.

Although guano of the best class has been derived from the neighbourhood
of Peru, deposits have also been found in many other parts of the
world--viz., in North America, West Indies, Australia, Asia, Africa, and
among the islands of the Pacific.[186]


_Variation in the Composition of different Guanos._

The guano found in these different deposits varies very considerably in
composition. This is due to the difference in the nature of the
prevailing climate of the places where these deposits occur. Where the
climate is dry and warm, as is the case in Chili and Peru, the
excrements dry quickly and remain very little changed, as one very
important condition of fermentation--viz., moisture--is absent.[187] In
a damp climate, on the other hand, speedy fermentation ensues, resulting
in the loss of nearly all the organic matter, including nitrogen, in
such volatile forms as carbonate of ammonia, carbonic acid gas, water,
&c. The soluble alkalies, the most important of which is potash, as well
as the soluble phosphates, are also, under such conditions, lost to the
guano by being washed out by the rain. We have thus a wide difference in
the quality of the different deposits, depending on the extent to which
decomposition has taken place. Guano thus ranges from the rich
nitrogenous Peruvian kind, which has undergone little or no change from
the time of its deposit, to the purely phosphatic kind (such as those of
Malden and Baker islands), in which everything of manurial value has
been lost except the insoluble phosphate of lime. Even among the
nitrogenous guanos we find a considerable difference in quality, some
deposits being partially impoverished by the action of the atmospheric
moisture, dew, spray or sea-water, but still containing a considerable
proportion of their nitrogen. Other deposits, again, are largely admixed
with sand, which has been blown in upon them to such an extent as to
make them unsaleable. We can divide guano, therefore, into two great
classes--viz., _nitrogenous_ and _phosphatic_.


I.--NITROGENOUS GUANOS.

(_a_) PERUVIAN.

By far the most valuable and abundant deposits as yet discovered have
been those on the Peruvian and Chilian coasts. As already pointed out,
guano seems to have been used in this country from a very early period;
and so impressed were the Incas with its importance as a manure, that
the penalty of death was imposed on any one guilty of killing the
sea-fowl during the breeding season in the vicinity of the deposits.

The occurrence of guano in Peru seems first to have been made known in
Europe in the beginning of the eighteenth century. It was not, however,
till the beginning of the present century--viz., 1804--that A. Humboldt,
the great German traveller, brought some of the wonderful fertiliser
home with him, and that its composition was able to be investigated by
chemical analysis. Shortly afterwards, its practical value was
demonstrated by experiments carried out on potatoes by General Beatson
in St Helena. To Lord Derby is due the credit of having first introduced
it into this country, the earliest importation into Liverpool being in
1840. Experiments were shortly afterwards instituted in different parts
of the country, prominent among which were those by Sir John Lawes and
Sir James Caird; and so striking were the results obtained, that the
manure rapidly found favour with the farming community--so much so, that
ten years later the importations into this country amounted to no less
than 200,000 tons, while in 1855 the total exports from the west coast
of South America reached the enormous amount of 400,000 tons. In all, it
has been estimated that since the year 1840 over 5,000,000 tons of
Peruvian guano have been imported into this country.


_Different Deposits._

Peruvian guano has been derived from various deposits occurring in
different parts of the coast, and from a number of small adjacent
islands. The richest of these was that found on Angamos, a rocky
promontory on the coast of Bolivia. Samples of this guano contained as
high as 20 per cent of nitrogen (equal to 24 per cent ammonia).[188]
Unfortunately, however, the quantity of this deposit was extremely
limited, and became rapidly exhausted. Next to this deposit in quality
was the guano found on the Chincha islands, three little islands off the
coast of Peru. These deposits were the largest which have ever been
discovered, and for a period of nearly thirty years were almost the sole
source of the Peruvian guano sold in commerce, over 10,000,000 tons
having been exported from them alone. Some of this guano contained 14
per cent of nitrogen (equal to 17 per cent ammonia); and although part
of the guano shipped from these islands was not quite so rich, yet it
was all of a high-class order. The deposits on these islands were in
many cases 100 to 200 feet in depth, and rested on rocks of granite. The
lower layers were consequently found to be poorer in quality, and mixed
with pieces of granite. The Chincha island deposits have been long
exhausted,[189] and the chief deposits of Peruvian guano since worked
have been those on Guanape and Macabi islands--a considerably inferior
guano, containing only 9 to 11 per cent of nitrogen (equal to 11 to 13
per cent of ammonia)--which in their turn have become exhausted; from
Ballestas, almost as rich as the Chincha island guano, also now
exhausted; and from Pabellon de Pica, Punta de Lobos, Huanillos,
Independence Bay, and Lobos de Afuera. Quite recently a deposit of very
high-class guano was discovered in Corcovado, and a good many cargoes
have already been shipped to this country. It is found to contain
nitrogen equal to from 10 to 13 per cent ammonia, 30 to 35 per cent
phosphates, and some potash, being thus a most valuable guano.


_Appearance, Colour, and Nature._

In colour it varies from a very light to a very dark brown, the richer
samples being generally lighter. Samples taken from even the same
deposit have been found to differ very considerably in appearance, those
taken from the lower and older layers being usually darker than those
taken from the more recent upper layers. It was soon found also to vary
very much in composition. After a deposit had been worked for some time,
the quality of guano it yielded was found to be inferior and coarser,
and in many cases mixed with pebbles or pieces of granite, porphyry, &c.
This led to the custom of screening it on arrival in this country,
before it was used as a manure. In the richer qualities--_e.g._, in the
Chincha guano--little round concretionary nodules, varying in colour
from pure white to dark brown, were occasionally found. Analysis showed
these nodules[190] to be composed chiefly of potash salts. Sometimes,
also, little crystals of almost pure ammonia salts were found. It soon
became customary, therefore, to prepare guano for the market by
separating the stones and reducing the whole to a fine uniform powder.
One of its most characteristic properties, and the one which seems to
have impressed the public most, was its pungent odour. Undue importance
was attached to this property, in the belief that it was caused by the
ammonia it contained. It may be doubted, however, whether the
characteristic smell of guano is due so much to its ammonia as to
certain fatty acids.


_Composition._

In composition it is of a most complex nature. It contains its nitrogen
in a great variety of forms, the chief of these being urate, oxalate,
ulmate, humate, sulphate, phosphate, carbonate, and muriate of ammonia;
and also in a rare form of organic nitrogen peculiar to guano, called
guanine. According to Boussingault, some guanos contain small quantities
of nitrates. Its phosphoric acid is present both in the soluble
state--viz., as phosphates of the alkalies (ammonia and potash)--and in
the insoluble state as phosphate of lime; and lastly, its potash is
present as sulphate and phosphate. The proportion in which these
different forms of nitrogen and phosphoric acid are present varies
considerably in different samples. The richer a sample, as a rule, the
more nitrogen in the form of uric acid it contains. The most of the
nitrogen is present as uric acid and ammonia. Damp guanos contain more
of their nitrogen as ammonia than dry ones, this being due to the
fermentation which goes on in the former. On an average, about a third
of its total nitrogen is soluble in water. Of its phosphates, on the
other hand, only about a fourth are soluble in water.

The following analyses of a sample of Chincha island guano by
Karmrodt[191] will illustrate this. (Sample dried at 212° Fahr.):--

1. _Constituents easily soluble in Water._

Urate of ammonium                                  12.74
Oxalate of ammonium                                13.60
Nitrogenous and sulphurous organic substances       3.61
Ammonium-magnesium phosphate                        4.00
Ammonium phosphate                                   .90
Ammonium sulphate                                   1.82
Ammonium chloride                                   1.55
Potassium sulphate                                  3.30
Sodium chloride                                     2.44
                                                   -----
                                                   43.96
                                                   -----

2. _Difficultly soluble in Water, soluble in Acids,
Alcohol, and Ether._

Uric acid                                          21.14
Resin                                               1.11
Fatty acids                                         1.60
Nitrogenous and sulphurous organic substances       2.29
Calcium phosphate                                  18.22
Phosphate of iron                                   1.04
Silica                                               .64
                                                   -----
                                                   46.04
                                                   -----

In the above analysis it will be noticed that none of the ammonia is
present as carbonate. In most samples, however, of Peruvian guano, the
ammonia in this form amounted to from 1 to 2 per cent. In the inferior
qualities, chiefly those which had been subjected to the action of
water, and consequently of fermentation, to a certain extent, this form
of ammonia was found to be most abundant. Such guanos were most liable
to loss of nitrogen by volatilisation.

The older Peruvian guano contained as high as 14 per cent of nitrogen
(equal to 17 per cent of ammonia), and of phosphoric acid 12 to 14 per
cent (equal to 26 to 28 per cent of phosphate of lime). It, however,
gradually deteriorated in quality as the deposits became worked out, the
percentage of nitrogen becoming year by year less, until latterly
Peruvian guano, as imported, contains only from 3 to 4 per cent of
nitrogen (equal to 4 to 5 per cent of ammonia). This guano is, however,
richer in phosphates, containing often 50 to 60 per cent of phosphate of
lime, and 3 to 4 per cent of potash.[192]


(_b_) OTHER NITROGENOUS GUANOS.

The guanos, other than those which come from Peru, are chiefly purely
phosphatic guanos, so that the term Peruvian has not unfrequently in the
past been used as a generic term synonymous with the term nitrogenous,
and consequently applied to all nitrogenous guanos independent of their
source. There are, however, a few deposits other than the Peruvian
which have yielded considerable quantities of valuable nitrogenous
guano. Of those, the richest in quality--in fact, the richest of any
deposits hitherto discovered--was the Angamos guano, which came from a
rocky promontory on the coast of Bolivia. The few samples of this which
have been analysed showed over 20 per cent of nitrogen. Unfortunately,
the deposit proved to be comparatively insignificant in amount, and has
long been exhausted.

Poorer in quality, but more abundant in quantity, were the deposits
found on the Ichaboe and other islands off the south-west coast of
Africa. These deposits were discovered shortly after the introduction of
Peruvian guano, and for a few years supplied considerable quantities of
valuable manure. The deposits first discovered were soon exhausted, so
that for a number of years Ichaboe guano ceased to be procurable. Fresh
deposits, however, were subsequently found, and considerable quantities
have of late years been used in agriculture.[193] Ichaboe guano is
inferior in value to Peruvian. It exemplifies the influence of small
quantities of rain on guano deposits in impoverishing them in their
nitrogen. In much of the Ichaboe guano imported into this country a
large amount of feathers is found. It also contains an abnormally large
quantity of insoluble matter.

Among the other nitrogenous guanos may be mentioned the Patagonian,
Falkland, and Saldanha Bay. They are, like the Ichaboa, of comparatively
recent origin, and are collected in small quantities after the breeding
season every year.


II.--PHOSPHATIC GUANOS.

Phosphatic guanos, as already pointed out, are similar in origin to
nitrogenous guanos. In their case, however, the nitrogen, alkalies, and
soluble phosphates which they originally contained have been almost
entirely lost by the decomposition of their organic matter and the
action of water.[194] Most of them still contain very small quantities
of nitrogen, amounting to a fraction of a per cent. Of these deposits
there are very many occurring on islands in different parts of the
world. In appearance the guano obtained from them is very different from
nitrogenous guano, being much lighter in colour, and of a fine powdery
nature. It forms a very rich phosphatic guano, containing in many cases
between 70 and 80 per cent of insoluble phosphate of lime. Such guanos
are largely used in the manufacture of high-class superphosphates, by
treating them with sulphuric acid. Being of an insoluble nature, they
are not very suitable for direct application to the soil. Of these
phosphatic guanos the following are the chief--those marked in italics
being still unexhausted:--

1. _Baker_, Jarvis, Howland, Starbuck, Flint, _Enderbury_, _Malden_,
Lacepede, _Browse_, _Huon_, _Chesterfield_, _Sydney_, _Phoenix_,
_Arbrohlos_, _Shark's Bay_, and _Timor_--all found on islands in the
Pacific Ocean.

2. _Mejillones_, on the coast of Bolivia.

3. Aves, _Tortola_, _Mona_, and other deposits in the West Indies.

4. _Kuria Muria_ islands, in the Arabian Gulf.

For further particulars as to the composition of these different guanos,
the reader is referred to the Appendix, Note V., p. 329.


_Inequality in Composition._

That guano was a substance of by no means uniform composition was a fact
early recognised in the history of the trade. Not only did guano from
different deposits show on analysis different percentages of the
manurial ingredients, but different samples of guano from the same
deposit were often found to differ very considerably from one another.
It soon became the custom, therefore, to sell it on chemical analysis,
each separate cargo being carefully analysed. But this custom did not
wholly obviate the difficulty, as the guano in even one cargo might
differ. In the case of the older and richer guanos, there was certainly
more uniformity in quality, but they were liable to differ in their
percentage of nitrogen.[195] As, however, the deposits became gradually
worked out, their lower layers were found more or less largely admixed
with stony and earthy matter, and their composition was naturally
rendered very variable. This state of matters was unsatisfactory to
buyers and sellers, and led to much friction between the two, as it was
found wellnigh impossible on the part of the seller to guarantee the
composition of his manure. The custom of preparing the material by
reducing it to a fine powder before sending it into the market, and the
custom, subsequently introduced, of treating it with sulphuric acid,
have done away with this difficulty to a large extent.


_"Dissolved" Guano._

The treatment of guano with sulphuric acid was first had recourse to in
the case of cargoes damaged with water. In such guano, as has been
already pointed out, fermentation has been permitted to take place, with
the result of the formation of volatile carbonate of ammonia in greater
or less quantity. By the addition of sulphuric acid the ammonia was
fixed, and the guano was prevented from losing its most valuable
constituent. It was soon found, however, that guano so treated possessed
greater activity as a manure. The result of the sulphuric acid was to
increase very materially the amount of its soluble phosphates, and also
its soluble nitrogen compounds.[196] It had, moreover, the effect of
producing a guano of uniform composition. The custom, first introduced
in 1864 by Messrs Ohlendorff & Co., was soon largely practised. The
guano is treated with 25 to 30 per cent sulphuric acid (sp. gr. 1.73).
After a short time the resulting hard mass is, by means of
disintegrators, reduced to a uniform powder.


_"Equalised" or "Rectified" Guano._

As guano decreased in its quality the demand for a high-class article
became more and more difficult to meet. This led to the custom of
"fortifying" or "rectifying"--as it is variously called--the natural
material with sulphate of ammonia. A manure closely resembling in the
percentage of its manurial constituents the older rich guanos is thus
obtained. Of these so-called "equalised" guanos, two qualities are at
present sold, the first being guaranteed to contain nitrogen equal to 8
to 9 per cent ammonia, 30 to 35 per cent phosphates, and 2 to 3 per cent
of potash; the second quality containing only about half as much
nitrogen, but more phosphates.

However valuable this fortified guano may be--and it is, undoubtedly, a
most valuable manure--its action cannot be supposed to be exactly
similar to the old Peruvian guano, which it resembles in the percentage
of its nitrogen, phosphates, and potash. Much of the distinctive value
of guano as a manure, as will be pointed out immediately, lies in the
fact that it contains its manurial ingredients in a variety of
differently soluble compounds, which are gradually rendered available in
the soil for the plant's needs. This undoubtedly is one of the reasons
why the action of guano among manures is quite unique; and there are
other reasons which we probably do not clearly understand. However
skilfully the composition of the guano may be artificially simulated, it
still remains an undoubted fact that the "equalised" guano is not
exactly similar in its action to the genuine article. Nevertheless, that
it is superior in its results to the poorer classes of guano at present
available, and to ordinary compound manures, there can be little doubt.
A great merit of the equalised guano is, however, that it is sold at a
lower price than guano as imported; and as the guano is sold on a
guaranteed analysis, the practice has done much to advance the true
interests of agriculture.


_Its Action as a Manure._

Next to farmyard manure, guano may be regarded as the most "general" of
all the commonly used manures; for in addition to nitrogen, phosphoric
acid, and potash, it contains nearly all the other plant ingredients,
such as lime, magnesia, &c. Its special value as a manure, however, does
not merely consist in the amount of valuable plant-food it contains.
Like farmyard manure, it owes much of its characteristic action to the
state of the intimate mixture of its manurial constituents, and also, as
has already been pointed out, to the fact that it contains those
constituents in a great variety of chemical forms, each of which differs
in its solubility, and consequently availability for the plant's needs.
Take, for example, the great number of different forms of nitrogen it
contains. Some are in the condition in which plants can immediately
absorb them, while the rest are in a series of less and less available
forms, which, however, are gradually converted into available forms as
the plant requires them. Like farmyard manure, again, it may be applied
with almost equally good results to all kinds of crops and on all kinds
of soils. We have in guano, in short, an admirable example of the value
of applying our manurial ingredients in different forms. That this is no
mere theory is abundantly proved by the large number of different
experiments which have in the past been carried out with guano, more
especially the well-known experiments made by Grouven, the German
chemist. In those well-known experiments, guano was tested against a
large variety of different fertilisers, and the tests were so arranged
that in most cases the amounts of nitrogen, phosphoric acid, and potash
were the same in the other manures used. In short, these experiments
prove in a very striking manner that a manure artificially made up out
of most valuable fertilisers, such as nitrate of soda, sulphate of
ammonia, superphosphate, &c., so as to closely resemble in its
composition guano, is by no means similar in its effects to the genuine
article. As in farmyard manure, so in guano: we must look to the
complexity of the composition of both these fertilisers in order to
fully estimate their worth. There is in the action of both manures much
that we cannot explain, or even, as yet, understand. The action of guano
is merely one of many problems in the science of manuring which
illustrate how unsatisfactory, despite the great amount of research
already carried out, is our knowledge of this most important department
of agriculture.[197]


_Proportion of fertilising Constituents in Guano._

Guano must be regarded as a nitrogenous and phosphatic manure, as the
quantity of potash it generally contains is small. In many soils, more
especially in such a country as Scotland, this deficiency in potash is
not of so much importance, as the value of potash as an artificial
manure is less than is the case with the other two ingredients. In
soils, however, lacking potash, guano ought to be supplemented with some
potash manure. With regard to the nitrogen and phosphoric acid, we may
ask if these two constituents are in the best proportions. This question
does not admit of a direct answer. In the first place, the proportion
in which these two ingredients are present is variable. In the old rich
Peruvian guanos, as we have above shown, the nitrogen was more abundant
than is the case at present. Such guanos, it was found, were best
supplemented with phosphatic manure when applied to the field. In the
"equalised" and "dissolved" guanos, which are now so largely sold,
manufacturers attempt to adjust the percentage of nitrogen and
phosphoric acid to what is considered the best proportion in most cases.
As, however, we have again and again to point out, regard must be had
both to the soil and the crop in determining what is the best proportion
of the manurial ingredients in a manure. For cereals it may be well
supplemented by nitrogenous manures, while for roots it may be well
supplemented by phosphatic manures.


_Mode of Application._

Like all manures, it is desirable to apply it in as fine a condition as
possible, so as to ensure as thorough a mixture with the soil-particles
as practicable. In order, furthermore, to prevent any risk of loss
through volatilisation of the ammonia, as well as to ensure even
distribution, it is best applied mixed with dry earth, ashes, sand, or
some other substance,--not lime, however. The custom of applying along
with the guano common salt, has been proved by numerous experiments to
be highly beneficial to the action of the guano as a manure. The exact
nature of the action of salt as an adjunct to manures is a point which
has elicited much discussion. Its action is probably to be ascribed to a
number of causes. For one thing, it probably acts as an antiseptic in
retarding the fermentative action which has a tendency to go on so
rapidly in such manures as guano. It further increases the power of the
manure to attract moisture from the air--a most important property in
the case of drought. Some experiments by Dr Voelcker illustrate this in
a striking manner. Two lots of guano--one pure and one mixed with
salt--were exposed to the action of the air for a month, and were then
tested as to the amount of water they contained, when it was found that
the lot containing the salt had absorbed 2 per cent more water than the
other.

Much stress has been laid on the importance of having the guano buried a
certain depth in the soil; and many experiments have been carried out to
prove how much better it acts when so applied. This is probably due to
the prevention of any loss of volatile ammonia, and the mixture of the
manure with the soil-particles before it comes in contact with the
plant-roots. This last precaution is an important one, for it has been
found that the raw material is apt to have a bad effect on the seed or
the plant's roots. This has been found to be especially the case in
regard to potatoes, the quality of which has been found to suffer when
the guano is brought into direct contact with the tubers. As guano is a
manure which is speedily available, it is desirable to apply it as
shortly before it is required by the plant as possible. It is therefore
generally best applied in spring, shortly before seed-time, or indeed at
the same time. Where farmyard manure is used, the guano has been
recommended to be used as a top-dressing in small quantities. In the
majority of cases it will be advisable, however, not to apply it as a
top-dressing, for the various reasons above-mentioned.


_Quantity to be used._

As to the quantity to be used, this of course will depend on the soil,
the crop, and the amount and nature of the other manures employed: 1 to
4 cwt. per acre have been the usual limits, but even heavier dressings
have been commonly resorted to, especially in Scotland, where 6 to 8 or
even 9 cwt. for turnips are often used. Sir J. B. Lawes and Sir James
Caird long ago, shortly after the introduction of guano, estimated, from
the experiments they carried out, that the application of 2 cwt. per
acre to the wheat crop gave an increase of 8 to 9 bushels in grain, and
added a fourth to the quantity of straw. The former authority recommends
2 to 3 cwt. per acre for wheat, to be sown broadcast and harrowed into
the land before sowing the seed. We have already stated that it may be
used in all soils and for all kinds of crops. While this is so, it has
been found to have specially favourable results when applied to the
turnip crop, when it may be used in larger quantities than in the case
of cereals. When applied to the turnip crop, it is well to use the more
phosphatic guanos or to supplement it with superphosphates. By applying
it in two lots, the larger portion before seed-time and the rest between
the drills after the turnips are up, excellent results have been
obtained. It has also proved an admirable manure for mangels. On the
whole, it gives best results on heavy soils and in a dampish climate.


_Adulteration of Guano._

Probably no artificial manure has been subjected to greater adulteration
in the past than guano. This has been due to the fact that the practice
of selling guano on analysis--especially among retail buyers--did not
largely obtain in the early years of the trade. A good deal of this
adulteration was probably caused by ignorant prejudice on the part of
the farmer, to whom the pungency of its smell and its colour were too
apt to be ranked as its most important properties. The variation in the
quality of different kinds of guano was too often not sufficiently
realised by the buyer, who not unfrequently was made to pay as high a
price for guano of an inferior quality as he ought to have paid for that
of the best quality. Indeed no manure illustrates the importance of
chemical analysis more than guano. Among the different forms of
adulteration practised may be mentioned the addition of such substances
as sawdust, rice-meal, chalk, sulphates of lime and magnesia, common
salt, sand, earth, peat, ashes of various kinds, and water. There can be
no doubt, however, that such adulteration has now long ceased to be
practised to any extent. Nevertheless, it may be of use to draw
attention to one or two of the tests by means of which some of the
commoner forms of adulteration may be detected. One or two are extremely
easily detected--as, for example, adulteration with sand or other
mineral substances. In such a case, the percentage of ash left on
burning a small portion of the guano will be found to be excessive. The
percentage of ash in a sample of genuine Peruvian guano should not
exceed from 50 to 60 per cent. The colour of the ash is another
important point, and may serve as a further indication of adulteration.
In the case of genuine guano, this should be whitish or greyish.
Red-coloured ash generally points to the adulteration of the guano with
some mineral substance containing iron--such, _e.g._, as Redonda
phosphate, a mineral phosphate of iron and alumina. Where the ash is
white, but excessive in quantity, adulteration with common salt,
sulphate of magnesia, gypsum, or chalk, may be suspected. The last-named
substance is easily detected by treating it with any of the common
acids, when brisk effervescence, due to the liberation of the carbonic
acid, will ensue.[198] A further point of importance with regard to the
ash is its solubility in water and in acids. A large insoluble residue
may be taken as indicating adulteration with sand. Adulteration with
water is also easily detected by heating a sample to the boiling
temperature and determining the loss it sustains. Of course the amount
of water varies in different samples. The appearance of the guano will
serve fairly well to detect whether it is abnormally moist. It may be
added, in conclusion, that Peruvian guano is extremely light; and while
this by itself is not a sufficient test of genuineness, it may serve to
confirm other tests.


III.--SO-CALLED GUANOS.

Before concluding this chapter, reference may be made to certain manures
which are commonly known under the name of guanos--such as "fish-guano,"
"flesh-guano," "meat-meal-guano," and "bat-guano,"--as well as to
manures which may more conveniently be described here--viz., "fowl and
pigeon dung."


_Fish-Guano._

The application of fish, not suited for other purposes, to the fields
as a manure is a practice which has obtained in certain parts of the
country for a number of years. In many districts on the sea-coast, where
fishing is the chief industry, the only way in the past of disposing of
a superabundant catch of herrings, for example, has been to utilise them
as a manure. From such a practice has sprung up what is now an important
and ever-increasing trade--viz., the manufacture of fish-guano.

This manufacture was first started, and is still most largely practised,
in Norway. The guano obtained varies very considerably in quality
according to the nature of the process employed, and as to whether the
guano is made from whole fish or merely from fish-offal. The latter
source is the common one. The manufacture is carried on at the
fish-curing stations, and the quality of the guano made from this source
is somewhat different from that made from whole fish, as a large
proportion of the fish-offal is made up of bones and heads. Large
quantities of Norwegian fish-guano are exported to various parts of
Europe.

The best quality of this guano may contain as much as 10 per cent of
nitrogen, but as a rule it is nearer 8 per cent. A very considerable
variation in the amount of phosphoric acid occurs for the reason above
stated, the guano made from fish-scrap being naturally much richer in
this ingredient than whole-fish guano. The phosphoric acid may be said
to range from 4 to 15 per cent, and there is also a small quantity of
potash present.

Guano is also manufactured in Norway from the carcasses of whales. Such
guano contains from 7-1/2 to 8-1/2 per cent of nitrogen, and about
13-1/2 per cent of phosphoric acid.

In America fish-guano is manufactured to a considerable extent--one
important source being the menhaddo, a coarse sort of herring. This fish
is caught for the sake of its oil, which is extracted by boiling, the
residue being manufactured, after pressing and drying, into guano.

In this country the manufacture of fish-guano is carried out to a
considerable and increasing extent. Formerly it was imported from Norway
to a larger extent than is now the case, the present annual imports
amounting only to 1000 or 2000 tons. The total annual production in the
United Kingdom is probably 7000 or 8000 tons.


_Value of "Fish-Guano."_

That fish-guano is a valuable manure there can be no doubt. What,
however, impairs its value is the fact that, as a rule, it contains a
certain amount of oil. The effect of this oil is to retard fermentation
and decomposition when the guano is applied to the soil, and thus render
its action slower than would otherwise be the case.

When applied to the soil, therefore, every opportunity ought to be
given to promote its fermentation. It is best applied some time before
it is likely to be used. It ought to be well mixed with the
soil-particles, and not allowed to lie on the top of the soil. Its best
effect will be on light well-cultivated soils, which permit of the
access both of sufficient moisture and of sufficient air for rapid
fermentation. Its value as a manure for hops, vines, grass, and
strawberries has been found to be considerable. It has been recommended
to be applied along with farmyard manure; and such a mode of application
is no doubt well suited to promote its decomposition. It has also been
used for mixing with superphosphate of lime. Professor Storer has
advocated a more general use of fish as a manure than is at present the
case. He suggests that even fish not suitable for edible purposes might
be caught for the purpose of conversion into manure. The difficulty of
preserving fish, however, is considerable; and he suggests the use of
potash salts, such as muriate of potash, or lime for this purpose. The
benefit of using potash would be twofold. In addition to acting as a
preservative, it would considerably enhance the value of the resulting
guano as a manure. There is much truth in Professor Storer's views; and
no doubt, as our sources of artificial nitrogenous manures grow more
limited, the manufacture of fish-guano will be carried on in the future
on a larger and more systematic scale than hitherto.


_Meat-meal Guano._

What is called "meat-meal guano" is generally that made from the refuse
of the carcasses of cattle after they have been treated for their
meat-extract according to Liebig's process. The meat-meal is used both
for feeding and manurial purposes. Considerable quantities[199] of this
guano are imported annually into this country from South America,
Queensland, and New Zealand,--that coming from Frey Bentos, in Uruguay,
being best known. It is a valuable manure, especially so for its
nitrogen, which varies from 4 to 8 per cent, while it contains of
phosphoric acid from 13 to 20 per cent. Some meat-meal guanos contain as
much nitrogen as 11 per cent.

In some parts of the world, more especially in Germany, the carcasses of
horses, as well as cattle, dogs, pigs, &c., which have died of disease,
are converted into a guano. They are subjected to treatment by steam in
digestors, by which means the fat and gelatine are separated and
utilised, while the remaining portion of the animal is converted into
guano. Other processes are also employed. The resulting manure contains
from 6 to 10 per cent of nitrogen, and from 6 to 14 per cent of
phosphoric acid.


_Value of Meat-meal Guano._

Meat-meal guano is a valuable nitrogenous manure. The same remarks
apply to it as to fish-guano, although it ferments probably very much
more quickly than the latter, and is undoubtedly a more valuable manure.


_Bat Guano._

In conclusion, we may consider bat guano. Bat guano, which is really a
very rare curiosity, has been found accumulated in hot climates in
caves.

The samples which have been analysed have differed very much in quality,
some containing as much as 9 per cent of nitrogen and 25 per cent of
phosphoric acid. Provided it could be obtained in any quantity, and of a
quality even approximating to the above analysis, it need scarcely be
pointed out that bat guano would be a most valuable manure.

A singular point about its composition is, that it has been found to
contain a considerable proportion of its nitrogen (as much as 3 per
cent) in the form of nitrates.


_Pigeon and Fowl Dung._

Pigeon dung is a manure which historically is of great importance. The
dung of pigeons was used as a manure by the ancient Romans; and even in
modern times, more especially in France, it was considered a most
important fertiliser. Despite these facts, pigeon dung is by no means a
rich manure, and its composition compares most unfavourably with that of
the guanos we have just been considering. According to Storer,[200] it
only contains from 1-1/4 to 2-1/2 per cent of nitrogen, and from 1-1/2
to 2 per cent of phosphoric acid, and a little over 1 per cent of
potash.

The dung of poultry is just about as poor, fowl dung containing from .8
to 2 per cent of nitrogen, 1-1/2 to 2 per cent of phosphoric acid, and a
little under 1 per cent of potash; while that of ducks and geese is even
poorer.[201]

From these statements it will be seen that the excrements of pigeons,
hens, and ducks do not form a rich manure. One thing about pigeon dung
which is to be noticed, is the fact that it ferments very quickly.

None of the pseudo-guanos, however rich they may be in manurial
ingredients, can be regarded as equal in their action to the genuine
article, for reasons which we have gone into already when considering
the action of guano.


FOOTNOTES:

[183] Bones, it is true, were in use long before guano; but popular as
they deservedly were, they had not been used, at the time of the
importation of guano, to any very considerable extent.

[184] The total annual imports at present may be taken at under 30,000
tons, whereas in 1855 they amounted to over 200,000 tons. For statistics
on this point the reader is referred to the Appendix, Note I., p. 327.

[185] With regard to the origin of certain guano deposits, which are of
very recent date--_e.g._, _Angamos_ and _Ichaboe_--there can be no doubt
whatever, because we can witness the process of formation still taking
place. It is not so, however, with regard to older deposits, for which
some have been inclined to claim mineral origin. The best proof that
such deposits owe their origin mainly to bird excrements is the
comparatively large quantity of _uric acid_ they contain. On the other
hand, the evidence in support of the belief that they are also formed
from the remains of the birds themselves and other animals, is to be
found in the large proportion of phosphates they contain, and the
presence in the deposits of feathers and the fossilised skeletons of the
animals above mentioned.

[186] A complete list of the various deposits will be found in the
Appendix, Note II., p. 327. It may be noticed that nearly all the
deposits lie within 10° to 20° north and south of the Equator.

[187] See Chapter on Farmyard Manure, p. 257.

[188] According to Nesbit, some of the cargoes of this guano contained
hard saline lumps of very little manurial value--over 50 per cent being
common salt.

[189] The salt exports were made in 1868.

[190] For analyses of these nodules and crystals, see Appendix, Note
III., p. 328.

[191] See Heiden, vol. ii. p. 356.

[192] See Appendix, Note IV., p. 329.

[193] The Ichaboe guano at present exported is a fresh deposit, and is
annually collected for shipment.

[194] Further chemical changes have occurred in certain cases between
the guano and the limestone rock beneath, resulting in the formation of
what is called a "crust" guano. Such guanos form a soft phosphatic rock,
and are extremely rich in phosphates. As examples of these "crust"
guanos may be mentioned Sombrero, Curaçao, Aruba, Mexico, and Navassa
phosphates.

[195] The presence in the old Peruvian guano of concretionary nodules
has already been referred to.

[196] According to Vogel the nitrogen as urates is converted by the
sulphuric acid into ammonia salts.

[197] See Appendix, Note VI. p. 330.

[198] It must be remembered, however, that even genuine guano contains a
certain quantity of carbonate of lime, and will give a slight amount of
effervescence when so treated.

[199] The annual imports may be stated at from 3000 to 4000 tons.

[200] Agricultural Chemistry, vol. i. p. 367.

[201] See Appendix, Note VII., p. 331.



APPENDIX TO CHAPTER VIII.


NOTE I. (p. 297).

PERUVIAN GUANO IMPORTED INTO THE UNITED KINGDOM, 1865-1893.

Year.          Tons.   |  Year.          Tons.
1865          213,024  |  1881           33,393
1870          247,028  |  1882           27,382
1871          144,735  |  1883           36,713
1872           74,964  |  1884           15,802
1873          135,895  |  1885              -
1874           94,346  |  1886           28,733
1875           86,042  |  1887            5,784
1876          158,674  |  1888           16,446
1877          111,835  |  1889           17,000
1878          127,813  |  1890           19,000
1879           45,475  |  1891           11,000
1880           58,631  |  1892           14,000


NOTE II. (p. 298).

GUANO DEPOSITS OF THE WORLD.

SOUTH AMERICA--

_Peru._--In various islands off the coast--viz., Chincha, Guanape,
Ballestas, Macabi, Lobos, and Patillos; and on different parts of the
coast--viz., Pabellon de Pica, Chipana, Huanillos, Punta de Patillos,
Independence Bay, and Lobos de Afuera.

_Columbia._--In different parts of the States of Venezuela, New Granada,
and Ecuador. Guano coming from these parts is often known as Columbian
guano, or according to the name of the State in which it is found.
Maracaïbo and Monks guanos come from the coast of Venezuela. Deposits
are also found on the Galapagos Islands, to the west of Ecuador.

_Bolivia._--Mejillones, Patagonia, Leon's.

NORTH AMERICA--Deposits have been found on the coasts of Mexico and
California; on the Raza and Patos Islands; and on the coasts of
Labrador. They have also been found on the Islands of Curaçao, Aruba,
and Navassa in the Gulf of Mexico.

AFRICA--On the west coast deposits have been found at Algoa
Bay, Saldanha Bay, and on the Island of Ichaboe.

AUSTRALIA--Shark's Bay and Swan Island.

WEST INDIES--Sombrero, Aves, and Cuba.

PACIFIC OCEAN--On the Islands of Baker, Jarvis, Howland,
Malden, Starbuck, Fanning, Enderbury, Lacepede, Browse, Huon, and
Surprise.

ASIA--Deposits at Kuria Muria on the Arabian coast, and on the
Sandwich Islands. (See Heiden's 'Düngerlehre,' vol. ii. p. 349.)


NOTE III. (p. 303).

COMPOSITION OF CONCRETIONARY NODULES.

(_Analyses by Karmrodt._)

No. 1.

Potassium sulphate                    7.49
     "    phosphate                   9.52
Sodium        "                       9.08
Ammonium      "                       7.57
Calcium sulphate                      3.40
Ammonium urate                        4.09
    "    oxalate                     41.28
Nitrogenous organic matter           10.17
Water                                 7.40
                                     -----
                                    100.00
                                    ------
          Nitrogen  14.84

No. 2.

Potassium sulphate                   45.64
Sodium       "                       13.22
Ammonium     "                       10.23
   "       oxalate                    9.14
Basic ammonium phosphate             12.09
Precipitated ammonium phosphate       4.78
Organic matter                         .94
  Insoluble                           1.90
  Water                               2.06
                                    ------
                                    100.00
                                    ------


NOTE IV. (p. 306).

The following analyses, being the average of a large number of different
samples analysed from time to time in the chemical laboratory of the
Pommritz Agricultural Experimental Station, show the gradual
deterioration of Peruvian guano, as regards its percentage of nitrogen,
during the years 1867-81:--

            Nitrogen.                  Nitrogen.
1867          13.16   |   1873           9.16
1868          11.98   |   1874           9.83
1869          13.66   |   1878           7.10
1870          12.37   |   1879           6.95
1871          10.04   |   1880           7.07
1872          10.72   |   1881           6.93


NOTE V. (p. 309).

COMPOSITION OF DIFFERENT GUANOS.

The following is a list of the more common nitrogenous and phosphatic
guanos which have been used in the past or are at present in use. Those
printed in italics are still being worked. As their value depends on
their nitrogen and phosphoric acid, these alone have been given. The
percentages must be taken as mere approximations, as the quality of
different cargoes from the same deposits varies very much. The table may
be found useful for reference.

_Nitrogenous Guanos._
                                            Phosphoric }   { Tricalcic
                   Nitrogen  =  Ammonia.       acid    } = { phosphate.
                   per cent.    per cent.    per cent.       per cent.
Angamos               20           24            5              11
Chincha               14           17           13              28
Ballestas             12           15           12              26
Egyptian              11           13           19              41
Guanape               11           13           -               -
Macabi                11           13           12              26
Corcovado             11           13           15              33
_Saldanha Bay_           9         11            9              20
_Ichaboe_                8         10            9              20
Independence Bay       7            9           12              26
_Pabellon de Pica_       7          9           14              31
_Punta de Lobos_         4          5           15              33
_Huanillos_              6          7           18              28
Penguin                5            6           11              24
Patagonian             4            5           18              39
Falkland Islands       4            5           14              31

_Phosphatic Guanos._

                                            Phosphoric }   { Tricalcic
                                               acid    } = { phosphate.
                                             per cent.       per cent.
Maracaïbo, or Monks                             42              92
Raza Island                                     40              87
Curaçao                                         40              87
_Baker Island_                                  39                85
Starbuck                                        38              83
_Enderbury_                                     37                81
Californian                                     35              76
_Aves_                                          34                74
Fanning Island                                  34              74
Howland                                         34              74
_Sidney Island_                                 34                74
Mejillones                                      33              72
Lacepede Island                                 33              72
_Malden Island_                                 32                70
Sombrero                                        32              70
_Browse Island_                                 31                68
_Huon Island_                                   28                61
Patos Island                                    24              52
Jarvis Island                                   20              44
Cape Vert                                       11              24


NOTE VI. (p. 314).

It may be of interest to refer to a theory put forward by Liebig as to
the action of oxalic acid in guano. This, he considered, had the effect
of gradually rendering the insoluble calcium phosphate soluble, and
giving rise to the formation of ammonium phosphate and calcium oxalate.
Such an action would probably take place were the guano allowed to
ferment by itself. We know, however, that when it is brought in contact
with the soil-particles, all its soluble phosphate is converted into
precipitated phosphate.


NOTE VII. (p. 326).

ANALYSES OF DUNG OF FOWLS, PIGEONS, DUCKS, AND GEESE. (Storer's
'Agricultural Chemistry,' vol. i. p. 367.)

                     Fowls.     Pigeons.    Ducks.    Geese.
Water                  56.00       52.00    56.60     77.10
Organic matter         25.50       31.00    26.20     13.40
Nitrogen                1.60        1.75     1.00       .55
Phosphoric acid     1.5-2.00    1.5-2.00     1.40       .54
Potash               .80-.90    1.0-1.25      .62       .95
Lime               2.00-2.50    1.5-2.00     1.70       .84
Magnesia                 .75         .50      .35       .20

According to a computation by a Belgian farmer, a pigeon yields about 6
lb. of dung in a year, a hen about 12 lb., a turkey or goose about 25
lb., and a duck 18 lb.



CHAPTER IX.

NITRATE OF SODA.


Nitrate of soda,[202] or, as it is more correctly designated from a
chemical point of view, sodium nitrate, now forms the chief artificial
nitrogenous manure in use. Along with sulphate of ammonia, it has taken
the place once held in the manure markets by the older Peruvian guano,
and may without doubt be reckoned, at present prices, one of the
cheapest and most valuable of the artificial sources of nitrogen for the
plant. It is some sixty-two years ago since it was first exported from
South America into this country. The total exports in that year amounted
to about 800 tons, and some indication of the enormous extent to which
the use of this valuable fertiliser has been developed since then will
be obtained from the statement that the total exports at present amount
to little less than 1,000,000 tons per annum, representing a monetary
value of 6 to 7 millions sterling. Of this quantity about 120,000 tons
are imported into Britain.[203] While its chief use is for manurial
purposes, it must not be imagined that it is only used for this purpose.
A certain amount is used in connection with various chemical
manufactures--for instance, that of nitric and sulphuric acid--and also
in the manufacture of saltpetre, the chief constituent of gunpowder.


_Date of Discovery of Nitrate Deposits._

The exact date of the discovery of the nitrate deposits seems to be a
point of considerable dubiety. The earliest published description of
them was written by Bollaert about the year 1820, in which year, it is
stated, the first shipment was made to England. It was not, however,
till some ten or twelve years later that the Peruvian Government, to
whom they then belonged,[204] seems to have recognised their value. The
most important deposits are found in the vicinity of the town of
Iquique, which is the chief nitrate port of South America. It is a
somewhat striking fact that this substance, which has conclusively
proved itself to be the most potent of all known artificial agents in
the promotion of vegetable growth, should be found in a district utterly
lacking the slightest traces of vegetation of any kind. Lest such a
statement should seem to savour of irony, we hasten to explain that the
singular barrenness of this part of the country is largely due to the
character of its climate, the deposits occurring in the midst of sandy
deserts,[205] on which rain never falls.


_Their Origin._

The origin of these nitrate-fields is a geological problem of very
considerable interest, the difficulty of which is greatly enhanced by
their altitude--3000 to 4000 feet above the sea-level--and their
distance inland, which amounts in some cases to eighty or ninety miles
from the sea-coast. The nitrate deposits are not the only saline
deposits found in Chili. According to the late David Forbes,[206] they
are not to be confused with other saline formations, which appear at
intervals scattered over the whole of that portion of the western coast,
on which no rain falls. The latter stretch from north to south for a
distance of more than 550 miles--their greatest development being
between latitudes 19° and 25° south. The depth to which they extend
downwards varies considerably. Most of them, however, are of a very
superficial character, and "they always show signs of their existence by
the saline efflorescence seen on the surface of the ground, which often
covers vast plains as a white crystalline incrustation, the dust from
which, entering the nostrils and mouth of the traveller, causes much
annoyance, whilst at the same time the eyes are equally suffering from
the intensely brilliant reflection of the rays of a tropical sun." These
saline incrustations, or _salinas_, as they are generally called, are
chiefly composed of salts of lime, soda, magnesia, alumina, and of
boracic acid. Their composition would lead one to attribute their origin
to the evaporation of salt water; for, with the single exception of
boracic acid,[207] all the mineral substances are such as would be
obtained by the evaporation of sea-water, or by the mutual reactions of
its salts with the constituents of the adjacent rocks. As there is
"indisputable evidence of the recent elevation of the whole of this
coast," volcanic upheaval might be reasonably held to explain their
altitude. Their comparative proximity to the coast would seem further to
favour this theory. On these grounds, therefore, Forbes is inclined to
think that they owe their origin to the evaporation, under the influence
of a tropical sun, of lagoons of salt water, the communication of which
with the sea had been cut off by the rising of the land.


_Forbes and Darwin on the Theory of their Origin._

The obvious difficulty of accounting for the formation of the larger
deposits by such a theory he meets by saying that it is only necessary
to suppose that, even after the partial isolation of the lagoons by the
elevations of the coast, they might still have maintained tidal or
occasional communication with the sea by means of lateral openings in
the chain of hills separating them from the ocean. In such cases there
would be a gradual accumulation of salts, very much greater in amount
than that due simply to the evaporation of the water originally
contained in the lagoons. The above theory of the origin of the lower
saline deposits may go to explain the mode of formation of the
nitrate-fields; but in this case several difficulties present
themselves. One is the much greater altitude of the latter, as well as
their greater distance inland. This difficulty, however, may be met by
assuming that they are of older origin than the lower deposits, and have
been subjected to a correspondingly greater amount of volcanic upheaval.
There is abundance of proof that this part of the continent has been the
scene in the past of such volcanic upheaval. Forbes is of opinion that
there is the fullest evidence to prove that, even since the arrival of
the Spaniards, a very considerable elevation of the land has taken place
over the greater part, if not the whole extent, of the line of coast;
while Darwin states that he has convincing proof that this part of the
continent has been elevated from 400 to 1200 feet since the epoch of
existing shells. Furthermore, elevations of the coast-line, amounting in
many cases to several feet, are known to have happened within recent
times, while earthquakes and volcanic disturbances of a less striking
nature are still of common occurrence. Successive lines, indicative of
old sea-beaches, can be distinctly traced stretching inland, one behind
the other; and patches of sea-sand and water-worn stone, found at a
great distance from the coast, both in valleys and at altitudes much
greater even than 4000 feet, point to the same conclusion.[208] The
difficulty, therefore, of altitude and distance from the coast cannot be
regarded as insuperable.


_Source of Nitric Acid._

A difficulty, however, which is not so easily met, is afforded by the
presence of the nitric acid which, in combination with the soda, forms
the nitrate of soda. It is scarcely necessary to inform our readers that
nitrogen--except, of course, in small quantities in the free state--is
not a normal constituent of salt water. The question, therefore, of
greatest interest in connection with the formation of these nitrate-beds
is, Whence has the nitric acid been derived? Several theories have been
put forward to account for it.


_Guano Theory._

One is to the effect that it owes its origin to huge guano deposits,
originally covering the shores of the large salt lakes which, by the
subsequent overflowing of their shores, effected the mixture of the
guano with the salts. In this way, by a slow process of decomposition,
nitrate of soda would be ultimately formed.[209] This theory, apart from
other considerations, seems at first sight extremely plausible, more
especially when we remember that it is on this very coast that the
greatest guano deposits have been found, and that the famous Chincha
Islands, which alone have yielded over 10 million tons of this valuable
fertiliser, are comparatively near the scene of the nitrate deposits.
What seems further to support this theory, is the actual occurrence in
the nitrate-fields themselves of small quantities of guano. But however
plausible it may appear at first sight, it does not bear closer
criticism. One very serious objection is the absence in these deposits
of phosphate of lime, which is the largest constituent of guano. If they
were really due to guano, how does it happen that the insoluble
phosphate of lime should have disappeared, while the easily soluble
nitrate of soda should alone be preserved? Again, assuming this theory
to be correct, we should naturally expect to be still able to find
evidence of the chemical changes which would under such circumstances
have taken place, in the shape of portions of the guano in the
transition stage. Such evidence, however, the most careful
investigations have failed to detect. Apart, however, from the above
objections, there seems to be little doubt, from evidence afforded by
traces of birds' nests, &c., that the guano found in the nitrate-beds
was deposited subsequent to the formation of the nitrate of soda.


_Nitric Acid derived from Sea-weed._

The most probable theory seems to be that put forward by Nöllner. The
origin of the nitric acid is, according to him, to be ascribed to the
decay of great masses of sea-weed, which, by means of hurricanes such as
are still prevalent in these districts, were driven into the lagoons.
The chief difficulty in the way of accepting this theory is the enormous
quantity of sea-weed required to produce the millions of tons of nitric
acid these deposits contain. It must be remembered, however, as bearing
upon this point, that the occurrence of gigantic masses of sea-weed in
the Pacific Ocean[210] is by no means uncommon even at the present time.
If, to understand the formation of coal, we must suppose the
Carboniferous period to be one during which exceptionally luxuriant
growth of vegetation took place, we may be permitted to suppose a
similar luxuriant growth of sea-weed during the formation of the nitrate
deposits. Very strong confirmation of the truth of this theory is
further afforded by the presence in large quantities, in the raw nitrate
of soda, of iodine, a substance characteristic of sea-weed; while pieces
of sea-weed still undecomposed are met with here and there. On the
whole, therefore, this theory, while not free from difficulties, seems
to be the most worthy of acceptance as regards the origin of the nitrate
deposits.[211]


_Appearance of Nitrate-fields._

Having thus discussed the origin of the nitrate-fields, we may now give
a more detailed description of their appearance. The chief deposits at
present being worked are those lying in the Pampa de Tamarugal, in the
province of Tarapaca. They stretch to a distance of thirty or forty
miles inland, from Pisagua southwards to somewhat beyond the town of
Iquique. This huge desert, as has been already indicated, seems to be
entirely destitute of all vegetation and animal life. Even in the
immediately adjoining country the only kind of vegetation that seems to
grow is a species of _acacia_. The few streams that are found in this
neighbourhood are entirely fed by the melting snow from the Cordilleras.
Darwin describes the appearance presented by these pampas as resembling
"a country after snow, before the last dirty patches are thawed." The
_caliche_, or raw nitrate of soda, is not equally distributed over the
pampas. The most abundant deposits are situated on the slopes of the
hills which probably formed the shores of the old lagoons. An expert can
tell from the external appearance of the ground where the richest
deposits are likely to be found. The _caliche_ itself is not found on
the surface of the plain, but is covered up by two layers. The
uppermost, known technically as _chuca_, is of a friable nature, and
consists of sand and gypsum; while the lower, the _costra_, is a rocky
conglomerate of clay, gravel, and fragments of felspar. The _caliche_
varies in thickness from a few inches to 10 or 12 feet, and rests on a
soft stratum of earth called _cova_.


_The Method of mining the Nitrate._

The mode in which the _caliche_ is excavated is as follows: A hole is
bored through the _chuca_, _costra_, and _caliche_ layers till the
_cova_ or soft earth is reached below. It is then enlarged until it is
wide enough to admit of a small boy being let down, who scrapes away the
earth below the _caliche_ so as to form a little hollow cup. Into this a
charge of gunpowder is introduced, and subsequently exploded. The
_caliche_ is then separated by means of picks from the overlying
_costra_ and carried to the refinery.


_Composition of Caliche._

Both in appearance and composition it varies very much. In colour it may
be snow-white, sulphur, lemon, orange, violet, blue, and sometimes brown
like raw sugar.

The _caliche_ found in the Pampa de Tamarugal contains generally about
30 to 50 per cent pure nitrate of soda; that in the province of Atacama
contains from 25 to 40 per cent. The subsequent refining processes,
which consist in crushing it by means of rollers and then dissolving it,
need not here be described. It may be sufficient to mention that the
process used is that known as systematic lixiviation, and is analogous
to the method introduced by Shanks in the manufacture of soda. The chief
impurity in the raw material is common salt: gypsum, sulphates of
potassium, sodium, and magnesium, along with insoluble matters, are the
other impurities. The manufacture of iodine, which, as has been already
noticed, is found in the nitrate-beds, is also carried on at these
_oficinas_.


_Extent of the Nitrate Deposits._

The question of the extent of the nitrate of soda deposits is naturally
one of very great interest, especially from the agricultural point of
view. M. Charles Legrange, a French writer, estimated a few years ago
that they still contained about 100,000,000 tons of pure nitrate of
soda. Opinions on this point differ very considerably, and it seems
wellnigh impossible to arrive at any very accurate estimate.

The number of years they will last will depend, of course, on the amount
of annual exportation. This, at present, falls little short of 1,000,000
tons. If this amount is maintained, they should last, according to
experts, some twenty or thirty years at least. A consideration which has
an important influence on this question, is the price obtained for the
article. If this should be increased, it may be possible to treat the
larger quantities of the inferior raw material (which at present prices
are allowed to accumulate) at a profit. Undoubtedly this is what will
ultimately take place, when the richer quality of the _caliche_ has been
exhausted.


_Composition and Properties of Nitrate of Soda._

As has already been pointed out, commercial nitrate of soda contains
about 95 per cent of pure nitrate of soda, or about 15-1/2 per cent of
nitrogen, which, if calculated as ammonia, would equal 19 per cent. It
is, next to sulphate of ammonia (which contains 24-1/2 per cent of
ammonia), the most concentrated nitrogenous manure, and further,
contains its nitrogen in the form most readily available for the plant's
use. Its most characteristic property is its great solubility, and
consequent speedy diffusion in the soil, and the inability of the
soil-particles to fix its nitrogen. In the latter respect it differs
very considerably from other forms of nitrogen. Ammonia salts, though
practically quite as soluble, do not diffuse in the soil so rapidly as
nitrate of soda does; for the ammonia is more or less tenaciously fixed
by the soil-particles, and retained till converted by the process of
_nitrification_ into nitrates.


_Nitrate of Soda applied as a Top-dressing._

On this account nitrate of soda is chiefly employed--and rightly so--as
a top-dressing. The risk of loss by drainage is thus minimised, and the
valuable nitrogen finds its rightful destination--viz., in the plant's
roots.


_Encourages deep Roots._

A special benefit which the diffusibility of nitrate of soda has been
held to confer on the plant, is to encourage the growth of deep roots,
by inducing the growing plant to send down its roots into the lower
layers of the soil after the nitrate of soda. The benefit of deep roots
is, of course, very great. They enable the plant to withstand the action
of drought, and at the same time increase the area whence the plant may
derive its nourishment. Although the value of the manure is practically
entirely due to the nitrogen it contains, it has been urged that the
soda exercises a beneficial effect on the mechanical properties of the
soil, by increasing its power of absorbing moisture, and in also
rendering it more compact. This would partly explain how its results in
dry seasons are so much better than those obtained from sulphate of
ammonia. This mechanical action of nitrate can scarcely be very great
when we remember the comparatively small quantity applied. Even in the
driest of seasons there will always be sufficient moisture to secure the
diffusion of the nitrate of soda, while the risk of loss by drainage
will be reduced to a minimum. Much ignorance, as well as prejudice, has
existed in the past as to the true nature of the action of nitrate of
soda. Nor is this prejudice even yet entirely dispelled.


_Is Nitrate an exhausting Manure?_

The common charge brought against it is, that it is what has been termed
an exhausting manure. This objection, to have any weight, must mean that
nitrate of soda produces a crop which takes out of the soil an
_abnormal_ quantity of fertilising matter. But, so far as the writer is
aware, no scientific evidence has ever been brought forward to support
this contention. That the indiscriminate use of a manure may produce a
crop in which the stem and leaves are unduly developed at the expense of
the grain, or in which the quality of the crop may suffer from too rapid
growth, is, of course, a well-known fact. But as this could also be
produced by an overdose of soluble phosphoric acid as well as ammonia
salts, it is not a property that belongs exclusively to nitrate of soda.
Probably nitrate of soda has in the past been often used in this
indiscriminate way so as to produce such results. The fault, therefore,
lies not in the manure, but in the mode of its application. A few
remarks, therefore, on this most important subject may prove
serviceable.


_Crops for which it is suited._

Opinions will naturally differ as to the crops to which it is profitable
to apply nitrate of soda. Its value as a manure for cereals is pretty
generally admitted. Its value as a manure for roots is not, however, so
universally admitted. Experiments would seem to show that such a crop as
the mangold derives just as much benefit as do the cereals; while in
Germany practical experience on a very large scale has demonstrated its
value as a manure for beetroots. It may be generally recommended as a
manure for all crops, except, perhaps, the so-called leguminous crops,
such as clover, beans, peas, &c, whose ability to obtain nitrogen for
themselves renders the application of expensive artificial nitrogenous
manures unadvisable.

An interesting point with regard to nitrate of soda is the curious
effect it seems to have on the colour of the leaves of plants. This
interesting fact has been strikingly demonstrated at the Rothamsted
Experimental Station, in the contrast in the colour of the leaves of
different experimental grass-plots, manured with nitrate of soda and
sulphate of ammonia respectively--the plots manured with nitrate of soda
being distinctly darker in hue, obviously owing to the greater
production of chlorophyll or green matter. Such a depth of colour would
seem to indicate a more healthy development.


_Method of Application._

While opinions, therefore, will naturally differ as to the crops to
which nitrate of soda will be most profitably applied, little difference
of opinion exists as to the method of its application. The inability of
the soil-particles to retain it, the frequency of rain, the costly
nature of the manure itself, and its immediate availability as a
plant-food, all point to the extreme advisability of using it as a
top-dressing. Even when used as a top-dressing, it may be advisable not
to apply the entire quantity all at one time. By applying it in
instalments, little risk is run that, through inclemency of weather, the
manure will be lost. Another point of importance in applying nitrate of
soda is to secure uniform distribution. This of course is applicable to
all artificial manures, but in a very special degree to nitrate of soda,
because of its great value and the comparatively small quantity
applied.

As the uniform distribution of one cwt. of any material over an acre of
soil is by no means an easy task, the mixing of nitrate of soda with
some diluent, such as dry loam, is consequently highly advisable. Common
salt is often applied along with nitrate of soda. The indirect value of
salt as a manure is considerable, and when applied along with nitrate,
ensures its more speedy diffusion in the soil, by increasing the soil's
capacity for absorbing moisture from the air.


_Must be a Sufficiency of other Fertilising Constituents._

A third point of importance in applying nitrate of soda, is to see that
the soil is sufficiently supplied with the other plant-foods--phosphates
and potash. This is a _sine qua non_, if the nitrate is to get a fair
chance. If it is desired to apply nitrate of soda along with
superphosphate of lime, a word of caution is necessary against making
the mixture long before it is used. The reason of this is, that a
chemical action is apt to ensue, resulting in the loss of the nitric
acid in the nitrate of soda. The nature of the soil is another important
consideration to be taken into account. In the case of extremely loose
and sandy soils, it is scarcely to be recommended as the most suitable
form in which to apply nitrogen. If applied to such soils, especial care
ought to be taken to minimise risk of loss. No hard-and-fast rules can
be laid down as to the quantity in which it ought to be applied. This
must be regulated very much by the crop, the nature of the soil, and
the quantity of other manures employed. From 1 to 1-1/4 cwt. may be
recommended as a suitable quantity for corn crops which are otherwise
liberally manured. On strong clay soils this quantity may be judiciously
increased up to 2 cwt. Dr Bernard Dyer, who has experimented largely on
its use as a manure for mangolds, is of opinion that an application of
from 3 to 4 cwt. an acre is likely to prove thoroughly profitable; and
the present writer has found in his experiments with turnips that a
top-dressing of 1 cwt. amply repaid itself.


_Conclusions drawn._

In conclusion, the nature and characteristics of nitrate of soda as a
manure may be briefly summed up as follows:--

1. It is a whitish, crystalline salt, extremely soluble, and is quickly
diffused in the soil. It should contain 95 per cent of pure nitrate of
soda--_i.e._, 15-1/2 per cent of nitrogen, equal to about 19 per cent of
ammonia.

2. Next to sulphate of ammonia, it is the most concentrated nitrogenous
manure; the relative quantities of nitrogen these two manures contain
being as three is to four.

3. It contains its nitrogen in the most valuable and readily assimilable
form--_i.e._, as _nitric acid_, the form into which all other forms of
nitrogen have first to be converted before they become available for
the plant's uses.

4. That, at present market prices, nitrate of soda may be safely
affirmed to be the cheapest form of nitrogenous manure.

5. That nitrate of soda, in addition to its direct value as a manure,
probably exercises a slight influence on the mechanical properties of
the soil, by increasing its compactness and water-absorbing capacities;
that it further tends to promote deep roots, and thus to increase the
soil area whence the plant may derive its nourishment, at the same time
rendering the plant more able to withstand the injurious influence of
drought.

6. That a plentiful supply of the other manurial constituents should be
present in the soil, if nitrate of soda is to exercise its full value.

7. That it may be profitably applied in the case of nearly all kinds of
crops, but that great care should be taken as to the mode of its
application. That this should be almost invariably as a top-dressing,
and that it should be applied in several doses if possible.

8. That its effects can be regarded as lasting only during the first
year after application.


FOOTNOTES:

[202] This substance is also largely known under the name Chili
saltpetre, to distinguish it from potassium nitrate or common saltpetre.

[203] See Appendix, p. 351.

[204] We may remind our readers that these nitrate deposits were largely
the cause of the late war between Chili and Peru, which resulted in the
cession to Chili by Peru of the province of Tarapaca, where the most
important deposits are situated.

[205] The other nitrate deposits are found in the provinces of
Antofagasta and Atacama, and a certain amount of the refined article is
exported from these places. The amount, however, is inconsiderable as
compared with that which comes from the province of Tarapaca.

[206] See his elaborate article on the Geology of Bolivia and Peru,
published in the 'Quarterly Journal of the Geological Society' for
November 1860.

[207] The source of the boracic acid is probably volcanic.

[208] A friend of the present writer, who has visited this part of the
west coast of South America, informs him that at one point of the coast
at Mejillones (in Bolivia) he could trace the remains of no fewer than
twelve distinct sea-beaches, situated at different distances from the
sea, and rising to an altitude of 2500 feet.

[209] In this change, lime derived from the sea-shells would play an
important part. Modern researches have shown, as we have already said in
a previous chapter, that, in the conversion of organic nitrogen into
nitrates, the presence of carbonate of lime is a necessary condition.

[210] The Gulf weed is an instance in point. Huge masses of floating
sea-weed are sometimes found, 500 to 600 miles in length, forming the
so-called Saragossa Sea.

[211] A difficulty which has not been referred to is the belief
entertained by geologists that "there has been a change of climate in
Northern Chili, and that there must have been more rain there formerly
than there is at present. Traces of human habitations are found high up
in the Cordilleras to-day. Cobs of Indian corn, axes and knives of
copper tempered to exceeding sharpness, arrow-heads of agate, even
pieces of cloth, are dug up in arid plains now without any trace of
water for many leagues in or around them" (Russell, 'The Nitrate-Fields
of Chili,' p. 290).



APPENDIX TO CHAPTER IX.


NITRATE OF SODA.

_Total Shipments from South America, 1830-1892._

Year.    Tons.  |  Year.    Tons.   | Year.      Tons.
1830       800  |  1870    131,400  | 1886      437,500
1835     6,200  |  1875    321,000  | 1887      680,600
1840    10,100  |  1880    217,300  | 1888      745,700
1845    16,800  |  1881    344,600  | 1889      930,000
1850    22,800  |  1882    477,800  | 1890    1,030,000
1855    41,800  |  1883    572,400  | 1891      790,000
1860    55,200  |  1884    540,900  | 1892      790,000
1865   109,000  |  1885    423,100  |

The following tables exhibit the total imports into Europe, and into the
United Kingdom from the years 1873-92:--

NITRATE OF SODA, 1873-1892.

_Imports into Europe._         _Imports into United Kingdom._

Year.    Tons.            |          Year.    Tons.
1873     225,000          |          1873     124,000
1874     230,000          |          1874     108,200
1875     280,000          |          1875     164,900
1876     300,000          |          1876     166,800
1877     208,000          |          1877      69,600
1878     250,000          |          1878     104,400
1879     205,000          |          1879      55,300
1880     140,000          |          1880      48,300
1881     230,000          |          1881      54,800
1882     335,000          |          1882      96,000
1883     440,000          |          1883     103,700
1884     505,000          |          1884     103,700
1885     380,000          |          1885     109,400
1886     330,000          |          1886      75,100
1887     440,000          |          1887      83,100
1888     640,000          |          1888     103,100
1889     760,000          |          1889     120,000
1890     784,000          |          1890     114,000
1891     851,000          |          1891     121,000
1892     795,000          |          1892     115,000



CHAPTER X.

SULPHATE OF AMMONIA.

_Value of Ammonia as a Manure._


The value of ammonia salts as a manure has been long recognised; indeed
till recently ammonia was thought to be the most valuable form in which
nitrogen could be applied as a plant-food--a view, we may mention, held
by Liebig. While the plant, no doubt, can absorb its nitrogen in the
form of ammonia,[212] as well as in other forms, as we have already
pointed out in previous chapters, it is now fully recognised that
ammonia salts, when applied to the soil, are converted into nitrates.
Nitric acid, then, must be regarded as the most valuable, inasmuch as it
is the most rapidly assimilated form of nitrogen for the plant; but
next to nitric acid in value comes ammonia. Of the different forms of
ammonia available for manurial purposes, the only one used to a large
extent is sulphate.


_Sources of Sulphate of Ammonia._

The oldest, and what is still the chief source of this valuable salt, is
the gas-works, where it is obtained as one of the bye-products in the
manufacture of gas. It is also obtained to a lesser extent from shale,
iron, coke, and carbonising works. Bones, horn, leather, and certain
other animal substances rich in nitrogen, when subjected to dry
distillation, as is the case in certain manufactures, such as the
manufacture of bone-charcoal for use in sugar-refineries, and the
distillation of horn, &c., in the manufacture of prussiate of potash,
also constitute less abundant sources.


_Ammonia from Gas-works._

Coal contains on an average from a half to one and a half per cent of
nitrogen. When it is subjected to dry distillation, as is done in the
gas-works, the nitrogen which it contains is chiefly converted into
ammonia, and, in the process of purification of the gas, is removed in
the "gas-liquor,"[213] which contains about one per cent of ammonia.
The ammonia recovered from this liquor by distillation is then absorbed
in sulphuric acid. It may be pointed out that nothing like all the
nitrogen contained in the coal is recovered as sulphate of ammonia. It
has been calculated that only from a fifth to a tenth is actually
recovered, and many processes have been patented with a view to
increasing the yield of ammonia in gas manufacture. The total production
of ammonia from gas-works may be placed at little over 100,000 tons per
annum for Great Britain. Mr L. Mond, F.R.S., recently drew attention to
the possibility of largely increasing our supply of sulphate of ammonia
from coal. As indicating what an enormous source of sulphate of ammonia
we have in coal, Mr Mond calculated that its annual consumption in this
country (estimated at 150,000,000 tons) would yield as much as 5,000,000
tons of sulphate of ammonia.


_Other Sources._

While the ammonia produced in the manufacture of gas has long been
collected, it is only of recent years that the other sources of ammonia
have been developed. Next to the gas-works, the shale-works of Scotland
form in this country the chief source of this valuable manure. In these
works the ammonia is obtained in distilling the paraffin shale by a
method somewhat similar to that in use in the gas-works. The amount of
sulphate of ammonia obtained from this source is between 20,000 and
30,000 tons per annum. Recently the ammonia has been recovered from the
blast-furnace gases in iron-works--some 6000 tons being annually
obtained in this way; while from coke and carbonising works the annual
production is about half that amount. The combined annual production
from all these sources may be put down at 140,000 tons, the total
production in Europe being probably little more than 200,000 tons. In
the Appendix further statistics will be found.[214]


_Composition, &c., of Sulphate of Ammonia._

Pure sulphate of ammonia is a whitish crystalline salt, extremely
soluble in water. The commercial article, however, is generally greyish
or brownish in colour, owing to the presence of slight quantities of
impurities. The pure salt should contain 25.75 per cent of ammonia; but
the commercial article is generally sold on a basis of 24.5 per cent. A
useful test of its purity is the fact that when subjected to a red-heat
it should almost entirely volatilise, leaving very little residue. The
chief impurities which it is likely to contain are an excess of
moisture, free acid, or the presence of insoluble matter. Certain
samples contain small quantities of ammonium sulphocyanate, an extremely
poisonous substance for plants. The presence of this dangerous impurity
is easily detected by adding ferric chloride, which, in presence of the
sulphocyanate, produces a blood-red colour. Sulphate of ammonia is thus
the most concentrated of all nitrogenous manures in common use, and is
for that reason the most expensive.


_Application._

For this reason, as well as from the fact that it contains a speedily
available form of nitrogen, sulphate of ammonia should only as a rule be
applied in comparatively small quantities--100 to 125 lb. per acre.[215]
It should also be applied before, but not too long before, the crop is
likely to require it. The reason of this is to give it time to be
converted into nitrates. The ability of the soil to retain ammonia has
already been pointed out. It is not safe, however, to rely too much on
the retentive power of the soil for ammonia, the conversion of ammonia
into nitrates going on very quickly under favourable circumstances. It
is most profitably used as a manure for cereals, and it has been found
by Lawes and Gilbert in their experiments, that an increase of one
bushel of wheat and a corresponding increase of straw have been obtained
for every 5 lb. of ammonia added to the soil. As has been pointed out in
the previous chapter, the respective merits of sulphate of ammonia and
nitrate of soda depend largely on the nature of the season during which
they are used. In wet seasons the sulphate is rather more favourable
than the nitrate, but, on an average, nitrate of soda is probably the
more valuable manure--_i.e._, due regard being had to the quantity of
nitrogen the two manures respectively contain. In one respect sulphate
of ammonia is a much more useful manure than nitrate of soda, as the
nature of its action when applied to the soil permits of it being used
as an ingredient of mixed manures.

Like nitrate of soda, but even to a greater extent, its most favourable
action is obtained when it is applied along with other manurial
ingredients. It should be applied at least a month earlier than nitrate.
It has been shown that in the case of chalky soils a certain loss of
ammonia in sulphate of ammonia is apt to take place, due to the action
of the lime; and this leads us to point out that, in preparing mixed
manures, care ought to be taken that it is not mixed with any compound
containing free lime or caustic alkali, as otherwise loss of ammonia
will ensue. It should never, for example, be used along with basic slag.


FOOTNOTES:

[212] From experiments by Lehmann and others with buckwheat and maize,
it would seem that certain plants may prefer, at certain stages of their
growth, ammonia to nitrates. In the case of maize, ammonia may be
preferred in the early stages of growth, while nitrates are preferred as
it becomes more mature. In view, however, of our present knowledge of
nitrification, it may well be doubted whether the conclusions arrived at
from Lehmann's experiments can be accepted.

[213] As the expense of converting the ammonia present in the ammoniacal
liquor is considerable, the practice of using the liquor itself as a
manure has been advocated; but as an objection to this it must be urged
that, besides being so bulky a manure, the liquor contains various
substances poisonous to plant-life.

[214] See Appendix, p. 358.

[215] Some crops, however, may with advantage be treated with larger
quantities of sulphate of ammonia, such as mangels and potatoes.



APPENDIX TO CHAPTER X.


NOTE (p. 355).

The following table will exhibit the production of sulphate of ammonia
in this country from 1870 to 1892:--

Year.              Tons.    |   Year.              Tons.
1870               40,000   |   1882               72,000
1871               41,000   |   1883               75,000
1872               42,000   |   1884               87,000
1873               43,000   |   1885               97,000
1874               45,000   |   1886              106,500
1875               46,000   |   1887              113,700
1876               48,000   |   1888              122,800
1877               52,000   |   1889              132,000
1878               55,000   |   1890              140,000
1879               57,000   |   1891              143,500
1880               60,000   |   1892              157,000
1881               65,000

The following table exhibits the sources, and the respective quantities
from each source, of the last seven years' production:--

              1886.    1887.     1888.    1889.    1890.    1891.    1892.

Gas-works    82,500   85,000    93,000  100,000  102,150  107,950  112,000
Iron-works    4,000    5,000     5,300    6,000    5,050    6,300   12,000
Shale-works  18,000   21,000    22,000   23,000   24,750   26,600   28,000
Coke and
 carbonising
 works        2,000    2,700     2,500    3,000    2,300    2,800    5,000



CHAPTER XI.

BONES


_Early Use of Bones._

A most important manure, and one to the history of which very peculiar
interest attaches, is Bones. Employed first in 1774, their use has
steadily increased ever since, and their popularity as a phosphatic
manure is among farmers in this country quite unrivalled. Like guano,
although to a less extent, the early practice of using bones has done
much to arouse interest in the problems of manuring, and to bring home
to farmers the principles underlying that practice. It was from bones
that Liebig first made superphosphate of lime, and the distinguished
veteran experimenter, Sir John Bennet Lawes, has told us that the
benefit accruing from the use of bones on the turnip crop first drew his
attention to the interesting problem connected with the application of
artificial manures. Bones were first used in Yorkshire. Shortly
afterwards they were applied to exhausted pastures in Cheshire. Soon
their use became so popular that the home supply was found inadequate;
and they were imported from Germany and Northern Europe, Hull being the
port of disembarkation. So largely were they used by English farmers,
that Baron Liebig considered it necessary to raise a warning protest
against their lavish application. "England is robbing all other
countries of the condition of their fertility. Already, in her eagerness
for bones, she has turned up the battle-fields of Leipzig, of Waterloo,
and of the Crimea; already from the catacombs of Sicily she has carried
away the skeletons of many successive generations. Annually she removes
from the shores of other countries to her own the manurial equivalent of
three millions and a half of men, whom she takes from us the means of
supporting, and squanders down her sewers to the sea. Like a vampire,
she hangs upon the neck of Europe--nay, of the entire world!--and sucks
the heart-blood from nations without a thought of justice towards them,
without a shadow of lasting advantage to herself."[216]


_Different Forms in which Bones are used._

It may be pointed out that bones have done much to alter our system of
farming, by helping to develop turnip culture. Used at first in
comparatively large pieces, experience gradually showed that a finer
state of division facilitated their action. Yet it was long before the
prejudice in favour of rough bones disappeared; and it was not till 1829
that Mr Anderson of Dundee introduced machinery for preparing 1/2-inch
and 1/4-inch bones and bone-dust. In the early days of their use, bones
were fermented before being used, in order to render their action more
speedy when applied to the soil; and this practice still obtains to the
present day in some parts of the country among farmers. This
fermentation was often effected simply by mixing the bones with water,
and allowing the heap to lie for a week or two. In other cases the bones
were mixed with urine or other refuse matter. The most important step,
however, in the history of the treatment of bones for manure was the
discovery in 1840, by Liebig, of the action of sulphuric acid on them--a
discovery which led to the institution of the manufacture of
superphosphate of lime by Sir John Lawes. The nature of this action will
be explained in the following chapter, so that we need only say here
that the efficacy of the manure by treatment with sulphuric acid is more
than doubled. Bones have thus been used, and still are used, in a
variety of conditions, such as in the raw or green state, bruised,
boiled, steamed, fermented, burned, dissolved, and broken or ground into
various states of fineness, to which the names of 1/2-inch, 1/4-inch
bones, bone-meal, bone-dust, and floated bones are given. We shall now
proceed to discuss the composition of bones, and investigate more
exactly the nature of their action.


_Composition of Bones._

The composition of bone-tissue varies considerably, and depends on the
age and kind of animal to which it belongs, as well as to the part of
the animal frame from which it is taken. Bones are made up of an organic
and an inorganic part. By steeping a piece of bone in a dilute acid
solution, the inorganic portion of the bone is dissolved out, and the
organic portion, which forms the framework of the bone, is alone left.
On the other hand, by submitting a bone to the action of great heat, the
organic portion of the bone is driven off, and all that remains is a
quantity of ash. The proportion of the organic to the inorganic matter
varies considerably in different bones. The bones of young animals
contain more organic matter than those of old animals. In compact bones,
also, the organic matter is greater than in spongy bones. The
thigh-bone, of all the bones, contains most inorganic matter. In short,
bones which have to bear the greatest strain are richest in inorganic
matter. Of the bones of animals, fish-bones exhibit the greatest variety
of composition, some being almost entirely made up of organic matter,
while others are similar in their composition to the bones of
quadrupeds.


_The Organic Matter of Bones._

The organic portion of bones is almost entirely made up of a substance
to which the name _ossein_ has been given, and which, when boiled for a
long time, is converted into gelatine. This ossein, which forms on an
average from 25 to 30 per cent of the weight of bones, is extremely rich
in nitrogen, containing over 18 per cent.


_Inorganic Portion of Bones._

The inorganic portion, which forms about 70 per cent, is made up chiefly
of phosphate of lime. The dry leg-bones of oxen and sheep, according to
Heintz, have the following percentage composition:--

                                 Per cent.
Phosphate of lime                58 to 63
Carbonate of lime                 6 to  7
Phosphate of magnesia             1 to  2
Fluoride of calcium                     2
Organic matter                   25 to 30

According to Payen and Boussingault, raw bones contain 6-1/4 per cent of
nitrogen and 8 per cent of water. Pure bones are thus seen to contain
about 29 per cent of phosphoric acid and 6-1/4 per cent of nitrogen. The
composition of the commercial article, however, differs very widely.
This is due to the fact that bones collected from India and America,
where they have been long exposed to atmospheric influences, have lost
much of their organic matter. The amount of sand and earthy impurities
also varies very considerably.


_Treatment of Bones._

Bones are used for the manufacture of glue and gelatine. These are
extracted from them by steaming the bones. The bones after treatment are
used as a manure. The improvement noted in the action of the bones thus
treated led to the introduction of the use of steamed bones as a manure.
Raw bones are now rarely used. The fat present in raw bones retards
their decomposition in the soil. Probably, as has been suggested, it
forms along with lime an insoluble soap which prevents the mineral
matter in the bone being dissolved by the carbonic acid of the soil. In
the process of boiling or steaming a certain loss of nitrogen takes
place, greater or less, according to the length of time they are boiled
or steamed, and in the latter case the pressure applied. A more
economical method for extracting the fat has been introduced by using
benzine, but this process is not used to any extent. The loss of
nitrogen in the former case is more than compensated for by their more
speedy action as a manure when applied to the soil. Bone-meal of good
quality contains from 45 to 55[217] per cent of phosphate of lime, and
3-1/2 per cent of nitrogen. Our present total consumption of bones is
probably little less than 100,000 tons per annum, of which about half is
obtained from home collections, over 20,000 tons being annually
collected in and around London alone.


_Action of Bones._

It is well known that bones are a slow-acting manure. They may be said
to possess both a mechanical and chemical action when applied to the
soil. When they putrefy, their nitrogen is slowly converted into
ammonia, and carbonic acid as well as various organic acids are formed,
which, acting upon the insoluble mineral matter in the bones, renders it
available for plant uses. Bones thus, when applied in large quantities,
may not merely act directly as suppliers of plant-food, but in the
course of their putrefaction may act upon a certain amount of the inert
fertilising matter of the soil and render it available. The more
readily, then, bones putrefy, the more speedy will be their effect. As
we have already pointed out, bones, in order to increase their
efficiency, are often fermented before application. The removal of the
fat is another means of increasing the rate of their action, but the
fineness to which they are ground determines this more than anything
else. Much ingenuity has been expended in perfecting machinery for
grinding bones. At one time in Germany they were pounded in stamps
similar to those used for ore. In America what has been called "floated
bone" has been prepared. This bone is so fine that it actually floats in
the air like flour-dust, and is made by whirling the bones against one
another. The action of bones prepared in this way is of course very
speedy, but the difficulty of applying a manure in such a fine state of
division to the soil is great. The expense of the process also is
considerable.

The ease with which bones when ground into a fine state of division
putrefy, is evidenced by the fact that bone-flour has to be salted in
order to enable it to keep. Another condition which determines the rate
at which the fertilising matters in bones become available is the nature
of the soil. Fermentation, as we have already seen, requires a plentiful
supply of air, and a certain amount, but not too much, of moisture.
Consequently bones act best in medium soils--soils which are "neither
too light and dry, nor too close and wet." There can be no doubt that
what gives to bones a peculiar value in the eyes of the farmer is the
fact that they form a manure of a lasting character. They give what has
been termed backbone to a soil. But the tendency of modern agricultural
practice is to use quick-acting manures rather than slow. This has been
admirably put by Professor Storer in the following words: "The old
notion, that those manures are best which make themselves felt through a
long series of years, is now recognised to be an error. The adage, that
'one cannot eat the cake and have the cake' is conspicuously true in
agriculture; and just as it is the part of prudence in household or
maritime economy to abstain from laying in at any one time more
provisions than can be properly disposed of in a year or during a
voyage, so should the farmer refrain from bringing to the land an
unnecessary excess of plant-food. Such food is liable to spoil withal
in the soil, as well as other kinds of provisions that are kept too long
in store. A just proportion of food, properly prepared, is the point to
be aimed at always."

In view, therefore, of what has just been said, it might seem best to
use bones in the form in which they are most speedily available--viz.,
as dissolved bones. This would be so if bones were the only source we
possessed for the manufacture of superphosphate of lime; but we now
have, in the various mineral phosphates, abundant and cheaper sources of
this valuable manure. The opinion of leading agriculturists and
agricultural chemists is rather in favour of applying bones in the
undissolved condition. For one thing, it seems far from economical to
utilise an expensive material such as bones for manufacturing an article
which can be equally well manufactured from cheaper materials; for once
the phosphate of lime is dissolved, it is equally valuable from whatever
source it may be derived. Of course this is not tantamount to saying
that dissolved bones as a manure are no more valuable than
superphosphate. In dissolved bones we have, in addition to soluble
phosphate, a considerable proportion of undissolved bone-tissue,
containing a certain quantity of nitrogen and organic matter; but so far
as the soluble phosphate is concerned, it seems only rational to
conclude that its efficacy is equally great, whether it be derived from
bone or mineral phosphate. Another reason is, that much of the
characteristic action of bones is lost by treating them with sulphuric
acid. As Dr Aitken has pointed out, the germ life in the soil and in the
bones gradually converts them into a form available for the nourishment
of plants; but to dissolve bones with sulphuric acid is to kill out the
germ life and retard the decay of any nucleus of bone in the dissolved
manure.


_Dissolved Bones._

Dissolved bones, however, are still manufactured. Formerly the manure
called dissolved bones was often a mixture of mineral superphosphate
along with undissolved bone-meal, but recent legislation has stopped the
continuance of this practice. The composition of dissolved bones varies
somewhat, the percentage of soluble phosphate being about 20 to 23 per
cent, the insoluble amounting to from 9 to 10 per cent, and the nitrogen
from 2-1/2 to 3-1/2 per cent.[218] Another reason against dissolving
bones is to be found in the difficulty experienced in dissolving their
phosphate. Bones, especially when raw, are not easily acted upon by
acids.


_Crops suited for Bones._

Bones are commonly regarded as being specially beneficial to
pasture-land, to which they are applied as a top-dressing. Turnips,
tobacco, potatoes, vines, and hops are also much benefited by the
application of bones. In America, mixed with wood-ashes (the chief
manurial constituent of which is potash), they have been extensively
used as a substitute for farmyard manure, and have been applied at the
rate of 5 to 6 cwt. per acre. In Saxony, according to Professor Storer,
1 cwt. of fine bone-meal is worth as much as 25 to 30 cwt. of farmyard
manure.


_Bone-ash._

The ash which is left on burning bones used to be an article of
considerable manurial importance. It is still imported from South
America in some quantity, and is used chiefly in the pottery industry.
It is occasionally still used to a limited extent for the manufacture of
high-class superphosphates. It is extremely rich in phosphate of lime,
of which it contains between 70 and 80 per cent; but of course it is
devoid of nitrogen.[219] Bone-ash is best used in the dissolved form, as
it possesses no characteristic action such as is possessed by bones.


_Bone-char or Bone-black._

When heated in a closed retort, bones are not converted into bone-ash,
but into a body called bone-char. This body is similar in composition to
bone-ash, except for a certain percentage of charcoal--amounting, on an
average, to 10 per cent. It contains but little nitrogen and other
organic matter. Bone-black or bone-char is an article which is prepared
in enormous quantities for use in sugar-refineries, where it is used in
the purification of sugar. After use it may be renovated by submitting
it to heat; but as this process gradually lessens the percentage of
carbon it contains, after a certain period it becomes too poor in this
substance for efficiently acting as a filter. When this takes place it
is technically known as spent char, and is used for the manufacture of
superphosphates. Spent char is a highly phosphatic substance, being very
little poorer than bone-ash, and containing about 70 per cent of
phosphate of lime.[220]


FOOTNOTES:

[216] It is only fair to Liebig to say that when he wrote these words
the practically boundless supply of mineral phosphates which we now know
to exist in many parts of the world was little dreamt of.

[217] See Appendix, Note I., p. 371.

[218] See Appendix, Note II., p. 371.

[219] See Appendix, Note III., p. 372.

[220] See Appendix, Note IV., p. 372.



APPENDIX TO CHAPTER XI.


NOTE I. (p. 364).

The following analysis will serve to show the composition of
bone-meal:--

 Moisture                            10.43
*Organic matter                      32.30
 Phosphate of lime                   48.40
 Carbonate of lime, magnesia, &c.     7.20
 Insoluble siliceous matter           1.67
                                    ------
                                    100.00
                                    ------
*Containing:--
   Nitrogen                           3.71
   Equal to ammonia                   4.51



NOTE II. (p. 368).

COMPOSITION OF DISSOLVED BONES.

The accompanying analysis may be taken as representing the average
composition of dissolved bones:--

 Moisture                                            10.10
*Organic matter and water of combination             29.34
 Monobasic phosphate of lime                         11.23
(Equal to tricalcic phosphate rendered "soluble"     17.58)
 Phosphate soluble in ammonium citrate               14.02
 Insoluble phosphate of lime                          1.88
 Calcium sulphate, magnesia, alkalies, &c.           30.23
 Sand                                                 3.20
                                                    ------
                                                    100.00
                                                    ------
*Containing:--
   Nitrogen                                           2.62
   Equal to ammonia                                   3.18


COMPOSITION OF COMPOUND BONES.

The following analysis illustrates the composition of compound bones:--

 Moisture                                             8.10
*Organic matter and water of combination             37.22
 Monobasic phosphate of lime                         13.68
(Equal to tricalcic phosphate rendered "soluble"     21.42)
 Insoluble phosphate of lime                         10.48
 Calcium sulphate, magnesia, alkalies, &c.           26.02
 Sand                                                 4.50
                                                    ------
                                                    100.00
                                                    ------
*Containing:--
   Nitrogen                                           1.90
   Equal to ammonia                                   2.30


NOTE III. (p. 369).

As showing the composition of bone-ash, the following analysis may be
quoted:--

 Moisture                              .25
 Organic matter                        .85
*Phosphoric acid                     35.56
 Lime                                47.09
 Magnesia, alkalies, &c.              9.80
 Sand                                 6.45
                                    ------
                                    100.00
                                    ------
*Equal to tricalcic phosphate        77.63


NOTE IV. (p. 370).

Composition of bone-char (on dry sample):--

Carbon                                                     10.51
Calcium and magnesium phosphates, calcium fluoride, &c.    80.21
Calcium carbonate                                           8.30
Calcium sulphate                                             .17
Ferric oxide                                                 .12
Silica                                                       .34
Alkaline salts                                               .35
                                                          ------
                                                          100.00
                                                          ------



CHAPTER XII.

MINERAL PHOSPHATES.


In this chapter we shall give an account of the more commonly occurring
mineral phosphates. In Chapter V., where we discussed the position of
phosphoric acid in agriculture, it was pointed out that mineral
phosphates were very abundant, and that large deposits of them were
found in many parts of the world.


_Coprolites._

Reference may first be made to the so-called coprolites or phosphatic
nodules which have been found in great abundance in the greensand
formation, in the crag of the eastern counties, and in the chalk
formation of the southern counties. These coprolites are rounded
nodules, and are composed of the fossil excrements and remains of
ancient animals. They are found in large quantities in Cambridgeshire,
and were discovered by Dr Buckland many years ago. The history of their
discovery is not a little curious. The manurial properties of
road-scrapings in parts of Cambridgeshire were noticed, and on being
examined were found to be in part composed of phosphate of lime, derived
from phosphatic nodules dug out of the underlying greensand, and used
for the purpose of repairing roads. Professor Henslow first drew
attention to them at a meeting of the British Association held in
Cambridge in 1845, and pointed out that they contained about 60 per cent
of phosphate of lime. They were also found in enormous quantities in
Suffolk, Norfolk, Bedfordshire, and Essex, and were for a long time
largely used in the manufacture of superphosphate, but of late years
have not been used to anything like the same extent, owing to the fact
that there are richer and cheaper sources of phosphate of lime
available. In 1887 about 20,000 tons of coprolites were raised. The
richest were those obtained in Cambridge, while those got from
Bedfordshire were about the poorest. Deposits have also been found in
France and other countries. The average amount of phosphate of lime in
English coprolites is between 50 and 60 per cent, while the French
contain about 45 per cent.


_Canadian Apatite or Phosphorite._

We have already referred in Chapter V. to large deposits of apatite or
phosphorite found in Canada. The Canadian mines commenced to be worked
about fifteen years ago, and the output now amounts to nearly 25,000
tons per annum.[221] A portion of this goes to the United States; the
rest, amounting to about 20,000 tons, being shipped to England, whence
it is again exported to Hamburg and other places.[222] It contains from
70 to 80 per cent of phosphate. Deposits are also found at Estremadura
in Spain, and in Norway.


_Estremadura or Spanish Phosphates._

Large deposits of phosphate have long been known to exist at Estremadura
in Spain, and the mines at Caceres have been worked on a large scale for
seventeen years, and about half a million tons have been raised. In 1882
the imports into this country amounted to over 56,000 tons; but latterly
they have only been about a fourth of this amount. Dr Dauberry visited
the deposits in 1843, and wrote a most interesting account of them. They
do not seem, however, to have been imported for purposes of
superphosphate manufacture till a number of years afterwards. Of
Estremadura phosphate there are three classes, containing respectively
50, 60, and 70 per cent of phosphate of lime, the lowest quality being
the commonest.[223]


_Norwegian Apatite._

This apatite has ceased to be imported of late years, owing to a duty on
exportation.


_Charleston or South Carolina Phosphate._

For a number of years these deposits have formed the chief source of
phosphate of lime used in the manufacture of mineral superphosphates in
this country (in fact they have furnished two-thirds of our phosphate
supply during recent years). Discovered twenty-five years ago, some four
to five million tons have already been shipped. About half a million
tons were raised in 1886 from these mines, which are the most abundant
in the world. There are two kinds--the so-called "land" and "river"
phosphates. The former contains more oxide of iron and alumina, and is
therefore less pure than the latter, in which the iron and alumina do
not exceed 2 per cent. The river phosphate is dredged from the Bull,
Coosaw, and Beaufort rivers. Of phosphate of lime it contains from 50 to
60 per cent. It is generally sold in three grades--50 to 52 per cent, 55
to 56 per cent, and 58 to 60 per cent of phosphate of lime. It will thus
be seen to be incapable of producing very high-class superphosphates
--_i.e._, containing more than 30 per cent "soluble" phosphate.
This point will be more intelligible when we describe the manufacture of
superphosphate. The demand for these phosphates in the United States has
increased enormously in recent years, owing to the increase in the
quantity of manure used.


_Belgian Phosphate._

Another very important source of mineral phosphates are deposits
discovered some years ago in Belgium near Mons. These phosphates are of
different qualities, and are found, some in layers near the surface in
pockets forming the richest class, and containing from 45 to 65 per cent
of phosphate, and some in the form of a friable phosphatic rock, the
so-called _craie-grise_ (phosphatic chalk), containing from 25 to 35 per
cent of phosphate of lime. The higher quality of Belgian phosphate is
pretty well exhausted, and it is the second class that forms the bulk of
the ordinary Belgian phosphate at present exported. The commercial
article contains about 35 to 40 per cent of phosphate, and about 45 per
cent of carbonate of lime. The fact of its poor quality, together with
the large percentage of carbonate of lime it contains, renders its
adoption alone in the manufacture of superphosphate unsuitable. Attempts
have been made to get rid of a portion of this carbonate of lime and to
raise the percentage of phosphate. For this purpose the phosphate has
been calcined, but this was soon found to be a great mistake. Other
means have been adopted, with the result that the percentage has been
increased to 50 per cent. It is consequently used in small quantities
as a drier, for which it is peculiarly suited on account of its
carbonaceous nature, along with the higher-class phosphates. In the year
1886 about 145,000 tons of this phosphate were raised, of which about
45,000 tons were imported into the United Kingdom.


_Somme Phosphate._

Still more recently a discovery of phosphate deposits has been made in
the Somme and Pas de Calais departments in the north of France,
adjoining, and similar in character to, the Belgian deposits. The only
difference between Belgian and French phosphates is, that the latter is
of a higher quality, and contains from 50 to 80 per cent of phosphate of
lime. A very large demand for these phosphates sprang up, and in 1888,
although they had only been worked for some two years, no less than
150,000 tons had been raised, of which about one-half contained from 70
to upwards of 75 per cent. There are four grades in the market,
containing 55 to 60, 60 to 65, 70 to 75, and 75 to 80 per cent of
phosphate of lime. The highest quality furnishes the chief material for
the manufacture of high-grade superphosphates.


_Florida Phosphate._[224]

During the last few years large quantities of phosphates have been
imported from Florida. These are of different qualities, the land rocks
now imported containing from 70 to 80 per cent of phosphate of lime, and
the river phosphate about 60 per cent. The latter class are similar in
composition to the best South Carolina river-phosphates, which they much
resemble.


_Lahn Phosphate._

Phosphate deposits were found at Nassau in Germany in 1864; but as the
phosphate contained a considerable proportion of iron and alumina, they
are not used in this country now, although they are in Germany for
double superphosphate manufacture.


_Bordeaux or French Phosphate._

Similar in quality to Lahn phosphate is that obtained in the
neighbourhood of Bordeaux.


_Algerian Phosphate._

Excellent phosphates are now being sent from Algeria--some cargoes being
as rich as 70 per cent.


_Crust Guanos._

We have already referred to the guanos in the chapter on Guano. They are
also known under the name of Caribbean phosphates, and come from the
West India Islands. The chief kinds are Aruba, Curaçao, Sombrero, and
Navassa, the Great Cayman, Redonda, and Alta Vela. Most of them are of
high quality, containing from 60 to 80 per cent of phosphate, and are
thus suited for the manufacture of high-class superphosphates. Some of
them, however, contain a considerable proportion of iron and alumina,
and are not suitable for this purpose. The Redonda and Alta Vela
phosphates consist chiefly of phosphate of alumina.


_Value of Mineral Phosphates as a Manure._

While it is commonly regarded as unadvisable to use mineral phosphates
directly as phosphatic manures, it may well be questioned how far such
an opinion is warranted by actual experience. Professor Jamieson of
Aberdeen, in his interesting and valuable experiments, has drawn
attention to the fact that coprolites in a fine state of division are an
extremely valuable source of phosphoric acid for crops, and are a more
quickly available source than is commonly supposed. Experiments
conducted elsewhere with ground coprolites and other mineral phosphates
corroborate Professor Jamieson's conclusions. The successful use of
Thomas-phosphate has drawn attention to the possibility of profitably
applying undissolved mineral phosphate to the soil; and no doubt the
practice may in future years be increased. At present, however, with the
exception of Thomas-phosphate, mineral phosphates alone are used for
conversion into superphosphate.


FOOTNOTES:

[221] Since the discovery of the Florida deposits of phosphate, the
working of the Canadian mines has been practically abandoned.

[222] See Appendix, p. 381.

[223] These phosphates are now no longer worked.

[224] These deposits were discovered a few years ago; and as they are of
considerable extent and high quality, have entirely revolutionised the
phosphate market. About 300,000 tons are now annually raised in
Florida.



APPENDIX TO CHAPTER XII.


NOTE (p. 375).

THE FOLLOWING TABLE SHOWS THE IMPORTS OF PHOSPHATES INTO THE UNITED
KINGDOM, AND THE COUNTRIES OF PRODUCTION, DURING THE YEARS 1885-92.

-----------------------+---------+---------+---------+---------+---------+
                       |  1885.  |  1886.  |  1887.  |  1888.  |  1889.  |
-----------------------+---------+---------+---------+---------+---------+
                       |  Tons.  |  Tons.  |  Tons.  |  Tons.  |  Tons.  |
 United States         | 138,844 | 144,623 | 165,275 | 111,369 | 122,554 |
 Canada                |  21,484 |  18,069 |  19,194 |  12,423 |  23,297 |
 Dutch West Indies     |         |         |         |         |         |
  (Curaçao, Aruba)     |  11,588 |  12,581 |   9,505 |  10,736 |  14,730 |
 British West Indies   |         |         |         |         |         |
  (Sombrero, &c.)      |   7,727 |   3,351 |   6,451 |  11,010 |   1,880 |
 Spain and Portugal    |  19,282 |   5,825 |  15,612 |   6,978 |   1,326 |
 Belgium               |  35,405 |  31,551 |  45,322 |  54,261 |  64,643 |
 Holland               |     865 |   2,194 |   4,778 |   4,137 |   2,270 |
 France                |   2,276 |   1,503 |  11,140 |  39,059 |  65,490 |
 Australia             |     -   |     200 |     350 |     -   |   1,250 |
 Germany               |     704 |     -   |     -   |     -   |     -   |
 Hayti (San Domingo)   |     -   |   2,175 |   3,044 |   6,238 |   4,094 |
 Brazil                |     -   |     -   |   1,200 |     -   |     -   |
 Venezuela and Guiana  |     -   |     -   |     405 |     -   |     -   |
 Norway                |     -   |     -   |     -   |     -   |     -   |
 Other countries       |     397 |   1,039 |   1,139 |   1,675 |     390 |
                       |         |         |         |         |         |
  *Florida phosphate.  |     -   |     -   |     -   |     -   |     -   |
   Carolina phosphate. |     -   |     -   |     -   |     -   |     -   |
-----------------------+---------+---------+---------+---------+---------+

-----------------------+---------+----------+----------
                       |  1890.  |   1891.  |   1892.
-----------------------+---------+----------+----------
                       |  Tons.  |   Tons.  |   Tons.
 United States         | 177,283 | *131,084 | *201,465
 Canada                |  21,089 |   15,918 |    7,814
 Dutch West Indies     |         |          |
  (Curaçao, Aruba)     |  14,763 |    8,851 |    6,648
 British West Indies   |         |          |
  (Sombrero, &c.)      |   3,970 |    1,960 |    2,473
 Spain and Portugal    |     -   |      320 |      971
 Belgium               |  82,096 |   70,723 |   65,079
 Holland               |   2,428 |    3,434 |    6,627
 France                |  35,659 |   18,325 |   18,239
 Australia             |     -   |      -   |      -
 Germany               |     -   |      -   |      -
 Hayti (San Domingo)   |    992  |    1,639 |    2,965
 Brazil                |    -    |      -   |      -
 Venezuela and Guiana  |    -    |      540 |      -
 Norway                |   4,151 |    1,495 |      305
 Other countries       |   1,070 |    1,483 |    1,594
                       |         |          |
  *Florida phosphate.  |     -   |   35,203 |   66,327
   Carolina phosphate. |     -   |   96,881 |  135,138
-----------------------+---------+----------+----------



CHAPTER XIII.


SUPERPHOSPHATES.

As was mentioned in the chapter on Bones, Liebig in the year 1840
discovered that the effect of adding oil of vitriol, or sulphuric acid,
to bones was to render the phosphate they contain soluble. This
discovery marked an epoch in the history of artificial manures, and laid
the foundation of the now enormous manufacture of superphosphate. In
1862 the juries of the London International Exhibition published an
elaborate report containing an interesting article on the manure trade
of Great Britain, in which it was stated that the annual quantity of
superphosphate then made amounted to from 150,000 to 200,000 tons. Now
it may be placed not far short of a million tons. Probably that made in
the United States is considerably more. In the first instance,
superphosphate was manufactured by Sir John Lawes from spent bone-char.
This was superseded by coprolites and Estremadura phosphorite, Suffolk
coprolites being for many years the chief material employed. This in
turn was succeeded by the richer Cambridge coprolites, but of late years
coprolites have practically ceased to be a source of superphosphate, the
other mineral phosphates mentioned in the previous chapter--such as the
South Carolina, Belgian, Somme, &c., phosphates--taking their place.


_Manufacture of Superphosphate._

The manufacture of superphosphate is of too technical a nature to permit
of discussion in a work of this kind. It is important, however, that the
general principles underlying the process of manufacture and the
chemical changes in the phosphate taking place during the process be
clearly understood. In the first place, great importance attaches in the
manufacture of the superphosphate to the fineness of division of the raw
material, and much ingenuity has been spent on apparatus designed for
this purpose. The difficulty of grinding the phosphate varies, of
course, with the nature of the material used--apatite, for example,
being much more difficult to reduce to the necessary fineness than
phosphatic guano. The finer the state of division, the more complete
will be the decomposition of the phosphate by the acid. Mr Warington
recommends that for first-class work the powder should be so fine as to
admit of it passing through a sieve of eighty wires to the inch. After
the phosphate is reduced to powder, it is mixed with acid. This takes
place in the mixer, which is generally in the form of an iron cylinder
furnished in the centre with a revolving shaft, the sulphuric acid used
being the ordinary chamber acid (sp. gr. 1.57). Whatever strength of
acid is used, there must be a certain quantity of water present to form
gypsum. It is to the formation of gypsum in the resulting product that
the dryness of the superphosphate is due. The proportion of sulphuric
acid used depends on the composition of the phosphate; and here it may
be pointed out that the presence of much carbonate of lime is a most
important factor in determining the quantity of acid required. The
reason of this is, that where carbonate and phosphate of lime are
present together, sulphuric acid first acts upon the carbonate, and it
is not till this is wholly decomposed that the phosphate can be acted
upon. Hence mineral phosphates with a large percentage of carbonate of
lime do not constitute such an economical material for the manufacture
of superphosphate as those in which the percentage of carbonate is
small.[225] A certain amount of heat is necessary for the purpose of
enabling a quick decomposition to take place. For this purpose the
sulphuric acid added has been previously heated. In the ordinary
manufacture of superphosphate, however, this is not considered
necessary, as the heat developed by the chemical action between the
phosphate and the acid is sufficiently great. The phosphate, after being
thoroughly mixed with the acid, is discharged into what is technically
known as the pit, a chamber built of brick or concrete. The mixture,
which is in a fluid state when it enters the pit, very soon hardens, and
is dug out in a day or two. It is next reduced to powder in a
disintegrator, and is then ready for use as a manure.


_Nature of the Reaction taking place._

In order to clearly understand the nature of the reaction which takes
place when sulphuric acid is added to a phosphatic material, it may be
well to say a word or two on the composition of the different compounds
of lime and phosphoric acid.


_Phosphates of Lime._

In the various phosphatic manures used in agriculture there are four
different kinds of phosphates. In the commonest form, popularly called
bone-phosphate, which is the form in which lime and phosphoric acid are
combined in bones, guano, and the ordinary mineral phosphates, the lime
and phosphoric acid are combined in the form of what is known as
tribasic phosphate of lime, or tricalcic phosphate--that is to say, for
every equivalent of phosphoric acid there are three equivalents of lime.
This may be represented as follows:--

Lime }
Lime >  Phosphoric acid.
Lime }

Or we may also say that for every 142 parts by weight of phosphoric acid
there are 168 parts by weight of lime in this form of phosphate. This is
the least soluble form of phosphoric acid,[226] and is the form
generally referred to in commercial analyses as insoluble phosphate.
When this phosphate is acted upon with sulphuric acid, a soluble
phosphate is formed, as Liebig first showed, to which the name
superphosphate has been given, and which is also known as monobasic
phosphate of lime, or monocalcic phosphate. This compound may be
represented as containing, instead of three equivalents of lime, only
one, the other two equivalents being replaced by water. This compound
may be represented as follows:--

Lime  }
Water >  Phosphoric acid.
Water }

In it, for every 142 parts of phosphoric acid, there are only 56 parts
of lime. It is soluble in water, and gives to the commercial article
known as superphosphate of lime its value. Intermediate in composition
between these two phosphates there is another known as precipitated
phosphate of lime, or dicalcic phosphate (the same as reverted
phosphate), which contains two equivalents of lime and one equivalent of
water as follows:--

Lime  }
Lime  >  Phosphoric acid.
Water }

This compound contains, for every 142 parts of phosphoric acid, 112
parts of lime; and in solubility occupies an intermediate position.
Lastly, there is a fourth compound of lime and phosphoric acid, which
only occurs in one phosphatic manure--viz., phosphatic slag, in which
indeed it was first discovered--which consists of four equivalents of
lime to one of phosphoric acid, to which the name tetrabasic phosphate
of lime or tetracalcic phosphate has been given. Its composition may be
illustrated as follows:--

Lime  }
Lime  >  Phosphoric acid.
Lime  >
Lime  }

Or, for every 142 parts of phosphoric acid, there are 224 parts of lime.
Contrary to what we might expect, this phosphate is less insoluble than
the ordinary tribasic or bone phosphate. This may be owing to the fact
that, in the tetrabasic phosphate, there is more lime present than that
which the phosphoric acid can retain with strong chemical affinity.[227]
In the manufacture of superphosphate the tribasic phosphate is converted
into the soluble phosphate--the lime, which was formerly in combination
with the phosphoric acid, uniting with the sulphuric acid, and forming
gypsum.[228] It was till recently supposed that soluble phosphate and
gypsum were the only two resulting products of this decomposition. It
has been recently shown, however, by Ruffle and others, that this is
not, strictly speaking, the case, and that probably a large proportion
of free phosphoric acid is formed; in fact, it seems probable that in
the first stage of the reaction, only phosphoric acid is produced, and
that this subsequently acts upon the undecomposed phosphate, with the
production of monocalcic phosphate.[229] The amount of sulphuric acid
which experience has shown it is necessary to add for the successful and
economical manufacture of superphosphate, depends on the composition of
the raw material employed. The larger the percentage of tribasic
phosphate, the larger the quantity of sulphuric acid required for its
decomposition; but sometimes even a poor phosphate consumes a large
amount of sulphuric acid. This is the case where much calcium carbonate
or fluoride is present in the raw phosphate, as both of these compounds
require a quantity of acid for their decomposition, which takes place
before the decomposition of the phosphate. Hence phosphates rich in
carbonate of lime are not well suited as economical materials from which
to manufacture superphosphate.


_Reverted Phosphates._

A change which is apt to take place in superphosphate after its
manufacture is what is known as reversion of the soluble phosphate. Thus
it is found that on keeping superphosphate for a long time the
percentage of soluble phosphate becomes less than it was at first. The
rate at which this deterioration of the superphosphate goes on varies in
different samples. In a well-made article it is practically
inappreciable, whereas in some superphosphates, made from unsuitable
materials, it may amount to a considerable percentage. The causes of
this reversion are twofold. For one thing, the presence of undecomposed
phosphate of lime may cause it. This source of reversion, however, is
very much less important than the other, which is the presence of iron
and alumina in the raw material. When a soluble phosphate reverts, what
takes place is the conversion of the monocalcic phosphate into the
dicalcic. Now in the first case, where reversion is due to the presence
of undecomposed phosphate, the action taking place may be represented as
follows:--

Lime }                     }     { lime }                  }
Lime } phosphoric acid     }     { water} phosphoric acid  }
Lime }                     }  +  { water}                  } =
(One molecule of insoluble }     { (One molecule of soluble}
    phosphate)             }     {     phosphate)          }

Lime }                     }     { lime }
Lime } phosphoric acid     }     { lime } phosphoric acid.
Water}                     }  +  { water}
(One molecule of reverted  }     { (One molecule of reverted
    phosphate)             }     {     phosphate.)

It may be mentioned, however, that reversion from this cause probably
takes place to a very slight extent in practice.[230] Where reversion is
due to the presence of iron and alumina in the raw material, the nature
of the reaction is not well understood, and is consequently not so
easily demonstrated as in the former case. Where iron is present in the
form of pyrites, or ferrous silicate, it does not seem to cause
reversion. It is only when it is present in the form of oxide--and in
most raw phosphatic materials it is generally in this latter
form[231]--that it causes reversion in the phosphate.


_Value of reverted Phosphate._

The value of reverted phosphate is a subject which has given rise to
much dispute among chemists. That it has a higher value than the
ordinary insoluble phosphate is now admitted; but in this country, in
the manure trade, this is not as yet recognised. At first it was thought
that it was impossible to estimate its quantity by chemical analysis.
This difficulty, however, has been overcome, and it is generally
admitted that the ammonium citrate process furnishes an accurate means
of determining its amount. Both on the Continent and in the United
States reverted phosphate is recognised as possessing a monetary value
in excess of that possessed by the ordinary insoluble phosphate. The
result is, that raw phosphates containing iron and alumina to any
appreciable extent are not used in this country, although they do find a
limited application in America and on the Continent.


_Composition of Superphosphates._

Superphosphates as manufactured may be divided, generally speaking, into
three classes--viz., low class, medium, and high class. The ordinary or
medium class contains from 25 to 27 per cent of soluble phosphate; and
here it may be pointed out that by soluble phosphate is meant the
percentage of tribasic phosphate which has been dissolved--not, as might
at first sight be supposed, the percentage of monocalcic phosphate. The
lower-class superphosphates are those containing less than 25 per cent,
generally 23 to 25 per cent, of soluble phosphate; while the high-class
superphosphate may contain from 30 to 45 per cent. For the manufacture
of high-class superphosphate only a certain number of raw phosphates are
available, such as Curaçao and Somme phosphates, phosphatic guanos,
bone-char, &c. Certain processes have been patented for the manufacture
of even more concentrated superphosphates, and by them phosphates
containing as much as 40 per cent of soluble phosphoric acid--_i.e._,
equal to 87 per cent of soluble phosphate--have been prepared. To this
class belongs the so-called double superphosphate, manufactured at
Wetzlar in Germany. Such a concentrated form of manure is naturally very
expensive to manufacture, and is hardly to be recommended for home
consumption. Where, however, manures have to be conveyed long distances,
and the freight is consequently very high, such a concentrated article
may be found most economical.


_Action of Superphosphates._

When superphosphate is applied to the soil it is converted into an
insoluble state. In short, the process of reversion is carried on on a
wholesale scale. This is due to the lime, iron, and alumina salts which
the soil contains. In all probability the phosphate is finally converted
into a hydrated ferric or aluminic phosphate, in which form it is
gradually acted upon by the sap of the plant-roots as required. This
being the case, it may be asked, Why is superphosphate so much more
rapid in its action than insoluble phosphate; or why should we be at the
trouble and expense of dissolving the phosphate if it has to become
insoluble again in the soil? This question is one of very great
importance, for the answer to it furnishes, in our opinion, the key to
the whole phosphate question. When superphosphate is added to the soil,
being soluble in water, it is soon dissolved and carried down by the
rain into its pores, and becomes thoroughly mixed with the
soil-particles. It is thus soon fixed in the soil, beyond the risk of
being washed away. The result is, that the phosphate is obtained in a
state of division infinitely more minute than could ever be obtained by
mechanical grinding, and is, further, most intimately mixed with the
particles of the soil. It is this intimate mixture of the phosphate with
the particles of the soil, and its minute state of division, that
constitute the only reason for rendering superphosphate superior in its
action to even the most finely ground insoluble phosphates. This opinion
is supported by the fact, that although the chemist has imitated nature
in this matter so far as to manufacture precipitated phosphate, he has
failed, as a rule, in getting as favourable results with it as with
superphosphate. Although the mechanical state of division of the
manufactured precipitated phosphate is probably as fine as that
obtained by nature from the superphosphate, it is impossible to obtain
so intimate a mixture with the soil-particles, and hence the results
obtained are different. For these reasons it will be easily seen that
the rate of action of the superphosphate must always be quicker than
that of any other form of phosphatic manure. The phosphate is everywhere
distributed in the soil. The plant-roots are thus furnished with a
continuous supply throughout their growth, and micro-organisms, which
require for their development a supply of this necessary plant-food, are
propagated. A regularity in the plant's growth is thus secured, which is
of great importance. But while admitting this, there are many cases in
which this greater quickness of action does not render soluble phosphate
the most economical form. The nature of the crop, as well as the nature
of the soil, may in many cases be such as to render the application of
the cheaper insoluble phosphate more economical. It is imperative that
the early growth of some crops be hastened as much as possible by a
ready supply of easily assimilable plant-food, in order to enable them
to successfully sustain the attack of certain pests to which they are
liable to succumb. This, for example, is notably the case with turnips.
In such a case there can be no doubt that the value of soluble phosphate
to the young plants is very great, as it enables them to survive this
critical period.


_Action of Superphosphate sometimes unfavourable._

But even in this case there may be other conditions which render
insoluble phosphate a preferable manure. Such a case is where the soil
is of a very light nature and is deficient in lime. In this case the
acid superphosphate, not having the necessary base to combine with, may
prove even hurtful to the young plants. According to the late Dr
Voelcker, a concentrated superphosphate may produce a smaller crop than
a fertiliser containing only a quarter as much soluble phosphoric acid,
when applied to root-crops on sandy soils, greatly deficient in lime.
Cases such as the above, however, are extremely rare; and we may say
that, in the case of root-crops generally, superphosphate must be
regarded as of special value.


_Application of Superphosphate._

In any case, superphosphate ought to be applied to a soil some time
before it is likely to be assimilated by the plant, in order to allow
neutralisation of its acid character to be fully effected before the
plant's roots come in contact with it. Thus Professor S. W. Johnson, one
of the greatest living American authorities, states it as his opinion
that recent investigations tend to show that soluble and reverted (or
precipitated) phosphates are, upon the whole, about equally valuable as
plant-food, and of nearly equal commercial value. But as Sir John Lawes,
in quoting Professor Johnson to the above effect, remarks, this opinion
is based on an experience of American agriculture, in which country
soluble phosphate is chiefly applied to cereal crops, while in this
country it is chiefly applied to turnips. In the case of cereal crops,
the importance of a speedy early growth is not so great, as we have
already pointed out, as it is in turnips, where the danger to the young
plants from the ravages of the turnip-fly is such that the growth of
even a day or two may make a very considerable difference.


_Value of Insoluble Phosphate._

A consideration of the action of superphosphate, then, throws a good
deal of light on the conditions which determine the value of insoluble
phosphates when applied to the soil, and shows that the state of
division, intimacy of mixture with soil-particles, and the nature of the
soil, are the determining factors. Insoluble phosphates, as we shall
have occasion to see when discussing basic slag, have their best action
on soils poor in lime and rich in organic matter. Tables have been drawn
up with a view to furnishing a guide for the value of phosphoric acid in
different manures. In the Appendix[232] we give those of Wolff for 1893,
and an American table, drawn up for 1892. The comparative values of
mineral phosphates, as well as Peruvian guano and bone-dust, will be
further referred to in the following chapter.


_Rate at which Superphosphate is applied._

The rate at which superphosphate is applied to the soil varies in
different parts of the country. In England 2 to 3 cwt. per acre is
considered an average dressing; whereas in many parts of Scotland it is
applied in as large quantities as 6 to 8 cwt. per acre to the turnip
crop. The reason why so much heavier dressings can be advantageously
given in northern parts of this country is owing to the much longer
period of unchecked growth. In the more southern districts, where the
rainfall is less, mildew is almost certain to appear when the sowing is
as early as required for a maximum crop. With it, as with other manures,
the quantity must be determined by the conditions of its application,
and the amount of other manure applied.


FOOTNOTES:

[225] This holds true, it may be mentioned, with regard to the
application of certain manures, such as bone-char, to the soil.
Bone-char was for a long time used in France as a manure without being
dissolved. The action of such a manure, containing a considerable
percentage of carbonate of lime, is slower than its action would be were
it pure phosphate of lime, as the carbonate of lime is first acted upon
(as in the case of superphosphate manufacture) by the soil acids.

[226] The solubility of tribasic phosphate, of course, is not always
equal in different manures. For example, the phosphate in apatite, owing
to the crystalline structure of that mineral, is not nearly so soluble
as the phosphate in phosphatic guanos, although in both cases its
chemical composition is practically the same.

[227] For formulæ of the different phosphates, see Appendix, Note I., p.
398.

[228] For chemical formulæ, showing reaction, see Appendix, Note II., p.
398.

[229] Of course it is well known that free phosphoric acid is obtained
by acting upon phosphate of lime with an excess of sulphuric acid; but
the point above referred to as having been recently discovered is, that
when phosphate of lime is acted upon, even by a small quantity of
sulphuric acid, free phosphoric acid is formed.

[230] For chemical formulæ showing this reversion, see Appendix, Note
III., p. 399.

[231] For chemical theories on reversion of soluble phosphate by iron
and alumina, see Appendix, Note IV., p. 399.

[232] See Appendix, Note V., p. 400.



APPENDIX TO CHAPTER XIII.


NOTE I. (p. 388).

The formulæ, and molecular and percentage composition, of the different
phosphates, are given in the following table:--

----------------------------------------------------------------------------
                             |          Composition in terms of--
                             +--------------------------+-------------------
                             |    Molecular weight.     |    Per cent.
---------------+-------------+-----+------+------+------+-----+------+------
               |             |     |      |Phos- |      |     |      |Phos-
    Name.      |   Symbol.   |Lime.|Water.|phoric|Total.|Lime.|Water.|phoric
               |             |     |      |acid. |      |     |      |acid.
---------------+-------------+-----+------+------+------+-----+------+------
Tri- or bone-  |3CaO,        |     |      |      |      |     |      |
 phosphate.    |  P_{2}O_{5} | 168 |   0  |  142 |  310 |54.19|  0.00| 45.81
Bi- or di-     |2CaO, H_{2}O,|     |      |      |      |     |      |
 phosphate.    |  P_{2}O_{5} | 112 |  18  |  142 |  272 |41.18|  6.61| 52.21
Mono- or super-|CaO, 2H_{2}O,|     |      |      |      |     |      |
 phosphate.    |  P_{2}O_5   |  56 |  36  |  142 |  234 |23.93| 15.39| 60.68
---------------+-------------+-----+------+------+------+-----+------+------


NOTE II. (p. 388).

When sulphuric acid is added to tricalcic phosphate, the following
reaction takes place:--

(1.)    3CaO, P_{2}O_{5}     +  2(H_{2}O, SO_{3})
      (Tricalcic phosphate),    (Sulphuric acid),

   =    2(CaO, SO_3)         +  CaO, 2H_{2}O, P_{2}O_{5}
         (Gypsum),              (Monocalcic phosphate).

(2.) 3CaO, P_{2}O_{5} + 3(H_{2}O, SO_{3}) = 3CaO, SO_{3} +
            3H_{2}O, P_{2}O_{5}, or 2H_{3}PO_{4}.


NOTE III. (p. 390).

This equation gives the chemical reaction taking place when soluble
phosphate is reverted, owing to the presence of undissolved
phosphate:----

   3CaO, P_{2}O_{5}      +  CaO, 2H_{2}O, P_{2}O_{5}
(Tricalcic phosphate),         Monocalcic phosphate,

=  2CaO, H_{2}O, P_{2}O_{5}  +  2CaO, H_{2}O, P_{2}O_{5}
     (Dicalcic phosphate),        (Dicalcic phosphate).


NOTE IV. (p. 390).

"Just what the reactions are which are produced by the iron and alumina
compounds has never been made out very clearly. But some idea of them
may be gained from the following suggestions, which were thrown out by
the English chemist Patterson. Suppose the sulphuric acid has dissolved
a quantity of iron or alumina, then we may have the reaction:----

Fe_{2}O_{3}, 3SO_{3} + CaO, 2H_{2}O, P_{2}O_{5} = Fe_{2}O_{3},
P_{2}O_{5} + CaO, SO_{3} + 2(H_{2}O, SO_{3}),

and the free acid thus formed would proceed to dissolve more iron or
alumina from the rock that had previously escaped decomposition, and the
reaction here formulated would occur again and again. Here we have a
cumulative process continually increasing the quantity of insoluble
Fe_{2}O_{3}, P_{2}O_{5}, and diminishing in the same proportion the
soluble P_{2}O_{5}. Again, we may have simply----

2Fe_{2}O_{3} + 3(CaO, 2H_{2}O, P_{2}O_{5}) = 2(Fe_{2}O_{3}, P_{2}O_{5})
+ 3CaO, P_{2}O_{5};

where three molecules of the soluble phosphoric acid are made to revert
to the insoluble state at one blow.

"In case the iron in the original rock were in the state of ferrous
oxide, perhaps the following reaction might occur:----

4(FeO, SO_{3}) + 2O + CaO, 2H_{2}O, P_{2}O_{5} + 3CaO, P_{2}O_{5} =
2(Fe_{2}O_{3}, P_{2}O_{5}) + 4(CaO, SO_{3}).

In all these equations, except the last, alumina would serve as well as
oxide of iron."--(_Vide_ Storer's 'Agricultural Chemistry,' vol. i. pp.
276, 277.)


NOTE V. (p. 396).

The following table shows the relative trade values of phosphoric acid
in different manures:--


I.--WOLFF, 1893.

Phosphate soluble in water (as in super)                       100
Precipitated phosphate, Peruvian guano                          92
Reverted phosphate, finest steamed bone-dust
  fish-guano, poudrette                                         83
Phosphatic guanos (Baker Island), wood-ashes                    75
Coarser bone-dust, powdered animal charcoal, bone-ash           67
Coarse fragments of bone, powdered phosphorite and
  coprolite, Thomas-slag, farmyard manure                       33


II.--AMERICAN, 1892.

Phosphate soluble in water                                     100
Phosphate soluble in ammonium citrate                           94
Fine bone-dust, powdered fish                                   94
Fine medium bone                                                74
Medium bone                                                     60
Coarse bone                                                     40



CHAPTER XIV.

THOMAS-PHOSPHATE OR BASIC SLAG.


In this substance we have a most important addition to our phosphatic
manures. It has been in the market since 1886, and the consumption alone
in Germany in 1887 amounted to nearly 300,000 tons. In this country it
is only now beginning to be used to any extent.


_Its Manufacture._

_Thomas-slag_ is a bye-product obtained in the manufacture of steel by
what is known as the "basic" process. In the year 1879 an improvement in
the well-known "Bessemer" process was patented by Messrs Gilchrist &
Thomas. It must be explained that in the manufacture of steel from
pig-iron certain impurities in the raw material have to be got rid of in
order to produce a good steel. Among these impurities one of the most
important is _phosphorus_. This is owing to the fact that even a very
small percentage of phosphoric acid in steel has the effect of
rendering it brittle. The extraction of the phosphorus from the raw
material was formerly, however, attended with very serious difficulties,
and had the effect naturally of rendering steel a costly article,
inasmuch as only the purer kinds of pig-iron could be used for the
purpose.

By the introduction in 1879, however, of the "Thomas-Gilchrist" or
"basic" process, these difficulties were very largely overcome, and the
employment of even such impure irons as the Cleveland (containing
comparatively a large percentage of phosphorus) was rendered possible,
and the price of steel consequently generally very much reduced. The
process consists of submitting the molten pig-iron to a very great heat
in a pear-shaped vessel (known technically as the "converter"). This is
open at the top, and is supported on hinges, which permit of its being
moved so as to pour off the scum which rises to the surface at the end
of the operation, and which, we may explain, consists of "basic slag."
In the original process the sides of the "converter" were lined with
fire-bricks, consisting largely of silica. This process was known as the
"acid" process. In the "Thomas-Gilchrist" process, however, the sides of
the "converter" are lined with _lime_ (dolomitic limestone being largely
used), lime being also added to the pig-iron. An air-blast is injected
through the molten mass, and the impurities are burnt, or oxidised as it
is chemically termed. The phosphorus in the iron is thus converted into
phosphoric acid, and, uniting with the lime, forms phosphate of lime,
which rises, as we have already said, to the surface in the form of a
scum, and is separated from the steel by being poured off.


_Not at first used._

This, then, is how the _Thomas-slag_ is obtained. It did not seem,
however, for some years after the introduction of this ingenious
process, to have struck any one that this rich phosphatic bye-product
might prove a valuable addition to our artificial fertilisers. The
result was, that the Thomas-slag was treated as another of the only too
numerous valueless bye-products which seem to be necessarily incidental
to most of our chemical and other manufactures, and was allowed to
accumulate in large quantities without being used for any purpose.


_Discovery of its Value._

In 1883 some short articles published in Germany on the subject were the
means of first drawing the attention of the public to its importance as
a manure. During the years 1884 and 1885 numerous experiments were
carried out on the subject in the same country; and from then up till
the present hour it has become more and more extensively used in
Germany, till in 1887, as already stated, its consumption amounted to
nearly 300,000 tons.


_Composition._

It consists mainly of phosphate of lime, silicate of lime, free lime,
free magnesia, and oxides of iron and manganese. Its composition, of
course, naturally varies; but the following may be taken as an average
analysis:[233]--

                                                 Per cent.
*Phosphoric acid                                    17
 Lime in combination with phosphoric, silicic,
   sulphuric, and carbonic acids                    40
 Free lime                                          15
 Oxides of iron                                     12
*Equal to tricalcic phosphate                       37

As a rule, the phosphoric acid varies considerably, ranging from 10 to
20 per cent--that is, from 22 to 44 per cent tricalcic phosphate. This
is owing to the difference in the percentage of phosphorus in the raw
material and the quantity of lime added. Attempts have been made in
Germany during the last two or three years to obtain a slag richer in
phosphoric acid than that obtained heretofore, and a process for this
purpose has been patented by Professor Scheibler. This consists of a
slight modification in the ordinary process. Instead of treating the
pig-iron with an excessive quantity of lime, the amount added is not
sufficient to effect the complete dephosphorisation of the iron. The
resulting slag is very rich in phosphoric acid, and is correspondingly
poor in iron. The iron is then again treated with fresh lime, and the
phosphorus completely removed, while the same lime may be used over
again. Such slag forms a very much more concentrated phosphatic manure
than the ordinary slag, and is known as _patent phosphate meal_.

A point which not only renders the slag a product of peculiar interest
from a chemical point of view, but has a most important bearing on its
value as a manure, is the nature of the compound formed by the union of
the lime with the phosphoric acid.

In the ordinary so-called raw phosphates, such as bone-meal, bone-ash,
coprolites, &c., the lime and phosphoric acid are combined in the form
of what is known, in chemical phraseology, as _tribasic phosphate of
lime_. That is to say, that for every equivalent of phosphoric acid
there are three equivalents of lime. Now it was naturally concluded at
first that the tribasic phosphate was the form in which these two
substances existed in the slag. This, however, was found out not to be
the case, in the following way. On allowing the slag to cool, it was
found that small but perfectly defined crystals were formed. These
crystals, by careful analysis, were shown, first by Hilgenstock, to
consist of a form of phosphate of lime hitherto unknown, in which four
equivalents of lime were combined with one equivalent of phosphoric
acid, and which was therefore called "tetrabasic phosphate."


_Processes for preparing Slag._

As soon as the idea of utilising the slag as a manure was suggested,
various plans for extracting its phosphoric acid, and rendering it
available as plant-food, were devised. These were deemed necessary, it
was thought, by the very insoluble nature of the phosphates in the slag,
as well as by the supposed injurious action which would be exerted on
plant-life by the protoxide of iron it contained. Accordingly, a large
number of patents were taken out, "covering almost every conceivable
method for treating the slag, whether practicable or not. They all in
the main are combinations or variations of the following processes:--

"1. _Preliminary preparation of the Slag._

    (_a_) By treating molten, or otherwise, with superheated steam, or
          cooling when hot with water, to reduce it to small pieces or
          to a fragile state.

    (_b_) Grinding.

    (_c_) Treating with water to wash out free lime, or with sugar
          solution.

    (_d_) Roasting in the air, or with some oxidising agent.

"2. _Solution of the Slag._

    (_a_) _Completely_ in weak or strong acids (hydrochloric, sulphuric,
          &c.)

    (_b_) _Partially_, so as to dissolve the phosphates and silicates of
          lime, and leave most of the iron and manganese oxides.

"3. _Precipitation_ of the phosphoric acid, with lime
or iron salts: or,

"Processes in which the slag is smelted with charcoal,
to reduce phosphates to phosphides, treated with
acid, and the phosphuretted hydrogen burnt to phosphoric
acid; and,

"Processes in which the slag is fused with soda or
potash salts,--caustic, chlorides, sulphates, carbonates,--with
or without steam being forced through, to form soluble alkaline
phosphates."[234]

Many of these processes were tried; but it was found by experiment that
the best and most economical way was by applying the slag direct to the
ground in a state of very fine powder. Experiments further showed that
it had _not_ the injurious effect on vegetation which it was feared it
would have from the protoxide of iron it contained. The discovery that
its phosphoric acid existed, as has been already explained, as a
tetrabasic phosphate of lime, has strengthened the opinion that this is
the best method of application.

A good deal has been found to depend upon the fineness of the ground
slag, with the result that it is now commonly sold on a mechanical as
well as a chemical analysis--_i.e._, the slag is guaranteed to pass
through a sieve of a certain fineness.


_Solubility of Slag._

Professor Wagner of Darmstadt has carried out some extremely interesting
experiments on the solubility of slag. He found that very finely
powdered slag was dissolved in carbonic acid water to the extent of 36
per cent, while, similarly treated, phosphorite only dissolved to the
extent of 8 per cent.[235] Another very important solvent is _citrate of
ammonia_. Reverted (or precipitated) phosphate is entirely soluble in
it, and phosphate soluble in it ought to be valued as worth more than
that which is not. Now, the solubility of Thomas-slag in citrate of
ammonia was found by Professor Wagner to be no less than 74 per cent,
while that of phosphorite only amounted to 4 per cent. These results
were corroborated by Professor S. W. Johnson, who found that of the
19.87 per cent of phosphoric acid contained in a sample of basic slag,
no less than 19.57 per cent was soluble in ammonium citrate, while a
finely ground sample of phosphatic rock yielded, on analysis, only 1.81
per cent soluble in citrate of ammonia, of a total of 29.49 per cent
phosphoric acid which it contained. Professor Fleischer has also tested
the comparative solubility of basic slag and phosphorite, by boiling
them in a solution of acetic acid. The former was found to have been
dissolved to the extent of 19 per cent, while the latter to only 5 per
cent. A highly interesting and most important experiment was performed
by Mr Heinrich Albert, of Biebrich. One gramme of basic slag and 100
grammes of peat were mixed together in a litre of water, and it was
found that, after standing for fourteen days, 79 per cent of the
phosphoric acid contained in the slag was rendered soluble.

In the above experiments it was found that the _fineness of grinding_
had a marked effect on the solubility of the slag, and that the finer it
was ground the greater was its solubility. This has been further
demonstrated in Professor Wagner's practical experiments. From these it
was found that finely ground slag has an action _four times_ as quick as
coarse slag; but that, as far as practical results were concerned, there
seemed to be a limit to the fineness to which it was advisable to grind
the slag, as slag above a certain fineness did not give better results
than a coarser slag. At any rate, he found that slag of a fineness so
great that it all passed through a gauze sieve, gave no better results
in his experiments than slag which left 17 per cent behind. We may say,
however, that the _finer the slag is ground, the greater will its
activity as a manure be_; and that a certain degree of fineness is
absolutely necessary to constitute it an active fertiliser. As
Professor Wagner's experiments are among the most valuable and complete
carried out on basic slag, we shall give a somewhat detailed account of
them.


_Darmstadt Experiments._

Professor Wagner's experiments were carried out on such different kinds
of crops as flax, rape, wheat, rye, barley, peas, and white mustard, and
the object of the experiments was to ascertain the comparative activity
as fertilisers of superphosphate, basic slag of different degrees of
fineness, Peruvian guano, damped bone-meal, and very finely ground
coprolites. In order to obtain a correct estimate of the relative value
of these different forms of phosphatic manures, it was necessary to
render the nitrogen in the bone-meal and the nitrogen and potash
contained by the Peruvian guano inactive--_i.e._, to limit the test
strictly to phosphoric acid. This was done by adding to the super, basic
slag, and coprolites, quantities of nitrogen and potash equal to those
contained by the other manures. There was further added to all the
experiments (the unmanured ones, of course, as well) an excess of
nitrogen and potash. In this way the increase in returns could only be
due to the phosphoric acid.

The general results obtained from these experiments may be summed up as
follows: Taking the activity of "super" to be represented by 100, then
the relative activity of--

Basic slag of No. 1[236] fineness is       61
Basic slag, No. 2[237]                     58
Peruvian guano                             30
Basic slag, No. 3[238]                     13
Bone-meal                                  10
Coprolites                                  9

From these results the value of the commercial article has been
attempted to be ascertained. As it contains 80 per cent or thereby of
fine meal and 20 per cent of coarse, its activity may be stated to be
50, or half as active as super. Thus 2 cwt. of basic slag is equal to 1
cwt. of super. This only refers to the first year's effect. Professor
Wagner has made further experiments as to the after-effects of the
different manures, with the result that he has found that the
after-effects of the basic slag are even _better_ than those of the
"super." This stands to reason, for if twice as much phosphoric acid be
added in the form of basic slag as is added in the form of "super," and
the effect of the first year is similar--that is, the same quantity of
phosphoric acid is assimilated by the plant from the soil in both
cases--there is naturally more phosphoric acid left behind in the soil
manured with basic slag than in that manured with superphosphate of
lime. For example, if 100 lb. of super has the same effect in the first
year as 200 lb. of basic slag, and it is found that only 60 lb. of the
super and the basic slag have been assimilated the first year by the
plant, it is only natural to conclude that the remaining 140 lb. of the
basic slag will have a better after-effect than the remaining 40 lb. of
super. This has been actually proved to have been the case in
Professor's Wagner's experiments. The following are the results of some
experiments which Professor Wagner has carried out on the after-effects
of different manures:--

Out of 100 parts of phosphoric acid, there was removed by the first
year's crop--

Super                            63
Peruvian guano                   22
Bone-meal                         7
Coprolites                        6
Thomas-meal--
  No. 1 fineness                 39
  No. 2  "                       43
  No. 3  "                       15

Out of 100 parts of phosphoric acid left by the first crop, there was
removed by the three succeeding crops--

Super                              30
Peruvian guano                      9
Bone-meal                          13
Coprolites                          6
Thomas-meal--
  No. 1 fineness                   14
  No. 2  "                         29
  No. 3  "                         24

Numerous other experiments have been carried out by various
experimenters in different parts of Germany which it is unnecessary to
cite here. None, however, are so complete as those of Professor Wagner.


_Results of other Experiments._

In this country experiments have been carried out at Rothamsted,
Cirencester, Downton, Bangor, and by Dr Aitken at the Highland and
Agricultural Society's stations, as well as elsewhere. The results of
these various experiments naturally differ considerably, this being
owing to the difference in the nature of the soils upon which the
experiments were carried out, as well as the different degrees of
fineness of the slag used. They all, however, serve to confirm Professor
Wagner's general results. The results obtained in Scotland by Dr Aitken
at the Highland Society's stations were especially favourable to basic
slag as a phosphatic manure. The experiments were carried out on
turnips, and it was found that the Thomas-slag was, weight for weight,
superior to superphosphate. It may be added that the slag used in these
experiments was rich in phosphoric acid, and was in an unusually fine
state of division. Experiments carried out by the author have proved
slag to be, on various Scottish soils, one of the most economical
phosphatic manures to apply to turnips.[239]

We will sum up, in conclusion, the deductions which may be fairly drawn
from the results of all the above-mentioned experiments as to the value
of basic cinder as a manure.


_Soils most suited for Slag._

Although its action is undoubtedly more favourable on some soils than
others, it may be broadly stated that generally its phosphoric acid is
_half as valuable_ as that in soluble phosphate. The soils on which it
will have the most marked effect will be those of a _peaty_ nature,
_poor_ in lime, but _rich_ in _organic matter_. The beneficial results
obtained by an application of lime to peaty soils are well known. As the
slag contains a large percentage of free lime, it thus performs on such
soils a double function. On meadow-lands, all kinds of pasture-lands (if
not of too dry a character), and clay soils poor in lime, its action has
been shown to be especially favourable. Of different kinds of crops,
those best suited to benefit from the slag as a phosphatic manure are
those of the leguminous kind. This arises from the fact that their
period of growth is longer than that of most other crops.


_Rate of Application._

As to the rate per acre at which the slag ought to be applied, there
will naturally be a difference of opinion. Professor Wrightson, of
Downton Agricultural College, recommends that it should be applied at
the rate of from 6 to 10 cwt. per acre. This, of course, is very
liberal manuring. We must remember, however, that phosphatic manures,
unlike nitrogenous manures, and to some extent potash manures, may be
applied in even excessive quantities without any risk of loss. It is
impossible to measure out our phosphate manures in the same accurate
manner as we measure out our nitrogen. It is safer, therefore, and on
that account more economical in the long-run, to apply our phosphate in
excessive quantity than the reverse. The reason of this may be shortly
explained. The phosphoric acid which is naturally present in most soils
is with difficulty soluble. Only a small quantity is yielded daily to
the plant. This quantity may, under favourable climatic conditions, be
sufficient; but these favourable influences never last very long at a
time.

For three weeks, perhaps, the plant may experience drought, and during
this period it takes up no phosphoric acid, and its growth practically
comes to a standstill; but this period of drought is followed by rain
and warm weather, and the plant, if it is to be ripe by harvest-time,
must make up for lost time. It must grow as much the next few days under
these favourable climatic conditions as it would have grown under normal
conditions in double or treble the time. In order to do so, however, it
must be able to obtain plenty of phosphoric acid, and this is only
possible where there is a decided excess of phosphoric acid present in
the soil.

The richness of a soil, therefore, in phosphoric acid, must be such
that it is not only able to supply the ordinary wants of the plant, but
to provide an excess when such an excess will be needed; for one must
remember that the amount of plant-substance formed in the course of a
few days under favourable conditions is very great, and that the amount
consequently of phosphoric acid which plants assimilate during that
period must also be very considerable.


_Method of Application._

In conclusion, as to the method of application of the slag,
agriculturists must be _warned against mixing it with sulphate of
ammonia_; for if this is done, a _considerable loss of ammonia_ will
ensue, set free from the sulphate by the action of the free lime which
the Thomas-slag contains. With nitrate of soda and potash salts it may
be freely mixed. Such mixtures, however, are apt to form themselves into
little balls, which soon become very hard. They should therefore only be
mixed shortly before use. To overcome this difficulty, Professor Wagner
recommends the mixture of a little peat or sawdust with the slag.


FOOTNOTES:

[233] See Appendix, p. 417.

[234] _Vide_ paper on "Basic Slag: Its Formation." By Stead and
Ribsdale. 'Journal of the Iron and Steel Institute,' 1887, p. 230.

[235] _Vide_ Professor Wagner's pamphlet, 'Der Düngewerth und die
rationelle Verwendung der Thomas Schlacke,' Darmstadt, 1888.

[236] No. 1 fineness was such as passed entirely through a fine gauze
sieve of 250 wires to the linear inch.

[237] No. 2 fineness was such as passed entirely through the regular
standard sieve--_i.e._, containing 120 wires to the linear inch.

[238] No. 3 was what would not pass through the standard sieve.

[239] 'Transactions of the Highland and Agricultural Society,' 1891;
'Chemical News,' 1893.



APPENDIX TO CHAPTER XIV.


NOTE (p. 404).

For those more particularly interested, we append a full analysis of the
slag, taken from Messrs Stead and Ribsdale's paper in the 'Journal of
the Iron and Steel Institute,' 1887, vol. i. p. 222:--

Lime                               41.58
Magnesia                            6.14
Alumina                             2.57
Peroxide of iron                    8.54
Protoxide of iron                  13.62
Protoxide of manganese              3.79
Protoxide of vanadium               1.29
Silica                              7.38
Sulphur   }                          .23
Calcium   }                          .31
Sulphuric anhydride                  .12
Phosphoric acid                    14.36
                                   -----
                                   99.93



CHAPTER XV.

POTASSIC MANURES.

_Relative Importance._


In Chapter VI. we pointed out that of the three manurial ingredients
potash was the one most abundantly occurring, and that, consequently,
the necessity of adding it in the form of an artificial manure existed
less frequently than in the case of nitrogen or phosphoric acid. It was
further pointed out that, under the ordinary conditions of agriculture,
a greater restoration to the soil of the potash removed in the crops was
made in the straw used in farmyard manure than was the case with regard
to the other two ingredients. Despite these facts, there are many cases
where the addition of potassic manures is of the highest importance in
increasing plant-growth. It will be well, therefore, to devote a little
space to considering our different potassic manures and their respective
action.


_Scottish Soils supplied with Potash._

Potassic manures are not so valuable in this country since experience
has shown that most Scottish soils are abundantly supplied with this
manurial ingredient. Moreover, under the conditions of most European
farming, there seems to be a steady gain to the soil of potash. In
America, however, the action of potash as a manure seems to be more
strikingly illustrated. Indeed, wherever forage crops or straw are sold
off the farm in large quantities, or where beets, cabbages, carrots,
potatoes, onions, &c., are also grown in large quantities, the necessity
for potash manuring generally arises.


_Sources of Potassic Manures._

The value of potash as a manure first came to be recognised from the
favourable action of wood-ashes. Of course their favourable action is
not due solely to potash, as they contain, in addition to the other ash
ingredients of the plant, phosphates; and their value as a manure may
also be said to depend not a little on their indirect action. They
contain a certain percentage of caustic alkali, which promotes the
decomposition of the nitrogenous matter of the soil. But making due
allowance for these other valuable properties, the chief value of
wood-ashes is undoubtedly due to the potash they contain. Hence the use
of the commercial article called _potash_, which is a mixture of
potassium carbonate and hydrate, and which is obtained from wood-ashes,
was formerly common to a considerable extent as a manure, especially for
clover. _Barilla_, a rich potassic manure prepared by burning certain
strand plants, especially the saltwort, was also in the past largely
exported from Sicily and Spain. _Kelp_, a product got by burning
sea-weed in Scotland, is also a rich potassic manure. Since, however,
the discovery of the Stassfurt mines, all potassic manures have come
from these.


_Stassfurt Potash Salts._

Huge salt deposits exist at Stassfurt in Germany. They have been formed
by the evaporation of an inland sea. Salt was first discovered in these
deposits in 1839, but for long the presence of potash salts was little
suspected, and it was not until 1862 that the potash salts were worked.
We have already, in the Appendix to Chapter VI., given a list of the
chief potash minerals occurring in the Stassfurt deposits. These
minerals are found in layers, the lowest layer consisting of almost pure
salt; while immediately above this we have a layer of salt mixed with
the mineral polyhallite (containing potassium sulphate) of about 100
feet thick. Above this last layer there is a layer of about 90 feet,
containing kieserite (magnesium sulphate) mixed with potassium and
magnesium chlorides; and above this again is a layer (90 feet) of
carnallite, which furnishes the chief source of the potash salts used
for manurial purposes.

At first the crude salts, as obtained direct from the deposits, were
sold as manures under the name of _Abraum_ salts. Now, however, they are
purified. Of potash salts in 1888 some 25,000 tons were exported from
Stassfurt for manurial purposes. Of these salts there may be mentioned,
viz., kainit, an impure form of the sulphate, containing on an average
about 12 per cent of potash, and the muriate and the sulphate--both
salts, in a more or less pure form, being used. A word or two may be
added on the effect of the two forms of potash--viz., as the sulphate
and as the muriate.


_Relative Merits of Sulphate and Muriate of Potash._

It is a well-known fact that muriate of potash, far from having a
beneficial effect on certain crops, is actually harmful. Of these,
sugar-beets, potatoes, and tobacco may be mentioned. In the case of
beets it seems to have an effect in lessening the percentage of
crystallisable sugar, while potatoes are rendered waxy. With regard to
the tobacco-plant, it seems to impair the value of the leaf from the
smoker's point of view. That this deleterious action is due to the form
in which the potash is present, and not to the potash itself, seems to
be pretty clear, since potash in the form of sulphate has not this
deleterious effect on these plants. Another objection which has been
urged against muriate of potash is that, when applied as a manure, it
is apt to give rise to the formation of calcium chloride,--a compound
which is distinctly hurtful to many plants. A similar charge cannot be
brought against sulphate of potash, since gypsum, which is the chief
compound it is likely to give rise to, is of much value, as we have
already pointed out, as an indirect manure. On the whole, therefore,
sulphate of potash seems to be the safest form in which to add potash.
Unfortunately, however, most of the commercial sulphates are very
impure, and contain generally considerable quantities of muriate. In
favour of the muriate, it may be said that it is the more concentrated
manure, and that it diffuses better in the soil than the sulphate--a
point of great importance. It has, moreover, been used without any bad
effect for clover, corn, grass, and some root crops.


_Application of Potash Manures._

The extreme tenacity with which the soil-particles fix potash salts,
when applied as manures, is a point which ought to be borne in mind in
their application. This, as we have just noticed, is greater in the case
of the sulphate than in the case of muriate, and it has been observed
that certain other fertilisers seem to exercise a considerable influence
in hindering their fixation. Among these may be mentioned bone-meal and
farmyard manure. Nitrate of soda also seems to increase the
diffusibility of potash salts. Conversely, potash salts seem to help to
fix ammonia.

For the above reasons potash manures ought to be applied to the soil a
considerable period before they are likely to be used by the crop. There
is little risk of any serious loss taking place owing to rain. Autumn
application is generally recommended. Even in very light soils it has
been proved in the Norfolk experiments that autumn application has an
immense advantage over spring application. It has been found that where
potash is applied as sulphate, little sulphuric acid is absorbed by the
plant.


_Soils and Crops suited for Potash Manures._

Of soils best suited for potash manures, it has been found that light
soils, and those largely charged with peaty organic matter (such as the
moorland soils of Germany), are most benefited; while on heavy clayey
soils the percentage of potash which these latter contain is already
sufficiently abundant for the needs of plants. At Flitcham the value of
potash on chalk soils has been strikingly demonstrated. Of crops, it is
now pretty generally acknowledged that those of the leguminous order are
most benefited by potash. Especially in the case of clover has potash
always proved itself a manure worth applying.


_Rate of Application._

Potash is best applied in small quantities. From 1 to 2 cwt. of the
muriate or sulphate is a common amount, and from 6 to 8 cwt. of kainit.



CHAPTER XVI.

MINOR ARTIFICIAL MANURES.


In addition to the manures which have been discussed in previous
chapters, there are a number of minor manures which are used to a very
much smaller extent--dried blood, hoofs, horns, &c.

Among these one of the most valuable is dried blood. Fresh blood,
containing 80 per cent of water, has from 2.5 to 3 per cent of nitrogen,
about .25 per cent of phosphoric acid, and about .5 per cent of
alkalies. When dried it forms a very concentrated and valuable
nitrogenous manure, which has long been used in France. The commercial
article contains, on an average, about 12 per cent of nitrogen, and
slightly over 1 per cent of phosphoric acid. When mixed with the soil it
ferments, and the nitrogen it contains is converted into ammonia.
Although not so quick-acting a manure as nitrate of soda or sulphate of
ammonia, it can by no means be described, as is done in ordinary
agricultural text-books, as a slow-acting manure. Its nitrogen may be
regarded as of equal value to that in Peruvian guano. It is peculiarly
suited for horticulture, and is chiefly used in this country as a manure
for hops. It has also been used with beneficial results for wheat,
grass, and turnips. As a manure it is best suited for sandy or loamy
soils. Considerable quantities are exported to the sugar-growing
colonies as a manure for sugar-cane. Manures are made from other animal
refuse. It may be mentioned that lean flesh (containing 75 per cent of
water) has about 3 to 4 per cent of nitrogen,.5 per cent of alkalies,
and .5 per cent of phosphoric acid; that is to say, a ton of lean flesh
would contain about 70 lb. of nitrogen and 10 lb. of phosphoric acid. In
air-dried flesh, according to Payen and Boussingault (containing 8-1/2
per cent of moisture), there is 13 per cent of nitrogen. Flesh,
therefore, is, when properly composted, a valuable nitrogenous manure.
Dried flesh is generally made into a manure called meat-meal guano, the
composition of which we have already referred to in the chapter on
Guano.[240]

Hoofs, horns, hair, bristles, and wool, wool-waste and the intestines of
animals, have been used as manures. Hoofs and horns form a regular
source of artificial nitrogenous manure; the latter being obtained as a
bye-product in the manufacture of combs and other articles. They are in
the form of a fine powder; and in order to increase their rate of
action, which is very slow, they are often composted in America with
horse-manure before use. They have also been composted with slaked lime.
There can be no doubt that such treatment increases very considerably
their value. Their percentage of nitrogen seems to vary very much
according to the kind of animal from which they are derived. In nine
samples of horn the nitrogen was found to vary from 7-1/2 to 14-1/4 per
cent; giving an average of 11-1/3 per cent. The nitrogen seems rarely to
exceed 15 per cent. The amount of phosphoric acid they contain has been
found by various investigators to range from 6 to 10 per cent. S. W.
Johnson found only from .08 to .15 per cent in buffalo-horn shavings. In
France what is known as "torrefied" horn has been used. This is horn
which has been subjected to the action of steam. The nitrogen in this
material is considered to be more active than in ordinary horn.
According to Way, horns have been used for the hop crop with good
results. Ground hoof is very similar in composition to horn, and
contains about 14 to 15 per cent of nitrogen. Considerable quantities
are now used. It must be remembered, however, that horns, hoofs, hair,
bristles, &c., although rich in nitrogen, possess a comparatively low
manurial value. The home production of these articles may be estimated
at 6000 to 7000 tons.


_Scutch._

Scutch is the name given to a manure made from the waste products
incidental to the manufacture of glue and the dressing of skins. It
contains about 7 per cent of nitrogen, and is manufactured in London to
the extent of several thousand tons annually.


_Shoddy and Wool-waste._

Shoddy, which is a manure made from waste-wool products, is a material
largely manufactured in this country, and which was formerly (it is now
used to a considerably less extent) used to a large extent as a manure.
Its annual production amounts to about 12,000 tons. There are three
qualities,--the first containing 8 to 12 per cent of nitrogen; the
second, 6 to 8 per cent; and the third, 5 to 8 per cent. Shoddy is by no
means a very valuable manure. Woollen-waste products were formerly much
richer in nitrogen than is now the case. This is due to the fact of the
adulteration with cotton, now so prevalent in the manufacture of woollen
goods. Pure woollen rags should contain 17 to 18 per cent of nitrogen.
It has been strongly recommended to treat woollen waste with caustic
alkali before being used as a manure, in order to render their nitrogen
more quickly available; and there is a good deal to recommend this
treatment. When wool-waste is applied as a manure, it should in every
case be in autumn, so as to allow as long a period as possible to elapse
before it is required for the plant's growth.

Leather has also been used as a manure. Its nitrogen may be stated at
from 4 to 6 per cent; and it may safely be described as of all materials
used as nitrogenous manures the least valuable. Leather is, from its
very nature, admirably adapted to resist decomposition when applied to
the soil, and unless it is reduced to a very fine condition, might be
trusted to remain undecomposed for a long period. Torrefied leather,
however, is probably of greater value. It is obtained in the same way as
torrefied horn, already referred to--namely, by treatment with steam.
The grease and fatty matters which so largely aid it in resisting
decomposition being extracted, it is much better suited for manurial
purposes than ordinary leather. Torrefied leather contains from 5 to 8
per cent of nitrogen.


_Soot._

A manure which has long been used and highly esteemed is soot. Obtained
in the usual way, it generally contains some 3 per cent of nitrogen,
chiefly in the form of sulphate of ammonia, and small quantities of
potash and phosphates. A varying proportion of the nitrogen is present
in the form of ammonia salts; and this undoubtedly confers upon soot its
manurial value. It has long been used as a top-dressing for young grain
and grass, and has been applied at the rate of from 40 to 60 bushels per
acre. It has an indirect value as a slug-destroyer.

Many of the above-mentioned manures, of comparatively low value, will
probably be less used in the future than they have been in the past,
owing to the more abundant supplies of nitrate of soda and ammonia salts
which are now available. Many of these substances have probably been
used in mixed manures.


FOOTNOTES:

[240] See p. 324.



CHAPTER XVII.

SEWAGE AS A MANURE.


The value of sewage as a manure has been in the past enormously
overrated, and much misunderstanding has existed on the part of the
public on the question of the profitableness of the disposal of town
sewage as an agricultural manure. Not a few of the erroneous opinions
prevalent in the past regarding sewage have been due to statements made
by scientific and other writers as to the enormous wealth lost to the
world by many of the present methods of sewage disposal. Fortunately,
however, the sewage question is now increasingly regarded as a question,
in the first instance, of sanitary interest. As much has been written on
the subject, and many schemes have been devised, at the expense of much
ingenuity, for utilising its manurial properties, it may be desirable
here to say a few words on the purely agricultural side of the question.

The two most important points about sewage are its enormous abundance
and its extremely poor quality. If the most important consideration
were not the sanitary one, but its manurial value, then indeed our water
system, so universally used in towns, must be regarded as a most
wasteful one; for by its means the value of the excrementitious matter
from which it derives its manurial ingredients is tremendously lessened.
When we reflect that a ton of sewage, such as is produced in many
European cities, contains only 2 or 3 lb. of dry matter, and that the
total amount of nitrogen in this is only an ounce or two, while the
phosphoric acid is considerably less, and that it is on those two
ingredients that its value as a manure entirely depends, we see very
strikingly how poor a manurial substance sewage is. Various methods have
been devised and experimented with for extracting these manurial
ingredients, and many methods are in operation in different parts of the
world. The methods of utilising sewage for agricultural purposes may be
broadly divided into two classes.


_Irrigation._

One of these, which may be classed under the heading of irrigation,
consists in pouring the sewage on to certain kinds of coarse green
crops. Sometimes the land is made to filter large quantities of sewage
by special arrangements of drains and ditches. The land is first
carefully and evenly graded down a gentle incline. At the top of the
field the sewage is conducted along an open ditch from which it is
permitted to escape, by the force of gravity, by several smaller
ditches running at right angles from the main ditch. By means of stops
which may be shifted at will, the sewage can be directed to flow over
different parts of the field. Modifications in this plan may be made so
as to suit the nature of the ground. In the case, for example, of a
steep incline, the field may be sewaged by means of what are known as
"catch-work" trenches running horizontally along the hill. In this way
the sewage is allowed to pass over the whole of the field, and is caught
at the bottom in a deep ditch, whence it is allowed to flow into the
nearest river or stream. This is the system which has been employed at
the famous Beddington Meadows, near Croydon.

Another method of distributing the sewage is by means of underground
pipes, which are laid in a sort of network over the ground to be
manured. At certain intervals pipes with couplings for hose are fitted
on, and by keeping a certain amount of pressure on the main pipes the
sewage may be distributed over the different parts of the field as it is
required.

A third modification is subsoil irrigation. This resembles the
last-named system, with this difference, that the pipes used are either
porous or perforated with small holes.

Total submersion can only be applied in the case of absolutely level
lands, and is practised to an enormous extent in Piedmont and Lombardy.

There has been little dispute as to the thorough efficiency of
irrigation--when conducted under favourable conditions--as a method of
purifying sewage and utilising to the full its constituents of manurial
value. It is the only method which has been conclusively shown to
extract from sewage that to which it owes most largely its value as a
manure--viz., ammonia; and from this fact it deserves a first place in
the consideration of agriculturists. For however admirable other methods
may be from a sanitary point of view, it is obvious that a method which
would allow the ammonia in sewage wholly, or at least to over 90 per
cent, to be lost, cannot claim the same place in the judgment of
agriculturists as a method which can extract for the soil not only the
whole of this valuable constituent, but all else in the sewage which in
any way is of value to plant-life.


_Effects of continued Application of Sewage._

When sewage is continuously applied to the same land, what generally
takes place is this: At first the sewage is purified, and the soil
derives corresponding benefit from the valuable fertilising ingredients
it thus extracts. After a time, however, the land becomes what has been
termed "sewage-sick." The pores in the soil become choked up by the
slimy matter the sewage contains in suspension; the aeration of the
soil, which, as we have already mentioned, is so necessary, is
consequently to a large extent stopped; and the result is, that the land
rapidly deteriorates, and the sewage is no longer purified.


_Intermittent Irrigation._

This is obviated to some extent by intermittent irrigation. The land,
instead of receiving sewage continuously, only receives it at intervals,
and is allowed some time to recover between each dose. It is, however,
the opinion of those who have given the subject much attention, that
land, even although intermittently sewaged, never recovers its original
efficacy.

Irrigation, therefore, under favourable conditions, is a most successful
method of utilising the manurial value of sewage; but the great
difficulty in practice is to obtain those favourable conditions. It has
long been known that if soil is properly to discharge its function as a
purifier of sewage water, it must be properly aerated; and we now know
that in every fertile soil the process of nitrification must be
permitted free development. Now the application of large quantities of
sewage to a soil is apt to prevent this free development. As we have
already seen, absence of air and the lowering of the temperature of the
soil distinctly tend to retard nitrification; and these two conditions
accompany the application of large quantities of sewage.


_Crops suited for Sewage._

Another objection to irrigation has been found in the alleged limited
number of crops sewaged land is suited to yield. It has been repeatedly
stated that rye-grass is about the only crop it is profitable to grow
on it. In opposition to this statement, however, is the opinion
expressed in the conclusions arrived at by the committee appointed by
the British Association for the consideration of the sewage question. A
vast number of experiments were carried out by them between the years
1868-72, and the result they arrived at was as follows: "It is certain
that all kinds of crops may be grown with sewage, so that the farmer can
grow such as he can best sell; nevertheless, the staple crops must be
cattle food, such as grass, roots, &c., with occasional crops of kitchen
vegetables and of corn." While, therefore, it is probably a mistake to
say that rye-grass is the only crop sewaged land is capable of growing
profitably, the bulk of experience goes to show that such a crop is best
suited for such land. This being so, the question naturally arises, What
is the farmer who uses sewage as a manure to do with the large green
crops he obtains from his land? He is, in most cases, unable to use them
himself or dispose of them at the time. And while this has hitherto
proved to be a most important drawback, now that we have in ensilage a
means of preserving our green crops in a condition suitable as fodder
for as long a time as is necessary, the grounds on which this objection
rests are almost entirely removed.

It will be obvious, of course, that some soils are naturally much better
fitted to perform purification of sewage than others; but it must be
frankly admitted that even the best of soils can only deal with a
certain quantity of sewage. Various calculations have been indulged in
as to the amount of sewage an acre of land can successfully deal with.
According to one of these, an acre can purify some 2000 gallons per day,
or that produced by 100 persons; while other calculations estimate it at
60 persons; and others, again, at 150. The capacity of a sandy soil in
this respect will be much greater than that of a heavier soil; and at
Dantzic an acre of the sand-dunes is regarded as being capable of
purifying the sewage of 600 persons. The late Dr Wallace has calculated
that, in order to treat the sewage of Glasgow, over twelve square miles
of land would be required. Of course, if the sewage is subjected to
previous treatment, which is often the case, by the method immediately
about to be described--namely, precipitation--the amount of sewage the
soil is capable of purifying will be correspondingly increased. A
difficulty which may also be pointed out in connection with irrigation
as a means of disposing of sewage, is the impossibility of carrying it
on during frosty weather, when the land is frost-bound. In warm climates
irrigation has much to recommend it as a means of sewage disposal. In
damp and cold climates, on the other hand, there are many objections.


_Treatment of Sewage by Precipitation, &c._

We now come to consider the methods grouped under this second heading.
Mechanical filtration, of course, only aims at purifying sewage to the
extent of removing all insoluble suspended matter which it contains.
Different substances have been used as filters, the most generally used
being charcoal. Charcoal mixed with burnt clay, gravel, sand, &c., has
also been used.

In chemical precipitation, however, we have a method which claims to do
more. Beyond the extracting of all solid matters in suspension, it
removes (at any rate most chemical precipitants do) nearly all the
phosphoric acid, which, next to the ammonia, is the most valuable
constituent the sewage contains. Of all precipitants, lime has been the
most universally used; and on the whole, it is perhaps the best, for it
is both cheap and obtainable almost anywhere. According to an analysis
by the late Professor Way, the difference in the percentages of
phosphoric acid, potash, and ammonia, before and after treatment with
lime, in a sample of sewage, was as follows:--

 _Grains per Gallon._

                   Before.  After.
Phosphoric acid     2.63      .45
Potash              3.66     3.80
Ammonia             7.48     7.50

From the above we see that while sludge caused by lime as a precipitant
contains nearly all the phosphoric acid, there is not a trace of the
potash or ammonia removed. Sulphate of alumina has also been used, both
alone and in conjunction with lime. The advantage claimed by it over
lime is, that the resulting precipitate is much less bulky. In other
respects, however, it does not seem to be any more efficient as a
precipitant. In the well-known A, B, C process, a mixture of alum, clay,
lime, charcoal, blood, and alkaline salts, in different proportions, has
been used. This mixture is said to extract, in addition to the
phosphoric acid, a certain proportion of the ammonia; but the amount is
so small as scarcely to be worth considering.

Numerous other chemical substances have been used, alone and also in
conjunction with one another, such as perchloride of iron, copperas,
manganese, &c. All alike, however, have failed to do more than effect
partial purification,--the best results, it may be added, being obtained
when the sewage thus treated was fresh. With regard to the manurial
value of the resulting sludges, much difference of opinion has existed.
The small percentage of phosphoric acid and nitrogen they contain has
prevented them from being used to any extent as a manure, as their value
did not admit of carriage beyond the distance of a few miles. By the
introduction a few years ago of the filter-press, their value has been
considerably enhanced. The old method of dealing with the sludge at
precipitation-works was to allow it to dry gradually by exposure to the
atmosphere. The result, however, of leaving sewage-sludge with over 90
per cent of water in it to dry in the air, was to encourage the rapid
decomposition and putrefaction of its organic matter, so that in many
cases the decomposing sludge proved to be as great a nuisance as the
unpurified sewage itself would have been. By the use of Johnson's
filter-press, however, a sludge containing 90 per cent of water was at
once reduced to 50 per cent or even less. By this means the percentage
of its valuable constituents was very much increased, and the
sludge-cake, besides being much more portable, was neither so
objectionable nor so liable to decomposition as before.


_Value of Sewage-sludge._

As to the value of this sludge-cake as a manure, we are happily in
possession of some very interesting and valuable experiments by
Professor Munro of Downton Agricultural College. The sludge experimented
upon was that produced by sulphate of alumina, lime, and sulphate of
iron, and contained, after being subjected to Johnson's filter-press,
from .6 to .9 per cent of nitrogen, and over 1 per cent of phosphoric
acid. It was found that the benefit resulting from the application of
the sludge was far from what in theory might have been expected. The
experiments were made with turnips; and the results obtained with
superphosphate and farmyard manure respectively, in the same field and
under exactly the same conditions, were contrasted with those obtained
with sludge. Thus it was found that 53 lb. of phosphoric acid as
superphosphate, or 60 lb. as farmyard manure, produced a considerably
larger crop than 240 lb. of phosphoric acid in the sludge. That is to
say, that the phosphoric acid in the sludge did not exert more than
one-fifth of its theoretical effect. The explanation of this somewhat
strange result Dr Munro finds in the unsuitable physical character of
the sludge-cakes. In farmyard manure we have a loose texture and a large
amount of soluble constituents when well rotted. It thus quickly
distributes its fertilising elements throughout the soil. In the case of
the sludge, on the other hand, its composing particles are closely
compacted together, and thus offer the greatest resistance to mechanical
and chemical disintegration. "As a matter of fact," says Dr Munro, "the
sludge-plots in my experimental series were all readily identified, when
the roots were pulled, by the presence of unbroken and undecomposed
clods of cake, which had evidently given up, at most, a small portion of
their valuable ingredients to the soil."

Briefly stated, therefore, the objections to chemical precipitation as a
means of dealing with sewage are these--viz., that while it relieves
sewage of all its organic matter, and to a large extent of its
phosphoric acid, it fails to extract any ammonia, which is thus lost;
that the resulting sludge is consequently so poor in fertilising matters
as scarcely to make it worth while to remove it any distance for
manuring purposes; and that, further, owing to its unfavourable
physical character, as at present made, even the small percentage of
plant-food it contains is not realisable, within, at any rate, anything
like a reasonable time, to its full theoretical extent.

The most profitable method of treating sewage must be determined by
various local conditions; and it must be clearly understood that the
question of sewage disposal is primarily a sanitary one, and that it
must be dealt with from the sanitary aspect. The most profitable way of
applying sewage as a manure, however, will doubtless be found by
combining chemical precipitation and land irrigation.



CHAPTER XVIII.

LIQUID MANURE.


The adoption of irrigation as a means of utilising sewage, suggests a
short consideration of the value of liquid manures. It has been a custom
on many farms to apply the liquid manure got from the oozings of
manure-heaps, the drainings of the farmyard, byres, stables, piggeries,
&c., directly to the soil. Indeed, so strongly has the belief in the
superiority of liquid manure over other manure been held by certain
farmers, that they have washed the solid animal excreta with water, in
order to extract from it its soluble fertilising constituents. The late
Mr Mechi was one of the foremost exponents of the value of liquid
manure. His farm of Tiptree Hall was fitted up with iron pipes for the
distribution of the manure over the different fields. Superphosphate, it
may also be added, as first made from bones by Baron Liebig, was applied
in a liquid form. As to the general merits of liquid manure, there can
be no doubt that it is the most valuable form in which to apply manure.
It secures for the manurial ingredients it contains a speedy and uniform
diffusion in the soil; but, on the other hand, the expense of
distributing it makes its application far from economical. The chief
ingredient in liquid manure is urine. Now the removal of urine from the
farmyard manure-heap entails a severe loss of the ingredient which is
most potent in promoting fermentation. Separation of the urine from the
solid excreta is on this very account not to be recommended. Urine, when
applied alone, is lacking in phosphoric acid, of which it contains mere
traces. It is not, therefore, suitable as a general manure. It has to be
pointed out, however, that the drainings from a manure-heap in this
respect are superior to pure urine, since they contain the soluble
phosphates washed out of the solid excreta. The objections against using
liquid manure may be summed up as follows:--

First, it is too bulky a form in which to apply the manure, and hence
too expensive; secondly, it is not advisable to deprive the solid
excreta of the liquid excreta, as the one supplements the other;
thirdly, fermentation is largely fostered in the solid excreta by the
presence of the liquid excreta--hence fermentation will not take place
properly in the solid excreta when deprived of the liquid excreta.

If, however, the production of liquid manure on the farm is in excess
of what can be used for the proper fermentation of farmyard manure, it
will be best to utilise it for composts. No better addition to a compost
can be made than liquid manure, as it induces speedy fermentation in
nearly all kinds of organic matter.



CHAPTER XIX.

COMPOSTS.


The use of composts is an old one. Before artificial manures were so
plentiful as they are at present, much attention was paid by farmers to
their preparation. A compost is generally made by mixing some substance
of animal origin which is rich in manurial ingredients with peat or
loam, and often along with lime, alkali salts, common salt, and indeed
any sort of refuse which may be regarded as possessing a manurial value.
Composting, in short, may be looked upon as a useful method of turning
to profitable use refuse of various kinds which accumulate on the farm.
The object of composting is to promote fermentation of the materials
forming the compost, and to convert the manurial ingredients they
contain into an available condition for plant needs. Composts often
serve a useful purpose in retaining valuable volatile manurial
ingredients, such as ammonia, formed in easily fermentable substances
like urine. In fact, we may say that farmyard manure is the typical
compost, and its manufacture serves to illustrate the principles of
composting.


_Farmyard Manure a typical Compost._

Farmyard manure as ordinarily made is not generally regarded as a
compost, but in the past it has been widely used for the purpose of
making composts. Thus the practice of mixing farmyard manure with large
quantities of peat has been in some parts of the world a common one.
Peat, as has already been pointed out in a previous chapter, is
comparatively rich in nitrogen. When it is mixed with urine or some
other putrescible substance, the peat undergoes fermentation, with the
result that its nitrogen is to a greater or less extent converted into
ammonia. The effect, therefore, of mixing peat with farmyard manure is
beneficial to both substances mixed: the escape of ammonia is rendered
impossible by the fixing properties of the peat, while the inert
nitrogen of the peat is largely converted by fermentation into an
available form. The proportion of peat which it is advisable to add in
composting farmyard manure will depend on the richness of the quality of
the manure: the richer the quality of the manure, the greater the amount
of peat it will be able to ferment. Composts of this kind are generally
made by piling up the manure in heaps, consisting of alternate layers of
peat and farmyard manure. From one to five parts of peat to every one
part of farmyard manure is a common proportion. The use of such a
manure, containing so much organic matter, will exercise its best effect
on light sandy soils.


_Other Composts._

But instead of farmyard manure, or in addition to farmyard manure,
various other substances may be added, as bones, flesh, fish-scrap, and
the offal of slaughter-houses. Sometimes leaves and the dried
bracken-fern are used for the manufacture of composts. Some of these
substances contain much nitrogen or phosphoric acid, but in their
natural condition ferment when applied to the soil at a slow rate. If
mixed together before application in pits with peat, leaves,
bracken-fern, or some other absorbent material, fermentation proceeds
evenly and rapidly. The addition of lime, potash, and soda salts has
been found to have a most beneficial effect in promoting fermentation.
These substances, as is well known, hasten putrefaction of organic
matter. Lime seems especially to be valuable in composting. This is no
doubt due to the fact that lime plays a valuable part in promoting the
action of various ferments, as has already been illustrated in the case
of nitrification. The effect of large quantities of sour organic acids
(humic and ulmic), which are the invariable products of the
decomposition of organic matter like peat, leaves, &c., is inimical to
micro-organic life. The action of lime is to neutralise these acids.
There can be no doubt that composting is a useful process for increasing
the fertilising properties of different more or less inert manurial
substances. But in view of the abundant supply of concentrated
fertilisers, the use of composts may considerably decrease in future.



CHAPTER XX.

INDIRECT MANURES.

LIME.


We now come to discuss those manures which we may class under the term
_Indirect_, because their value is due, not to their direct action as
suppliers of plant-food--like those manures we have hitherto been
engaged in discussing--but to their indirect action. Of these by far the
most important is lime.


_Antiquity of Lime as a Manure._

Lime is one of the oldest and one of the most popular of all manures. It
is mentioned, and its wonderful action commented on, in the works of
several ancient writers, more especially Pliny. Of late years, perhaps,
its use has become restricted; and, as we shall point out by-and-by, it
is well that it is so.


_Action of Lime not thoroughly understood._

Despite the fact of the long-established and almost universal use of
lime, it can scarcely be said that we as yet clearly understand the
exact nature of its action. Much light, however, has been thrown of late
years on the subject by the great advance which has been made in our
knowledge of agricultural chemistry. Nevertheless, there are many points
connected with the action of lime on the soil which are still obscure.
Perhaps one reason for the conflicting ideas prevalent with regard to
the value of this substance in agriculture is to be found in the fact
that it acts in such a number of different ways, and that the nature of
the changes it gives rise to in the soil is most complicated. The
experience of agriculturists with lime in one part of the country often
seems contradictory to the experience of those in other parts of the
country. Its action on different soils is very dissimilar. For these
reasons, therefore, the discussion of the value of lime as a manure is
by no means an easy one.


_Lime a necessary Plant-food._

Lime, as we have already pointed out in a former chapter, is a necessary
plant-food, and were it present in the soil to a less extent than is
actually the case, would be just as valuable a manure as the different
nitrogenous and phosphatic manures; and in certain circumstances this is
the case. There are soils, though they are by no means of common
occurrence, which actually lack sufficient lime for supporting
plant-growth, and to which its addition directly promotes the growth of
the crop. Poor sandy soils are often of this nature. Another class of
soils are also apt to be lacking in lime--at any rate their surface-soil
is. These are permanent pasture-soils. Originally there may have been an
abundance of lime in the surface portion of the soil; but, as is well
known to every practical farmer, lime has a tendency to sink down in the
soil. This tendency in ordinary arable soils is largely counteracted by
ordinary tillage operations, such as ploughing, &c., by means of which
the lime is again brought to the surface. In permanent pasture-soils,
however, no such counteracting action takes place, hence impoverishment
of the surface-soil in lime eventually results. It is for this
reason--partly at any rate--that permanent pasture benefits in an
especial degree by the application of lime. We say _partly_, for there
are other important reasons. One is, that lime seems to have a striking
effect in improving the quality of pastures by inducing the finer
grasses to predominate. It has also a very favourable action in
promoting the growth of white clover. Another reason for the favourable
effect of lime on pasture-soils is doubtless on account of the action it
has in setting potash free from its compounds. Soils, however, which
directly benefit from the application of lime in the same way as they
benefit from the application of nitrogenous manures, may be safely said
to be rare. In the great majority of soils lime exists, so far as the
demands of plant-life are concerned, in superabundance.


_Lime of abundant Occurrence._

Indeed limestone is one of the most abundant of all rock substances, and
it has been calculated that it forms not less than one-sixth of the
rock-mass of the earth's crust. Nearly all the commonly occurring
minerals contain it, and in the course of their disintegration furnish
it to the soil. Vast tracts of country are composed of nothing but
limestone; and we have examples, even in this country, of so-called
chalk-soils, where it is the most abundant constituent. Nor can it be
classed amongst the insoluble mineral constituents of the soil; for
although insoluble in pure water, it is soluble in water--such as the
soil-water--which contains carbonic acid. This is proved by the fact
that it is the chief dissolved mineral ingredient in all natural waters.


_Lime returned to the Soil in ordinary Agricultural Practice._

It may be further pointed out, as bearing upon the true function of lime
when applied as a manure, that in ordinary agricultural practice nearly
all the lime removed from the soil in crops finds its way back again to
the farm in the straw of the farmyard manure. For these reasons, then,
it is clear that the true function of lime is as an indirect manure.

Let us now proceed to discuss its action. Before doing so, however, it
is important that we should clearly understand the different chemical
forms in which it occurs.


_Different Forms of Lime._

Lime occurs chiefly as carbonate of lime in the forms of limestone,
marble, or chalk, which are all chemically the same. It occurs also as
sulphate of lime or gypsum, as well as in the forms of phosphate and
fluoride. In agriculture it is only used--if we except the phosphate,
which is applied not on account of its lime, but its phosphoric acid--in
the form of the carbonate or _mild_ lime as it is commonly called,
burnt, caustic, or quick lime, and as gypsum. As the value of gypsum as
a manure is of such importance, and depends not entirely on its being a
compound of lime, we shall consider it by itself. Hence we have only to
consider here the action of mild and caustic lime.


_Caustic Lime._

When limestone or mild lime is submitted to a great heat, such as is
practically done on a large scale in lime-kilns, it is converted into
caustic lime or lime proper. Limestone is made up, as we have just
mentioned, of lime and carbonic acid. The latter ingredient is expelled
in the form of a gas, and the lime is left behind. Lime never occurs
naturally as caustic lime, for the simple reason that it is impossible
for it to remain in this state, owing to the great affinity it has both
for water and carbonic acid.

When lime is burnt, and before it is applied to the field, some time is
allowed to elapse in order to permit of its absorbing moisture--or
becoming slaked, as it is technically called. This it does more or less
slowly by absorbing moisture from the air. As, however, the process
would take too long, and as, moreover, the absorption of carbonic acid
gas would also take place at the same time, lime is generally slaked in
another way. This can be done by simply adding water. An objection to
this method is, that the lime is not so uniformly slaked as is
desirable. It becomes gritty. The usual method is to cover it up with
damp earth in heaps, and allow the moisture of the earth to effect the
slaking. When lime absorbs water a new chemical compound is formed,
known as lime hydrate; and so rapidly does the lime unite with water,
that a great deal of heat is evolved in the operation, the temperature
produced being considerably above that of boiling-water. The conversion
of slaked lime into carbonate of lime or mild lime is a slower process.
Sooner or later, however, it takes place, whether the lime is left on
the surface of the soil or buried in it.

A knowledge of these elementary chemical facts is necessary in order
clearly to understand the nature of the action of lime in agriculture.

The respective action of quicklime and mild lime is, on the whole,
similar, although the former is in every case very much more powerful
in its effects than the latter.


_Lime acts both mechanically and chemically._

Lime may be said to act on the soil both mechanically and chemically. It
alters the texture of the soil, and affects its mechanical properties,
such as its absorptive, retentive, and capillary powers with regard to
water. It acts upon its dormant fertility, and decomposes its mineral
substances as well as its organic matter. Lastly, its influence on the
micro-organic life of the soil, which plays such an important part in
the preparation and elaboration of plant-food, is of the highest
importance. We cannot do better, therefore, than discuss its properties
under the headings _mechanical_, _chemical_, and _biological_.


I. MECHANICAL FUNCTIONS OF LIME.


_Action on Soil's Texture._

The effect of lime upon the texture of a soil is among its most striking
properties. Every farmer knows well what a transformation is effected in
the texture of a stiff clay soil by the application of a dressing of
lime. The adhesive property of the soil--its objectionable tendency to
puddle when mixed with water--is greatly lessened, and the soil is
rendered very much more friable when it becomes dry. Several reasons
exist for this change. In the first place, the tendency to puddle in a
clayey soil is due to the fine state of division of the soil-particles.
The way in which lime counteracts this adhesive property is by causing a
coagulation of the fine soil-particles. This flocculation or aggregation
of the fine clay-particles, when mixed with water by lime, is strikingly
demonstrated by adding to some muddy water a little lime-water. The
result will be that the water will speedily be rendered clear, the fine
clay-particles coming together and sinking to the bottom of the vessel.
Even a very small quantity of lime will effect this change. This
property possessed by lime, we may mention, is utilised in the treatment
of sewage. As it is the fine clay-particles that are the chief cause of
the puddling of clay soils, their flocculation does much to destroy this
objectionable property. Another reason why lime renders a clay soil more
friable when dry is, that lime does not undergo any shrinkage in dry
weather. As clay soils shrink very much in drying, the mixture with such
a substance as lime tends to minimise this tendency to cake in hard
lumps. The effect of even a very small addition of lime to a clay soil,
in the way of increasing its friable nature, is very striking, and can
be easily illustrated by taking two portions of clay, into one of which
a small percentage of lime is introduced, and working both into a
plastic mass with water, and then allowing them to dry. It will be found
that while the one is hard and resists disintegration, that portion to
which the lime has been added crumbles away easily to a powder. This
effect which lime has in "lightening" heavy soils has been known to last
for years. The disintegrating effect of quicklime when applied to heavy
soils is also due, it may be added, to the change undergone by the lime
itself from the caustic state to the mild state.


_Lime renders light Soils more cohesive._

Although it may seem somewhat paradoxical, lime, it would appear, in
some cases exercises an effect upon the soil exactly the reverse of what
has just been stated. That lime should act as a binding agent is only
natural when we reflect on the way in which it acts when used as mortar.
It is quite to be understood, therefore, that its action on light
friable soils should be to increase their cohesive powers, and at the
same time to increase the capillary power of the soil to absorb water
from the lower layers. The extent of this action, of course, would
depend on the form in which the lime is applied, and the amount. A
striking example of the binding power of lime is to be found in certain
soils extremely rich in lime, in which what is known as a lime-pan has
been formed at some distance from the surface.


II. CHEMICAL ACTION OF LIME.

But more important probably than even its mechanical action is the
chemical action of lime. It is a most important agent in unlocking the
inert fertility of the soil. This it does by decomposing different
minerals and setting free the potash they contain. The disintegrating
power of lime in this respect depends, of course, on its chemical
condition, the caustic form being much more potent than the other forms.
Its action in decomposing vegetable matter and rendering the inert
nitrogen it contains available for the plant's use, is also one of its
most important properties, and accounts for its beneficial action when
applied to soils, such as peaty soils, rich in organic matter. Again,
its use as a corrective for sour lands has long been practically
recognised. The presence of acidity in a soil is hurtful to vegetable
life. Lime, by neutralising this acidity, removes the sourness of the
land, and does much to restore it to a condition suitable for the growth
of cultivated crops. The generation of sourness in a soil is almost sure
to give rise to certain poisonous compounds. Lime, therefore, in
sweetening a soil, prevents the formation of these poisonous compounds.
Badly drained and sour meadow-lands, as every farmer knows, are
immensely benefited by the application of this useful manure; for not
merely is their sourness removed and their general condition
ameliorated, but many of the coarser and lower forms of plant-life,
which alone flourish on such soils, are killed out, and the more
nutritive grasses are allowed to flourish instead. The action of lime in
promoting the formation of a class of compounds of great importance in
the soil--viz., hydrated silicates--is worthy of notice. According to
the commonly accepted theory, much of the available mineral fertilising
matter of the soil is retained in the form of these hydrated silicates.
Hence lime, by increasing these compounds, not merely adds to the amount
of the available fertility in the soil, but also increases its
absorptive power for food-constituents.


III. BIOLOGICAL ACTION OF LIME.

The last way in which lime acts is what we have termed biological. By
this we mean the important _rôle_ lime plays in promoting or retarding,
as the case may be, the various kinds of fermentative action which go on
so abundantly in all soils. The presence of carbonate of lime in the
soil is a necessary condition for the process of nitrification. Lime is
the base with which the nitric acid, when it is formed, combines; and as
we have seen, when discussing nitrification, soils of a chalky nature
are among those best suited to promote the natural formation of
nitrates. This is one of the reasons for the beneficial effects produced
by lime when applied to peaty soils. Not merely does it help to
decompose the organic matter so abundant in such soils, but it also
furnishes the base with which the nitric acid may combine when it is
formed. But while the action of lime is to promote fermentation, it must
not be forgotten that there may be cases in which its action is rather
the reverse of this. Fermentation of organic matter goes on when there
is a certain amount of alkalinity present; while, on the other hand, the
presence of acidity seems to retard and check it. Too great an amount of
alkalinity, however, would, in the first instance, retard fermentation
as much as too great acidity. It has been claimed that the addition of
caustic lime to fresh urine may act in this way; and if this were so,
the addition of lime to farmyard manure might, to a certain extent, be
defended. The experiment, however, would be a hazardous one and not to
be recommended, as loss of ammonia would most likely ensue.


_Action of Lime on Nitrogenous Organic Matter._

The action of lime on nitrogenous organic matter is of a very striking
kind, and is by no means very clearly understood. As we have pointed
out, it sometimes acts as an antiseptic or preservative; and this
antiseptic or preservative action has been explained on the assumption
that insoluble albuminates of lime are formed. Its action in such
industries as calico-printing, where it has been used along with casein
for fixing colouring matter; or in sugar-refining, where it is used for
clarifying the sugar by precipitating the albuminous matter in solution
in the saccharine liquor; or lastly, in purifying sewage,--has been
cited in support of this theory. While, however, there may be
circumstances in which lime, especially in its caustic form, acts as an
antiseptic, its general tendency is to promote these fermentative
changes, such as nitrification, so important to plant-life.

An important use of lime in agriculture is in preventing the action of
certain fungoid diseases, such as "rust," "smut," "finger-and-toe," &c.,
as well as in killing, as every horticulturist and farmer knows, slugs,
&c.


_Recapitulation._

We may, in conclusion, sum up in a single paragraph the different ways
in which lime acts. Its action is mechanical, chemical, and biological.
It acts on the texture of the soil, rendering clay soils more friable,
and exerting a certain binding effect on loose soils. It decomposes the
minerals containing potash and other food-constituents, and renders them
available for the plant's needs. It further decomposes organic matter,
and promotes the important process of nitrification. It increases the
power of a soil to fix such valuable food-constituents as ammonia and
potash. It neutralises sourness, and prevents the formation of poisonous
compounds in the soil. It increases the capillary condition of the soil,
prevents fungoid diseases, and promotes the growth of the more nutritive
herbage in pasture-land.



CHAPTER XXI.

INDIRECT MANURES--GYPSUM, SALT, ETC.


GYPSUM.

In the previous chapter mention was made of gypsum as a compound of
lime, but no reference to its action as a manure was made. In the past,
gypsum was used extensively and highly valued. It was found to be of
especial value for clover; and there is a story told of Benjamin
Franklin which illustrates the very striking nature of its action on
this crop. It is related that he once printed with gypsum the words
"This has been plastered" on a field of clover, and that for a long time
afterwards the legend was plainly discernible on account of the
luxuriance of the clover on the parts of the field which had been thus
treated.


_Mode in which gypsum acts._

Despite the fact that gypsum is a most ancient manure, it is only of
late years that we have come to understand the true nature of its
action. For long it was believed that the reason of its striking effect
in promoting clover was due to the fact that, as clover was a
lime-loving plant, the action of gypsum was owing to the lime it
contained. That, however, the action of gypsum is not due to the fact
that it supplies lime to the plant, seems evident when it is stated that
were this so, any other form of lime would have the same beneficial
effect. It is well known, however, that this is not so. Besides, as we
have already pointed out, lime is not a constituent which most soils
lack, so far as the needs of the crop are concerned. There is a certain
amount of truth in the old belief that gypsum enriches the soil in
ammonia by fixing it from the air. The power that gypsum has as a fixer
of ammonia has already been referred to in the chapter on Farmyard
Manure; but in this case the gypsum is brought in contact with the
ammonia. The origin of this old belief was due to a misconception as to
the amount of ammonia in the atmosphere. No doubt gypsum greatly
increases the power of a soil to absorb ammonia from the air; but the
quantity of ammonia in the air is so very trifling, that its action in
this respect is hardly worth considering. The true explanation of the
action of gypsum is to be found in its effect on the double silicates,
which it decomposes, the potash being set free. Its action is similar to
that of other lime compounds, only more characteristic. As a manure,
therefore, its action is indirect, and its true function is to oust the
potash from its compounds. Its peculiarly favourable action on clover is
due to the fact that clover specially benefits by potash, and that
adding gypsum practically amounts to adding potash. Of course it should
be borne in mind that the soil must contain potash compounds if gypsum
is to have its full effect. Now, however, that potash salts suitable for
manuring purposes are abundant, it may well be doubted whether it is not
better to apply potash directly. Further, it must be borne in mind that
gypsum is applied to the soil whenever it receives a dressing of
superphosphate of lime, as gypsum is one of the products formed by
treating insoluble phosphate of lime with sulphuric acid.

It is possible that gypsum may act as an oxidising agent in the soil,
just as iron in the ferric condition does. It has a large quantity of
oxygen in its composition, and under certain conditions may act as a
carrier of oxygen to the lower layers of the soil. When it is used, it
should be applied some months before the crop is sown.

Gypsum, therefore, although it contains two necessary
plant-constituents, lime and sulphuric acid, cannot be regarded as a
direct manure; and as its action comes to be more fully understood, its
use, which was never very abundant in this country, will probably
decrease. We have already, in the chapter on Nitrification, referred to
the action of gypsum in promoting nitrification.


SALT.

The action of salt as a manure presents a problem which is at once of
the highest interest and surrounded with the greatest difficulties. In
view of the large quantities now used for agricultural purposes, a
somewhat detailed examination of the nature of its action is not out of
place in a work such as the present.


_Antiquity of the Use of Salt._

The recognition of the manurial functions of salt dates back to the very
earliest times. Its use among the ancients is testified by numerous
allusions in the Old Testament; while, according to Pliny, it was a
well-known manure in Italy. The Persians and the Chinese seem also to
have used it from time immemorial, the former more especially for
date-trees.


_Nature of its Action._

Despite, however, the great antiquity of its use, much difference of
opinion seems always to have existed as to the exact method of its
action, and as to its merits as a manure in promoting vegetable growth.
It furnishes, in fact, a good example of the difficulty which exists in
the case of many manures, whose action is chiefly indirect, of fully
understanding their influence on the soil and on the crop. In fact, the
action of salt is probably more complicated than that of any other
manurial substance.


_Salt not a necessary Plant-food._

We have already seen that neither sodium nor chlorine--the two
constituent elements of salt--are in all probability absolutely
necessary plant-foods. If they are necessary, the plant only requires
them in minute quantities. Despite this fact, soda is an ash-constituent
of nearly every plant, and in many cases one of the most abundant. In
amount it is one of the most variable of all the ash-constituents, being
present in some plants only in minute quantities, while in others it
occurs in large quantities. Mangel and plants of the cabbage tribe may
be cited as examples of plants containing large amounts of soda in their
composition. But the plants which contain it in largest quantity are
those which thrive on the sea-coast, and it has been thought that for
them at least salt is a necessary manure. This, however, does not seem
to be the case. In fact, the amount of soda in a plant seems to be
largely a matter of accident. It may be added that the succulent
portions of a plant are generally richest in soda.


_Can Soda replace Potash?_

Again, it has been believed that soda is capable of replacing potash in
the plant; but this does not seem to be the case to any extent. The view
that soda is able to replace potash, it has been thought, is supported
by the variation which exists in the proportion of soda and potash in
different plants. It must be remembered, however, that it is highly
probable that most plants contain a larger quantity of ash-constituents
than is absolutely necessary for their healthy growth. Especially is
this the case with such a necessary plant-food as potash, of which there
is generally present, in all likelihood, an excess. The variation in the
quantity of potash and soda present in many plants under different
circumstances can scarcely, therefore, be regarded as furnishing a proof
of the replacement of potash by soda. Incidentally we may mention, as a
fact worthy of notice, that cultivated plants have more potash and less
soda in their composition than wild plants. What has been said of soda
may be held to apply equally to chlorine, as it seems to be chiefly in
the form of common salt that soda enters the plant. The amount of salt,
therefore, present in plants must be regarded as largely accidental and
dependent on external circumstances, such as the nature of the soil, &c.


_Salt of universal Occurrence._

But even were salt a necessary plant-food, its occurrence in the soil is
already of sufficient abundance to obviate any necessity for its
application. It may be said to be of almost universal occurrence. Even
the air contains it in traces. That this is the case in the
neighbourhood of the sea-coast is well known; but even in air far
inland, accurate analysis of the air would probably demonstrate its
presence in greater quantity than is commonly believed. It is a wise
provision that plants absorb salt, for it increases their efficiency as
food,--the function of salt as a constituent of animal food being of the
very highest importance. It is an indispensable food-ingredient for
animal life. With regard to ordinary farm-stock, the amount of salt
which naturally occurs in their food is quite sufficient. In the case,
however, of pastures in countries far removed from the sea, the custom
of specially supplying stock with salt is common. This is done by
placing a piece of rock-salt in the fields.


_Special Sources of Salt._

The salt of commerce is obtained from various sources. Besides the sea,
we have ample sources of salt in the large saline deposits found in many
parts of Europe, especially in Austria, and in England in Cheshire.


_The Action of Salt indirect._

From what has been said above, it is clear that the action of salt as a
manure is indirect and not direct. What the nature of that indirect
action is we shall now proceed to discuss.

In considering the evidence of the manurial value of salt, we are at
once brought face to face with the fact that the experience of its
action in the past has as often been unfavourable as favourable. Salt,
it is well known, is both an antiseptic and a germicide. It is, indeed,
one of the most commonly used of preservatives. When applied in large
quantities to the soil, it has a most deleterious action on vegetation.
This hurtful action of salt has long been known; and it is as often
mentioned in the writings of antiquity on account of its unfavourable as
on account of its favourable action. Thus, for example, among the
ancient Jews it was customary, after the conquest of a hostile town, to
strew salt on the enemy's fields, for the purpose of rendering them
barren and unfertile. And again, among the Romans, for the same purpose,
salt was often spread on a spot where some great crime had been
committed.

While, therefore, its unfavourable action has long been known, the fact
that there are circumstances under which its action is, on the contrary,
favourable for promoting vegetable growth has also been long recognised.
The difficulty for the agricultural student is to reconcile these two
seemingly contradictory experiences. For the English agriculturist the
subject possesses especial interest, since in England it has been in the
past most generally used and its action most discussed since the time of
Lord Bacon, who discusses in his writings the action of solutions of it
on different plants.

The true explanation of salt being so different in its action is to be
found in the quantity applied, the nature of the soil, the crop to which
it is applied, and the conditions under which it is applied--_i.e._,
whether it is applied alone or along with other manures.


_Mechanical Action on Soils._

In the first place, it must be noted that salt exerts a mechanical
action on the soil of a very similar kind to that exercised by lime.
When applied to clay soils it causes a flocculation or coagulation of
the fine clay-particles, and thus prevents the soil from puddling to the
same extent as would otherwise be the case. In fact, an example of this
action of salt when in solution causing the precipitation of fine
suspended clayey matter, is afforded by the formation of deltas at the
mouths of rivers. The power of clarifying muddy water is common indeed
to saline solutions. Schloesing attributes the clarifying power of a
soil to the presence of the saline matters it contains; and from this
point of view it would appear that manures containing any saline
substance may exert an important mechanical influence on the soil.


_Solvent Action._

But a much more important property of salt is its solvent action on the
plant-food present in the soil. Its action in decomposing the minerals
containing lime, magnesia, potash, &c., is similar to the action of
gypsum. By acting upon the double silicates it liberates these necessary
plant-foods. It is not only on the basic substances upon which it acts,
but also on the phosphoric and silicic acids, which it sets free. Its
power of dissolving ammonia from the soil is considerable. Experiments
with a weak solution of salt on a soil by Peters and Eichhorn to test
its solvent power, showed that the salt solution dissolved more than
twice as much potash and nearly thirty times as much ammonia as an equal
quantity of pure water did. When applied to the soil, it seems chiefly
to liberate lime and magnesia. The exact nature of the chemical action
taking place is a point of some dubiety. According to some, it is
changed into nitrate of soda; according to others, into carbonate of
soda. The latter theory seems to be the more probable one. Its action on
the lime and magnesia compounds is to convert them into chlorides; and
this chemical reaction explains the action that salt has in increasing
the water-retaining and water-absorbing power of the soil; for the
chlorides of magnesia and lime are salts which have a great power of
attracting water from the air.

Again, the very fact that salt acts as an antiseptic may serve to
explain its beneficial action in certain cases where it prevents
rankness of growth. No doubt this was its function when applied along
with Peruvian guano. This it might do by preventing too rapid
fermentation (nitrification) of the manure, or by actually weakening the
plant. Its action when applied with farmyard manure may also be similar.
But while its effect in many cases may be towards retarding
fermentation, on the other hand its action, when applied along with lime
to compost-heaps, is towards promoting more rapid decomposition.
Probably a reaction takes place between the lime and the salt, the
result of which is the formation of caustic soda.

Such are some of the ways in which salt may act. It must at once be seen
how its action in one case will be favourable and in another case
unfavourable. There must be fertilising matter present in the soil if it
is to act favourably. Again, it will only be under such circumstances,
where rankness of growth is likely to ensue, that its antiseptic
properties will act favourably and not unfavourably.


_Best used in small Quantities along with Manures._

Probably it is for these reasons that its action has been found to be
most favourable when applied along with other manures and not alone.
Applied along with nitrate of soda, as is commonly done, it doubtless
increases the efficiency of the nitrate. Some plants seem to be
undoubtedly benefited by salt: of these flax may be mentioned. The
application of salt to plants of the cabbage tribe seems also to be
highly beneficial. On mangels, along with other manures, it has also
been found to have a very favourable effect. But with many crops its
action has been proved to be less favourable.


_Affects Quality of Crop._

Although salt has often been found to increase the quantity of a crop,
the quality of the crop has been made to suffer. Its action on beetroot
has been more especially studied. The effect of its application is to
lessen the total quantity of dry matter and sugar in the plant. This has
been found to be the case both when the salt was applied alone and along
with nitrate of soda and other manures. On potatoes, again, its action
has been found to be deleterious, lessening their percentage of starch.
The deleterious action of chlorides on the quality of potatoes is also
seen when potassium chloride is applied. It is for this reason that
potash should never be applied to the potato crop in the form of
chloride.

In the late Dr Voelcker's opinion, the conditions under which salt had
the most favourable action on the mangel crop was in the case of a light
sandy soil, and applied at the rate of 4 to 5 cwt. per acre. Its action
when applied to clay soils was not so favourable.


_Rate of Application._

Lastly, the rate at which it may be applied will naturally vary. From 1
cwt. and even less, up to 6 cwt. or even more, has been the rate at
which it has been commonly applied in the past. From what has been said,
it will be seen that it is more likely to exert a favourable influence
when applied only in small quantities.



CHAPTER XXII.

THE APPLICATION OF MANURES.


The conditions which regulate the application of manures are many and
varied, and the subject, it must be admitted, despite the large amount
of investigation already carried out, is most imperfectly understood.
For these reasons it is impossible to do little more than lay down
certain general principles which may be of service to the agriculturist
in guiding him in carrying out the manuring of his crops.


_Influence of Manures in increasing Soil-fertility._

In the first place it may be asked, How far can what we may call the
permanent fertility of a field be influenced by the application of
manures? And to this question the answer must be made, that the
influence of manuring in increasing soil-fertility is very slight and
only very gradually felt. This is illustrated by the difficulty
experienced in attempting to restore to a fertile condition a soil which
has long been treated by an exhaustive system of cultivation. In such a
case it will be found impossible to restore the fertility of the soil,
except very gradually. Farmers who farm in new countries, and in rich
virgin soils, little realise sometimes how quickly they may impoverish
the fertility of their soils by exhaustive treatment, and how slow the
process of restoration is. Nor is this strange when we reflect on the
relatively small quantities of fertilising ingredients we are in the
habit of adding to the soil by the application of manures, and the
nature of their action. The small rate at which they are applied, and
the impossibility of distributing them equally in the soil, explain how
comparatively limited their action must necessarily be. Some manures, it
is true--viz., those which are soluble--are more equally distributed;
but then such manures, from their very nature, are little likely to
affect the permanent fertility of the soil.


_Influence of Farmyard Manure on the Soil._

Of manures which have the best effect in improving a soil's permanent
fertility, farmyard manure is undoubtedly the most important. This is
owing partly to the fact that it is applied in such large quantities,
and partly on account of its composition. Liberal manuring with farmyard
manure, systematically carried out, will in time do much to build up a
soil's fertility. But liberal manuring with artificial manures will also
effect the same end. This it does in an indirect manner by means of the
increased crop residues obtained under such treatment. Indeed one of the
speediest methods of bringing a soil into good condition is by heavily
manuring certain green crops, and then ploughing them in.


_Farmyard Manure v. Artificials._

The question how far farmyard manure may be supplanted by artificials is
one often discussed. We have already referred to this question in the
chapter on Farmyard Manure. It is possible that, with our increasing
knowledge of agricultural science, we may in the future be able to
dispense with farmyard manure, and make shift to do with artificials
alone. At present, however, all our experience points to the fact that
the most satisfactory results are obtained from manures by using
artificials in conjunction with farmyard manure. It is better both for
farmyard manure and artificial manures to be applied together,[241] so
that they may mutually act as supplementary the one to the other. While
this is so, there may be circumstances in which it will be best to use
artificials alone. Where, for example, fields, owing to their situation,
are inaccessible, and where the expense of conveying the bulky farmyard
manure would be very considerable, it may be found more economical to
apply the more concentrated artificial manures. With few exceptions,
however, it will be found most desirable to use artificial manures as
supplementary to farmyard manure, and not as substitutes for it.


_Farmyard Manure not favourable to certain Crops._

While the above is true, it may be well to point out one or two facts
regarding the nature of the influence of farmyard manure on certain
crops. For instance, it has long been recognised as inadvisable in
strong rich soils to apply it directly to certain grain crops, such as
barley and wheat, since such a practice is apt to encourage rankness of
growth--an undue development of straw at the expense of the grain. It is
consequently customary to apply farmyard manure to the preceding crop.
The direct application of farmyard manure to wheat, however, according
to Sir J. B. Lawes, is not fraught with unfavourable results where the
soil is a light one; it is only when the soil is of a heavy nature that
it is best to apply it to the preceding crop. Potatoes are another crop
to which it is best not to apply it directly. On the other hand, many
are of the opinion that mangels seem to be able to benefit from large
applications of farmyard manure.


_Conditions determining the Application of Artificial Manures._

In the application of artificial manures a large number of
considerations have to be taken into account. Among these may be
mentioned the nature of the manure itself, and its mechanical and
chemical condition; the nature of the soil and its previous treatment
with manures, as well as the nature of the climate, the nature of the
crop, and the previous cropping. It may be well, therefore, to examine
somewhat in detail some of these considerations.


_Nature of the Manure._

Nitrogen, phosphoric acid, and potash exist in the common manures, as
has already been pointed out, in different states of availability.
Nitrogen, for example, may exist in a soluble or insoluble condition, as
nitrates, as ammonia, or in various organic forms. Phosphoric acid,
similarly, may exist in a soluble form, as it does in superphosphate of
lime, or in an insoluble form, as it does in bones or basic slag.
Potash, on the other hand, exists--or should exist--in artificial
manures only in a soluble form. Now a correct knowledge of the behaviour
of these different forms of the common manurial ingredients when applied
to the soil is, in the first place, necessary for their successful and
economical use.


_Nitrogenous Manures._

Thus our knowledge of the inability of the soil-particles to retain
nitrogen in the form of nitric acid, as well as our knowledge of the
fact that nitrogen is in this form immediately available for the
plant's needs, teaches us that nitrate of soda should never be applied
before the plant is ready to utilise it--in short, that it should only
be applied as a top-dressing; and further, that the use of such a
fertiliser in a damp season is less likely to be economical than in a
dry one. Again, with regard to nitrogen in the form of ammonia salts,
our knowledge of the fact that ammonia is retained by the
soil-particles, and that before it becomes available for the plant's
needs it has to undergo the process of nitrification, teaches us the
desirability of applying it a short time before it is likely to be used.
While, lastly, with regard to the nitrogen in the various organic forms
in which it occurs, our knowledge of the rate at which these are
converted into an available form in the soil will determine when they
are best applied. Some forms of organic nitrogen are in a soluble
condition, and are quite as speedy in their action as sulphate of
ammonia. This is the case with a considerable proportion of the
different organic forms of nitrogen present in guano. Other forms of
organic nitrogen are only slightly less so--as, for example, dried
blood, which ferments very speedily. With regard, therefore, to nitrates
and ammonia salts, as well as the more quickly available organic forms
of nitrogen, they should either be applied as a top-dressing after the
plant has started growth, or only shortly before seed-time. Bones,
shoddy, and the various so-called native guanos, should be applied a
considerable period before they are likely to be required--not later
than the previous autumn.


_Phosphatic Manures._

With regard to phosphatic manures the same considerations hold good.
Inasmuch as phosphoric acid, whether applied in the soluble condition,
as in superphosphate, or the insoluble form, as in bones, basic slag,
&c., is not liable to be washed out of the soil, the risk of loss is
very slight, and need not be taken into account. As we have pointed out
in considering the action of superphosphate, phosphoric acid in this
latter form is more speedily available to the crop, and the necessity of
applying it much before it is likely to be used does not exist. Hence
superphosphate and manures which contain any appreciable amount of
soluble phosphoric acid, such as guano, should only be applied shortly
before seed-time. Bones, basic slag, or mineral phosphate ought to be
applied, on the other hand, a long time before they are likely to be
used. Hence an autumn application is to be recommended in the case of
such manures.


_Potash Manures._

Lastly, with regard to potash manures, as these are soluble, there is no
necessity for applying them much before they are likely to be absorbed
by the plant. Some are of the opinion that potash is, except in the case
of sandy soils, best applied some little time before it is likely to be
used, so as to permit of its being washed down into the soil--a process
which takes place only comparatively slowly. As potash manures have
often been found to give a better result on pastures during the second
year than during the first, they are best applied in the autumn.

The above statement as to the behaviour of the different fertilisers
when applied to the soil, has a not unimportant bearing on the
quantities in which they may safely be respectively applied. The rate at
which manures may be applied depends, as we shall immediately see, on
other conditions; but what it is here desirable to point out is, that it
is not safe to apply such manures as nitrate of soda, or, for that
matter, sulphate of ammonia, in large quantities at a time. In fact
these manures, especially the former, will best be applied in very small
quantities, and rather in several doses. With regard to other manures,
more especially phosphatic manures, the same reasons for small
application do not exist.

The truth of the above statements is so obvious that it may be regarded
as superfluous to make them. As, however, their clear apprehension is
essential to understanding the conditions of successful manuring, no
apology need be made for making them.


_Nature of Soil._

Another condition which has to be taken into account in considering the
application of manures is the nature of the soil, as well as its
previous treatment. Soils poor in organic matter are those which are
most likely to be benefited by the application of nitrogenous manures.
Soils of a dry light character require less phosphoric acid than they do
of nitrogen and potash; while on a damp and heavy soil phosphatic
manures are more likely to be beneficial than nitrogenous or potassic
manures. Lastly, a soil rich in organic matter generally requires
phosphates, and possibly potash. A point of considerable importance to
notice is, that a soil rich in lime can stand a larger application of
phosphoric acid than one poor in lime. As a rule, it will be found that
the best results with potash will be obtained when applied to a sandy
soil. The nature of the soil is an important consideration in
determining how far it is advisable to apply readily soluble manures. To
a very light and non-retentive soil the risk of loss in applying an
easily soluble manure is considerably increased. The nature of the
climate is also of importance. Thus, in a dry climate, manures of a
soluble nature will have a better effect than in a wet climate, while
the opposite will be the case with the more slowly acting manures.


_Nature of previous Manuring._

A consideration of equal importance is the previous treatment of the
soil with manure. For example, where a soil has been liberally treated
with farmyard manure, it has been found that mineral manures have a
very inferior effect to that obtained by nitrogenous manure. Lawes and
Gilbert have found this to be strikingly the case in their experiments
on the growth of wheat. In these experiments it was found that the
application of mineral manures was accompanied with little or no benefit
to the crop, whereas very striking results followed the application of
nitrogen. This they attributed to the fact that the supply of mineral
fertilisers in the straw of the farmyard manure is largely in excess of
the supply of nitrogen. The nature of the action of the manure
previously applied is also to be taken into account in determining how
long its influence may probably last. Where, for example, the manure has
been nitrate of soda or sulphate of ammonia, it may be safely concluded
that its direct influence is no longer felt a year after application.
The influence of superphosphate of lime, while scarcely so temporary,
may be said to last only for a comparatively short time.[242] On the
other hand, when the manure applied is of a slow-acting nature, such as
bones or basic slag, its influence will probably be felt for a number of
years.


_Nature of the Crop._

But more important than any of the above-mentioned conditions is the
nature of the crop itself. Our knowledge of the requirements of the
different farm crops is still very imperfect. A very wide experience,
however, of the effect of different manures on different crops, has
conclusively proved that their manurial requirements differ very
considerably. The subject is complicated by other considerations, such
as the nature of the soil, &c.; but notwithstanding this fact, certain
points seem to be pretty well established.

In seeking to understand the respective requirements of the different
crops for different fertilisers, two important considerations must be
borne in mind. These are--(1) _the quantities of the three fertilising
ingredients--nitrogen, phosphoric acid, and potash--which different
crops remove from the soil;_ and (2) _the different power crops possess
of assimilating these ingredients._


_Amounts of Fertilising Ingredients removed from the Soil by different
Crops._

The most convenient way of instituting a comparison between the
requirements of the different crops in this respect is by calculating
the amount, in pounds, of nitrogen, phosphoric acid, and potash, which
average amounts of the different crops remove per acre. The following
table shows this for the common crops:--


                 FERTILISING INGREDIENTS REMOVED FROM SOIL.
----------------------------+-----------+------------+----------
                            | Nitrogen. | Phosphoric |  Potash.
                            |           |    Acid.   |
----------------------------+-----------+------------+----------
Mangels {Root, 22 tons      |    87     |    36.4    |   222.8
        {Leaf               |    51     |    16.5    |    77.9
                            |-----------|------------|----------
             Total crop     |   138     |    52.9    |   300.7
                            |-----------+------------+----------
Turnips {Root, 17 tons      |    63     |    22.4    |   108.6
        {Leaf               |    49     |    10.7    |    40.2
                            |-----------+------------+----------
             Total crop     |   112     |    33.1    |   148.8
                            |-----------+------------+----------
Beans   {Grain, 30 bushels  |    77     |    22.8    |    24.3
        {Straw              |    29     |     6.3    |    42.8
                            |-----------+------------+----------
             Total crop     |   106     |    29.1    |    67.1
                            |-----------+------------+----------
Red clover hay, 2 tons      |   102     |    24.9    |    83.4
                            |-----------+------------+----------
Swedes  {Root, 14 tons      |    70     |    16.9    |    63.3
        {Leaf               |    28     |     4.8    |    16.4
                            |-----------+------------+----------
             Total crop     |    98     |    21.7    |    79.7
                            |-----------+------------+----------
Oats    {Grain, 45 bushels  |    38     |    13.0    |     9.1
        {Straw              |    17     |     6.4    |    37.0
                            |-----------+------------+----------
             Total crop     |    55     |    19.4    |    46.1
                            |-----------+------------+----------
Meadow hay, 1-1/2 ton,      |    49     |    12.3    |    50.9
                            |-----------+------------+----------
Wheat   {Grain, 30 bushels  |    33     |    16.0    |     9.8
        {Straw              |    15     |     4.7    |    25.9
                            |-----------+------------+----------
             Total crop     |    48     |    20.7    |    35.7
                            |-----------+------------+----------
Barley  {Grain, 30 bushels  |    35     |    16.0    |     9.8
        {Straw              |    13     |     4.7    |    25.9
                            |-----------+------------+----------
             Total crop     |    48     |    20.7    |    35.7
                            |-----------+------------+----------
Potatoes, 6 tons            |    47     |    21.5    |    76.5
                            |-----------+------------+----------
Maize   {Grain, 30 bushels  |    28     |    10.0    |     6.5
        {Stalks, &c.        |    15     |     8.0    |    29.8
                            |-----------+------------+----------
             Total crop     |    43     |    18.0    |    36.3
----------------------------+-----------+------------+----------

From the table it will be seen that the crops which remove the largest
quantities of all three fertilising ingredients are the root
crops--mangels and turnips; that beans remove twice as much nitrogen as
the cereals--oats, barley, and wheat--which, in this respect,
practically differ very little from one another; while potatoes remove
about the same quantity of nitrogen as the cereals. It will further be
noticed that the amounts of phosphoric acid removed by the different
crops differ very much less than those of nitrogen and potash. Mangels
remove slightly more, and turnips slightly less, than double the amount
removed by cereals. Meadow-hay, it will be seen, of all crops removes
the least phosphoric acid.

In looking at the amounts of potash, we are at once struck by their
great discrepancy. Such a crop as mangels removes more than six times as
much potash from the soil as the cereals. Turnips also make large
demands on this ingredient, removing over four times as much as the
cereals. Leguminous crops, such as red clover and beans, remove about
twice as much.


_Capacity of Crops for assimilating Manures._

Instructive though these figures undoubtedly are, _they must not be
regarded, as often erroneously they are, as furnishing by themselves
sufficient data upon which to base the practice of manuring_. A
consideration which is of much greater importance is the capacity that
different crops possess for assimilating the various manurial
ingredients from the soil. Considered from the point of view of absolute
amount, there is in most soils an abundant supply of plant-food; but of
this amount only a small proportion is available. Further, the amount of
this available plant-food will vary with different crops--one crop being
able to grow where another crop would starve. As illustrative of this,
in the Norfolk experiments it was found that the turnip was able to
assimilate potash from a soil on which the swede was practically
starved. It is on this fact more than any other that the principles of
manuring are based. Several explanations of the different capacities
crops possess of assimilating their food may be put forward. And we may
here point out that crops belonging to the same class exhibit, on the
whole, a certain amount of similarity in their manurial requirements.
Thus, for example, we may say that _gramineous crops_ so far resemble
one another in possessing _small capacity for assimilating nitrogen_,
_root crops for assimilating phosphoric acid_, and _leguminous crops for
assimilating potash_, and that, consequently, these crops are generally
most benefited by the application, respectively, of nitrogen, phosphoric
acid, and potash. But while a certain general resemblance exists, crops
belonging to the same class differ in many cases very considerably, as
we shall immediately see.


_Difference in Root Systems of different Crops._

One explanation of the different capacity possessed by different crops
for absorbing plant-food from the soil is to be found in the difference
of their root systems. Every agriculturist knows that crops in this
respect differ very widely. Crops having deep roots will naturally have
a larger surface of soil from which to draw their food-supplies than
crops having shallower roots. Such crops as red clover, wheat, and
mangels are able to draw their food-supplies from the subsoil to an
extent not possessed by shallower-rooted crops, such as barley, turnips,
and grass. Crops having surface-roots, on the other hand, have often
greater capacity for assimilating nitrogen,--this ingredient, as has
already been pointed out, being chiefly located in the surface-soil. The
tendency of growing shallow-rooted crops will therefore be towards
impoverishing the surface-soil; whereas the occasional growth of a
deep-rooted crop brings the plant-food in the subsoil into requisition.
In this connection it may be well to draw attention to the singular
capacity possessed by certain crops for absorbing nitrogen. Of these the
case of clover is the most striking, and has long puzzled
agriculturists. The discovery, which has been repeatedly referred to in
these pages, that the leguminous order of crops, to which clover
belongs, have the power of absorbing the free nitrogen of the air
through the agency of micro-organic life in the plant and in the soil,
has furnished an explanation of this long-debated problem.


_Period of Growth._

A further reason is the difference in the period of a crop's growth. A
crop which grows quickly, and consequently occupies the ground during a
comparatively short period, will naturally require a richer soil, and
therefore a more liberal treatment with manure, than one whose growth is
more gradual.

Another consideration is the season of the year during which active
growth of the crops takes place. For example, in the case of the wheat
crop, active growth takes place in spring and ceases early in the
summer. Since, however, nitrification goes on right through the summer,
and nitrates are most abundant in the soil in late summer and autumn,
such a crop as wheat is ill suited to obtain any benefit from this
bountiful provision of nature, and is consequently particularly
benefited by the application of nitrogenous manures. Root crops, on the
other hand, sown in summer, continue their active growth into autumn,
and are thus enabled to utilise the nitrates formed in the process of
nitrification. The custom of sowing a quickly growing green crop, such
as rye, mustard, rape, &c., after a wheat crop, is a practice which aims
at conserving the nitrates and preventing their loss by autumn and
winter rains. The name "catch crop" has been applied to such a crop. By
ploughing under the green crop, the nitrogen removed from the soil in
the form of easily soluble nitrates is restored in an insoluble organic
form, and the soil is at the same time enriched by the addition of much
valuable organic matter.[243]

It is chiefly the above facts that form the scientific basis of the
long-pursued practice of the rotation of crops.


_Variation in Composition of Crops._

A point of considerable interest is the influence exerted by manures on
the composition of crops. It has been assumed in the preceding pages
that the composition of crops of the same plant is uniform; but this is
not strictly the case, as it has been proved that not merely the manure
and soil have an appreciable influence on the crop's composition, but so
also has the climate.


_Absorption of Plant-food._

The laws regulating the absorption of plant-food are most interesting,
although, unfortunately, very imperfectly understood as yet. The
fertilising ingredients are capable of considerable movement in the
plant, and are only absorbed up to a certain period of growth. This in
many plants is reached when they flower. After this period they are no
longer capable of absorbing any more food. The popular belief that
plants in ripening exhaust the soil of its fertilising matters is
consequently a fallacy.


_Fertilising Ingredients lodge in the Seed._

The tendency of fertilising matters is to move upward in the plant as it
matures, and finally to become lodged in the seed. It is for this reason
that the cereals prove such an exhaustive crop. That nature, however,
can in certain cases be very economical of her food-supplies, is
strikingly illustrated by the fact that much of the fertilising matter
contained in the mature leaves in autumn passes back into the tree
before the leaves fall from it.


_Forms in which Nitrogen exists in Plants._

The form in which nitrogen is present in the plant is chiefly as
albuminoids. As, however, albuminoids belong to that class of bodies
known as colloids, which cannot easily pass through porous membranes
like those forming the walls of plant-cells, they are changed during
certain periods of the plant's growth into amides, which are
crystalloids, and consequently able to move freely about in the plant.
Amides are most abundant in young plants during the period of their most
active growth, and as the plant ripens the amides seem to be largely
converted into albuminoids.

While the subject is not very clearly understood, it would seem to be
pretty conclusively proved that there is a direct relation between the
amount of the phosphoric acid and of the nitrogen absorbed.


_Bearing of above Facts on Agricultural Practice._

The bearing of these facts upon practice is obvious. In the first place,
they show how important it is that plants should be well fed when they
are young, and that in the practice of green manuring it is best to
plough in the crop when it is in flower, as no additional benefit is
gained by allowing it to ripen, seeing that no further absorption of
fertilising ingredients takes place after the period of flowering.


_Influence of excessive Manuring of Crops._

The influence of large quantities of manures is seen in the case of
certain root crops. It is found, in such a case, that while the roots
are larger, they are more watery in composition and of less nutritive
value. Again, it seems to be a fact pretty generally known to practical
men, that nitrate of soda seems to have a bad effect on the quality of
hay. It would seem, further, that the influence of nitrogenous
fertilisers on cereals is to increase the percentage of nitrogen in the
grain, but that they have no such influence in the case of leguminous
crops. Phosphatic manures, on the other hand, in the case of leguminous
crops, seem to have the effect of diminishing the amount of nitrogen in
the seed.


FOOTNOTES:

[241] Though not necessarily at the same time or to each succeeding
crop. There may be comparatively long intervals between the applications
of farmyard manure in many cases.

[242] Of course what is meant here is the direct influence of such
manures. Their indirect value may be shown in the soil by the increased
crop residues they give rise to.

[243] This is very concisely and clearly put in Mr Warington's admirable
'Chemistry of the Farm.'



CHAPTER XXIII.

MANURING OF THE COMMON FARM CROPS.


In this chapter we shall attempt to summarise briefly the results of
experiments on the manuring of some of the commoner crops, and we shall
start with the manuring of cereals.


CEREALS.

As we have already pointed out, a certain similarity in the manurial
requirements of the different members of this class exists. They are
characterised, for one thing, by the comparatively small quantity of
nitrogen they remove from the soil--less than either leguminous or root
crops. Of this nitrogen the larger proportion--amounting to
two-thirds--is contained in the grain, the straw only containing about a
quarter of the total amount of nitrogen in the plant. The amount of
phosphoric acid they remove from the soil is not much less than that
removed by the other two classes of crops; but this, again, is also
chiefly in the grain. It is on this account that the cereals may be
regarded, in a sense, as exhaustive crops, seeing that the grain is
almost invariably sold off the farm. But, on the other hand, owing to
the comparatively small demands they make on fertilising ingredients,
cereals will continue to grow on poor land for a longer period than most
crops,--a fact of very great importance for mankind.


_Especially benefited by Nitrogenous Manures._

Despite the fact that cereals remove comparatively little nitrogen from
the soil, it is somewhat striking to find that they are chiefly
benefited by the application of nitrogenous manures. This fact may be
explained by the shortness of the period of their growth, and the fact
that they assimilate their nitrogen in spring and early summer, and are
thus unable to utilise to the full the nitrates which accumulate in the
soil during later summer and autumn. As they seem to absorb their
nitrogen almost exclusively in the form of nitrates, they are especially
benefited by the application of nitrate of soda.


_Power of absorbing Silicates._

A characteristic feature in the composition of cereals is the large
amount of silica they contain. In common with the grasses, they seem to
possess a power, not possessed by other crops, of feeding upon
silicates.

The special manure, therefore, required for cereals is a nitrogenous
manure, and that, as a rule, of a speedily available character, such as
nitrate of soda or sulphate of ammonia. Furthermore, certain members of
the group are also specially benefited by phosphatic manures.

We shall now consider individually a few of the more important cereal
crops.


BARLEY.

Of cereal crops barley deserves to be considered first, owing to the
fact that it is, of all grain crops, the most widely distributed. In
England, in amount, it comes next to wheat among cereals. Its habits
have also been studied in a very elaborate and careful manner, and have
been made the subject of many experiments, both in this country and
abroad.


_Period of Growth._

The first point to notice about barley is the fact that its period of
growth is a short one. This has a most important bearing on its
treatment with manure. It may be said to ripen, on an average, in
thirteen or fourteen weeks in this country; although in Norway and
Sweden its period of growth is much less--viz., from six to seven weeks.
Indeed no fewer than three crops have been obtained in one year in
certain districts in these countries, and two crops are common. With
regard to the period of its growth, it differs from wheat, which in its
general manurial requirements it resembles. Wheat, which is largely sown
in autumn, has four or five months' start of barley. From the fact that
it is a short-lived crop, and that its roots are shallower than wheat,
and draw their nourishment chiefly from the surface-soil, it benefits to
a greater extent from liberal manuring than wheat, which is more
independent of artificial supplies of fertilisers.


_Most suitable Soil._

Again, while wheat does well on a heavy soil, and does not require a
fine surface-tilth, barley does best on a light, rich, friable soil. It
has, however, been very successfully grown on a heavy soil after wheat.
Barley benefits more than wheat does from the application of
superphosphate of lime, or some other readily available phosphatic
manure. This may be accounted for by its shorter period of growth and
shallower root system, which thus prevent it drawing much mineral
sustenance from the subsoil. In fact, spring-sown crops, as a rule,
benefit more from superphosphate than autumn-sown crops. The exhaustion
of a soil under barley is essentially, as in the case of wheat, one of
nitrogen, as Sir J. Henry Gilbert has pointed out.[244]


_Farmyard Manure not suitable._

It has been urged, with some show of reason, that farmyard manure is not
suitable for barley, as its action is too slow to have much influence on
so short-lived a plant, and that only quick-acting manures should be
used. Where farmyard manure is applied, it should be to the preceding
crop; and this is advisable for more reasons than one.


_Importance of uniform Manuring of Barley._

The use to which barley is put--viz., for malting purposes--renders the
uniformity of its composition a point of great importance. Since its
quality is very largely influenced by its treatment with manures,
special care has to be exercised in their application. Grown as it
generally is after roots, fed off with sheep, its quality, it is
alleged, is apt to suffer from the unequal distribution of the manure
applied in this way. It has consequently been recommended, in order to
avoid this inequality, rather to grow a wheat crop immediately preceding
the barley.


_Norfolk Experiments on Barley._

Mr Cooke, in summing up the results of the interesting Norfolk
experiments on barley, points out that in these experiments barley
always was benefited by nitrogenous manures, sometimes by superphosphate
of lime, and more rarely by potash; that of nitrogenous manures those
of quickest action exerted the best influence. On an average it was
found that 1 cwt. nitrate of soda per acre gave an increase of 8 bushels
of barley, and 2 cwt. gave 14 bushels; while 3/4 cwt. sulphate of
ammonia (_i.e._, the amount containing the same quantity of nitrogen as
1 cwt. nitrate of soda) gave only 5-1/2 bushels of an increase, and
1-1/2 cwt. (= 2 cwt. nitrate of soda) gave 10 bushels.

Mr Cooke recommends the following manures for the barley crop. From 1/4
to 1 cwt. of nitrate of soda, according to previous treatment of soil;
from 1 to 2 cwt. super; and where it is required, from 1/2 to 1 cwt.
muriate of potash.


_Proportion of Grain to Straw._

Professor Hellriegel, the distinguished German investigator, has carried
out most elaborate experiments on a small scale, with a view to
investigating the habits of the barley plant. In the most perfectly
developed of these plants, grown under the most favourable conditions,
he found that the grain and straw were about equal in weight. Such a
proportion of grain is, however, never realised in practice, the
proportion of 2 of grain to 3 of straw being probably the common one.


WHEAT.

Wheat occupies the first position amongst cereals, in respect of extent
of cultivation, in England. As a rule it is sown in autumn, although it
is also sown in spring. It is generally taken after rotation grasses or
a leguminous crop, such as peas or beans, or after potatoes or roots.

Unlike barley, it does best on a clay soil, or at any rate on a firm
soil, and requires a moist seed-bed. From the fact that wheat is often
sown after such a crop as potatoes or a root crop to which a liberal
application of manure has been given, it is not so necessary to manure
it except with a top-dressing of nitrate of soda. In short, it is
usually considered highly desirable to get land into "good heart" before
wheat, so that the wheat may obtain its nourishment from the residue of
the previous crop and the farmyard manure previously applied.

Although, therefore, as a rule, the only manure it will be found
necessary to add to wheat is a nitrogenous manure, such as nitrate of
soda or sulphate of ammonia, still there are circumstances in which it
will be well to supplement these by phosphatic or even potassic manures.
On a light soil it may be advisable to add superphosphate of lime,
guano, or bone-meal, in quantities of 2 to 3 cwt. per acre, in addition
to a nitrogenous manure.


_Rothamsted Experiments on Wheat._

Of experiments carried out on the growth of wheat, those which have now
been in progress for over half a century at Rothamsted are the most
valuable and famous. In these experiments the comparative value of
nitrogen and mineral manures on this crop was strikingly exemplified.
The former gave a most marked increase in the crop, while with the
latter little or no increase was obtained. A combination of nitrogenous
and mineral manures, on the other hand, gave the most striking results.
An explanation of these results may be afforded by the fact that in
ordinary farming an excess of mineral matter, as compared with nitrates,
is returned to the soil in the crop residues and in the straw of the
farmyard manure.

Of nitrogenous manures, nitrate of soda, on the whole, showed better
results than sulphate of ammonia.


_Continuous Growth of Wheat._

The possibility of growing fair crops of wheat year after year for fifty
years on the same land, and that without any manure whatever, is among
the most striking of the results of these famous Rothamsted wheat
experiments.


_Flitcham Experiments._

In conclusion, we may refer to Mr Cooke's Flitcham experiments. These
were carried out for the purpose of ascertaining the most suitable
manure for the wheat crop under different conditions.

It will be sufficient here to give the recommendations made by Mr Cooke
as the practical outcome of these experiments.

He recommends the application of 10 tons of farmyard manure on light or
mixed soils, after rotation seeds, ploughed in in the autumn, with from
1/4 to 1 cwt. of nitrate of soda, sown in the spring. In certain cases
farmyard manure will be sufficient without the nitrate of soda. When
farmyard manure is not available, the most effective and economical
substitute is 4 cwt. per acre of rape-cake, ploughed in in the autumn,
or 1 cwt. of sulphate of ammonia, sown in the spring, with, in either
case, 1 cwt. of nitrate of soda as a spring top-dressing. In addition to
the above, on land in doubtful agricultural condition, or exceptionally
deficient in one or other of these ingredients, Mr Cooke recommends the
addition of 2 cwt. superphosphate, or 1 cwt. muriate of potash, or both
of these manures, ploughed or harrowed in in autumn.


OATS.

Like barley, oats are generally sown in spring, and, like barley, may be
described as a shallow-rooted crop. They require, therefore, manures
which are readily available, and their demands on the different
fertilising ingredients are very similar to barley. The manures which
will pay best, consequently, for oats, are nitrate of soda, used as a
top-dressing, and superphosphate of lime, applied along with the seed.
Probably upon no other crop is nitrate of soda so safe and so effective
as upon oats. In some respects, however, oats differ strikingly from
barley.


_A very hardy Crop._

In the first place, oats are a much hardier crop than barley or wheat.
They can grow on a wonderfully wide range of soil, and under
comparatively adverse circumstances, both of climate and situation. They
are better suited for a damp climate such as our own than a warm
climate. They may be described as of all crops the least fastidious, and
will flourish on sandy, peaty, or clayey soils. While this is so, they
show a preference for soils rich in decayed vegetable matter. It is for
this reason that they flourish so well on soils freshly broken up from
pasture, and are often the first crop to be grown on such soils.


_Require mixed Nitrogenous Manuring._

Stoeckhardt has found, in experiments on the manuring of the oat crop,
that they greedily absorb nitrogen during nearly the whole period of
their growth, and that, consequently, it is desirable to manure them
with a mixed nitrogenous manure which shall contain nitrogen, both in a
readily available form to supply the plant during the early stages of
its growth, and in a less available form for the later stages of
growth. He was of the opinion that in this way a continuous and
satisfactory growth of the crop would be promoted.


_Arendt's Experiments._

The oat-plant has been made the subject of many elaborate
investigations. Of these, those carried out by Arendt are the most
elaborate and best known. In these experiments the composition of the
oat-plant at different stages of growth was investigated. It was found
that the oat-plant increased during the whole period of its life, and
that two-thirds of the nitrogen absorbed was absorbed during the later
period of growth. It has since been shown, however, that the absorption
of nitrogen is very much influenced by circumstances. Indeed its
composition is peculiarly susceptible to the influence of manures, and
especially the influence of weather. Thus Arendt found that the
assimilation of nitrogen is checked by cold wet weather; while, on the
other hand, it is promoted by warm dry weather. The grain of oats grown
in warm seasons is better developed, and in composition more nutritious
(_i.e._, contains more nitrogen), than that of oats grown in wet
seasons, while the reverse is the case with the straw.


"_Avenine._"

A point of considerable interest in connection with the composition of
oats is the fact that it contains a body which exerts a strikingly
stimulating effect on the nervous system of the animal, and to which the
name "avenine" has been given.


_Quantities of Manures._

The quantities of manures which may be applied to the oat crop are
similar in amount to those which ought to be applied to barley--from 1/2
to 1 cwt. of nitrate of soda, and from 2 to 3 cwt. superphosphate of
lime. Very often, however, the oat crop receives directly little or no
manure. In the Highland and Agricultural Society of Scotland's
experiments, sulphate of ammonia was found to be of very much less value
than nitrate of soda as a manure for oats. Potash manures, especially
muriate of potash, had a very beneficial effect. The general conclusions
drawn from these experiments were, that the treatment of the land should
be such as to accumulate organic matter in it, to prevent too great a
loss of moisture, and to provide the young plant with manures that come
speedily into operation.


GRASS.

The manuring of grass is a question of very great interest and
importance, but is, at the same time, beset with peculiar difficulties.
Grass is grown under two conditions--first, that grown on soils
exclusively set apart for its continuous growth (permanent pasture);
and secondly, that grown for the purpose of being converted into hay and
of providing pasture in the ordinary rotation of crops (rotation seeds).
The manuring of the former is somewhat different from the manuring of
the latter.


_Effect of Manure on Herbage of Pastures._

The nature of the herbage growing on pasture is very much influenced by
the manure applied. This, indeed, is one of the most noteworthy features
connected with the manuring of grass, and has been especially observed
in the Rothamsted experiments, where the influence of the different
manures on the various kinds of herbage has been investigated with great
care. The herbage constituting pasture is, as every farmer knows, of a
varied description. We have in pastures a mixture of plants belonging
both to the gramineous and leguminous classes, as well as a variety of
weeds. Now the result of the application of different manures tends
respectively to foster the different kinds of grasses. Thus when one
kind of manure is applied, grasses of one kind tend to predominate and
crowd out grasses of another. It has been found that _the more highly
pasture-land is manured the simpler is the nature of its herbage_ (that
is, the fewer are the different kinds of herbage growing on it).
_Unmanured pasture, on the other hand, is more complex in its herbage._
The result is, that the application of manure to pasture-land is
attended with certain dangers. To maintain good pasture it is desirable
to effect a proper balance between the different kinds of grasses. For
this reason permanent pasture may be said to be, of all crops, the least
commonly manured. As a rule it is only manured by the droppings of the
cattle and sheep feeding upon it.


_Influence of Farmyard Manure._

It is found that the influence of farmyard manure upon the composition
of the pasture does not tend, to the same extent, to the undue
development of one type of herbage over another; and in this respect it
is probably to be preferred to artificial manures.

The same reasons, however, do not hold with regard to rotation seeds,
where an abundant growth is desired, and complexity of herbage is not so
important. A further reason which exists for the manuring of meadow-land
is the greater impoverishment of the soil taking place under such
conditions. As illustrating the influence of different manures on
different kinds of herbage, it may be mentioned that in New England
wood-ashes, a manure commonly used there, have been observed, when
applied to pasture, to bring in white clover, and that the application
of gypsum had the same effect. An explanation of this fact may be found
in the influence of potash on leguminous crops. The chief value of
wood-ashes as a manure is due to the large percentage of potash they
contain, while the value of gypsum is probably to be accounted for by
the fact that it has an indirect action, and sets free potash from its
inert compounds in the soil. In the Rothamsted experiments this point
has been verified, and potash has been shown to increase the proportion
of leguminous plants on a grass-field. Nitrogenous manures, on the other
hand, more especially sulphate of ammonia, have been found to increase
the proportion of grasses proper, and to diminish the proportion of
leguminous plants. The effect of farmyard manure, while less marked in
inducing simplicity of herbage, has a similar effect to sulphate of
ammonia; while phosphates and other mineral manures exercise an
influence similar to that of potash. Mixtures of mineral and nitrogenous
manures gave the largest returns obtained, but their influence was to
increase the proportion of grasses proper. Sewage irrigation also tends
chiefly to develop grasses.


_Influence of Soil and Season on Pastures._

Manures are not the only factors influencing the quality of pastures.
The nature of the soil, as well as the age of the pasture and the
character of the season, exert a very considerable influence. Grass
growing on damp or badly drained soil is invariably of poor quality, the
coarser grasses predominating. Old pastures, again, are generally of
better quality than new ones.


MANURING OF MEADOW-LAND.

Nitrate of soda is a common manure for grass grown for hay. It is often
applied at the rate of 2 or 3 cwt. per acre. It is best, however, to
apply it in smaller doses. On soils where lime is abundant,
superphosphate may be applied, if necessary, at the rate of 2 or 3 cwt.
per acre, or bones at a similar rate. Basic slag has been found to meet
with good results as a manure for grass-land, especially where the soil
is rich in organic matter.


_Bangor Experiments._

Mr Gilchrist of University College, Bangor, as a result of numerous
experiments carried out in different parts of Wales, recommends for
rye-grass and clover hay on land in good condition 1 cwt. of nitrate of
soda or sulphate of ammonia per acre, the former being applied about the
middle of April, the latter during March. For land in poor condition,
the addition of 2 cwt. of superphosphate is recommended--this to be
applied some time between December and March. Farmyard manure may be
usefully applied to young grass and clover seeds in the autumn, more
especially on light soils. For meadow-land which is growing hay every
year, Mr Gilchrist further recommends the following 4-course rotation of
manuring:--

First year, 15 tons farmyard manure, applied in the autumn.

Second year, 1 cwt. nitrate of soda.

Third year, 4 cwt. basic slag or 3 cwt. superphosphate and 1 cwt.
nitrate of soda.

Fourth year, 1 cwt. nitrate of soda.


_Norfolk Experiments._

Mr Cooke, from his Norfolk experiments, recommends the following manures
for rotation seeds:--

One to 1-1/2 cwt. nitrate of soda as a top-dressing in early spring.
Where the clover plant is a good one, and it is particularly desired to
cultivate it, he recommends as a dressing 1 cwt. of muriate of potash
per acre, to be applied immediately after the clover is sown. The
practice of dressing growing seeds in their first winter has, so far as
the experiments in Norfolk go, less to recommend it than the earlier
dressing.


MANURING OF PERMANENT PASTURES.

In this case the manure should be applied so as not to impair the
quality of the herbage. Slow-acting manures are consequently best, such
as basic slag or bones, which have been found to be of special value. On
wet or marshy land after draining, lime is perhaps one of the best
manures to apply in the first instance. As we have already said,
farmyard manure will do more to maintain the quality of pasture than any
kind of artificial manure. Mr Cooke is of opinion that no system of
manuring yet discovered will both thicken and improve the herbage at
all equally in success to the careful and regular feeding upon the grass
of cattle or sheep, the animals having a good allowance of decorticated
cotton-cake, or even of linseed-cake.


ROOTS.

Of all crops roots may be said to require the most liberal application
of manure, and to respond most freely to it. They contain large
quantities of the fertilising ingredients--nitrogen, phosphates, and
potash--and may be regarded as exceedingly exhaustive crops. This is
especially the case with regard to mangels, which make particularly
large demands on a soil's fertilising ingredients.

Turnips are characterised by the large amount of sulphur they contain;
and, according to some, this explains the beneficial effect which gypsum
has when applied to them as a manure. This, however, is more probably to
be explained by the indirect action of gypsum in setting free the potash
of the soil. The fact that the successful cultivation of root crops
depends on the application of large quantities of manure, is recognised
in practice, as they receive the most manure of any crop of the
rotation. Roots flourish best on a light soil which is neither too wet
nor too dry; but with liberal manuring and careful tillage, they may be
said to do well on any soil. Mangels are generally more benefited by
the application of nitrogenous manures than are turnips or swedes,
which, it would seem, have a greater power of absorbing nitrogen from
the soil than the first-named crop; but it is a mistake to suppose that
any of the root crops are not dependent on a ready supply of nitrogen;
and the fact that large crops of turnips can often be grown by the
application of superphosphate alone, may be taken as a proof that the
soil contains plenty of nitrogen. Mangels are, from their deeper roots,
more capable of drawing their supply of phosphoric acid from the soil
than turnips. They respond, therefore, as a rule, less freely than
turnips or swedes to an application of superphosphate. Generally
speaking, we may say that the characteristic manure for turnips is
superphosphate, and that for mangels is a nitrogenous manure such as
nitrate of soda or sulphate of ammonia.

A special reason for manuring root crops is the fact that they are more
liable to disease than other crops; and this is especially the case in
the early stages of their growth. One of the great benefits conferred on
the turnip crop by an application of superphosphate, is the help it
gives the crop to pass safely the critical period of its growth. The
superphosphate is best drilled in with the seed, in quantities varying
from 3 to 5 cwt. In Scotland, it may be well to point out, the manure
applied to this crop is very much in excess of the amount customarily
applied in England; for in the former country larger applications of
manure may be profitably employed. Roots generally receive a large
dressing of farmyard manure. Salt has been found in some districts to
have a very good effect on the mangel crop, and potash is often found to
amply repay application.


_Influence of Manure on Composition._

A most interesting point in connection with the manuring of roots is the
effect of manure on their composition. This has been most elaborately
investigated at Rothamsted and elsewhere. Thus it has been found that
the effect of the application of excessive quantities of nitrogenous
manures is to produce too great a development of leaves at the expense
of the roots.


_Nitrogenous Manures increase Sugar in Roots._

Nitrogenous manures also tend to increase the proportion of sugar and
diminish the proportion of nitrogenous matter in roots. This has an
important bearing on the treatment of roots which are cultivated for
their sugar, such as beets, in the growth of which nitrate of soda is
the chief artificial manure applied.[245]

The leaf, it may be pointed out, contains a larger percentage of dry
matter, both in swedes and in turnips, than the root.


_Amount of Nitrogen recovered in Increase of Crop._

With regard to the amount of nitrogen recovered in the increased crop of
mangel and roots when manured with different nitrogenous manures, it was
found at Rothamsted, as an average of six years, that the following
percentages of nitrogen were recovered: When nitrate of soda was
applied, 60 per cent of the nitrogen it contained was recovered in the
increased crop; when ammonia salts were applied, 52 per cent; when
rape-cake was used, 50 per cent; and when a mixture of rape-cake and
ammonia salts was used, 46 per cent.

It may be pointed out that the influence of season and climate on the
composition of root crops is very great--greater, indeed, than on any
other crop. Like oats, turnips grow better in Scotland than in England,
the moister climate of the former country being more suitable for their
maximum development, and hence the economy of maximum dressings in
Scotland.


_Norfolk Experiments._

In conclusion, a few words may be said on the Norfolk experiments,
carried out under the direction of Mr Cooke for the purpose of
ascertaining the best and most economical manure for mangels and swedes
on different Norfolk soils. In most of these experiments it was found
that superphosphate had not much effect in producing increase of crop in
the case of mangels; that the best nitrogenous manure was nitrate of
soda; and that on the whole it was not economical to apply farmyard
manure at the rate of more than 10 tons per acre. It was further found
that, although either potash or common salt gave a decided increase in
weight of roots, it was not necessary to give both these manures at
once, either of them being about as effective as the other.

Mr Cooke recommends the following manures as best suited for
mangels--viz., 2 cwt. nitrate, 3 cwt. common salt, and 2 cwt.
superphosphate. Upon certain soils peculiarly adapted to mangels, and in
warm localities where larger crops than 25 to 30 tons per acre are
habitually grown, it would probably pay to increase or to double the
above quantity of nitrate of soda. Ten tons of farmyard manure may, if
preferred, be substituted for all or a part of the nitrate of soda, or
may even be used in addition to it, according to the resources of the
farmer in respect of it, and the return he desires to get from the dung
in the first year of application or in future ones. It is best to apply
the nitrate of soda in two instalments--half at the time of seeding, and
half as a top-dressing immediately after the first hand-hoeing of the
roots. A third dressing may often be given with advantage a month later.


_Manure for Swedes._

As a complete and economical dressing for swedes in Norfolk, Mr Cooke
recommends 3 to 4 cwt. superphosphate, 1 cwt. sulphate of ammonia, and
1/2 cwt. of muriate of potash. Occasionally it may be found advisable to
reduce the quantity of sulphate of ammonia, or to leave it out
altogether; and in other cases the potash may be judiciously omitted.
The entire mixture should be sown at the time of drilling the turnips.
If farmyard manure is used--and if used it should be applied in a
well-decomposed state--no other manure than 3 cwt. of superphosphate
will be required.


_Highland Society's Experiments._

Valuable experiments have been carried out on the subject of manuring of
turnips by Dr A. P. Aitken, for the Highland and Agricultural Society of
Scotland. The following are some of the results to be gathered from
these experiments. The effect of a dissolved phosphate as compared with
a ground phosphate is to produce a turnip of less feeding value.
Superphosphate had a better effect when applied in April than when
applied with the seed in June. It was further found that when the
nitrogenous manure was given entirely in the form of nitrate of soda or
sulphate of ammonia, the latter produced a denser and sounder turnip.
Lastly, with regard to the application of potash, it was found that the
best way was to apply it several months before sowing. The effect of
potash manures is to increase the amount of turnips, but to retard the
ripening of the bulbs. The effect of excessive potash manuring is to
greatly injure the crop.


_Manuring for rich Crops of Turnips._

In Dr Aitken's own words: "In order to grow a large and at the same time
a healthy and nutritious crop of turnips, such a system of manuring or
treatment of the soil, by feeding or otherwise, should be practised as
will result in the general enriching and raising of the condition of the
land, so that the crop may grow naturally and gradually to maturity. For
that purpose a larger application of slowly acting manures, of which
bone-meal may be taken as the type, is much better suited than smaller
applications of the more quickly acting kind. A certain amount of
quickly acting manure is very beneficial to the crop, especially in its
youth; but the great bulk of the nourishment which the crop requires
should be of the slowly rotting or dissolving kind, as uniformly
distributed through the soil as possible."


_Experiments by the Author._

Experiments by the author on turnip-manuring, carried out in different
parts of the South and West of Scotland, showed that while farmyard
manure is valuable in giving the crop a good start and bringing it well
forward during the period of germination and early growth, by supplying
a certain amount of easily assimilable plant-food, and in the case of
dry weather attracting a quantity of moisture, its application in
quantities of 20 or even 10 tons per acre can scarcely be regarded as
profitable, giving to farmyard manure a nominal value of a few shillings
a ton. In these experiments slag proved itself a most valuable manure,
indeed one of the most economical of all the manures experimented with.
They further showed that heavy dressings with superphosphate, amounting
to as much as 8 cwt. per acre, are, from an economical point of view, as
a rule justifiable in Scotland; and that nitrate of soda and sulphate of
ammonia possess practically equal value as a manure for turnips. In
almost every one of the experiments the benefit of supplementing
superphosphate with nitrogenous manure was shown. Potash was also found
in many cases to be a thoroughly paying manure for the turnip crop, when
it was applied along with nitrogen and phosphates; but when applied
alone, far from exercising any appreciable benefit, it seemed to exert
an injurious action.


POTATOES.

Potatoes are often classed along with the root crops, and in their
manurial requirements they offer many points of similarity. Next to root
crops, they may be said to make the most exhaustive demands on the soil,
and therefore require a liberal general manuring. A point of importance
in the manuring of potatoes is a good tilth in the soil, so as to enable
a free expansion of the tubers to take place. They may be said to grow
best on deep warm soils; but, like roots, if liberally manured, they may
be successfully grown on any kind of soil. Farmyard manure has long been
regarded as specially valuable for the potato crop. In many parts of
Scotland it is applied in enormous quantities, ranging from 20 to even
40 tons per acre. There can be little doubt that the value of farmyard
manure, as well as other bulky manures, for the potato crop, is partly
due to their mechanical influence on the soil. Potatoes are
surface-feeders, and require their food in a readily available
condition. It is found desirable, therefore, to supplement farmyard
manure by readily available artificial manures. Potatoes repay the
application of a mixed manure containing all the fertilising
ingredients--nitrogen, phosphoric acid, and potash--better than most
crops.


_Highland Society's Experiments on Potatoes._

The nitrogen is, according to the Highland Society's experiments, best
applied in the form of nitrate of soda. Sulphate of ammonia does not
seem, when farmyard manure is also applied, to have an equally valuable
effect, as it influences the size of the tuber, producing an undue
proportion of small potatoes. When no farmyard manure is applied,
however, sulphate of ammonia seems to have a good effect, especially in
wet seasons.

With regard to the nature of the phosphatic manure to be applied,
superphosphate is to be preferred. Potatoes make large demands on
potash, and consequently require potassic manures. In consequence of the
fact that they receive large applications of farmyard manure, the
necessity for adding potash in the form of artificial manures does not
generally exist. Potash, if applied in too large quantities, has been
found to exert a deleterious effect. We have already pointed out that
muriate of potash tends to produce a waxy potato.


_The Rothamsted Experiments with Potatoes._

The Rothamsted experimenters have very fully investigated the conditions
of the manurial requirements of potatoes. In these experiments potatoes
were grown year after year in the same field. It was found that the
effect of mineral manures alone was greater than the effect of
nitrogenous manures alone, and that of mineral manures phosphates, as a
rule, had a better effect than potash; that under the action of the
growth of potatoes a greater exhaustion of phosphates than of potash
takes place in the soil; and lastly, that it is essential to have an
abundant supply of the different fertilising ingredients in order to
grow successful crops. In the Rothamsted experiments, the slow action of
farmyard manure in supplying fertilising ingredients to the potatoes is
strikingly demonstrated. Thus, although farmyard manure was applied at
such a rate that more than 200 lb. of nitrogen were added to the soil,
the result was inferior to that obtained from the application of 86 lb.
of nitrogen applied in the form of readily available artificial manure.


_Effect of Farmyard Manure on Potatoes._

It may be said, in this respect, that the potato is less able to utilise
the fertilising ingredients of farmyard manure than any other of the
farm crops. Yet, despite this fact, farmyard manure has been found to be
one of the best manures to apply. The reconciliation of these seemingly
contradictory statements depends on the influence exerted by the
farmyard manure on the mechanical condition of the soil, rendering it
more porous and easily permeable to the surface-roots, upon the
development of which the success of the crop so much depends. The
beneficial effect of farmyard manure is also doubtless due to the
increased temperature which large applications of it produce in a soil.

Sir J. Henry Gilbert, in his well-known Cirencester Lecture on the
Growth of Potatoes, cites several examples of the manurial treatment of
potatoes in different parts of the country. In Forfarshire, farmyard
manure or stable manure is largely employed (at the rate of 12 to 14
tons, and in some cases even 20 tons per acre), and it is also largely
supplemented by artificial manures. These latter are applied to the
extent of about 10 cwt., and consist of superphosphate, dissolved bones,
and potash salts. Six tons of potatoes are considered a fair crop. In
East Lothian the manuring is similar, with the exception that farmyard
manure is applied in even larger quantities--30 to 40 tons being often
used. Sometimes potatoes are grown with artificial manures alone. It
would seem that the usual crop of potatoes ranges from 4 to 8 tons per
acre.


_Manuring of Potatoes in Jersey._

The manuring of the potato crop, so largely grown in Jersey in the
Channel Islands, is of interest. Potatoes are there grown two or three
years, then corn, then grass for a few years, then potatoes again, no
special rotation of crops being followed. Either farmyard manure or
sea-weed is applied at the rate of 25 to 30 tons per acre, supplemented
by 8 to 12 cwt. of artificial manures.

These statements show how prevalent the practice of heavily manuring the
potato crop is.


_The Influence of Manure on the Composition of the Potato._

The influence of manure on the composition of the potato crop is of much
interest. Potatoes grown without manure, just as in the case of roots,
are found to have a larger percentage of nitrogen than potatoes grown
with manure. The effect of manuring, therefore, is to increase the
proportion of starch, which is the most important constituent of the
potato. Mineral manures have a greater effect in increasing the
percentage of starch than purely nitrogenous manures; but when used
together, a still greater increase is obtained than when used singly.
The effect of nitrogenous manures on the composition of roots and
potatoes is thus seen to be similar. In the case of both crops the
effect is to increase the proportion of the characteristic carbohydrate
constituent, which in roots is sugar, and in potatoes starch. Potatoes,
like roots, are also much influenced by the season. The effect of season
and manuring on the potato disease is worthy of notice. Wet seasons are
favourable to the development of the disease. It has been found that in
a highly nitrogenous manured crop the proportion of diseased tubers is
greater than in a non-manured crop.


LEGUMINOUS CROPS.

We have already referred to the manuring of crops of the leguminous
class in discussing the manuring of meadows and permanent pasture. It
was there pointed out that the tendency of certain manures was to
encourage the growth of the leguminous plants of the herbage, while
other manures had the effect of encouraging those of the gramineous
class. It was pointed out that a manure which had this effect was
potash, or any manure which owed its characteristic action to the fact
that it supplied potash to the soil or set it free in the soil.


_Leguminous Plants benefit by Potash._

This is one of the most important points to notice in manuring
leguminous plants. Just as we can say that nitrogenous manures are
specially beneficial to cereals, and phosphatic manures to roots, so
potash is the special manure for leguminous crops.


_Nitrogenous Manures may actually be hurtful._

But we have, further, an even more striking characteristic of leguminous
crops to notice. We have seen that, with regard to the crops already
discussed, while there are cases in which a fertilising ingredient may
be of no value, or may positively exert a hurtful action on the crops,
such cases are only exceptional. With regard to leguminous crops,
however, we find that almost invariably they derive little or no benefit
from the use of artificial nitrogenous manures. And this is all the more
striking since they contain large quantities of nitrogen in their
composition--twice as much as the cereals. The fact, which has long been
noticed with regard to certain members of this class of plants, such as
clover, that not only do they contain a large amount of nitrogen, but
that by growing them on a soil the soil is largely enriched in this
valuable fertilising constituent, has long waited for a satisfactory
explanation, which at last has been forthcoming. The discovery that
leguminous crops can draw on the boundless store of nitrogen present in
the air has done much to clear up the mystery. There are, however, other
problems with regard to the growth of leguminous plants which still
await solution.


_Clover-sickness._

One of these is the fact that land on which a leguminous crop like
clover has been growing for a number of years becomes unfit to support
its growth any longer. Such a soil is termed "clover-sick"; and many
have been the theories put forward to explain the phenomenon, but none
of them can be regarded as satisfactory.

The knowledge that leguminous plants have the power of deriving their
nitrogen from the air, furnishes us with an economical means of
enriching our soils in nitrogen. By growing leguminous crops alternately
with cereals, for example, the air should be made to furnish the
necessary nitrogenous manure. As a matter of fact, modified forms of
such a practice have long been in use--indeed the ordinary rotations of
crops are, to a certain extent, adaptations of this practice.


_Alternate Wheat and Beans Rotation._

An interesting experiment carried out at Rothamsted may be here cited
which illustrates in a striking manner the truth of the above statement.
Wheat and the leguminous crop beans were grown alternately. It was
found that almost as much wheat (containing nearly as much nitrogen) was
yielded in eight crops of wheat so grown as was yielded by sixteen crops
of wheat grown consecutively in an adjoining field.

The most commonly cultivated leguminous crops are clover, beans, and
peas. Clover having been already discussed, we need only say a word or
two on the manuring of beans and peas.


BEANS.

Beans do best on strong land, and, unlike some of the crops considered,
do not require a particularly fine tilth. They are generally grown after
cereals, and as a rule are sown in spring. More rarely, however, they
are sown in autumn. Spring-sown beans take about seven months to come to
maturity. They are much affected, like other crops, but to a greater
extent, by the nature of the season--a wet season inducing an undue
development of straw.


_Manure for Beans._

In common practice the manure used for the bean crop is farmyard manure,
applied to the soil in autumn after the harvest of the wheat, barley, or
other cereal crop grown. So common is this practice, that the belief
commonly exists that farmyard manure is necessary for a successful bean
crop. But experiments conducted at the Highland Society's Experiment
Station at Pumpherston show that full crops of beans may be grown with
the aid of artificial manures on soils which have received no
application of farmyard manure for ten years.


_Relative Value of Manurial Ingredients._

In the Appendix[246] will be found a table giving the results of
manurial experiments with the nitrogenous, phosphatic, and potash
manures on beans, carried out by Dr A. P. Aitken at the Highland
Society's Experiment Station. From these experiments it will be seen
that the application of phosphates and nitrogenous manures, either alone
or together, exerted a comparatively small effect in increasing the
yield of beans compared with that obtained with potash, either alone or
combined with phosphates. As Dr Aitken says, "Without potash in the
manure, the other two ingredients are of very little use, unless,
indeed, the land be very rich in potash."


_Gypsum._

Gypsum has a good effect on the bean crop, both on account of the lime
it contains, and of its indirect action in setting free potash.

Superphosphate is a much better manure than insoluble phosphates, and
similarly, in the few cases where nitrogenous manures are beneficial,
the speediest acting are best. Hence nitrate of soda is to be preferred
to other nitrogenous manures. When it is applied, it should be applied
in small quantities. A slow-acting nitrogenous manure is positively
injurious; so also, according to Dr Aitken, is nitrate of soda, applied
as a top-dressing to the crop.

Of potash manures, the muriate seems to be more effective than the
sulphate.


_Effect of Manure on Composition of Crop._

Lastly, we may refer to the effect of manures on the composition of the
crop. This is, on the whole, very slight, especially when compared with
the effect manures exert on the composition of such crops as turnips or
potatoes. It is the quantity and not the quality of the crop which the
manure affects in the case of beans.


PEAS.

Peas are not grown to anything like the same extent as beans. As a rule,
when they are cultivated it is along with beans, when they are
necessarily manured in a similar manner. If grown alone, however, it may
be well to point out that peas do best, unlike beans, on light, friable,
chalky loam. When grown in clay they tend to develop an undue amount of
straw. The effect of season on the crop is similar to that exerted on
the bean crop. In conclusion, it may be pointed out that it is alleged
that the effect of farmyard manure on peas is to force the straw.

In concluding this chapter a word or two may be said on the manuring of
two other crops which are cultivated to a considerable extent in this
country--viz., hops and cabbages.


HOPS.

The requirements of the hop crop in the matter of manures are rather
singular. It has been pointed out that in the case of most crops
quick-acting manures are to be preferred to slow-acting manures. With
hops, however, the case is very different; for they require, and cannot
be successfully cultivated without, slow-acting manures. Hops are
especially benefited by bulky nitrogenous manures--such as shoddy,
horn-meal, hide-scraps, hoofs, rape-dust, &c.; and it is only when
quick-acting manures are applied along with such slow-acting manures
that they will exercise their full influence. It is best to manure hops
twice a-year,--in spring with farmyard manure, supplemented by a
slow-acting nitrogenous manure, such as shoddy; and again in summer with
a more quickly acting manure. The dressings applied to hops are enormous
relatively to those used on other farm crops.


CABBAGES.

Cabbages belong to that class of crops known as gross feeders, to which
any sort of manure, applied in almost any quantities, does not come
amiss. Cabbages grow best on good loams with a well-drained porous
subsoil, although they also do well on clay soils. The quantity of
fertilising ingredients, especially potash, which a large crop of
cabbage removes from the soil is very great. They consequently require
large quantities of manure, and are especially benefited by saline
manures--such as kainit and common salt--and liberal doses of nitrate of
soda, which may be regarded as the most effective of manures for all the
cabbage tribe. Farmyard manure may be applied with benefit in larger
quantities than it would be applied to any other crop.


FOOTNOTES:

[244] See his Lecture on the Growth of Barley.

[245] Small roots are found to contain a larger proportion of sugar than
large roots.

[246] See Note I., p. 530.



APPENDIX TO CHAPTER XXIII.


NOTE I. (p. 526).

EXPERIMENTS ON THE MANURING OF BEANS.

Experiments with beans carried out at the Highland and Agricultural
Society's Experiment Station at Pumpherston, showing the effect of
potash:--

No. of                                             Bushels dressed
plots.                Kind of manure.              grain, per acre.

 27.     No manure                                       2-1/2
 12.     Phosphate (bone-ash)                            5-1/6
 18.     Nitrate                                         6-1/4
 21.     Phosphate and nitrate                           5-1/3
 22.     _Potash_                                       26-1/2
 17.     _Potash_  and phosphate                        42-1/3
 10.     _Potash_, phosphate, and nitrate               45-1/2
 38.     _Potash_, phosphate, nitrate, and gypsum       51



CHAPTER XXIV.

ON THE METHOD OF APPLICATION AND ON THE MIXING OF MANURES.


Having considered the manuring of the different crops, we may now pass
on to the consideration of some points in the method of application and
on the mixing of manures.


_Equal Distribution of Manures._

A most important object in applying manures is to effect equal
distribution of the manure in the soil. This is often, however,
unusually difficult to do, especially in the case of artificial manures,
where the quantity to be spread over a large area of the soil is
extremely small. The difficulty in the case of farmyard or other very
bulky manure is not so great. In order to overcome this difficulty in
the case of artificial manures, it is often advisable to mix them with
some such substance as sand, ashes, loam, peat, or salt. The manure is
thus diluted in strength, and a very much larger bulk of substance is
obtained to work with. Circumstances must decide which of these
substances to use. If the soil be a heavy clay, the addition of sand or
ashes may have an important mechanical effect in improving its texture;
while, on the other hand, if it be a light soil, the addition of peat
may improve its mechanical condition. It must also be remembered that
peat itself contains a large amount of nitrogen, and thus forms a manure
of some value. In using loam or peat to mix with artificial manures,
they should be first dried and then riddled; while if ashes be used,
they should be previously reduced to a fine state. Wood-ashes, however,
must be used with caution, and ought not to be mixed with ammoniacal
manures, as they are apt to contain caustic alkali, which would tend to
drive off the ammonia in a volatile state.

It has been recommended, in order to save trouble and effect equal
distribution, that the manure to be applied should always be made up to
the same amount, so that the farmer by experience may ascertain the rate
at which to apply it. And here it may be well to say a word or two on
the subject of mixing manures--a subject with which the farmer is not
always so conversant as it is desirable in the interests of his own
pocket he should be.


_Mixing Manures._

It is to be feared that not unfrequently indiscriminate mixing may cause
very serious loss in the most valuable constituent of a manure. It may
be well, therefore, to point out one or two of the causes of the loss
that is apt to ensue on the mixing of different kinds of manures
together.

As the subject depends for its clear comprehension on certain chemical
elementary principles, it may be well for the benefit of non-chemical
readers to state these pretty fully.


_Risks of Loss in Mixtures._

The risks of loss which may occur from the mixing of artificial manures
together may be of different kinds. One is the risk of actual loss of a
valuable ingredient through volatilisation; another is the risk of the
deterioration of the value of a mixture through change of the chemical
state of a valuable ingredient. Undoubtedly the most common and most
serious source of loss is the former. Of the three valuable manurial
ingredients--nitrogen, phosphoric acid, and potash--only the first is
liable to loss by volatilisation, and this generally only when the
nitrogen is either in the form of ammonia or nitric acid.


_Loss of Ammonia._

Ammonia, when uncombined, is a very volatile gas with a pungent smell, a
property which enables its escape from a manure mixture to be very
easily detected. It belongs to a class of substances which are known
chemically as bases, and which have the power of combining with acids
and forming salts. Sulphate of ammonia is a salt formed--as its name
indicates--by the union of the base, ammonia, with the acid, sulphuric
acid. Now when ammonia unites with sulphuric acid and forms sulphate of
ammonia, it is no longer volatile and liable to escape as a gas, but
becomes "fixed," as it is called.

Although most salts are more or less stable bodies--not liable to
change--if left alone, and not submitted to a high temperature or
chemical action, they can be easily decomposed if they are heated or
brought into contact with some other substance which will give rise to
chemical action. Sulphate of ammonia is a salt that is very easily
decomposed. This is due to the fact that its base, ammonia, is very
volatile, and not capable of being held very firmly by an acid, even by
sulphuric, which is among the least volatile of all the common acids.
If, therefore, sulphate of ammonia be heated above the boiling-point of
water, or brought in contact with any other substance which will give
rise to chemical action, it is easily decomposed. Now a salt may be
acted upon by a base or an acid or another salt. When it is brought in
contact with a base, if the base with which it is brought in contact be
a stronger base than the base of the salt, the salt is decomposed, and a
new salt is formed. The acid, in short, exchanges its old base for the
new one.


_Effect of Lime on Ammonia Salt._

This is exactly what takes place when the base lime comes in contact
with an ammonium salt, such as sulphate of ammonia. The sulphuric acid
exchanges its old base, ammonia, for the stronger base, lime, and
sulphate of lime is formed, and ammonia is set free as a gas, and
escapes and is lost. Sulphate of ammonia, or any substance in which
there is an ammonia salt, must never be brought in contact with free
lime, otherwise the ammonia will be lost, and should be harrowed in on
chalky soils for this reason.

It is different entirely with gypsum--which is sulphate of lime--or
phosphate of lime, both of which may be safely mixed with sulphate of
ammonia without any danger of escape of ammonia. It follows from the
above that a mixture which must on no account be tried is slag phosphate
and sulphate of ammonia. This is because the slag phosphate contains a
large percentage of free lime, which would at once, on being brought in
contact with the sulphate of ammonia, decompose it, and cause the
ammonia to be lost. For this same reason guano must not be mixed with
slag. It is perhaps unnecessary, however, to warn one against so doing,
as it is not likely such a mixture would be made, as the ratio of
phosphoric acid to nitrogen in guanos is generally greater than is
required. If it be desired to mix the slag with a quickly available form
of nitrogen, nitrate of soda is not liable to loss; although for other
reasons it is not desirable to apply nitrate of soda along with the
slag, as the former manure should be applied almost always as a
top-dressing.


_Loss of Nitric Acid._

The risks of the loss of nitrogen in the form of nitric acid, although
not so great as they are in the case of ammonia, are still considerable.
As nitric acid is not a base but an acid, what is to be avoided in
mixing nitrates is bringing them in contact with any other manure which
contains another free and stronger acid--as, for example,
superphosphate. The free acid present in superphosphate has the tendency
to drive out the nitric acid from the nitrate and usurp its place. The
risk of loss of expulsion in the above cases is always augmented by the
rise of temperature which invariably accompanies chemical action of any
kind; and although the loss of nitrogen, in the form of nitric acid,
caused by mixing superphosphate and nitrate of soda, might, under
ordinary circumstances, amount to very little, yet, if the mixture were
to be allowed to stand any time, and the temperature of the mass to be
heightened, the loss which would undoubtedly then ensue would be
considerable.

The nitrogen salt which it is safe to mix with superphosphate is
sulphate of ammonia.


_Reversion of Phosphates._

But, as has already been mentioned, there is another loss which may
result from the mixing of manures. This is the deterioration of the
value of an ingredient by reason of change of chemical condition. This
is a source of loss that was little suspected a number of years ago, but
it is now well known that superphosphate of lime, under certain
conditions, is changed from its soluble to an insoluble form. We have
already referred to the reversion of phosphate in the chapter on the
Manufacture of Superphosphates.[247] It was there pointed out that
reversion is often caused by the presence of iron and alumina or
undissolved phosphate, and that the risk of reversion is therefore very
much less in a well-made article, made from pure raw material, than in
one made from a raw phosphate containing much iron and alumina.
Superphosphates containing a large percentage of insoluble phosphates
ought not to be kept too long before being used as a manure, otherwise
much of the labour and expense involved in their manufacture will be
lost by the reversion of their soluble phosphate. Further, it is highly
inadvisable to mix superphosphates with basic slag, which contains a
large percentage of both iron and free lime. Lastly, if it is desired to
mix superphosphate with insoluble phosphate, the mixture ought to be
made just previous to application.


_Manurial Ingredients should be applied separately._

The question of applying manure in mixtures is one on which considerable
difference of opinion may exist. For many reasons manures are often
better applied in the unmixed condition. For example, a mixture of a
quickly acting nitrogenous manure with a slowly acting phosphatic manure
is not suitable. In such a case either the nitrogenous manure will be
applied too long before it is required by the plant, and thus suffer
from risk of loss, or the phosphatic manure will not be applied long
enough before it is likely to be used. By applying manures in an unmixed
condition the chances are that a more economical use of them is made
than would otherwise be the case. On the other hand, while the
application of the separate constituents may be desirable from the
scientific point of view, it involves a considerable amount of extra
trouble. Of course a further consideration is the desirability in many
cases of having a complete manure. The above hints, therefore, on the
risks of loss which exist in mixing manures, may be of service to the
agricultural student.


FOOTNOTES:

[247] See p. 389.



CHAPTER XXV.

ON THE VALUATION AND ANALYSIS OF MANURES.


_Value of Chemical Analysis._

The value of a manure to the farmer depends on the proportion of
_nitrogen_, _phosphoric acid_, and _potash_ it contains, as well as--and
this is hardly less important--the condition in which the ingredients
are present. Since these facts can alone be determined by a chemical
analysis, it is obvious that manures should always be purchased with a
chemical analysis. It is unfortunate, however, that very often a
chemical analysis, even when procured, is unintelligible. It may be of
advantage, therefore, to say a word or two on the correct interpretation
of the significance of the data furnished in the ordinary chemical
analysis of manures.


_Interpretation of Chemical Analysis._

The first thing that the farmer ought to look for in the analysis of a
manure is the amount of nitrogen, phosphoric acid, and potash which the
manure contains.


_Nitrogen._

The percentage of nitrogen in a manure is generally stated as equal to
its equivalent percentage of ammonia. Very often, indeed, in the older
analyses, its equivalent of ammonia was alone stated. Now this statement
does not necessarily imply that the nitrogen in a manure is actually
present in the form of ammonia. Thus, for example, when it is stated in
an analysis of bone-meal that it contains 3.5 per cent of nitrogen,
equal to 4.20 per cent of ammonia, it is not to be inferred that
bone-meal actually contains nitrogen in the form of ammonia. In point of
fact the nitrogen is present in an insoluble, slowly available, organic
form, which possesses a manurial value very inferior to that possessed
by ammonia. This custom is a most unfortunate one, and is much to be
regretted, as it is often liable to give rise to serious
misunderstanding. It must be remembered, therefore, that an ordinary
chemical analysis does not always specify the exact form in which
nitrogen is actually present. It is nevertheless of importance for the
farmer to know this, of which the nature of the manure analysed is
generally a good indication. Unfortunately this is not shown in the case
of _mixed_ manures; and this constitutes one of the reasons why mixed
manures are sometimes to be regarded with suspicion.


_Phosphoric Acid._

The amount of phosphates present in a manure is usually stated in its
analysis as so much phosphoric acid, while in a footnote the quantity of
tricalcic (or ordinary bone) phosphate this amount is equivalent to is
also given, this being the unit of valuation. When the phosphates are in
a soluble condition they are stated as such, and at the same time a
statement is made as to the quantity of tricalcic phosphate which would
be required to furnish this amount by treatment with sulphuric acid.
Thus, for example, in an analysis of a superphosphate of lime, the
statement, _monocalcic phosphate, 17.3 per cent, equal to tricalcic
phosphate rendered "soluble," 27.2 per cent_, means that it would
require 27.2 per cent of tricalcic phosphate to furnish 17.3 per cent of
soluble phosphate. Paradoxically enough, the former amount is called
_"soluble" phosphate_, and such a superphosphate as the above would be
described as containing 27.2 per cent of "soluble" phosphate.

Again, there are different forms of the so-called "insoluble"
phosphates,[248] although they are often not distinguished in a chemical
analysis. As we have already pointed out in the chapter on Basic Slag,
phosphoric acid occurs in the slag in the form of tetrabasic phosphate
of lime, although it is invariably stated in analysis as so much
tricalcic phosphate. Then we have the so-called dibasic phosphate of
lime, the form into which soluble phosphate in superphosphate is
converted when "reversion" takes place. Hitherto it has not been
customary in this country--although the custom is prevalent both on the
Continent and in America--to distinguish in the analysis of a
superphosphate the "reverted" phosphate from the undissolved phosphate;
since the superior value of the former as a manure is not recognised in
the manure-trade.[249]


_Importance of Mechanical Condition of Phosphate._

A further point to which it is desirable to draw attention is the
_mechanical_ condition of the different insoluble phosphates, which has
an important influence on their value. A very wide difference, for
example, exists between the value of phosphate of lime in such a manure
as Malden guano and in the crystalline mineral apatite; although,
chemically considered, the form in which the phosphoric acid is present
is the same in both substances.


_Potash._

Potash ought only to occur in a soluble form in manures. It is generally
stated as so much potash, and in a footnote the equivalent amount of
muriate or sulphate of potash is given, the former being the more
concentrated form of potash.

For purposes of reference a table will be found in the Appendix[250]
giving some useful factors for converting different forms of nitrogen,
phosphoric acid, and potash into one another.


_Other Items in the Chemical Analysis of Manures._

The other items in the analysis of a manure are of comparatively
secondary importance compared with those already named. Among them may
be mentioned the _moisture_, the _insoluble matter_, and the _organic
matter_. The amount of moisture and the amount of sand are two items of
importance, since, if these are excessive, they afford presumption that
the manure has been adulterated.


_Fertilisers and Feeding Stuffs Act._

An Act was passed, and came into operation in January 1894, for the
purpose of compelling every vendor of manure manufactured in this
country or imported from abroad to give to the purchaser "an invoice
stating the name of the article, and whether it is an artificially
compounded article or not, and what is at least the percentage of the
nitrogen, soluble and insoluble phosphates, and potash, if any,
contained in the article, and this invoice shall have the same effect
as a warranty by the seller of the statements contained therein."


_Different Methods of Valuing Manures._

The monetary value of a manure depends upon a number of more or less
complicated commercial considerations, such as the questions of supply
and demand, &c., which need not here be discussed, and which similarly
regulate the monetary value of any other article of commerce.


_"Unit" Value of Manurial Ingredients._

For the purpose of affording data for ascertaining the approximate value
of a manure, tables have been drawn up giving what is called the "unit"
value of the different manurial ingredients in various manures. This is
obtained by dividing the market value of a manure per ton by the
percentage of nitrogen, phosphoric acid, and potash it contains. Thus,
for example, sulphate of ammonia of 97 per cent purity contains 25 per
cent of ammonia, and at present (Dec. 1893) is valued at £13, 15s. per
ton. In order to obtain the unit value of ammonia in sulphate of
ammonia, we have only to divide £13, 15s. by 25, which gives us 11s. The
value of such tables depends on the competence of those drawing them up,
and they require to be subjected to constant revision. In the Appendix
will be found two of these tables, taken from the 'Transactions of the
Highland and Agricultural Society of Scotland.'[251]


_Intrinsic Value of Manures._

But there is another way of valuing manures, and that is by attempting
to ascertain what their intrinsic worth is in producing an increase in
the returns of the crops. Of course it may be said that the intrinsic
worth of manure affects directly its market value. This is doubtless
true, but it is not the only factor in determining the market value of a
manure.

Again, the intrinsic worth of a manure may be said to vary according to
the soil to which it is applied and the climatic conditions. This being
so, it is important for every farmer to try and ascertain for himself
what the intrinsic value of different manures is on the soil of his
farm; and this can only be done by carrying out manuring experiments for
himself. This leads us to say a word or two on the important subject of


_Field Experiments._

It is impossible that every farm should be able to support an experiment
station for the purpose of carrying out elaborate experiments on the
effect of different manures on different crops. Nevertheless it is
possible and highly desirable for _every_ farmer who is engaged in
arable farming on any scale to carry out simple experiments for the
purpose of ascertaining the characteristic manurial requirements of his
soil. This can be done at the expenditure of a little time and trouble,
and should be carried out in the following way. The field on which it is
desired to carry out the experiments should be divided into the
requisite number of experiment plots. These, which may be the tenth,
twentieth, or fortieth of an acre in extent, should be, if possible, on
a level piece of ground--all of them equally free from the shelter of
hedge or tree, and otherwise subjected to the same conditions. The
nature of the soil of the different plots, as well as its past
treatment, should be similar. It is desirable, in order to minimise
experimental error as much as possible, to carry out the experiments in
duplicate, or even triplicate. In the first place, there should be what
is called a _nothing_ plot--_i.e._, a plot receiving no manure. The
produce obtained from this plot, compared with the produce obtained from
the other manured plots, will thus furnish data for estimating the
respective amounts of increase obtained by different manures. One very
simple kind of experiment is what is called the "seven-plot" test. It
consists in testing the results obtained by using nitrogenous,
phosphatic, and potash manures alone and in different combinations. Thus
the plots would be manured respectively as follows:--

No.                    No.
 1. Nothing plot.   |   5. Nitrogen and phosphates.
 2. Nitrogen.       |   6. Nitrogen and potash.
 3. Phosphates.     |   7. Phosphates and potash.
 4. Potash.

The subjects of other experiments might be such as the respective values
of nitrogen in the different forms of sulphate of ammonia and nitrate of
soda; phosphoric acid as superphosphate, and in an undissolved form as
Thomas-slag; the relative importance of artificial and farmyard manure;
the effect of manures applied at different times, as well as the effect
of different quantities of the same manure; the most economical manures
for different kinds of crops; and numerous other interesting problems
connected with the practical application of manures.

In carrying out these experiments, care should be taken not to have the
experimental plots _immediately_ adjoining one another, as the manure
applied to the one plot may, by soaking through the soil, affect the
result on the adjoining plot. Especial note ought to be taken of the
weather during the progress of the experiment. In order to make such
experiments as valuable as possible, they ought to be continued year
after year. At the conclusion of the experiment the produce obtained
from each plot should be carefully weighed.


_Educational Value of Field Experiments._

The educative value of such experiments is very great, and in this
connection the remarks made by Mr F. J. Cooke, in a recent lecture
delivered to the London Farmers' Club, are worthy of most careful
consideration.

"Local experiments," he says, "teach the simple principles which should
determine the selection of manures, as well as scientific accuracy and
method in their use. The value of experiments is thus brought home to
men who would not go far to discover it; and the practice of a few
simple trials upon a correct system, each on his own farm, is
encouraged. That such trials may be conducted with very little expense
to the farmer, or other difficult qualifications, and yet to his great
practical advantage, I will venture to assert on the ground of my own
personal experience. For some twenty years I have annually conducted
private experiments on a very humble scale, and am not aware of any
other separate practice which has been so useful to me. It has been
pursued upon two light-land farms in different parts of the same county.
Yet, in respect of manurial requirements, the proper treatment for one
of them has differed so essentially from the other that a common
practice upon both would have been simply ruinous."


_Value of Manures deduced from Experiments._

Tables have been constructed for the purpose of showing the comparative
value of different kinds of manures as deduced from such experiments,
and may be fittingly compared with the tables giving the trade prices.
We have already quoted some of these tables in the Appendix to the
chapter on Mineral Phosphates. These tables show the relative intrinsic
value of different forms of phosphatic manures. In the Appendix[252] to
this chapter tables showing the relative value of different kinds of
nitrogenous and potash manures will be found.


_Value of Unexhausted Manures._

A subject which has had much attention devoted to it of late years is
the question of the value of unexhausted manures in the soil. In the
Agricultural Holdings Act special provision is made for giving
compensation to the out-going tenant of a farm for unexhausted manures
in the soil. The Act has given rise to endless disputes between landlord
and tenant, owing to the extreme difficulty of arriving at a
satisfactory estimate of what the value of the unexhausted manures in
reality is. The difficulty arises from the fact that we have not
sufficient data available for guiding us in estimating this value, which
further varies under different conditions. The fertilising ingredients
of a soil are present in the soil for the most part in an inert
condition, from which they are only slowly converted into an available
form.


_Potential Fertility of a Soil._

As indicating the total amount of the more important mineral ingredients
present in a soil, it may be mentioned that it has been calculated, in
the case of a poor sandy soil, _that the amount of potash it contains_
(_provided it were in an available condition_) would be sufficient to
yield three or four average crops of potatoes; of phosphates, nineteen
average crops; and of lime, seventy-three. But then only a very small
amount of this fertilising matter is in a readily available form.

It is for this reason that artificial manures, although added in such
small amounts, exercise so striking an influence in increasing plants'
growth. Their effect, however, is to a large extent only of a temporary
nature; and in attempting to assess the unexhausted value of a manure a
year or two after its application, we must remember this fact.

Some manures are very speedily taken up by plants, and some are very
easily washed out of the soil. Others, again, it would seem highly
probable, have a tendency to become converted into a more or less inert
condition after a while. This remark may be especially applied to the
fertilising constituents (chiefly nitrogen) in farmyard manure.[253] The
whole question, however, is little understood. One or two points may be
drawn attention to. In the first place, it may be safely affirmed that
little direct effect can be expected from such quickly available and
easily soluble forms of nitrogenous manures as nitrate of soda and
sulphate of ammonia a year after application. Potash and phosphates, on
the other hand, may exercise an effect for a considerably longer period;
and what the length of this period may be will depend on their amount
and condition. Thus it is not likely that superphosphate will have much
effect more than two years after application. On the other hand, such
manures as bones, basic slag, and farmyard manure may exert an
appreciable influence for a number of years. How long exactly, it is
wellnigh impossible to say, the rate at which they are applied and the
nature of the soil having an important influence.


_Tables of Value of Unexhausted Manures._

Numerous tables have been drawn up for the purpose of guiding farmers in
estimating this unexhausted value at different periods after
application, and in the case of different manures. Such tables, as a
rule, furnish only very rough approximations, and are little better than
mere guess-work. Still more complicated is the attempt to assess the
manurial value of foods consumed by the stock of the farm. Lawes and
Gilbert have devoted much attention to the elucidation of this difficult
question, and have drawn up most elaborate and valuable tables,
furnishing data for calculating unexhausted manure value in the case of
commonly used foods. These tables are given in the Appendix.[254] In
them will be found the manurial value of different cattle-foods,
calculated on the basis of numerous experiments carried out at
Rothamsted.

Thus these experiments have demonstrated that, on an average, probably
not more than _one-tenth_ of the nitrogen, phosphoric acid, and potash a
food contains is removed from the food in its passage through the animal
system. The exact amount will obviously depend on a variety of
conditions, referred to already in a previous chapter.[255]

In explanation of these tables, it may be pointed out that Table I.
gives the total quantities of the three fertilising ingredients in
various foods; while Table II. shows the proportion retained in the
animal body and the proportion voided in the manure, as well as the
manurial value of the food, assuming that it exercises its full
theoretical effect. As this, however, is never fully realised, it is
necessary to make some deduction. The deduction suggested by the
Rothamsted experimenters, on the basis of their wide experience, is 50
per cent for food consumed within the last year. That is to say, the
manurial value of food consumed during the last year is _only one-half
its theoretical value_. For food consumed within the last year but one,
they suggest a deduction of one-third of the allowance for last year;
while for food consumed three years back, a deduction of one-third from
this latter sum should be made; and so on for whatever number of years,
down to eight, may be taken.


FOOTNOTES:

[248] The term _insoluble phosphates_ is an unfortunate one, as the word
insoluble is purely relative in its significance. _Undissolved_
phosphates would be a better term.

[249] The amount of "reverted" phosphate is estimated by _the ammonium
citrate process_.

[250] See Note I., p. 553.

[251] See Note II., p. 554.

[252] See Note III., p. 556.

[253] See Chapter on Farmyard Manure, p. 271.

[254] See Note IV., p. 557.

[255] See Chapter on Farmyard Manure, pp. 224-236.



APPENDIX TO CHAPTER XXV.


NOTE I. (p. 543).

USEFUL FACTORS FOR CALCULATING THE PERCENTAGE OF IMPORTANT MANURIAL
INGREDIENTS IN A MANURE INTO THEIR DIFFERENT COMPOUNDS. (From the
'Transactions of the Highland and Agricultural Society.')

----------------------------+------------+----------------------
                            | Multiplied | Gives corresponding
        Amount of           |     by     |      amount of
----------------------------+------------+----------------------
Nitrogen                    |   1.214    | Ammonia.
   "                        |   6.3      | Albuminoid matter.
Ammonia                     |    .824    | Nitrogen.
   "                        |   3.882    | Sulphate of ammonia.
   "                        |   3.147    | Muriate of ammonia.
   "                        |   3.706    | Nitric acid.
   "                        |   5.0      | Nitrate of soda.
Potash (anhydrous)          |   1.85     | Sulphate of potash.
   "                        |   1.585    | Muriate of potash.
Phosphoric acid (anhydrous) |   2.183    | Phosphate of lime.
       "             "      |   1.4      | Biphosphate.
       "             "      |   1.648    | Soluble phosphate.
Soluble phosphate           |   1.325    | Phosphate of lime.
Biphosphate                 |   1.566    |         "
Lime                        |   1.845    |         "
 "                          |   1.786    | Carbonate of lime.
Chlorine                    |   1.648    | Chloride of sodium.
----------------------------+------------+----------------------


NOTES II. (p. 545).

UNITS TO BE USED IN DETERMINING THE COMMERCIAL VALUE OF
MANURES.

_For Season 1893._

A: Ichaboe.
B: Peruvian (riddled).
C: Fish guano.
D: Frey Bentos guano.
E: Steamed bone-flour.
F: Dissolved or vitriolated bones.
G: Superphosphates.
H: Genuine.
I: Genuine.
J: Average.

-----------------+-----------+-----------------+-----------+
                 |  Guanos.  |  Scrap manures. |           |
 Items to be     +-----+-----+-----+-----------+ Bone-meal.|
  valued.        |  A  |  B  |  C  |    D      |           |
-----------------+-----+-----+-----+-----+-----+-----+-----+
  Classes        |  H  |  I  |     |_a._ |_b._ |_a._ |_b._ |
-----------------+-----+-----+-----+-----+-----+-----+-----+
Phosphates       |     |     |     |     |     |     |     |
  Dissolved      |  -  |  -  |  -  |  -  |  -  |  -  |  -  |
  Undissolved--  |  2/-|  2/-|  1/5|  1/6|  1/4|  1/4|  1/3|
  Ammonia        | 16/-| 17/6| 10/-| 11/6| 10/-| 10/-|  9/6|
  Potash         |  -  |  3/6|  -  |  -  |  -  |  -  |  -  |
                 |     |     |     |     |     |     |     |
Prices per ton,  |     |     |     |     |     |     |     |
 March 1893--    |     |     |     |     |     |     |     |
  From           |250/-|230/-|130/-|150/-|120/-|105/-|100/-|
  To             |270/-|290/-|150/-|180/-|140/-|115/-|110/-|
-----------------+-----+-----+-----+-----+-----+-----+-----+

-----------------+-----+-----+-----+-----------------
                 |     |     |     |
 Items to be     |  E  |  F  |  G  |  Dissolved
  valued.        |     |     |     |  Compounds.
-----------------+-----+-----+-----+-----+-----+-----
  Classes        |_a._ |     |     | From|  To |  J
-----------------+-----+-----+-----+-----+-----+-----
Phosphates       |     |     |     |     |     |
  Dissolved      |  -  |  2/6|  -  |  2/-|  2/6|  2/3
  Undissolved--  |  1/5|  1/6| 1/11|  1/3|  1/9|  1/6
  Ammonia        | 10/-| 11/6|  -  | 10/-| 12/-| 11/-
  Potash         |  -  |  -  |  -  |  3/4|  3/8|  3/6
                 |     |     |     |     |     |
Prices per ton,  |     |     |     |     |     |
 March 1893--    |     |     |     |     |     |
  From           | 95/-| 95/-| 45/-|  -  |  -  |  -
  To             |110/-|110/-| 60/-|  -  |  -  |  -
-----------------+-----+-----+-----+-----+-----+-----


CASH PRICES OF DIFFERENT MANURES, MARCH 1893.

---------------------------------+-----------+----------+--------------
                                 |           |Price per |
            MANURES.             | Guarantee.|   ton.   |    Unit.
---------------------------------+-----------+----------+--------------
                                 | Per cent. |_£  s. d._|
Sulphate of ammonia, 97 per cent |  24  Am.  | 11 10  0 |  Am. = 9/7
Nitrate of soda, 95 per cent     |  19  "    | 10  5  0 |  "  = 10/9
Castor-cake dust                 |  5.5 "    |  3 10  0 |  "  = 12/9
Horn-dust                        |  15  "    |  8 10  0 |  "  = 11/4
Dried blood                      |  15  "    |  8  0  0 |  "  = 10/7
Muriate of potash, 80 per cent   |  50 Pot   |  8 15  0 |  Pot.= 3/6
Sulphate of potash, 50 per cent  |  27  "    |  5  5  0 |  "   = 3/10
Kainit, 23 per cent              |  12  "    |  2  0  0 |  "   = 3/4
Nitrate of potash, 73 per cent   | {14  Am.} | 14 10  0 | {Am. = 10/}
                                 | {40 Pot.} |          | {Pot.= 3/9}
Ground Charleston phosphate      |  57 Phos. |  3  0  0 |  Phos.=1/
Belgian phosphate                |  50  "    |  2  5  0 |  "   = 0/11
Thomas-slag (fine) Scotch        |  30  "    |  1 16  0 |  "   = 1/2
Thomas-slag (fine) English       |  37  "    |  2  3  0 |  "   = 1/2
Phosphatic guano                 | {67  "  } |  5  0  0 | {"   = 1/4}
                                 | {1  Am. } |          | {Am. = 10/}
---------------------------------+-----------+----------+--------------


NOTE III. (p. 549).

TABLES SHOWING RELATIVE MANURIAL VALUE OF NITROGEN AND POTASH IN
DIFFERENT SUBSTANCES.

                              _Wolff, 1893._

Nitrogen in form of ammonia and nitrates, and easily
         decomposable organic compounds, as dried blood,
         flesh-meal, meat-meal, Peruvian guano, and as urate       100
   "     in fine steamed bone-meal, fish-guano, oilcakes, and
         better kinds of artificial guano                           85
   "     in fine bone-meal and horn-meal                            77
   "     in coarse bones and horn-shavings, woollen refuse,
         farmyard manure, and poudrette                             61

                             _American, 1892._

   "     in ammonia salts                                          100
   "     as nitrates                                                86
   "     in dry and fine-ground fish, meat, and blood               91
   "     in cotton-seed meal, and castor pomace                     86
   "     in fine bone and tankage                                   86
   "     in medium bone and tankage                                 68
   "     in coarser bone and tankage                                43
   "     in hair and horn-shavings, and coarse fish scrap           40
Potash as high-grade sulphate, and in forms free from muriates
         (or chlorides)                                            100
   "   as muriate                                                   82

Professor Wagner has drawn up, from numerous experiments, the relative
manurial values of different nitrogenous manures, which he rates as
follows:--

Nitrate of soda                                             100
Sulphate of ammonia                                          90
Blood-meal, horn-meal, and green vegetable matter            70
Finely ground steamed bone-meal, fish-meal, and meat-meal
 guano                                                       60
Farmyard manure                                              45
Shoddy                                                       30
Leather-meal                                                 20


NOTE IV. (p. 551).

TABLE I.--AVERAGE COMPOSITION, PER CENT AND PER TON, OF
CATTLE-FOODS.

-----+-------------------+-----------------------------------------------+
     |                   |               PER CENT.                       |
 NO. |     FOODS.        +---------+--------+---------+--------+---------+
     |                   | Dry     | Nitro- | Mineral | Phos-  | Potash. |
     |                   | Matter. | gen.   | Matter  | phoric |         |
     |                   |         |        | (Ash).  | Acid   |         |
-----+-------------------+---------+--------+---------+--------+---------+
     |                   |  per    |  per   |  per    |  per   |  per    |
     |                   |  cent.  |  cent. |  cent.  |  cent. |  cent.  |
  1  | Linseed           |  90.00  |  3.60  |  4.00   |  1.54  |   1.37  |
  2  | Linseed-cake      |  88.50  |  4.75  |  6.50   |  2.00  |   1.40  |
  3  | Decorticated      |         |        |         |        |         |
     |   cotton cake     |  90.00  |  6.60  |  7.00   |  3.10  |   2.00  |
  4  | Palm-nut-cake     |  91.00  |  2.50  |  3.60   |  1.20  |   0.50  |
  5  | Undecorticated    |         |        |         |        |         |
     |   cotton-cake     |  87.00  |  3.75  |  6.00   |  2.00  |   2.00  |
  6  | Cocoa-nut-cake    |  90.00  |  3.40  |  6.00   |  1.40  |   2.00  |
  7  | Rape-cake         |  89.00  |  4.90  |  7.50   |  2.50  |   1.50  |
-----+-------------------+---------+--------+---------+--------+---------+
  8  | Peas              |  85.00  |  3.60  |  2.50   |  0.85  |   0.96  |
  9  | Beans             |  85.00  |  4.00  |  3.00   |  1.10  |   1.30  |
 10  | Lentils           |  88.00  |  4.20  |  4.00   |  0.75  |   0.70  |
 11  | Tares (seed)      |  84.00  |  4.20  |  2.50   |  0.80  |   0.80  |
-----+-------------------+---------+--------+---------+--------+---------+
 12  | Indian corn       |  88.00  |  1.70  |  1.40   |  0.60  |   0.37  |
 13  | Wheat             |  85.00  |  1.80  |  1.70   |  0.85  |   0.53  |
 14  | Malt              |  94.00  |  1.70  |  2.50   |  0.80  |   0.50  |
 15  | Barley            |  84.00  |  1.65  |  2.20   |  0.75  |   0.55  |
 16  | Oats              |  86.00  |  2.00  |  2.80   |  0.60  |   0.50  |
 17  | Rice-meal*        |  90.00  |  1.90  |  7.50   | (0.60) |  (0.37) |
 18  | Locust-beans*     |  85.00  |  1.20  |  2.50   |   -    |    -    |
-----+-------------------+---------+--------+---------+--------+---------+
 19  | Malt-combs        |  90.00  |  3.90  |  8.00   |  2.00  |   2.00  |
 20  | Fine pollard      |  86.00  |  2.45  |  5.50   |  2.90  |   1.46  |
 21  | Coarse pollard    |  86.00  |  2.50  |  6.40   |  3.50  |   1.50  |
 22  | Bran              |  86.00  |  2.50  |  6.50   |  3.60  |   1.45  |
-----+-------------------+---------+--------+---------+--------+---------+
 23  | Clover-hay        |  83.00  |  2.40  |  7.00   |  0.57  |   1.50  |
 24  | Meadow-hay        |  84.00  |  1.50  |  6.50   |  0.40  |   1.60  |
-----+-------------------+---------+--------+---------+--------+---------+
 25  | Pea-straw         |  82.50  |  1.00  |  5.50   |  0.35  |   1.00  |
 26  | Oat-straw         |  83.00  |  0.50  |  5.50   |  0.24  |   1.00  |
 27  | Wheat-straw       |  84.00  |  0.45  |  5.00   |  0.24  |   0.80  |
 28  | Barley-straw      |  85.00  |  0.40  |  4.50   |  0.18  |   1.00  |
 29  | Bean-straw        |  82.50  |  0.90  |  5.00   |  0.30  |   1.00  |
-----+-------------------+---------+--------+---------+--------+---------+
 30  | Potatoes          |  25.00  |  0.25  |  1.00   |  0.15  |   0.55  |
 31  | Carrots           |  14.00  |  0.20  |  0.90   |  0.09  |   0.28  |
 32  | Parsnips          |  16.00  |  0.22  |  1.00   |  0.19  |   0.36  |
 33  | Swedish turnips   |  11.00  |  0.25  |  0.60   |  0.06  |   0.22  |
 34  | Mangel-wurzels    |  12.50  |  0.22  |  1.00   |  0.07  |   0.40  |
 35  | Yellow turnips*   |   9.00  |  0.20  |  0.65   | (0.06) |  (0.22) |
 36  | White turnips     |   8.00  |  0.18  |  0.68   |  0.05  |   0.30  |
-----+-------------------+---------+--------+---------+--------+---------+


-----+-------------------+-------------------------------
     |                   |       PER TON.
     |                   +-----------+---------+---------
 NO. | FOODS.            | Nitrogen. | Phos-   | Potash.
     |                   |           | phoric  |
     |                   |           | Acid    |
-----+-------------------+-----------+---------+---------
     |                   |    lb.    |   lb.   |   lb.
  1  | Linseed           |   80.64   |  34.50  |  30.69
  2  | Linseed-cake      |  106.40   |  44.80  |  31.36
  3  | Decorticated      |           |         |
     |   cotton cake     |  147.84   |  69.44  |  44.80
  4  | Palm-nut-cake     |   56.00   |  26.88  |  11.20
  5  | Undecorticated    |           |         |
     |   cotton-cake     |   84.00   |  44.80  |  44.80
  6  | Cocoa-nut-cake    |   76.16   |  31.36  |  44.80
  7  | Rape-cake         |  109.76   |  56.00  |  33.60
-----+-------------------+-----------+---------+---------
  8  | Peas              |   80.64   |  19.04  |  21.50
  9  | Beans             |   89.60   |  24.64  |  29.12
 10  | Lentils           |   94.08   |  16.80  |  15.68
 11  | Tares (seed)      |   94.08   |  17.92  |  17.92
-----+-------------------+-----------+---------+---------
 12  | Indian corn       |   38.08   |  13.44  |   8.29
 13  | Wheat             |   40.32   |  19.04  |  11.87
 14  | Malt              |   38.08   |  17.92  |  11.20
 15  | Barley            |   36.96   |  16.80  |  12.32
 16  | Oats              |   44.80   |  13.44  |  11.20
 17  | Rice-meal*        |   42.56   | (13.44) |  (8.29)
 18  | Locust-beans*     |   26.88   |    -   |    -
-----+-------------------+-----------+---------+---------
 19  | Malt-combs        |   87.36   |  44.80  |  44.80
 20  | Fine pollard      |   54.88   |  64.96  |  32.70
 21  | Coarse pollard    |   56.00   |  78.40  |  33.60
 22  | Bran              |   56.00   |  80.64  |  32.48
-----+-------------------+-----------+---------+---------
 23  | Clover-hay        |   53.76   |  12.77  |  33.60
 24  | Meadow-hay        |   33.60   |   8.96  |  35.84
-----+-------------------+-----------+---------+---------
 25  | Pea-straw         |   22.40   |   7.84  |  22.40
 26  | Oat-straw         |   11.20   |   5.38  |  22.40
 27  | Wheat-straw       |   10.08   |   5.38  |  17.92
 28  | Barley-straw      |    8.96   |   4.03  |  22.40
 29  | Bean-straw        |   20.16   |   6.72  |  22.40
-----+-------------------+-----------+---------+---------
 30  | Potatoes          |    5.60   |   3.36  |  12.32
 31  | Carrots           |    4.48   |   2.02  |   6.27
 32  | Parsnips          |    4.93   |   4.26  |   8.06
 33  | Swedish turnips   |    5.60   |   1.34  |   4.93
 34  | Mangel-wurzels    |    4.93   |   1.57  |   8.96
 35  | Yellow turnips*   |    4.48   |  (1.34) |  (4.93)
 36  | White turnips     |    4.03   |   1.12  |   6.72
-----+-------------------+-----------+---------+---------
* In the case of neither rice-meal, locust-beans, nor yellow turnips have
records of ash analyses been found. For rice-meal the same percentages of
phosphoric acid and potash as in Indian corn, and for yellow turnips the
same as in swedes, are provisionally adopted; but in all the Tables the
assumed results are given in parentheses.  For locust-beans no figure has
been assumed, and the columns are left blank.


NOTE IV.

TABLE II.--LAWES' & GILBERT'S TABLES FOR CALCULATING UNEXHAUSTED
VALUE OF MANURES.

KEY:
A - Fattening Increase in Live Weight (Oxen or Sheep).
B - In Food.
C - In Fattening Increase (at 1.27 per cent).
D - In Manure.
E - Food to 1 Increase.
F - Increase per ton of Food.
G - Per cent.
H - Per ton.
I - From 1 ton of Food.
J - Per cent of total consumed.
K - Total remaining for Manure.
L - Nitrogen equal Ammonia.
M - Value of Ammonia at 6d per lb.

---+---------------+-----------+----------------------------------------------+
   |               |           |                  NITROGEN.                   |
   |               |           |------------+-----------+---------------------|
   |               |     A     |     B      |     C     |          D          |
   | DESCRIPTION   |-----+-----+-----+------+-----+-----+------+-----+--------|
NO.|   OF FOOD.    |  E  |  F  |  G  |   H  |  I  |  J  |   K  |  L  |   M    |
---+---------------+-----+-----+-----+------+-----+-----+------+-----+--------+
   |               |     | lb. |  %  |  lb. | lb. |  %  |  lb. | lb. |£ s. d. |
 1 |Linseed        |  5.0|448.0| 3.60| 80.64| 5.69| 7.06| 74.95| 91.0|2  5  6 |
 2 |Linseed-cake   |  6.0|373.3| 4.75|106.40| 4.74| 4.45|101.66|123.4|3  1  8 |
 3 |Decorticated   |     |     |     |      |     |     |      |     |        |
   |  cotton-cake  |  6.5|344.6| 6.60|147.84| 4.38| 2.96|143.46|174.2|4  7  1 |
 4 |Palm-nut-cake  |  7.0|320.0| 2.50| 56.00| 4.06| 7.25| 51.94| 63.1|1 11  7 |
 5 |Undecorticated |     |     |     |      |     |     |      |     |        |
   |  cotton-cake  |  8.0|280.0| 3.75| 84.00| 3.56| 4.24| 80.44| 97.7|2  8 10 |
 6 |Cocoa-nut-cake |  8.0|280.0| 3.40| 76.16| 3.56| 4.67| 72.60| 88.2|2  4  1 |
 7 |Rape-cake      |(10) |(224)| 4.90|109.76| 2.84| 2.59|106.92|129.8|3  4 11 |
   |               |-----+-----+-----+------+-----+-----+------+-----+--------+
 8 |Peas           |  7.0|320.0| 3.60| 80.64| 4.06| 5.03| 76.58| 93.0|2  6  6 |
 9 |Beans          |  7.0|320.0| 4.00| 89.60| 4.06| 4.53| 85.54|103.9|2 11 11 |
10 |Lentils        |  7.0|320.0| 4.20| 94.08| 4.06| 4.32| 90.02|109.3|2 14  8 |
11 |Tares (seed)   |  7.0|320.0| 4.20| 94.08| 4.06| 4.32| 90.02|109.3|2 14  8 |
   |               |-----+-----+-----+------+-----+-----+------+-----+--------+
12 |Indian corn    |  7.2|311.1| 1.70| 38.08| 3.95|10.37| 34.13| 41.4|1  0  9 |
13 |Wheat          |  7.2|311.1| 1.80| 40.32| 3.95| 9.80| 36.37| 44.2|1  2  1 |
14 |Malt           |  7.0|320.0| 1.70| 38.08| 4.06|10.66| 34.02| 41.3|1  0  8 |
15 |Barley         |  7.2|311.1| 1.65| 36.96| 3.95|10.69| 33.01| 40.1|1  0  1 |
16 |Oats           |  7.5|298.7| 2.00| 44.80| 3.79| 8.46| 41.01| 49.8|1  4 11 |
17 |Rice-meal      |  7.5|298.7| 1.90| 42.56| 3.79| 8.91| 38.77| 47.1|1  3  6 |
18 |Locust-beans   |  9.0|248.9| 1.20| 26.88| 3.16|11.76| 23.72| 28.8|0 14  5 |
   |               |-----+-----+-----+------+-----+-----+------+-----+--------+
19 |Malt-combs     |  8.0|280.0| 3.90| 87.36| 3.56| 4.08| 83.80|101.8|2 10 11 |
20 |Fine pollard   |  7.5|298.7| 2.45| 54.88| 3.79| 6.91| 51.09| 62.0|1 11  0 |
21 |Coarse pollard |  8.0|280.0| 2.50| 56.00| 3.56| 6.35| 52.44| 63.7|1 11 10 |
22 |Bran           |  9.0|248.9| 2.50| 56.00| 3.16| 5.64| 52.84| 64.2|1 12  1 |
   |               |-----+-----+-----+------+-----+-----+------+-----+--------+
23 |Clover-hay     | 14.0|160.0| 2.40| 53.76| 2.03| 3.78| 51.73| 62.8|1 11  5 |
24 |Meadow-hay     | 15.0|149.3| 1.50| 33.60| 1.90| 5.65| 31.70| 38.5|0 19  3 |
   |               |-----+-----+-----+------+-----+-----+------+-----+--------+
25 |Pea-straw      | 16.0|140.0| 1.00| 22.40| 1.78| 7.95| 20.62| 25.0|0 12  6 |
26 |Oat-straw      | 18.0|124.4| 0.50| 11.20| 1.58|14.11|  9.62| 11.7|0  5 10 |
27 |Wheat-straw    | 21.0|106.7| 0.45| 10.08| 1.36|13.49|  8.72| 10.6|0  5  4 |
28 |Barley-straw   | 23.0| 97.4| 0.40|  8.96| 1.24|13.84|  7.72|  9.4|0  4  8 |
29 |Bean-straw     | 22.0|101.8| 0.90| 20.16| 1.29| 6.39| 18.87| 22.9|0 11  6 |
   |               |-----+-----+-----+------+-----+-----+------+-----+------- +
30 |Potatoes       | 60.0| 37.3| 0.25|  5.60| 0.47| 8.39|  5.13|  6.2|0  3  1 |
31 |Carrots        | 85.7| 26.1| 0.20|  4.48| 0.33| 7.37|  4.15|  5.0|0  2  6 |
32 |Parsnips       | 75.0| 29.9| 0.22|  4.93| 0.38| 7.71|  4.55|  5.5|0  2  9 |
33 |Swedish turnips|109.1| 20.5| 0.25|  5.60| 0.26| 4.64|  5.34|  6.5|0  3  3 |
34 |Mangel-wurzels | 96.0| 23.3| 0.22|  4.93| 0.30| 6.09|  4.63|  5.6|0  2 10 |
35 |Yellow turnips |133.3| 16.8| 0.20|  4.48| 0.21| 4.69|  4.27|  5.2|0  2  7 |
36 |White turnips  |150.0| 14.9| 0.18|  4.03| 0.19| 4.71|  3.84|  4.7|0  2  4 |
---+---------------+-----+-----+-----+------+-----+-----+------+-----+--------+



KEY:
 N - In Food.
 O - In Fattening Increase at (0.86 per cent).
 P - In Manure.
 Q - In Food.
 R - In Fattening Increase at (0.11 per cent).
 S - In Manure.
 T - Per cent.
 U - Per ton.
 V - From 1 ton of Food.
 W - Per cent of total consumed.
 X - Total remaining for Manure.
 Y - Value at 3d per lb.
 Z - Per cent.
AA - Per ton.
BB - From 1 ton of Food.
CC - Per cent of total consumed.
DD - Total remaining for Manure.
EE - Value at 2-1/2d. per lb.
FF - Total original Manure value per ton of Food consumed.

---+---------------+------------------------------------------+
   |               |              PHOSPHORIC ACID.            |
   |---------------+--------------+------------+--------------+
   |               |       N      |     O      |      P       |
   |  Description  |------+-------+----+-------+-------+------+
No.|    of Food.   |   T  |  U    | V  |   W   |   X   | Y    |
---+---------------+------+-------+----+-------+-------+------+
   |               |   %  |  lb.  | lb.|   %   |  lb.  |s. d. |
 1 |Linseed        | 1.54 | 34.50 |3.85| 11.16 | 30.65 | 7  8 |
 2 |Linseed-cake   | 2.00 | 44.80 |3.21|  7.17 | 41.59 |10  5 |
 3 |Decorticated   |      |       |    |       |       |      |
   |  cotton-cake  | 3.10 | 69.44 |2.96|  4.26 | 66.48 |16  8 |
 4 |Palm-nut-cake  | 1.20 | 26.88 |2.75| 10.23 | 24.13 | 6  0 |
 5 |Undecorticated |      |       |    |       |       |      |
   |  cotton-cake  | 2.00 | 44.80 |2.41|  5.38 | 42.39 |10  7 |
 6 |Cocoa-nut-cake | 1.40 | 31.36 |2.41|  7.68 | 28.95 | 7  3 |
 7 |Rape-cake      | 2.50 | 56.00 |1.93|  3.45 | 54.07 |13  6 |
   |               |------+-------+----+-------+-------+--- --+
 8 |Peas           | 0.85 | 19.04 |2.75| 14.44 | 16.29 | 4  1 |
 9 |Beans          | 1.10 | 24.64 |2.75| 11.10 | 21.89 | 5  6 |
10 |Lentils        | 0.75 | 16.80 |2.75| 16.37 | 14.05 | 3  6 |
11 |Tares (seed)   | 0.80 | 17.92 |2.75| 15.36 | 15.17 | 3  9 |
   |               |------+-------+----+-------+-------+--- --+
12 |Indian corn    | 0.60 | 13.44 |2.68| 19.94 | 10.76 | 2  8 |
13 |Wheat          | 9.85 | 19.04 |2.68| 14.08 | 16.36 | 4  1 |
14 |Malt           | 0.80 | 17.92 |2.75| 15.35 | 15.17 | 3  9 |
15 |Barley         | 0.75 | 16.80 |2.68| 15.95 | 14.12 | 3  6 |
16 |Oats           | 0.60 | 13.44 |2.57|(19.12)| 10.87 | 2  8 |
17 |Rice-meal      |(0.60)|(13.44)|2.57|(19.12)|(10.87)|(2  8)|
18 |Locust-beans   |  -   |   -   |2.14|   -   |   -   |   -  |
   |               |------+-------+----+-------+-------+------+
19 |Malt-combs     | 2.00 | 44.80 |2.41|  5.38 | 42.39 |10  7 |
20 |Fine pollard   | 2.90 | 64.96 |2.57|  3.96 | 62.39 |15  7 |
21 |Coarse pollard | 3.50 | 78.40 |2.41|  3.07 | 75.99 |19  0 |
22 |Bran           | 3.60 | 80.64 |2.14|  2.65 | 78.50 |19  8 |
   |               |------+-------+----+-------+-------+------+
23 |Clover-hay     | 0.57 | 12.77 |1.38| 10.81 | 11.39 | 2 10 |
24 |Meadow-hay     | 0.40 |  8.96 |1.28| 14.28 |  7.68 | 1 11 |
   |               |------+-------+----+-------+-------+------+
25 |Pea-straw      | 0.35 |  7.84 |1.20| 15.31 |  6.64 | 1  8 |
26 |Oat-straw      | 0.24 |  5.38 |1.07| 19.89 |  4.31 | 1  1 |
27 |Wheat-straw    | 0.24 |  5.38 |0.92| 17.10 |  4.46 | 1  1 |
28 |Barley-straw   | 0.18 |  4.03 |0.84| 20.84 |  3.19 | 0  9 |
29 |Bean-straw     | 0.30 |  6.72 |0.88| 13.10 |  5.84 | 1  5 |
   |               |------+-------+----+-------+-------+------+
30 |Potatoes       | 0.15 |  3.36 |0.32|  9.52 |  3.04 | 0  9 |
31 |Carrots        | 0.09 |  2.02 |0.22| 10.89 |  1.80 | 0  5 |
32 |Parsnips       | 0.19 |  4.26 |0.26|  6.10 |  4.00 | 1  0 |
33 |Swedish turnips| 0.06 |  1.34 |0.18| 13.43 |  1.16 | 0  4 |
34 |Mangel-wurzels | 0.07 |  1.57 |0.20| 12.74 |  1.37 | 0  4 |
35 |Yellow turnips |(0.06)| (1.34)|0.14|(10.78)| (1.20)|(0  4)|
36 |White turnips  | 0.05 |  1.12 |0.13| 11.61 |  0.99 | 0  3 |
---+---------------+------+-------+----+-------+-------+------+

---+---------------+---------------------------------------+---------
   |               |               POTASH.                 |
   |---------------+-------------------------+-------------+
   |               |      Q      |     R     |     S       |
   |  Description  |------+------+----+------+------+------+
No.|    of Food.   |   Z  | AA   | BB |  CC  | DD   |  EE  |   FF
---+---------------+------+------+----+------+------+------+---------
   |               |   %  | lb.  | lb.|   %  | lb.  |s. d. | £ s. d.
 1 |Linseed        | 1.37 |30.69 |0.49| 1.60 |30.20 | 6  3 | 2 19  5
 2 |Linseed-cake   | 1.40 |31.36 |0.41| 1.31 |30.95 | 6  5 | 3 18  6
 3 |Decorticated   |      |      |    |      |      |      |
   |  cotton-cake  | 2.00 |44.80 |0.38| 0.85 |44.42 | 9  3 | 5 13  0
 4 |Palm-nut-cake  | 0.50 |11.20 |0.35| 3.13 |10.85 | 2  3 | 1 19 10
 5 |Undecorticated |      |      |    |      |      |      |
   |  cotton-cake  | 2.00 |44.80 |0.31| 0.69 |44.49 | 5 11 | 3  5  4
 6 |Cocoa-nut-cake | 2.00 |44.80 |0.31| 0.69 |44.49 | 9  3 | 3  0  7
 7 |Rape-cake      | 1.50 |33.60 |0.25| 0.74 |33.35 | 6 11 | 4  5  4
   |               |------+------+----+------+------+------+---------
 8 |Peas           | 0.96 |21.50 |0.35| 1.63 |21.15 | 4  5 | 2 15  0
 9 |Beans          | 1.30 |29.12 |0.35| 1.20 |28.77 | 6  0 | 3  3  5
10 |Lentils        | 0.70 |15.68 |0.35| 2.23 |15.33 | 3  2 | 3  1  4
11 |Tares (seed)   | 0.80 |17.92 |0.35| 1.95 |17.57 | 3  8 | 3  2  1
   |               |------+------+----+------+------+------+---------
12 |Indian corn    | 0.37 | 8.29 |0.34| 4.10 | 7.95 | 1  8 | 1  5  1
13 |Wheat          | 0.53 |11.87 |0.34| 2.86 |11.53 | 2  5 | 1  8  7
14 |Malt           | 0.50 |11.20 |0.35| 3.13 |10.85 | 2  3 | 1  6  8
15 |Barley         | 0.55 |12.32 |0.34| 2.76 |11.98 | 2  6 | 1  6  1
16 |Oats           | 0.50 |11.20 |0.33| 2.94 |10.87 | 2  3 | 1  9 10
17 |Rice-meal      |(0.37)|(8.29)|0.33|(4.00)|(7.96)|(1  8)|(1  7 10)
18 |Locust-beans   |  -   |   -  |0.27|   -  |   -  |   -  |    -
   |               |------+------+----+------+------+------+---------
19 |Malt-combs     | 2.00 |44.80 |0.31| 0.69 |44.49 | 9  3 | 3 10  9
20 |Fine pollard   | 1.46 |32.70 |0.33| 1.01 |32.37 | 6  9 | 2 13  4
21 |Coarse pollard | 1.50 |33.60 |0.31| 0.92 |33.29 | 6 11 | 2 17  9
22 |Bran           | 1.45 |32.48 |0.27| 0.83 |32.21 | 6  8 | 2 18  5
   |               |------+------+----+------+------+------+---------
23 |Clover-hay     | 1.50 |33.60 |0.18| 0.54 |33.42 | 7  0 | 2  1  3
24 |Meadow-hay     | 1.60 |35.84 |0.16| 0.45 |35.68 | 7  5 | 1  8  7
   |               |------+------+----+------+------+------+---------
25 |Pea-straw      | 1.00 |22.40 |0.15| 0.67 |22.25 | 4  8 | 0 18 10
26 |Oat-straw      | 1.00 |22.40 |0.14| 0.63 |22.26 | 4  8 | 0 11  7
27 |Wheat-straw    | 0.80 |17.92 |0.12| 0.67 |17.80 | 3  8 | 0 10  1
28 |Barley-straw   | 1.00 |22.40 |0.11| 0.49 |22.29 | 4  8 | 0 10  1
29 |Bean-straw     | 1.00 |22.40 |0.11| 0.49 |22.29 | 4  8 | 0 17  7
   |               |------+------+----+------+------+------+---------
30 |Potatoes       | 0.55 |12.32 |0.04| 0.32 |12.28 | 2  7 | 0  6  5
31 |Carrots        | 0.28 | 6.27 |0.03| 0.48 | 6.24 | 1  4 | 0  4  3
32 |Parsnips       | 0.36 | 8.06 |0.03| 0.37 | 8.03 | 1  8 | 0  5  5
33 |Swedish turnips| 0.22 | 4.93 |0.02| 0.41 | 4.91 | 1  0 | 0  4  7
34 |Mangel-wurzels | 0.40 | 8.90 |0.03| 0.34 | 8.93 | 1 10 | 0  5  0
35 |Yellow turnips |(0.22)|(4.93)|0.02|(0.34)|(4.91)|(1  0)|(0  3 11)
36 |White turnips  | 0.30 | 6.72 |0.02| 0.30 | 6.70 | 1  5 | 0  4  0
---+---------------+------+------+----+------+------+------+---------



CHAPTER XXVI

THE ROTHAMSTED EXPERIMENTS.


Reference has been so repeatedly made in the preceding pages to the
Rothamsted experiments on manures, that it may form a fitting conclusion
to the present treatise to give a short account of these famous
experiments.

In describing these experiments, the author has remarked elsewhere[256]
"that, in respect of their wide scope, dealing as they have done with
almost every department of farming, the elaborate care and accuracy with
which they have been carried out, the length of time they have been in
progress, and, lastly, in respect of the important bearing their results
have had on agricultural practice, these famous experiments may be
justly described as unrivalled by any other similar ones."

Started on a small scale in 1837 by Sir John (then Mr) Lawes, they were
placed on a systematic basis in 1843, in which year Sir John Lawes
associated with himself Sir (then Dr) J. Henry Gilbert. They have thus
been in progress for a period of fifty years--a fact which was
celebrated a few months ago by the presentation of numerous
congratulatory addresses from various learned and agricultural societies
to the distinguished investigators, and the erection of a memorial
granite slab at Rothamsted. What increases the feeling of gratitude due
to Sir John Lawes by the agricultural community, is the fact that the
entire expense of conducting these experiments has been borne by
himself, and he has further most generously handed over to the nation a
large sum of money and a certain area of land for carrying them on in
perpetuity.


_Nature of Experiments on Crops and Manures._

The earliest systematic experiments were on turnips, and since then
almost every common crop has been experimented on. Table I. (p. 562) is
a list of the different experiments, with their duration, area, and
number of plots.


_Soil of Rothamsted._

Before describing the more striking results of these experiments, it may
be advisable to say that the elevation of the land at Rothamsted is
about 400 feet above sea-level; that the average rainfall is about 28
inches per annum; and that the surface-soil is a heavy loam, and the
subsoil a stiff clay, resting on chalk.


TABLE I.--LIST OF ROTHAMSTED FIELD EXPERIMENTS.

----------------------------------+------------+----------+------------
       Crops.                     |  Duration. |  Area.   |   Plots.
----------------------------------+------------+----------+------------
                                  |    Years.  |  Acres.  |
Wheat (various manures)           |     50     |   11     | 34 (or 37)
Wheat alternated with fallow      |     42     |    1     |      2
Wheat (varieties)                 |     15     |  4-8     |  about 20
Barley (various manures)          |     42     |    4-1/4 |     29
Oats (various manures)            |     10[1]  |    0-3/4 |      6
Beans (various manures)           |     32[2]  |    1-1/4 |     10
Beans (various manures)           |     27[3]  |    1     |      5
Beans, alternated with wheat      |     28[4]  |    1     |     10
Clover (various manures)          |     29[5]  |    3     |     18
Various leguminous plants         |     16     |    3     |     18
                                  |            |          |
Turnips (various manures)         |     28[6]  |    8     |     40
Sugar-beet (various manures)      |      5     |    8     |     41
Mangel-wurzel (various manures)   |     18     |    8     |     41
                                  +------------+          |
      Total root crops            |     51     |          |
                                  +------------+          |
                                  |            |          |
Potatoes (various manures)        |     18     |    2     |     10
Rotation (various manures)        |     46     |    3     |     12
Permanent grass (various manures) |     38     |    7     |     22
----------------------------------+------------+----------+------------
1. Including one year fallow.

2. Including one year wheat and five years fallow.

3. Including four years fallow.

4. Including two years fallow.

5. Clover, twelve times sown (first in 1848), eight yielding crops, but
   four of these very small, one year wheat, five years barley, twelve
   years fallow.

6. Including barley without manure three years (eleventh, twelfth, and
   thirteenth seasons).


WHEAT EXPERIMENTS.

The first experiments we shall refer to are those on _wheat_, since they
are among the oldest, and their results the most striking of any.


_Unmanured Plots._

Wheat has been continuously grown year after year on three plots for
fifty years, without the application of any manure whatever.

We shall first give the results of the first eight years as illustrating
the effect of season, which accounts for the irregular results obtained.
But for the difference in seasons, we should expect to find a steady
decrease in the amount of produce; and this is shown in taking the
average of groups of years, as we shall do in the next table.


WHEAT GROWN CONTINUOUSLY ON SAME LAND (unmanured).

TABLE II.--(a.) _Remits of first Eight Years (1844 to 1851)._

Year.              Bushels.   |   Year.              Bushels.
1844                15        |   1849.               19-1/4
1845                23-1/4    |   1850.               15-7/8
1846                18        |   1851.               15-7/8
1847                16-7/8    |                       ------
1848                14-3/4    |   Average of 8  years 17-3/8
                              |                       ------

TABLE III.--(b.) _Results of subsequent Forty Years (1852 to
1891)._

                          Grain       Weight per     Straw
                        (bushels).      bushel.     (cwts.)
20 years (1852-1871)      14-1/2        57-5/8       13
20  "    (1872-1891)      11-1/2        58-3/4        8-5/8
40  "    (1852-1891)      13            58-1/4       10-5/8
49th season   (1891)       9-3/8        59-1/2        7-1/2

It is interesting to notice the comparatively slight decrease which has
taken place in the yield of wheat during these fifty years. With such
wide variations, due to season, it is extremely difficult, as Sir J.
Henry Gilbert has pointed out, to estimate rate of decline due to
exhaustion. Excluding the very bad seasons, this may be reckoned at from
one-fourth to one-third of a bushel per acre per annum. _The return of
the first year is 15 bushels, while the yield of the forty-ninth season
is 9-3/8 bushels._ The average of the returns obtained during these
fifty years is really in _excess of the average yield of the principal
wheat-producing countries in the world_. This is truly a most astounding
result.

The next experiments we shall describe are those on the influence of
farmyard manure on the wheat crop when grown continuously.


TABLE IV.--WHEAT GROWN CONTINUOUSLY WITH FARMYARD MANURE (14
tons per annum).

                                      Weight per
                         Bushels.       bushel       Straw
                                         (lb.)      (cwts.)
 8 years (1844-1852)      28              -            -
20 "     (1852-1871)      35-7/8        60           33-7/8
20 "     (1872-1891)      33-1/2        60-3/8       31-3/8
40 "     (1852-1891)      34-7/8        60-1/4       32-5/8

It will be seen from the above results, which contain merely a selection
from a very much greater number of experiments, that farmyard manure
gives as good an average over the forty years as most of the artificial
mixtures do. That this is due to the nitrogen it contains, is strikingly
illustrated by the fact that mixed mineral manures alone give less than
half the return, and also by the fact that ammonia salts alone give a
return twice as great as mineral mixtures; while, lastly, the mixture of
mineral manures and ammonia salts gives but a slight increase over that
obtained with ammonia salts alone.

The remaining results, selected from a much larger number, need no
comment, and we shall give them in tabular form.


Table V.--WHEAT GROWN CONTINUOUSLY WITH ARTIFICIAL MANURES, FARMYARD
MANURE, AND UNMANURED.

_Average of Forty Years (1852-91)._

-------------------------------------+--------------------------------------+
                                     | PRODUCE PER ACRE--AVERAGE PER ANNUM. |
                                     +--------------------------------------+
                                     |            Dressed grain.            |
    MANURES PER ACRE PER ANNUM.      +--------------------------------------+
                                     |               Quantity.              |
                                     +------------+------------+------------+
                                     |  20 years, |  20 years, |  40 years, |
                                     |  1852-71.  |  1872-91.  |  1852-91.  |
-------------------------------------+------------+------------+------------+
                                     |    bush.   |    bush.   |    bush.   |
Farmyard manure, 14 tons per annum   |            |            |            |
    since 1843                       |    35-7/8  |    33-1/2  |    34-7/8  |
Unmanured continuously               |    14-1/2  |    11-1/2  |    13      |
Mixed mineral manures[1] and 3-1/2   |            |            |            |
    cwt. superphosphate              |    17      |    12-7/8  |    15      |
Mixed mineral manures, 3-1/2 cwt.    |            |            |            |
    superphosphate, 200 lb. ammonium |            |            |            |
    salts                            |    26-1/2  |    21-3/4  |    24-1/8  |
Mixed mineral manures and 3-1/2 cwt. |            |            |            |
    superphosphate, 600 lb. ammonium |            |            |            |
    salts                            |    38-1/4  |    34-3/4  |    36-1/2  |
Mixed mineral manures, 3-1/2 cwt.    |            |            |            |
    superphosphate, 275 lb. nitrate  |            |            |            |
    of soda                          |    36-7/8  |    34      |    35-3/8  |
275 lb. nitrate of soda              |    26      |    19-3/8  |    22-3/4  |
400 lb. ammonium salts every year    |            |            |            |
    since 1845                       |    22-1/2  |    19      |    22-1/2  |
400 lb. ammonium salts, 3-1/2 cwt.   |            |            |            |
    superphosphate                   |    28      |    22-1/4  |    25-1/8  |
Mineral manure, 3-1/2 cwt.           |            |            |            |
    superphosphate, 400 lb. ammonium |            |            |            |
    salts in autumn                  |    31-5/8  |    29-1/2  |    30-1/2  |
-------------------------------------+------------+------------+------------+
----------------------------------------------------------------------------+
                                     | PRODUCE PER ACRE--AVERAGE PER ANNUM. |
                                     +--------------------------------------+
                                     |            Dressed grain.            |
    MANURES PER ACRE PER ANNUM.      +--------------------------------------+
                                     |               Quantity.              |
                                     +------------+------------+------------+
                                     |  20 years, |  20 years, |  40 years  |
                                     |  1852-71.  |  1872-91.  |  1852-91.  |
-------------------------------------+------------+------------+------------+
                                     |     lb.    |     lb.    |     lb.    |
Farmyard manure, 14 tons per annum   |            |            |            |
    since 1843                       |    60      |    60-3/8  |    60-1/4  |
Unmanured continuously               |    57-5/8  |    58-3/4  |    58-1/4  |
Mixed mineral manures and 3-1/2      |            |            |            |
    cwt. superphosphate              |    58-7/8  |    59      |    58-7/8  |
Mixed mineral manures, 3-1/2 cwt.    |            |            |            |
    superphosphate, 200 lb. ammonium |            |            |            |
    salts                            |    59-3/8  |    60      |    59-5/8  |
Mixed mineral manures and 3-1/2 cwt. |            |            |            |
superphosphate, 600 lb. ammonium     |            |            |            |
    salts                            |    59      |    60      |    59-1/2  |
Mixed mineral manures, 3-1/2 cwt.    |            |            |            |
superphosphate, 275 lb. nitrate of   |            |            |            |
   soda                              |    58-3/8  |    59-5/8  |    59      |
275 lb. nitrate of soda              |    56-5/8  |    56-5/8  |    56-5/8  |
400 lb. ammonium salts every year    |            |            |            |
    since 1845                       |    58      |    57-3/8  |    57-5/8  |
400 lb. ammonium salts, 3-1/2 cwt.   |            |            |            |
    superphosphate                   |    57-3/8  |    58      |    57-5/8  |
Mineral manure, 3-1/2 cwt.           |            |            |            |
    superphosphate, 400 lb. ammonium |            |            |            |
    salts in autumn                  |    59-1/2  |    60      |    59-3/4  |
-------------------------------------+------------+------------+------------+
-------------------------------------+--------------------------------------
                                     |  PRODUCE PER ACRE--AVERAGE PER ANNUM.
                                     +--------------------------------------
    MANURES PER ACRE PER ANNUM.      |              Total straw.
                                     +--------------------------------------
                                     |  20 years, |  20 years, |  40 years
                                     |  1852-71.  |  1872-91.  |  1852-91.
-------------------------------------+------------+------------+------------
                                     |     cwt.   |     cwt.   |     cwt.
Farmyard manure, 14 tons per annum   |            |            |
    since 1843                       |    33-7/8  |    31-3/8  |    32-5/8
Unmanured continuously               |    13      |     8-5/8  |    10-5/8
Mixed mineral manures and 3-1/2      |            |            |
    cwt. superphosphate              |    15      |     9-3/4  |    12-3/8
Mixed mineral manures, 3-1/2 cwt.    |            |            |
    superphosphate, 200 lb. ammonium |            |            |
    salts                            |    24-1/2  |    19-1/8  |    21-7/8
Mixed mineral manures and 3-1/2 cwt. |            |            |
superphosphate, 600 lb. ammonium     |            |            |
    salts                            |    41-3/8  |    39-5/8  |    40-1/2
Mixed mineral manures, 3-1/2 cwt.    |            |            |
superphosphate, 275 lb. nitrate of   |            |            |
    soda                             |    41-1/2  |    37-3/4  |    39-5/8
275 lb. nitrate of soda              |    28-1/4  |    18-1/2  |    23-3/8
400 lb. ammonium salts every year    |            |            |
    since 1845                       |    24-3/4  |    16-1/4  |    20-1/2
400 lb. ammonium salts, 3-1/2 cwt.   |            |            |
    superphosphate                   |    26-3/8  |    21      |    23-3/4
Mineral manure, 3-1/2 cwt.           |            |            |
    superphosphate, 400 lb. ammonium |            |            |
    salts in autumn                  |    31-1/4  |    28-3/8  |    29-3/4
-------------------------------------+------------+------------+------------
1. By the term mixed mineral manures is meant a mixture of mineral
   fertilisers, not including phosphates.


Table VI.--EXPERIMENTS ON THE GROWTH OF BARLEY FOR FORTY YEARS, 1852-91.

-------------------------------------+--------------------------------------+
                                     | PRODUCE PER ACRE--AVERAGE PER ANNUM. |
                                     +--------------------------------------+
                                     |            Dressed grain.            |
    MANURES PER ACRE PER ANNUM.      +--------------------------------------+
                                     |               Quantity.              |
                                     +------------+------------+------------+
                                     |  20 years, |  20 years, |  40 years, |
                                     |  1852-71.  |  1872-91.  |  1852-91.  |
-------------------------------------+------------+------------+------------+
                                     |    bush.   |    bush.   |    bush.   |
Unmanured continuously               |    20      |    13-1/4  |    16-1/2  |
3-1/2 cwt. superphosphate of lime    |    25-1/2  |    17-3/4  |    21-3/4  |
Mixed mineral manures                |    22-1/2  |    13-1/2  |    18      |
Mixed mineral manures, 3-1/2 cwt.    |            |            |            |
    superphosphate                   |    27-1/2  |    17-1/4  |    22-3/8  |
200 lb. ammonium salts               |    32-1/2  |    25-5/8  |    29      |
200 lb. ammonium salts, 3-1/2 cwt.   |            |            |            |
    superphosphate                   |    47      |    38-1/2  |    42-3/4  |
200 lb. ammonium salts,              |            |            |            |
    mixed mineral manures            |    35      |    27-3/4  |    31-3/8  |
Manures, 3-1/2 cwt.                  |            |            |            |
    superphosphate of lime           |    46-1/4  |    40-3/4  |    43-1/2  |
275 lb. nitrate of soda              |    37      |    28-3/8  |    32-3/4  |
275 lb. nitrate of soda, 3-1/2 cwt.  |            |            |            |
    superphosphate                   |    49-1/4  |    42-1/4  |    45-3/4  |
275 lb. nitrate of soda,             |            |            |            |
    mixed mineral manures.           |    37-3/8  |    29-1/2  |    33-1/2  |
275 lb. nitrate of soda,             |            |            |            |
    mixed mineral manures,           |            |            |            |
  3-1/2 cwt. superphosphate          |    49-3/4  |    41-1/4  |    45-1/2  |
1000 lb. rape-cake                   |    45-1/4  |    37-1/8  |    41-1/4  |
1000 lb. rape-cake, 3-1/2 cwt.       |            |            |            |
    superphosphate                   |    46-3/4  |    40      |    43-3/8  |
1000 lb. rape-cake,                  |            |            |            |
     mixed mineral manures           |    43-5/8  |    35-5/8  |    39-1/2  |
1000 lb. rape-cake,                  |            |            |            |
    mixed mineral manures, and       |            |            |            |
    3-1/2 cwt. superphosphate        |    47-3/8  |    39      |    43-1/4  |
Farmyard manure, 14 tons every year  |    48-1/4  |    49      |    48-5/8  |
-------------------------------------+------------+------------+------------+

----------------------------------------------------------------------------+
                                     | PRODUCE PER ACRE--AVERAGE PER ANNUM. |
                                     +--------------------------------------+
                                     |            Dressed grain.            |
    MANURES PER ACRE PER ANNUM.      +--------------------------------------+
                                     |               Quantity.              |
                                     +------------+------------+------------+
                                     |  20 years, |  20 years, |  40 years  |
                                     |  1852-71.  |  1872-91.  |  1852-91.  |
-------------------------------------+------------+------------+------------+
                                     |     lb.    |     lb.    |     lb.    |
Unmanured continuously               |    52-3/8  |    51-3/4  |    52      |
3-1/2 cwt. superphosphate of lime    |    53-1/4  |    53      |    53-1/8  |
Mixed mineral manures                |    53      |    51-7/8  |    52-1/2  |
Mixed mineral manures, 3-1/2 cwt.    |            |            |            |
    superphosphate                   |    53-3/8  |    52-3/8  |    53      |
200 lb. ammonium salts               |    52-1/8  |    52      |    52      |
200 lb. ammonium salts, 3-1/2 cwt.   |            |            |            |
    superphosphate                   |    53-3/8  |    52-1/4  |    52-7/8  |
200 lb. ammonium salts,              |            |            |            |
    mixed mineral manures            |    52-3/4  |    52-1/2  |    52-5/8  |
Manures, 3-1/2 cwt.                  |            |            |            |
    superphosphate of lime           |    54      |    54-1/8  |    54      |
275 lb. nitrate of soda              |    52      |    52-1/8  |    52      |
275 lb. nitrate of soda, 3-1/2 cwt.  |            |            |            |
    superphosphate                   |    53-3/8  |    53-1/4  |    53-1/4  |
275 lb. nitrate of soda,             |            |            |            |
    mixed mineral manures.           |    52-1/4  |    52-3/4  |    52-1/2  |
275 lb. nitrate of soda,             |            |            |            |
    mixed mineral manures,           |            |            |            |
  3-1/2 cwt. superphosphate          |    53-3/8  |    54      |    53-5/8  |
1000 lb. rape-cake                   |    53-3/4  |    53-7/8  |    53-7/8  |
1000 lb. rape-cake, 3-1/2 cwt.       |            |            |            |
    superphosphate                   |    53-7/8  |    54-3/8  |    54-1/8  |
1000 lb. rape-cake,                  |            |            |            |
     mixed mineral manures           |    53-3/4  |    54-1/8  |    54      |
1000 lb. rape-cake,                  |            |            |            |
    mixed mineral manures, and       |            |            |            |
    3-1/2 cwt. superphosphate        |    53-5/8  |    54-1/4  |    53-7/8  |
Farmyard manure, 14 tons every year  |    54-3/8  |    54-1/4  |    54-1/4  |
-------------------------------------+------------+------------+------------+

-------------------------------------+--------------------------------------
                                     |  PRODUCE PER ACRE--AVERAGE PER ANNUM.
                                     +--------------------------------------
    MANURES PER ACRE PER ANNUM.      |              Total straw.
                                     +--------------------------------------
                                     |  20 years, |  20 years, |  40 years
                                     |  1852-71.  |  1872-91.  |  1852-91.
-------------------------------------+------------+------------+------------
                                     |     cwt.   |     cwt.   |     cwt.
Unmanured continuously               |    11-3/4  |     6-7/8  |     9-3/8
3-1/2 cwt. superphosphate of lime    |    13-3/8  |     8-1/4  |    10-3/4
Mixed mineral manures                |    12-1/4  |     7      |     9-5/8
Mixed mineral manures, 3-1/2 cwt.    |            |            |
    superphosphate                   |    14-3/8  |     8-3/8  |    11-3/8
200 lb. ammonium salts               |    18-1/2  |    13-1/2  |    16
200 lb. ammonium salts, 3-1/2 cwt.   |            |            |
    superphosphate                   |    27-5/8  |    20-1/8  |    23-7/8
200 lb. ammonium salts,              |            |            |
    mixed mineral manures            |    20-1/4  |    15-1/8  |    18
Manures, 3-1/2 cwt.                  |            |            |
    superphosphate of lime           |    28-1/2  |    23-3/8  |    25-7/8
275 lb. nitrate of soda              |    22-1/8  |    15-7/8  |    19
275 lb. nitrate of soda, 3-1/2 cwt.  |            |            |
    superphosphate                   |    30-1/2  |    23-3/8  |    27
275 lb. nitrate of soda,             |            |            |
    mixed mineral manures.           |    23-7/8  |    17-1/2  |    20-3/4
275 lb. nitrate of soda,             |            |            |
    mixed mineral manures,           |            |            |
  3-1/2 cwt. superphosphate          |    32-3/8  |    24-1/2  |    28-1/2
1000 lb. rape-cake                   |    26-7/8  |    20      |    23-3/8
1000 lb. rape-cake, 3-1/2 cwt.       |            |            |
    superphosphate                   |    28-3/8  |    21-1/2  |    24-7/8
1000 lb. rape-cake,                  |            |            |
     mixed mineral manures           |    27-1/8  |    19-7/8  |    23-1/2
1000 lb. rape-cake,                  |            |            |
    mixed mineral manures, and       |            |            |
    3-1/2 cwt. superphosphate        |    29-3/4  |    21-7/8  |    25-5/8
Farmyard manure, 14 tons every year  |    28-1/4  |    29-3/4  |    29
-------------------------------------+------------+------------+------------


TABLE VII.

EXPERIMENTS ON THE GROWTH OF OATS, 1869-78.

-------------------------------------------+----------------------------
                                           |     AVERAGE PER ANNUM.
                                           |      5 YEARS, 1869-73.
                                           +----------------------------
       MANURES PER ACRE PER ANNUM.         |   Dressed grain.  |
                                           +----------+--------+ Total
                                           | Quantity.| Weight | straw.
                                           |          |  per   |
                                           |          | bushel.|
-------------------------------------------+----------+--------+--------
                                           | Bushels. |   lb.  |  cwt.
Unmanured                                  |  19-7/8  | 33-3/4 | 10-3/8
200 lb. sulphate potash, 100 lb. sulphate  |          |        |
  soda, 100 lb. sulphate magnesia, and     |          |        |
  3-1/2 cwt. superphosphate of lime        |  24-1/2  | 35     | 13-3/8
400 lb. ammonium salts                     |  47      | 35-7/8 | 28-1/2
400 lb. ammonium salts, 200 lb. sulphate   |          |        |
  potash, 100 lb. sulphate soda, 100 lb.   |          |        |
  sulphate magnesia, and 3-1/2 cwt.        |          |        |
  superphosphate                           |  59      | 37     | 41-1/8
550 lb. nitrate of soda                    |  47-1/8  | 35-1/2 | 27-1/2
550 lb. nitrate of soda, 200 lb. sulphate  |          |        |
  potash, 100 lb. sulphate soda, 100 lb.   |          |        |
  sulphate magnesia, and 3-1/2 cwt.        |          |        |
  superphosphate                           |  57-1/2  | 35-3/4 | 35
-------------------------------------------+----------+--------+--------
                                           |     AVERAGE PER ANNUM.
                                           |     4 YEARS, 1874-78.
-------------------------------------------+----------------------------
                                           | Bushels. |   lb.  |  cwt.
Unmanured                                  |  13-3/4  | 31-1/4 |  6
200 lb. sulphate potash, 100 lb. sulphate  |          |        |
  soda, 100 lb. sulphate magnesia, and     |          |        |
  3-1/2 cwt. superphosphate of lime        |  13-1/8  | 31-5/8 |  6-1/8
200 lb. ammonium salts.                    |  28-7/8  | 33-1/4 | 14-1/8
200 lb. ammonium salts, 200 lb. sulphate   |          |        |
  potash, 100 lb. sulphate soda, 100 lb.   |          |        |
  sulphate magnesia, and 3-1/2 cwt.        |          |        |
  superphosphate                           |  38      | 35-1/2 | 20
275 lb. nitrate of soda                    |  26-3/8  | 31-5/8 | 11-1/8
275 lb. nitrate of soda, 200 lb. sulphate  |          |        |
  potash, 100 lb. sulphate soda, 100 lb.   |          |        |
  sulphate magnesia, and 3-1/2 cwt.        |          |        |
  superphosphate                           |  28-1/2  | 34-1/8 | 14
-------------------------------------------+----------+--------+--------

     TABLE VIII.--EXPERIMENTS ON ROOT CROPS: SWEDISH TURNIPS.
_Fifteen Seasons_, 1856-70.[1] Roots and Leaves carted off the Land.
-----+--------------------------------------------+---------------------+
     |                                            |      SERIES 1.      |
     |                                            |                     |
     |                                            |   Standard manures  |
     |                                            |        only.        |
     |                                            |                     |
     |                                            |                     |
     |                                            |                     |
     |                                            |                     |
     |                                            |                     |
     |                                            |                     |
PLOT.|              STANDARD MANURES.             |                     |
-----+--------------------------------------------+---------------------+
     |                                            |  Roots.  | Leaves.  |
     |                                            |----------+----------+
     |                                            |Tons. cwt.|Tons. cwt.|
  1  |Farmyard manure, 14 tons                    |  6     4 |  0    17 |
  2  |Farmyard manure, 14 tons, and superphosphate|  6     7 |  0    16 |
  3  |Without manure, 1846, and since             |  0    11 |  0     3 |
  4  |Superphosphate, each year; sulphate potash, |          |          |
     |  soda, and magnesia, 1856-60               |  2    16 |  0     8 |
  5  |Superphosphate, each year                   |  2    12 |  0     9 |
  6  |Superphosphate, each year; sulphate potash, |          |          |
     |  1856-60                                   |  2     7 |  0     7 |
  7  |Superphosphate, each year; sulphate, potash,|          |          |
     |  and 36-1/2 lb. ammonium salts, 1856-60    |  2    12 |  0     7 |
  8  |Unmanured 1853, and since; previously part  |          |          |
     |  unmanured; part superphosphate            |  1     3 |  0     4 |
-----+--------------------------------------------+----------+----------+

_Note._--Sulphate of ammonia is estimated to contain 23 per cent
ammonia, and muriate of ammonia 27 per cent. Ammonium salts, in each
case, equal parts sulphate and muriate of ammonia of commerce; and the
mixture is estimated to contain 25 per cent ammonia. The 328 lb. nitric
acid (sp. gr. 1.35) mixed with sawdust, and used as  a cross-dressing on
the plots of Series 2 from 1856-60, were estimated to contain nitrogen =
50 lb. ammonia.

1. The crops of 1859 and 1860 failed, and were ploughed in; but as the
manures were applied, and there would be accumulation with the soil for
the succeeding crops, the average produce is calculated as for fifteen
years--that is, the produce of the thirteen years is, in each case,
divided by 15.

-----+-----------------------+-----------------------
     |       SERIES 2.       |       SERIES 3.
     |                       |
     |   Standard manures,   |   Standard manures,
     |   and cross-dressed   |   and cross-dressed
     |   with--              |   with--
     |                       |
     |5 years, 1856-60,      |5 years, 1856-60,
     |3000 lb. sawdust, and  |200 lb. ammonium
     |328 lb. nitric acid.   |salts
     |                       |
     |10 years, 1861-70,     |10 years, 1861-70,
     |550 lb. nitrate soda.  |400 lb. ammonium salts.
PLOT.|                       |
-----+-----------+-----------+-----------+-----------
     |   Roots.  |  Leaves.  |   Roots.  |  Leaves.
     +-----------+-----------+-----------+-----------
     |Tons.  cwt.|Tons.  cwt.|Tons.  cwt.|Tons.  cwt.
  1  |  7      9 |  1      2 |8      8   |  1      4
  2  |  7     13 |  1      3 |8      5   |  1      5
  3  |  0     19 |  0      4 |0     13   |  0      3
  4  |  5      2 |  0     16 |4     12   |  0     14
     |           |           |           |
  5  |  4     13 |  0     18 |3     16   |  0     15
  6  |  4     11 |  0     14 |4      5   |  0     13
     |           |           |           |
  7  |  4     13 |  0     14 |4     12   |  0     14
     |           |           |           |
  8  |  1     13 |  0      5 |1      2   |  0      5
     |           |           |           |
-----+-----------+-----------+-----------+-----------

-----+-----------------------+-----------------------
     |       SERIES 4.       |       SERIES 5.
     |                       |
     |   Standard manures,   |   Standard manures,
     |   and cross-dressed   |   and cross-dressed
     |   with--              |   with--
     |                       |
     |5 years, 1856-60,      |5 years, 1856-60,
     |200 lb. ammonium salts,|3000 lb. sawdust.
     |and 3000 lb. sawdust.  |
     |                       |10 years, 1861-70,
     |10 years, 1861-70,     |2000 lb. rape-cake.
     |400 lb. ammonium salts,|
PLOT.|and 2000 lb. rape-cake.|
-----+-----------+-----------+-----------+-----------
     |   Roots.  |  Leaves.  |   Roots.  |  Leaves.
     +-----------+-----------+-----------+-----------
     |Tons.  cwt.|Tons.  cwt.|Tons.  cwt.|Tons.  cwt.
  1  |  8     16 |  1      9 |  8      0 |  1      4
  2  |  8     14 |  1      9 |  7     16 |  1      2
  3  |  3      6 |  0     14 |  3      8 |  0     13
  4  |  6     12 |  1      5 |  5      8 |  0     17
     |           |           |           |
  5  |  5     16 |  1      7 |  5      0 |  0     19
  6  |  6      6 |  1      2 |  5      3 |  0     16
     |           |           |           |
  7  |  6     15 |  1      4 |  5      9 |  0     17
     |           |           |           |
  8  |  3     19 |  0     18 |  3     14 |  0     19
     |           |           |           |
-----+-----------+-----------+-----------+-----------


TABLE IX.--EXPERIMENTS ON MANGEL-WURZEL.

_Average of Sixteen Seasons_, 1876-92. Manures per Acre per Annum.

-----+--------------------------------------------+---------------------
     |                                            |      SERIES 1.
     |                                            |
     |                                            |   Standard manures
     |                                            |        only.
     |                                            |
     |                                            |
PLOT.|            STANDARD MANURES.               |
-----+--------------------------------------------+----------+----------
     |                                            |  Roots.  | Leaves.
     |                                            |----------+----------
     |                                            |Tons. cwt.|Tons. cwt.
  1  |Farmyard manure, 14 tons                    | 16     6 |  2    17
  2  |Farmyard manure, 14 tons, and 3-1/2 cwt.    |          |
     |  superphosphate                            | 16    12 |  2    18
  3  |Without manure, 1846, and since             |  4     9 |  1     8
  4  |3-1/2 cwt. superphosphate, 500 lb. sulphate |          |
     |  of potash and 400 lb. mixed mineral manure|  5     8 |  1     1
  5  |3-1/2 cwt. superphosphate                   |  5     0 |  1     1
  6  |3-1/2 cwt. superphosphate, and 500 lb.      |          |
     |  sulphate of potash                        |  4     9 |  0    18
  7  |3-1/2 cwt. superphosphate, 500 lb. sulphate |          |
     |  of potash, and 36-1/2 lb. ammonium salts  |  5    17 |  1     8
-----+--------------------------------------------+----------+----------

-----+-----------------------+-----------------------+
     |       SERIES 2.       |       SERIES 3.       |
     |                       |                       |
     |Standard manures,      |Standard manures,      |
     |and cross-dressed      |and cross-dressed      |
     |with 550 lb.           |with 400 lb.           |
     |nitrate of soda.       |ammonium salts.        |
PLOT.|                       |                       |
-----+-----------+-----------+-----------+-----------+
     |   Roots.  |  Leaves.  |   Roots.  |  Leaves.  |
     +-----------+-----------+-----------+-----------+
     |Tons.  cwt.|Tons.  cwt.|Tons.  cwt.|Tons.  cwt.|
  1  | 22     11 |  4      2 | 22      3 |  5      7 |
  2  | 23     12 |  4     14 | 21      8 |  5      6 |
     |           |           |           |           |
  3  | 13      7 |  3      4 |  6     14 |  2     18 |
  4  | 12     17 |  3     15 | 16      2 |  3      0 |
     |           |           |           |           |
  5  | 15     13 |  3      5 |  8     10 |  3      1 |
  6  | 15     15 |  2     18 | 14      6 |  2     16 |
     |           |           |           |           |
  7  | 16      0 |  3      1 | 16      3 |  3      0 |
     |           |           |           |           |
-----+-----------+-----------+-----------+-----------+

-----+-----------------------+-----------------------
     |       SERIES 4.       |       SERIES 5.
     |                       |
     |Standard manures,      |Standard manures,
     |and cross-dressed      |and cross-dressed with
     |with 2000 lb. rape     |2000 lb. rape-cake.
     |cake, and 400 lb.      |
PLOT.|ammonium salts.        |
-----+-----------+-----------+-----------+-----------
     |   Roots.  |  Leaves.  |   Roots.  |  Leaves.
     +-----------+-----------+-----------+-----------
     |Tons.  cwt.|Tons.  cwt.|Tons.  cwt.|Tons.  cwt.
  1  | 24     11 |  6      1 | 23      7 |  4      6
  2  | 23     12 |  6      1 | 23      1 |  4      6
     |           |           |           |
  3  | 10     11 |  3     17 | 11      2 |  3      0
  4  | 24     18 |  5      7 | 20      4 |  3      9
     |           |           |           |
  5  | 11      7 |  4      2 | 12      3 |  3      2
  6  | 21      6 |  5      7 | 16     14 |  2     15
     |           |           |           |
  7  | 21      6 |  5      9 | 17     10 |  3      3
     |           |           |           |
-----+-----------+-----------+-----------+-----------

TABLE X.--Experiments with different Manures on Permanent Meadow-land.

_Thirty-six Years_, 1856-91.

-------------------------------+--------------------------+--------------------
                               |      Produce per Acre, weighed as Hay.
                               +--------------------------+--------------------
                               |   Average per Annum,     | Average per annum,
                               |   20 years, 1856-75      | 16 years, 1876-91
                               |   (1st crops only).      | (1st and 2d crops).
  Manures per Acre per Annum.  +--------+--------+--------+------+------+------
                               |10 years|10 years|20 years| 1st  |  2d  |
                               |1856-65.|1866-75.|1856-75.|crops.|crops.|Total.
-------------------------------+--------+--------+--------+------+------+------
                               |  cwt.  |  cwt.  |  cwt.  | cwt. | cwt. | cwt.
Unmanured continuously         | 22-1/2 | 20     | 21-1/4 |18    | 8-1/2|26-1/2
3-1/2 cwt. superphosphate of   |        |        |        |      |      |
  lime                         | 23-1/4 | 21-1/4 | 22-1/4 |18    | 9    |27-1/2
3-1/2 cwt. superphosphate of   |        |        |        |      |      |
  lime, and 400 lb. ammonium   |        |        |        |      |      |
  salts                        | 33-7/8 | 30-1/2 | 32-1/4 |30-3/4|10-1/2|41-1/4
400 lb. ammonium salts         | 30-1/2 | 22     | 26-1/4 |18-1/4|10-1/8|27-3/8
275 lb. nitrate of soda, 3-1/2 |        |        |        |      |      |
  cwt. superphosphate, and     |        |        |        |      |      |
  mixed mineral manure         | 45-1/4 | 47-5/8 | 46-1/2 |41-1/8|12-1/8|53-1/4
275 lb. nitrate of soda        | 34-1/4 | 33-1/2 | 33-7/8 |30-1/8|10    |40-1/8
-------------------------------+--------+--------+--------+------+------+------

TABLE XI.--EXPERIMENTS ON THE GROWTH OF POTATOES.

_Average of Five Seasons, 1876-80._[1]
----+-------------------------------+-------------------------------------------
    |                               |         PRODUCE PER ACRE--TUBERS.
Plot|  MANURE PER ACRE PER ANNUM.   |----------+----------+----------+----------
    |                               |   Good.  |   Small. | Diseased.| Total.
----+-------------------------------|----------+----------+----------+----------
Plot|                               |Tons cwt. |Tons cwt. |Tons cwt. |Tons cwt.
  1 |Unmanured                      | 1  18    | 0   6-1/4| 0   2-1/4| 2   6-1/2
  2 |Farmyard manure (14 tons)      | 3  19-3/8| 0   7-5/8| 0   6-5/8| 4  13-5/8
  3 |Farmyard manure (14 tons), and |          |          |          |
    |  3-1/2 cwt. superphosphate    | 4   9-1/2| 0   8    | 0   8-3/4| 5   6-1/4
  4 |Farmyard manure (14 tons),     |          |          |          |
    |  3-1/2 cwt. superphosphate,   |          |          |          |
    |  and 550 lb. nitrate of soda  |  5  8    | 0   7    | 0  19-1/2| 6  14-1/2
  5 |400 lb. ammonium salts         |  1 19-1/2| 0   7-1/8| 0   3-1/2| 2  10-1/8
  6 |550 lb. nitrate of soda        |  2 11-7/8| 0   6-7/8| 0   5-1/4| 3   4
  7 |400 lb. ammonium salts, 3-1/2  |          |          |          |
    |  cwt. superphosphate, 300 lb. |          |          |          |
    |  sulphate potash, 100 lb.     |          |          |          |
    |  sulphate soda, 100 lb.       |          |          |          |
    |  sulphate magnesia            | 5  14-1/4| 0   8-1/4| 0  14-3/4| 6  17-1/4
  8 |550 lb. nitrate of soda, 3-1/2 |          |          |          |
    |  cwt. superphosphate, 300 lb. |          |          |          |
    |  sulphate potash, 100 lb.     |          |          |          |
    |  sulphate soda, 100 lb.       |          |          |          |
    |  sulphate magnesia            | 5  19-7/8| 0   7-7/8| 0  19-1/8| 7   6-7/8
  9 |3-1/2 cwt. superphosphate      | 3  0-3/4 | 0   8    | 0   4-5/8| 3  13-3/8
 10 |3-1/2 cwt. superphosphate, 300 |          |          |          |
    |  lb. sulphate potash, 100 lb. |          |          |          |
    |  sulphate soda, and 100 lb.   |          |          |          |
    |  sulphate magnesia            | 3  4-1/2 | 0   6-1/2| 0   4-7/8| 3  15-7/8
----+-------------------------------+----------+----------+----------+----------
1. In each year the tops were spread on the respective plots.


TABLE XII.--EXPERIMENTS ON THE GROWTH OF POTATOES--_Continued_.

_Average of Twelve Seasons, 1881-92._

----+------------------------------+-------------------------------------------
    |                              |         PRODUCE PER ACRE--TUBERS.
Plot|  MANURE PER ACRE PER ANNUM.  |----------+----------+----------+----------
    |                              |   Good.  |   Small. | Diseased.| Total.
----+------------------------------|----------+----------+----------+----------
    |                              |Tons cwt. |Tons cwt. |Tons cwt. |Tons cwt.
  1 |Unmanured in 1876, and each   |          |          |          |
    |  year since                  |  1  3-3/4|  0  3-3/4|  0  0-1/4|  1  7-3/4
  2 |Unmanured in 1882, and since; |          |          |          |
    |  previously farmyard manure  |          |          |          |
    |  (14 tons)                   |  2 14-1/4|  0  4-3/4|  0  2    |  3  1
  3 |Farmyard manure (14 tons)     |          |          |          |
    |  alone, 1883, and since;     |          |          |          |
    |  previously 3-1/2 cwt.       |          |          |          |
    |  superphosphate also         |  4  3-1/4|  0  4-1/4|  0  4-1/2|  4 12
  4 |Farmyard manure (14 tons)     |          |          |          |
    |  alone, 1883, and since. In  |          |          |          |
    |  1882 and previously 3-1/2   |          |          |          |
    |  cwt. superphosphate, and in |          |          |          |
    |  1881 and previously 550 lb. |          |          |          |
    |  nitrate of soda also        |  4  6-1/4|  0  4-1/2|  0  4-3/4|  4 15-1/2
  5 |400 lb. ammonium salts        |  1  2-3/4|  0  4-3/4|  0  0-1/2|  1  8
  6 |550 lb. nitrate of soda       |  1 17-3/4|  0  3-3/4|  0  0-3/4|  2  2-1/4
  7 |400 lb. ammonium salts, 3-1/2 |          |          |          |
    |  cwt. superphosphate, 300 lb.|          |          |          |
    |  sulphate of potash, and 200 |          |          |          |
    |  lb. mixed mineral manure    |  5  6-3/4|  0  5    |  0  4-1/2|  5 16-1/4
  8 |550 lb. nitrate of soda, 3-1/2|          |          |          |
    |  cwt. superphosphate, 300 lb.|          |          |          |
    |  sulphate of potash, and 200 |          |          |          |
    |  lb. mixed mineral manure    |  5  7-1/2|  0  4-1/4|  0  3-3/4|  5 15-1/2
  9 |3-1/2 cwt. superphosphate     |  2 17-3/4|  0  3-1/4|  0  1    |  3  2
 10 |3-1/2 cwt. superphosphate, 300|          |          |          |
    |  lb. sulphate of potash, and |          |          |          |
    |  200 lb. mixed mineral manure|  3  2-1/4|  0  3-1/4|  0  1-1/4|  3  6-3/4
----+------------------------------+----------+----------+----------+----------


FOOTNOTES:

[256] See Sir John Bennet Lawes, Bart., and the Rothamsted Experiments.
By C. M. Aikman. ('Scottish Farmer' Office, Glasgow.)



INDEX.


 Abraum salts, 421.

 Absorptive power of soils for water, 67, 98;
   how to increase, 74.

 Acidity in soils neutralised by lime, 458.

 Acids fixed by soil, 58.

 Adametz on organisms in soil, 92.

 Adulteration of guano, 318-320.

 Africa, guano from, 298, 328.

 Agricultural chemistry, historical introduction to, 3-61;
   Liebig's researches on, 23-32;
   Liebig's services to, 31.

 Agronomy, 56.

 Air, ammonia in, 48, 118;
   nitrates in, 118; nitrites in, 118;
   nitrogen in, 116;
   organic nitrogen in, 118.

 Aitken, Dr, experiments with basic slag, 413
     --with beans, 526, 530;
   on germ-life in bones, 368;
   on manuring of turnips, 515.

 Albert, Heinrich, on solubility of basic slag, 409.

 Albite, composition of, 103.

 Albuminates, 460.

 Albuminoids, in plants, 491;
   of milk, nitrification in, 182;
   phosphorus in, 205.

 Algerian phosphate, 379.

 Algoa Bay, guano deposits at, 328.

 Alkalies, in cow-dung, 226, 227
     --cow-urine, 230
     --horse-dung, 226, 227
     --horse-urine, 230
     --pig-dung, 226, 227
     --pig-urine, 230
     --sheep-dung, 226, 227
     --sheep-urine, 230.

 Alkalinity necessary for nitrification, 172.

 Alumina, in ash of plants, 55;
   compounds, reversion caused by, 388, 400;
   salts, in _salinas_, 335.

 America, virgin soils of, 133.

 American farming, 86.

 Amides, 501.

 Ammonia, absorbed by soil, 81;
   amount dissolved in rain, 49;
   amount in air, 48;
   amount in soil, 127;
   amount supplied to soil by rain, 155;
   converted into nitrates in soil, 50;
   converted into nitrous acid, 167;
   fixed by soil, 58;
   from  decomposition of farmyard manure, 258;
   from gas-works, 353;
   lost in mixing manures, 533;
   relation of, to plants, 48-50;
   salts, most easily nitrifiable, 191;
   salts of, in farmyard manure, 257;
   sulphate of, 352-358;
   value of, as a manure, 352.

 Ammonium chloride in Chincha guano, 305.

 Ammonium-magnesium phosphate in Chincha guano, 305.

 Ammonium oxalate in concretionary nodules, 328.

 Ammonium phosphate, in Chincha guano, 305;
   in concretionary nodules, 328.

 Ammonium sulphate, 352-358;
   in Chincha guano, 305;
   in concretionary nodules, 328.

 Ammonium sulphocyanate, 355.

 Ammonium urate in concretionary nodules, 328.

 Amphibole, potash in, 220.

 Analysis, of manures, 539-554;
   of soils, value of, 90.

 Anderson, Dr, analyses of minerals by, 103, 105-107;
   on nitrogen in soil, 121, 124.

 Angamos, guano from, 301, 329.

 Animals, phosphoric acid in, 205;
   potash in, 205;
   solid excreta of, 224;
   urine of, 228.

 Apatite, Canadian, 201, 374;
   composition of, 210;
   most abundant form of phosphoric acid, 200;
   varieties of, 200.

 Application of manures, 474-492.

 Arabian coast, guano deposits on, 328.

 Arable soil, absorptive power of, 98.

 Arbrohlos Island guano, 309.

 Arendt, experiments on oats by, 503.

 Aristotelian doctrine of chemical elements, 4.

 Artificial soil, 54.

 Aruba phosphate, 308, 328, 379.

 Ash, constituents of plants, 53-55;
   of rye, phosphoric acid in, 204;
   of wheat, phosphoric acid in, 204;
   of farmyard manure, composition of, 287, 288.

 Ashes, an adulterant of guano, 319;
   mixed with manures, 532.

 Asia, guano from, 298.

 Asparagin, nitrification in, 182.

 Atacama, nitrate of soda deposits at, 342.

 Atmosphere, ammonia in, 48, 81;
   relation of, to plants, 39.

 Atwater on nitrogen in plants, 44.

 Augite, 105.

 Australia, guano from, 298;
   virgin soils of, 133.

 Avenine in oats, 503.

 Aves guano, 309, 328;
   phosphoric acid in, 330.


 Bacilli, 94.

 Bacon, Lord, on salt as a manure, 469.

 Bacteria, in soil, 92;
   different classes of, 93-96.

 Baker Island guano, 309, 328;
   phosphoric acid in, 330.

 Ballestas, guano from, 302, 327;
   nitrogen in, 329;
   phosphoric acid in, 329.

 Barilla, potash in, 420.

 Barley, farmyard manure not suited for, 497;
   fertilising ingredients removed from soil by, 485;
   manurial constituents in, 282;
   manuring of, 495, 498;
   nitrogen removed in crop of, 145;
   Norfolk experiments on, 497;
   period of growth of, 495;
   period of ripening of, 495;
   Rothamsted experiments on growth of, 566;
   soils suited for, 496;
   uniform manuring of, 497.

 Barley soils, amount of nitrates and nitrogen in, 158.

 Barley-straw, composition of, 238;
   manurial constituents in, 282.

 Basalt, phosphoric acid in, 202, 210.

 Bases fixed by soil, 58.

 Basic ammonium phosphate in concretionary nodules, 328.

 Basic process of steel-smelting, 400.

 Basic silicates, 103.

 Basic slag, 401-417;
   after-effects of, 412;
   application of, method of, 416
     --rate of, 414;
   compared with other manures, 410-414;
   composition of, 404, 417;
   Darmstadt experiments with, 410;
   discovery of value of, 403;
   manufacture of, 401;
   preparation of, processes for, 406;
   relative activity of, 411;
   soils best suited for, 414;
   solubility of, 408;
   Wagner's experiments with, 408-413.

 Bat guano, 320, 325;
   nitrogen in, 325;
   phosphoric acid in, 325.

 Beans, fertilising ingredients removed from soil by, 485;
   good effect of gypsum on, 526;
   manurial constituents in, 282;
   manuring of, 525-527;
   phosphorus in, 205;
   relative value of manurial ingredients to, 526;
   source of nitrogen, 153.

 Bean-straw, manurial constituents in, 282.

 Beatson, General, experiments of, with Peruvian guano, 301.

 Beddington meadows, irrigation at, 432.

 Bedfordshire, coprolites from, 374.

 Belgian phosphate, 377.

 Berthelot on sources of plant-nitrogen, 42.

 Biological properties of soil, 92-96.

 Blood corpuscles, potash in, 217.

 Blood, dried, 424;
   composition of, 424;
   manure for sugar-cane, 425;
   potash in, 217, 219;
   rate of nitrification in, 192;
   source of nitrogen, 152;
   suited for horticulture, 425.

 Bohemia, phosphoric acid removed from, 206.

 Bolivia, guano deposits at, 327.

 Bollaert on nitrate deposits, 333.

 Bone-ash, 369;
   composition of, 372.

 Bone-black, 369;
   composition of, 372.

 Bone-char, 369;
   composition of, 372.

 Bone-dust, 360.

 Bone-meal, 361, 364;
   composition of, 371;
   condition of nitrogen in, 540.

 Bone-phosphate, 385.

 Bones, 359-372;
   action of, 365;
   boiled, 361;
   bruised, 361;
   capable of nitrification, 182;
   collected in Britain, 353, 362;
   composition of, 362, 371;
   compound, 372;
   crops suited for, 368;
   dissolved, 368, 371;
   early use of, 359;
   fermentation of, 361;
   floated, 365;
   forms of, 360;
   grinding of, 365;
   imports of, 151;
   inorganic matter in, 363;
   nitrogen in, 151;
   organic matter in, 363;
   putrefaction of, 365, 366;
   raw, 361;
   source of nitrogen, 151;
   treatment of, 364.

 Bonnet, Charles, discovery of source of plant's carbon by, 11.

 Boracic acid in _salinas_, 335.

 Bordeaux phosphate, 379.

 Boussingault, on dry matter in horse-manure, 243;
   early researches of,
   in agricultural chemistry, 21;
   experiments by, on nitrification, 185, 198;
   on excrements of pig, 250;
   on nitrates in guano, 304;
   on nitrogen in plants, 41, 42;
   on nitrogen in soil, 124;
   on nitrogen in excreta, 234;
   on nitrogen lost during fermentation, 245.

 Bracken-fern, analyses of, 283;
   as litter, 241.

 Bran, manurial constituents in, 282.

 Bretschneider on sources of plant-nitrogen, 42.

 Brewers' grain, manurial constituents in, 282.

 Bromine in ash of plants, 55.

 Browse Island guano, 309, 328;
   phosphoric acid in, 330.

 Brüstlein and Peters on fixation of bases and acids by soil, 59.

 Buckland, Dr, discovery of coprolites by, 373.

 Buckwheat absorbs ammonia, 352.

 Bull River, phosphates from, 376.


 Cabbages, benefited by saline manures, 529;
   manuring of, 528-529;
   soils suited for, 529.

 Caird, Sir James, experiments by, with Peruvian guano, 301.

 Calcareous earth, absorptive power of, 98;
   stones, phosphoric acid in, 211.

 Calcium phosphate in Chincha guano, 305.

 Calcium sulphate in concretionary nodules, 328.

 _Caliche_, composition of, 342;
   occurrence of, 341.

 California, guano deposits at, 328.

 Cambridgeshire, coprolites from, 373.

 Cameron, Sir Charles, on assimilation of urea by plants, 46.

 Canadian apatite, 201, 374.

 Cape Vert guano, phosphoric acid in, 330.

 Carbolic acid, action of, on nitrifying organisms, 177.

 Carbon, fixation of, by plants, 37;
   in plants, discovery of source of, 11.

 Carbon bisulphide, effect of, on nitrification, 166, 176.

 Carbonate of ammonia formed in fermentation of dung, 247, 258.

 Carbonic acid, absorbed by plants, 12
     --by soil, 81;
   in ash of plants, 55;
   oxidation of, by bacteria, 95;
   produced in decomposition of farmyard manure, 258.

 Carbonising-works, ammonia from, 353, 358.

 Carburetted hydrogen, produced in decomposition of farmyard manure, 258.

 Caribbean phosphates, 379.

 Carnallite, 420.

 Carolina phosphate, 376.

 Carrots, manurial constituents in, 282.

 Catacombs, bones from, 360.

 Catch-cropping, 138, 489.

 Cattle foods, average composition of, 557.

 Caustic lime, 453.

 Cereals, manuring of, 493-504;
   nitrogenous manures benefit, 494;
   potash in, 217;
   silicates absorbed by, 494;
   value of nitrate of soda as manure for, 346.

 Chalk, an adulterant of guano, 319;
   lias, phosphoric acid in, 211.

 Charcoal, a filter for sewage, 437.

 Charleston phosphate, 376.

 Chemical analysis of manures, interpretation of, 539;
   value of, 539.

 Chemical composition of soil, 87-92.

 Cheshire, bones used in, 360.

 Chesterfield Island guano, 309.

 Chili and Peru, chief source of nitrate of soda, 162.

 Chincha Island guano, 302, 303, 327;
   composition of, 305;
   nitrogen in, 329;
   phosphoric acid in, 329.

 Chipana, guano deposits at, 327.

 Chlorapatite, composition of, 210.

 Chlorine in ash of plants, 55.

 Chloroform prevents nitrification, 166, 176.

 Chlorophyll, organisms destitute of, 169;
   produced by nitrate of soda, 347;
   relation of, to fixation of carbon by plants, 37.

 Chuca, 341.

 Citrate of ammonia, solubility of phosphates in, 408.

 Clay, absorptive power of, 68;
   analysis of, 107;
   grey, evaporation of water from, 99;
   loamy, evaporation of water from, 99;
   sandy, absorptive power of, 98
     --evaporation of water from, 99;
   soils, benefited by basic slag, 414
     --puddling in, 455;
   stiffish, evaporation of water from, 99;
   strong absorptive power of, 98.

 Clover-hay, fertilising ingredients removed from soil by, 486;
   manurial constituents in, 282;
   manuring of, 522.

 Clover-sickness, 522.

 Coal, nitrogen in, 353.

 Coke-works, ammonia from, 353, 358.

 Colloids, 491.

 Colour of soil, 80;
   difference in temperature due to, 80.

 Columbia, guano deposits at, 327.

 Composts, 113, 445-448;
   farmyard manure a typical, 446;
   manufacture of, 445;
   object of, 445;
   purposes served by, 445;
   substances used for, 447.

 Compound bones, 372.

 Concretionary nodules, composition of, 328.

 Conglomerate, 341.

 Connecticut, experimental station at, 33.

 Cooke, F. J., on, farmyard manure, 272, 277;
   field experiments. 547;
   manuring of barley, 497
     --of mangels, 514
     --of meadow-land, 509
     --of swedes, 514
     --of wheat, 501.

 Copper, oxide of, in plants, 55.

 Copperas, as a fixer, 246, 247.

 Coprolites, 373;
   percentage of phosphates in, 201:
   occurrence of, 201, 373.

 Corcovado guano, nitrogen in, 329;
   phosphoric acid in, 329.

 Cordilleras, 340, 341.

 Costra, 341.

 Cotton-cake, decorticated, manurial constituents in, 282.

 Cotton-cake, undecorticated, manurial constituents in, 282.

 Cotton-seeds, imports of, 153.

 Cova, 341.

 Covered manure, potatoes grown with, 289;
   wheat grown with, 289.

 Cow-dung, alkalies in, 226;
   composition of, in dry state, 227;
   cool, 225;
   nitrogen in, 226;
   phosphoric acid in, 226;
   water in, 226.

 Cow-manure, 247;
   amount voided per day, 248;
   amount voided per year, 248;
   analysis of, 286;
   dry matter in, 248;
   fermentation in, slow, 248;
   mineral matter in, 248;
   mucilaginous matter in, 248;
   nitrogen in, 248;
   resinous matter in, 248.

 Cow-urine, alkalies in, 230;
   composition of, in dry state, 231;
   fertilising ingredients in, for food consumed, 232;
   nitrogen in, 230;
   phosphoric acid in, 230;
   water in, 230.

 Cows, percentage of food voided in excrements of, 281;
   solid excrements of, 280;
   urine voided by, 280.

 Cress, experiments with, 41.

 Crimea, bones from, 360.

 Cropped soils, nitrates in, 157
     --lost by drainage in, 141.

 Crops, capacity of, for assimilating manures, 486;
   difference in root-systems of, 488;
   manuring of common farm, 493-530;
   period of growth of, 489;
   potash removed in, 218;
   suited for sewage, 434;
   variation in composition of, 490.

 Crusius on phosphoric acid removed from the farm, 207.

 Crust guanos, 308, 379.

 Crystalloids, 491.

 Curaçao phosphates, 308, 330, 379.



 Darmstadt experiments with basic slag, 410-413.

 Darwin on origin of nitrate-fields, 335.

 Daubeny on mineral sources of phosphoric acid, 200.

 Davy, Sir Humphry, lectures of, on agricultural chemistry, 17-19;
   on heat and water absorbing and retaining properties of soils, 57;
   on hygroscopic power of soils, 99.

 Dehérain, on nitrification, 52;
   on nitrification in sulphate of ammonia, 191;
   on rate of nitrification, 186.

 Denitrification, 177;
   conditions favourable for, 178;
   effected by bacteria, 178.

 Derby, Lord, introduction of Peruvian guano by, 301.

 Detmer on humus in soil, 47.

 Dew, action of, on guano, 300;
   explanation of, 77;
   most abundant in summer, 78.

 Dicalcic phosphate, 387;
   formula of, 398;
   molecular composition of, 398;
   percentage composition of, 398.

 Digby, Sir Kenelm, on value of nitrates to plants, 45;
   theory of, on plant-food, 6-8.

 Diorite, phosphoric acid in, 202, 211.

 Direct manures, 113.

 Dissolved-bone compound, 372.

 Dissolved bones, 368;  composition of, 371.

 Dissolved guano, 310.

 Dolerite, phosphoric acid in, 202, 211.

 Dolomite, phosphoric acid in, 202, 211.

 Downton experiments on sewage-sludge, 439.

 Drainage, average of thirteen years, 160;
   nitrates in, 160;
   nitrates lost by, 140;
   phosphoric acid lost by, 206;
   potash lost by, 217.

 Drainings of manure-heaps, analysis of, 290.

 Dried blood, 424;
   composition of, 424;
   manure for sugar-cane, 425;
   potash in, 219;
   rate of nitrification in, 192;
   source of nitrogen, 152;
   suited for horticulture, 425.

 Dried flesh, 425;
   nitrogen in, 425.

 Dried leaves, as litter, 242;
   composition of, 242;
   nitrogen in, 242;
   phosphoric acid in, 242;
   potash in, 242.

 Ducks' dung, analysis of, 331.

 Duhamel and Hales, theory of, on plant-growth, 8.

 Dundonald, Earl, treatise by, on agricultural chemistry, 13.

 Dung and urine, composition of, 234.

 Dutrochet on absorption of plant-food, 55.

 Dyer, Dr Bernard, analyses of stable manure by, 283;
   experiments on peat as litter, 240;
   on nitrate of soda as manure for mangolds, 349.


 Earth, an adulterant of guano, 319;
   composition of solid crust of, 102.

 Ecuador, guano deposits at, 327.

 Egyptian guano, nitrogen in, 329;
   phosphoric acid in, 329.

 Elbe, waters of, phosphoric acid in, 206;
   potash in, 217.

 Elm-tree, water transpired by, 71.

 Enderbury Island guano, 309, 328;
   phosphoric acid in, 328.

 Endosmosis, 55.

 English farming, 86.

 Equalised guano, 311.

 Essex, coprolites from, 374.

 Estremadura phosphate, 375.

 Ethylamine, nitrification in, 182.

 Evaporation from soil, 71, 72, 98.

 Excreta, amount of nitrogen in, 149, 292;
   composition of, 226, 292;
   difference in amount of, for food consumed, 279;
 liquid, in farmyard manure, 224;
   solid, in farmyard manure, 224;
   solid, undigested food in, 224;
   solid, voided by cows, 280, 292;
   solid, voided by horse, 292;
   solid, voided by oxen, 280;
   solid, voided by sheep, 280, 292.


 Factors for calculating manurial ingredients
   into their different compounds, 553.

 Falkland guano, 308;
   nitrogen in, 330;
   phosphoric acid in, 330.

 Fallow-fields, nitrates formed in, 188.

 Fanning Island guano, 328;
   phosphoric acid in, 330.

 Farmyard manure, 223-292;
   action of, on soils, 273;
   ammonia in, 258;
   amount produced on farm per year, 252;
   analyses of, 259, 286;
   application of, 264;
   ash of, 287, 288;
   carbonic acid gas in, 258;
   classes of constituents of, 224;
   compared with artificials, 476;
   composition of, 259;
   denitrification in, 179;
   depth to plough to, 267;
   effect of, on potatoes, 520;
   fertilising matter in, 270;
   fire-fang in, 264;
   fresh, composition of, 286, 288;
   functions of, 268;
   heat in fermentation of, 78, 253;
   humates in, 259;
   humic acid in, 258;
   inadequate source of nitrogen to soil, 271;
   indirect influence of, 273;
   influence of, on soil, 475;
   Lawes, Sir John, on composition of, 291;
   Lord Kinnaird's experiments with, 289;
   marsh-gas in, 258;
   mineral matter in, 260;
   moisture in, 260;
   nitric acid in, 259;
   nitrogen in, 260;
   ratio of, to ash ingredients, 271;
   organic matter in, 260;
   phosphoretted hydrogen in, 258;
   phosphoric acid in, 260;
   potash in, 260;
   products of decomposition of, 257;
   rate of application of, 275;
   retrogression of nitrogen in, 142;
   rotten, composition of, 287, 288
     --value of, 261;
   rotting, effects of, on, 262;
   solid excreta in, 224;
   sulphuretted hydrogen in, 258;
   supplemented with nitrogen, 271;
   supplemented with phosphoric acid, 272;
   temperature, effect of, on soil, 79, 274;
   typical compost, 446;
   ulmates in, 259;
   ulmic acid in, 258;
   unfavourable to certain crops, 477;
   urine in, 228;
   value of, 268;
   variation in composition of, 223;
   water in, 258.

 Fatty acids in guano, 305.

 Felspars, 103;
   albite, 103;
   composition of, 103;
   labradorite, 220;
   oligoclase, 103, 214, 220;
   orthoclase, 103, 214, 220;
   phosphoric acid in, 211;
   potash manures, 213;
   potash in, percentage of, 213, 220.

 Ferment, aerobic, 173, 255;
   anaerobic, 255.

 Fermentation, ammonium carbonate formed during, 245;
   in bones, 365;
   heat of, 79;
   of farmyard manure, 253;
   of guano, 299;
   temperature of, 256.

 Fern, bracken, as litter, 241.

 Ferric chloride, test for sulphocyanates, 355.

 Fertilisers and Feeding Stuffs Act, 543.

 Fertilising ingredients, amount of soluble, in soil, 90;
   amounts removed by different crops, 484, 485;
   chemical condition of, in soil, 89;
   lodge in seed, 491;
   in soil, 87.

 Fertility, of the soil, 65-97;
   potential, of soil, 214, 549;
   properties necessary for, 66;
   supply of oxygen necessary for, 81.

 Field experiments, 545, 548;
   educational value of, 547;
   on rate of nitrification, 187.

 Finger-and-toe prevented by lime, 461.

 Fire-fang in farmyard manure, 264.

 Fischer on absorption of plant-food, 55.

 Fish-guano, 320-323;
   application of, 323;
   consumption of, 152;
   manufacture of, 321;
   nitrogen in, 321;
   phosphoric acid in, 321;
   production of, 322;
   source of nitrogen, 152;
   value of, 322.

 Fixers, 246;
   chemical reactions with, 284.

 Fleece, potash in, 217.

 Fleischer, Professor, on solubility of phosphates, 408.

 Flesh-guano, 320.

 Flint Island guano, 309.

 Flitcham experiments on growth of wheat, 500.

 Floated bones, 362, 365.

 Florida phosphate, 378.

 Fluorapatite, composition of, 210.

 Food, consumed by pigs, 281;
   dry matter of, voided in dung, 228;
   percentage of, in excrements, 281.

 Food-constituents, plant, necessary for nitrification, 170.

 Forbes, David, on nitrate-fields of Chili, 334.

 Forest-soils, absence of nitrification in, 193.

 Fowl-dung, 320, 326;
   analysis of, 331.

 Fownes on phosphoric acid in rocks, 202.

 Frankland, P. F., experiments on nitrification, 52, 167, 198.

 Franklin, Benjamin, experiment of, with gypsum, 462.

 Frey Bentos, meat-meal guano from, 324.


 Galapagos Islands, guano deposits at, 327.

 Garden earth, absorptive power of, 98;
   ammonia in, 128.

 Gas-liquor, ammonia in, 353.

 Gas-works, ammonia from, 353, 358.

 Gases, absorbed by soils, 81;
   present in soil, 100.

 Gazzeri on retention by soil of plant-food, 57.

 Geese-dung, analysis of, 331.

 Geic acid in humus, 47.

 Gelatin, nitrification in, 182;
   from bones, 364.

 Germany, agricultural research in, 32;
   bones imported from, 360;
   manufacture of meat-meal guano in, 324.

 Germination, influence of temperature on, 76;
   oxygen necessary for, 81.

 Gilbert, Sir J. Henry, on barley-manuring, 496;
   on Liebig's mineral theory, 28;
   on manuring of potatoes, 520;
   Presidential address of, 61;
   and see Lawes and Gilbert.

 Glauber on artificial production of nitre, 164.

 Glue, 364.

 Glycin, assimilated by plants, 47.

 Glycocoll, experiments with, 46.

 Gneiss, 106;
   phosphoric acid in, 207.

 Grandeau, Professor, on forms of plant-food in soil, 107;
   on loss of phosphoric acid, 207.

 Granite, 105;
   in guano, 303;
   phosphoric acid in, 202, 211;
   potash in, 214.

 Grass, Bangor experiments on, 508;
   effect of manure on, 505;
   influence of farmyard manure on, 506;
   manuring of, 504-510.

 Gray, Asa, on transpiration by plants, 71.

 Great Cayman guano, 379.

 Green manures, 113.

 Grouven on guano, 313.

 Guanape Island guano, 302, 327;
   nitrogen in, 329;
   phosphoric acid in, 329.

 Guanine, 304;
   experiments with, 46.

 Guano, 293-331;
   action of, as a manure, 312;
   adulteration of, 318;
   application of, 315;
   bat, 325;
   composition of, 305, 329;
   crust, 308;
   deposits of the world, 327;
   dissolved, 310;
   equalised, 309;
   fermentation of, 299;
   fertilising constituents in, 314;
   fish, 320-323;
   importance of, in agriculture, 293;
   inequality in composition of, 309;
   influence of, on farming, 294;
   meat-meal, 324;
   mode of application of, 315;
   nitrification in, rate of, 192;
   nitrogenous, 300-308;
   origin of, 297;
   Peruvian, 300-306;
   phosphatic, 308;
   quantity to apply, 317;
   rectified, 311;
   so-called, 320;
   source of phosphoric acid, 202;
   source of potash, 219;
   value of, as a manure, 296;
   variation in composition of, 299.

 Gulf of Mexico, guano deposits at, 328.

 Gulls, guano from, 297.

 Gunning on sources of plant-nitrogen, 42.

 Gunpowder, exports of, 149;
   nitrogen lost in, 149;
   production, annual, of, 149;
   saltpetre in, 149, 333.

 Gypsum, 462-464;
   absorptive power of, 98;
   action of, mode of, 462--on nitrification, 173;
   an adulterant of guano, 319;
   as a fixer, 246, 247, 285;
 decomposes double silicates, 463;
   favourable to clover, 464;
   as an oxidising agent, 464.


 Hales, Stephen, theory of, on plant-growth, 8.

 Hampe, Dr, on nitrogen in plants, 46.

 Harting on sources of plant-nitrogen, 42.

 Heat, of soils, 76-78;
   of fermentation, 78.

 Heiden, Dr, on application of farmyard manure, 265;
   on fixation of bases and acids by soil, 59;
   on loss of ammonia from dung, 249;
   on percentage of food voided by animals, 253;
   on straw as litter, 244, 249.

 Hellriegel, on amount of water in soils, 75;
   on barley, 498;
   on nitrogen in plants, 44.

 Helmont, Van, theory of, on source of plant-food, 4.

 Henslow, Professor, on coprolites, 374.

 Heraüs on organisms in soil, 95.

 Herbage, effect of manure on, 505.

 Herrings as manure, 321.

 Hervé-Mangon, experiments on action of light on plants by, 38.

 Hilgenstock on tetracalcic phosphate, 405.

 Hippuric acid, experiments with, 46;
   in farmyard manure, 257.

 Hire, De la, on evolution of gases by plants, 11.

 Hofmeister on horse excrements, 243.

 Hoof-guano, source of nitrogen, 152.

 Hoofs and horns, manure from, 425.

 Hops, manuring of, 528;
   potash removed by, 217;
   slow-acting manures benefit, 528.

 Horn, capable of nitrification, 182;
   as manure, 425;
   nitrogen in, 426;
   phosphoric acid in, 426.

 Hornblende, 105.

 Horse-dung, alkalies in, 226;
   composition of, in dry state, 227;
   hot, 225;
   nitrogen in, 225, 226;
   phosphoric acid in, 226;
   water in, 225, 226.

 Horse-manure, 242;
   amount produced per day, 243;
   amount produced per year, 243;
   analyses of, 283;
   dry matter in, 243;
   dry nature of, 245;
   fermentation rapid in, 245;
   mineral matter in, 243;
   nitrogen in, 243, 244.

 Horse-urine, alkalies in, 230;
   composition of, in dry state, 231;
   fertilising ingredients in, 232;
   nitrogen in, 230;
   phosphoric acid in, 230;
   water in, 230.

 Hosäus on assimilation of ammonia, 50.

 Howland Island guano. 309, 328;
   phosphoric acid in, 330.

 Huanillos, guano from, 302, 327;
   nitrogen in, 330;
   phosphoric acid in, 330.

 Huano, 297.

 Hueppe on organisms in soil, 95.

 Hughes, John, on bracken-fern as litter, 241;
   on composition of bracken, 283.

 Humates in farmyard manure, 259.

 Humboldt, A., discovery of Peruvian guano by, 300.

 Humic acid in farmyard manure, 258;
   in humus, 47.

 Humin in humus, 47.

 Humus, absorptive power of, 68, 98;
   evaporation from, 99;
   nature of, in soil, 47;
   soils improved by addition of, 273.

 Huon Island guano, 309, 328;
   phosphoric acid in, 330.

 Huxtable and Thompson on retention of plant-food by soil, 57.

 Hydrated silicates, 107, 459.

 Hydrochloric acid as a fixer, 245.

 Hydrogen, amount of, in plants, 40;
   source of, in plants, 40.

 Hygroscopic power of soils, 75.


 Ichaboe guano, 307;
   nitrogen in, 329;
   phosphoric acid in, 329.

 Independence Bay guano, 302, 327;
   nitrogen in, 329;
   phosphoric acid in, 329.

 India, nitre soils of, 162.

 Indirect manures, 113, 114, 449-473.

 Ingenhousz, John, experiments by, on nitrogen in plants, 41;
   on oxygen evolved by plants, 12.

 Insoluble phosphate, 386;
   value of, 396.

 Iodine, in ash of plants, 55;
   in nitrate of soda, 340, 342.

 Iquique, nitrate of soda from, 333.

 Iron in ash of plants, 54;
   necessary for plant-growth, 55;
   reversion in superphosphates caused by, 390, 399.

 Iron-works, ammonia from, 353, 355, 358.

 Irrigation, 431-433;
   intermittent, 434;
   subsoil, 432.


 Jamieson, Professor, experiments with coprolites, 380.

 Jarvis Island guano, 309, 328;
   phosphoric acid in, 330.

 Jersey, manuring of potatoes in, 521.

 Johnson, Professor S. W., on application of superphosphate, 395;
   on Earl Dundonald, 13;
   on nitrogen in buffalo-horn shavings, 426;
   on nitrogen in soils, 123;
   on solubility of basic slag, 408;
   value of organic nitrogen to plant, 46.

 Jürgensen on nitrogen in excreta, 234.


 Kainit, as a fixer, 247;
   potash in, percentage of, 214, 220, 421;
   rate of application of, 423.

 Kaolin clay, analysis of, 104.

 Karmrodt, analysis of Chincha Island guano, 305;
   of concretionary nodules, 328.

 Karnallite, potash in, 220.

 Kellner, experiments on nitrification by, 52.

 Kelp, potash in, 420.

 Kieserite, 420.

 Kinnaird, Lord, experiments by, with farmyard manure, 289.

 Kitchen-garden soil, nitrogenous matter in, 122.

 Knop on condition of nitrates in soil, 138.

 Koosaw River, phosphates from, 376.

 Kreatin assimilated by plants, 47.

 Kuria Muria guano, 309, 328.


 Labrador, guano deposits at, 328.

 Labradorite, 214;
   potash in, 220.

 Lacepede Island guano, 309, 328;
   phosphoric acid in, 330.

 Lahn phosphate, 379.

 Lava, phosphoric acid in, 202, 211.

 Lawes, Sir J. B., and Gilbert, early researches of, at Rothamsted, 34;
   experiments with farmyard manure, 271;
   experiments with Peruvian guano, 301;
   inauguration of Rothamsted experiments by, 33;
   on composition of farmyard manure, 291;
   on manuring of wheat, 483;
   on motion of plant's sap, 56;
   on percentage of food in excreta, 233;
   on rate of nitrification, 186;
   on sources of plant-nitrogen, 43;
   on sulphate of ammonia, 356;
   on unexhausted manures, 550, 557-559.

 Lawes, Sir J. B., experiments with guano by, 301;
   manufacture of superphosphate by, 382;
   on application of superphosphate, 395;
   on bones, 359;
   on composition of farmyard manure, 291;
   on farmyard manure, 477;
   on loss of nitrates, 142;
   on sources of nitrogen, 154.

 Leather, as manure, 428;
   nitrogen in, 428.

 Leaves, dried, as litter, 242;
   nitrogen in, 242;
   phosphoric acid in, 242;
   potash in, 242.

 Legrange, Charles, on extent of nitrate-fields, 343.

 Leguminous plants, benefited by basic slag, 414
     --by potash, 523;
   fixation of free nitrogen by, 42;
   gain of nitrogen with, 135;
   manuring of, 522-527, 530;
   nitrogenous manures hurtful to, 523.

 Lehmann on ammonia as plant-food, 50, 352.

 Leipzig, bones from, 361.

 Leones, guano deposits at, 327.

 Leucite, potash in, 220.

 Lias chalk, phosphoric acid in, 211.

 Liebig, criticism of humus theory by, 25;
   dissolved bones discovered by, 361;
   first report to British Association, 24;
   manufacture of superphosphate from bones by, 359;
   mineral theory of, 26-29;
   on ammonia as a manure, 352;
   on importation of bones by Britain, 360;
   researches of, in agricultural chemistry, 23-32;
   services of, to agricultural chemistry, 31;
   theory of manures by, 29;
   theory of, on rotation of crops, 29.

 Light, action of, on plant-growth, 38.

 Lime, 449-461;
   abundant occurrence of, 452;
   action of, 461
     --contradictory, 450
     --not thoroughly understood, 449
     --on nitrogenous organic matter, 460
     --on soil's texture, 455;
   antiquity of, as a manure, 449;
   binding effect of, 457;
   biological action of, 459;
   caustic, 453;
   chemical action of, 457;
   decomposes minerals, 458;
   different forms of, 453;
   effect of, on soils, 112;
   fixed by soils, 58;
   in ash of plants, 54;
   mechanical functions of, 455;
   mild, 453;
   necessary for nitrification, 171, 459
     --for plant-growth, 55, 450;
   neutralises acidity in soils, 458;
   phosphates of, 385-388;
   pig excrements contain, 281;
   prevents clay puddling, 456;
   returned to soil, 452;
   soils contain, 450-452.

 Limestone, analyses of, 106;
   evaporation of water from, 99;
   occurrence of, 452.

 Linseed, imports of, 153;
   manurial constituents of, 282.

 Linseed-cake, manurial constituents of, 282.

 Liquid manure, 442-444.

 Lithia in ash of plants, 55.

 Litter, loam as, 239;
   peat as, 240;
   straw as, 236;
   uses of, 236.

 Lloyd on fattening animals, 253.

 Loam, as litter, 239;
   evaporation of water from, 99;
   poor in fertilising matter, 239.

 Lobos, guano deposits at, 327.

 Lobos de Afuera guano, 302, 327.


 Macabi Island guano, 302, 327;
   nitrogen in, 329;
   phosphoric acid in, 329.

 Maercker, Professor, on destruction of nitrifying organisms, 177.

 Magnesia, fixed by soils, 58;
   in ash of plants, 54;
   in pig excrements, 281;
   necessary for nitrification, 171;
   necessary for plant-growth, 55;
   sulphate of, as a fixer, 246, 285.

 Maize, absorbs ammonia, 352;
   fertilising ingredients removed from soil by, 485;
   manurial constituents in, 282;
   source of nitrogen, 153.

 Malden Island guano, 309, 328;
   phosphoric acid in, 330.

 Malpighi on importance of atmospheric air for germination, 39.

 Malt-dust, manurial constituents in, 282.

 Manganese, oxide of, in ash of plants, 54.

 Mangels, fertilising ingredients removed from soil by, 485;
   guano a manure for, 318;
   manurial constituents in, 282;
   manuring of, 346, 511, 513, 514;
   Rothamsted experiments on growth of, 568.

 Manitoba soils, nitrogen in, at various depths, 156;
   rate of nitrification in, 186.

 Manure, cow, 247;
   farmyard, 223-292;
   horse, 243;
   liquid, 442-444;
   meaning of word, 109;
   pig, 250;
   sewage, 430-441;
   sheep, 251;
   stable, from peat-moss, 283
     --wheat-straw, 283.

 Manures, action of, 61;
   analysis of, interpretation of, 539-544;
   application of, 474-492;
   method of, 531-538; cash prices of, 555;
   equal distribution of, 531;
   functions of, 109, increase soil-fertility, 474;
   intrinsic value of, 545;
   lasting effects of, 483;
   methods of valuing, 544;
   minor artificial, 424-429;
   mixing of, 531-538;
   nitrogenous, 293-359;
   phosphatic, 359-417;
   potassic, 418-423;
   quantities of, applied to oats, 504;
   unexhausted, 549-552, 558;
   units for determining commercial value of, 554;
   valuation of, 539-559;
   value of, deduced from experiments, 548;
   various classes of, 111-114.

 Manurial constituents of various foods, 282.

 Manurial ingredients, unit value of, 544.

 Manuring of, barley, 495-498;
   beans, 525-527, 530;
   cabbages, 528;
   cereals, 493-504;
   clover, 524;
   common farm crops, 493-530;
   grass, 504-510;
   hops, 528;
   leguminous crops, 522-528;
   mangels, 511, 513, 514;
   oats, 493-504;
   peas, 527;
   potatoes, 517-522;
   roots, 510-517;
   turnips, 510, 511, 513-517;
   wheat, 499-501.

 Maracaïbo guano, nitrogen in, 330;
   phosphoric acid in, 330.

 Marl, phosphoric acid in, 211.

 Marsh-gas from farmyard manure, 258.

 Meadow-hay, fertilising ingredients removed from soil by, 485;
   manurial constituents in, 282;
   Rothamsted experiments on manuring of, 570.

 Meadow-land, benefited by basic slag, 414, 508;
   manuring of, 508;
   Norfolk experiments on, 509.

 Meat-meal guano, 320, 324;
   composition of, 152;
   imports of, 324;
   manufacture of, 324;
   nitrogen in, 324;
   phosphoric acid in, 324;
   rate of nitrification in, 192;
   source of nitrogen, 152;
   value of, 324.

 Mechi on liquid manure, 442.

 Mejillones guano, 309, 327;
   phosphoric acid in, 330.

 Mène, on sources of plant-nitrogen, 42.

 Menhaddo, guano manufactured from, 322.

 Mexico phosphate, 308, 328.

 Mica, analysis of, 105;
   potash in, 214, 220.

 Micro-organisms, convert ammonia into nitrous acid, 167;
   convert nitrous acid into nitric acid, 168;
   effect fermentation, 80;
   effect fixation of free nitrogen, 44;
   effect nitrification, 161;
   oxidising power of, 197.

 Mild lime, 453.

 Milk, nitrification in albuminoids of, 182;
   nitrogen removed in, 147;
   phosphoric acid removed in, 207;
   potash removed in, 218.

 Mineral phosphates, 373-381;
   value of, as a manure, 380.

 Mineral salts necessary for nitrification, 52.

 Minor artificial manures, 424-429.

 Mixing manures, 532-538;
   ammonia lost in, 533;
   nitric acid lost in, 536;
   phosphates reverted in, 536.

 Moisture, atmospheric, action on guano, 300;
   in farmyard manure, 260;
   in manures, 543;
   necessary for nitrification, 52, 176.

 Molds, 94.

 Mona guano, 309.

 Mond, Ludwig, on nitrogen in coal, 354.

 Monks guano, 327;
   phosphoric acid in, 330.

 Monocalcic phosphate, 386;
   formula of, 398;
   molecular composition of, 398;
   percentage composition of, 398;
   reversion of, with iron and alumina compounds, 399
     --with tricalcic phosphate, 399.

 Mulder on humus in soil, 47, 126.

 Müller, A., on nitrogen in soil, 121, 124.

 Munro, Dr J. M. H., on nitrification, 52;
   on sewage-sludge as manure, 439;
   on urine voided, 292.

 Müntz, on ammonia in air, 118;
   on nitrifying organisms in soil, 180;
   on oxidising power of micro-organisms, 197.

 Muriate of potash, application of, 423;
   forms calcium chloride, 422;
   harmful effects of, 421;
   more concentrated than sulphate, 422.

 Mustard, 139.


 Navassa phosphate, 308, 328, 379.

 Nesbit on composition of guano, 301.

 New Granada, guano deposits at, 327.

 New Zealand, meat-meal guano from, 324.

 Nile, nitrates in waters of, 159.

 "Nitraries," 163.

 Nitrate-fields, appearance of, 340;
   origin of, 334.

 Nitrate of soda, 332-351;
   amount exported from Chili, 151, 332, 351;
   amount imported into Britain, 151, 351;
   appearance of fields of, 340;
   application of, 347;
   Chili and Peru chief source of, 161;
   composition of, 343;
   crops suited by, 346;
   discovery of deposits of, 333;
   extent of deposits of, 342;
   encourages deep roots, 344;
   formation of fields of, 334-340;
   method of applying, 347;
   method of mining, 341;
   nitric acid in, source of, 337;
   nitrogen in, percentage of, 343;
   not an exhausting manure, 345;
   origin of fields of, 334;
   properties of, 343;
   quantity to apply, 348;
   shipments of, 351;
   soils benefited by, 348;
   source of
 nitrogen, 150;
   top-dressing with, 344.

 Nitrates, amount lost by drainage, 140;
   amount produced at different times, 189;
   amount in soil, 129;
   conditions diminishing loss of, 139;
   constantly formed in soil, 138;
   in barley-soils, 158;
   in cropped soils, 130, 157;
   in drainage-waters, 160, 188;
   in fallow-soils, 129;
   in manured wheat-soils, 131, 157;
   in soil, 129, 162;
   lost by drainage, 137;
   most formed in summer, 139;
   nitrogen as, in Rothamsted soils, 198;
   position of, in soil, 188;
   quantity formed in fallow-fields, 188.

 Nitre, beds, 163;
   occurrence of, 162;
   soils of India, 162.

 Nitric acid, amount of, supplied to soil by rain, 155;
   derived from sea weed, 337;
   formed from ammonia, 118;
   formed from nitrous acid, 168;
   in farmyard manure, 259;
   in soil, 128;
   lost in mixing manures, 536;
   most important nitrogen compound for plants, 161;
   relation of, to plants, 50;
   source of, in nitrate of soda, 337.

 Nitrification, 51, 52, 161-198;
   action of gypsum on, 173;
   alkalinity necessary for, 172;
   in asparagin, 182;
   bearing of, on agriculture, 193;
   in bones, 182;
   cause of, 165;
   conditions favourable for, 170;
   denitrification, 177-179;
   effected by micro-organisms, 51, 167;
   in ethylamine, 182;
   in fallow-fields, 184;
   food-constituents necessary for, 170;
   field experiments on rate of, 187;
   in gelatin, 182;
   in horn, 182;
   laboratory experiments on rate of, 185;
   in manures, 190, 192;
   in milk albuminoids, 182;
   mineral salts necessary for, 52;
   moisture necessary for, 52, 176;
   old theories on, 196;
   organic matter not necessary for, 169, 196;
   oxygen necessary for, 52, 173;
   plant-roots promote, 181;
   in rape-cake, 182;
   rate of, 183;
   rotation of crops, bearing of, on, 195;
   soil best suited for, 192;
   in subsoils, conditions favourable for, 181;
   substances capable of, 181;
   in summer, 183;
   sunlight, effect of, on, 176;
   temperature necessary for, 52, 175;
   in thiocyanates, 182;
   in urea, 182;
   in wool, 182.

 Nitrifying organisms, depth found at in soil, 180;
   distribution of, in soil, 179;
   effect of poisons on, 176;
   organic matter not required by, 169.

 _Nitrobaeter_, 167.

 Nitrogen, 115-160;
   absorbed by soil, 81, 131;
   accumulates in pastures, 134;
   in air, 116;
   as ammonia in soils, 127;
   amount of, in plants, 40;
   amount of, in soil, 123;
   artificial supply of, 150;
   in bat guano, 325;
   in bones, 363, 364;
   combined, in air, 118;
   combined, in rain, 119, 155;
   condition of, in manures, 540;
   converted into nitrates in soil, 51;
   in cow-dung, 226-228;
   in cow excrements, 278;
   in cow-urine, 230;
   difference between surface and subsoil, 126;
   different forms of, 45, 116;
   dissolved in rain, 131;
   in dried blood, 424;
   in farmyard manure, 260;
   in fish-guano, 321;
   fixation of free, 136;
   forms of, in plants, 491;
   free, relation of, to plant, 117;
   gain of, with leguminous crops, 135;
   in guanos, 329;
   in hoofs and horns, 426;
   in horse-dung, 226-228;
   in horse-manure, 243;
   in horse-urine, 230;
   importance of, in soil, 88;
   in lean flesh, 424;
   in leather, 428;
   least abundant of manurial ingredients in soil, 271;
   loss of, artificial sources of, 144;
   loss of, by crops, 144;
   loss of, on farm, 146;
   loss of, sources of, 137-150;
   loss of, total amount of, 142;
   lost in the arts, 148;
   lost in free condition, 141;
   lost in treating farmyard manure, 146;
   lost in milk, 147;
   lost by retrogression, 142;
   in Manitoba soils, 156;
   in meat-guano, 324;
   nature of, in soil, 124;
   as nitrates in soil, 128;
   as nitrates in cropped soils, 130, 157;
   as nitrates in Rothamsted soils, 198;
   as nitrates in wheat-soils, 157;
   in nitrate of soda, 343;
   nitric, in soil, 128;
   organic, absorbed by plants, 47;
   organic, in soil, 125;
   original source of, in soil, 133;
 in oxen excrements, 280;
   in pasture-lands, 158;
   peat-soils richest in, 123;
   in  Peruvian guano, 302, 306, 307, 329;
   in pig-dung, 226-227;
   position of, in agriculture, 115-160;
   relative manurial value of, 556;
   Rothamsted experiments on, 115;
   in scutch, 427;
   in sewage, 431;
   in sewage-sludge, 439;
   in sheep-dung, 226-228;
   in sheep excrements, 280;
   in sheep-urine, 230;
   in soil, 120;
   in soil, portion of, easily nitrifiable, 187;
   in soils at various depths, 156;
   in soot, 428;
   source of, in plants, 15, 16, 40-52;
   sources of soil, 131-137;
   in straw, 237, 243;
   in subsoil, 121;
   in surface-soil, 121;
   in swine-urine, 230;
   in woollen rags, 427.

 Nitrogenous guano, 300-308, 329.

 Nitrogenous manures, application of, 478;
   benefit cereals, 494;
   hurtful to leguminous crops, 523.

 Nitrogenous organic substances, in Chincha guano, 305;
   in concretionary nodules, 328.

 _Nitrosomonas_, 167.

 Nitrous acid, converted into nitric acid, 168;
  formed from ammonia, 167.

 Nobbe, on fixation of free nitrogen, 136;
   on potash in soil, 108.

 Nöllner on origin of nitrate-fields, 339.

 Norfolk, coprolites from, 374;
   experiments on barley, 497
     --on meadow-land, 509
     --on turnips, 513.

 North America, guano from, 298, 328.

 Norwegian apatite, 375.


 Oak-tree, water transpired by, 71.

 Oat-straw, composition of, 238;
   manurial constituents in, 282.

 Oats, Arendt's experiments with, 503;
   avenine in, 503;
   fertilising ingredients removed from soil by, 485;
   hardy crop, 502;
   manurial constituents in, 282;
   manuring of, 501-504;
   nitrogen removed in crop of, 148;
   require mixed nitrogenous manures, 502;
   source of nitrogen, 153;
   Rothamsted experiments on growth of, 567.

 _Oficinas_, 342.

 Ohlendorff, introduction of dissolved guano by, 311.

 Oilcakes, imports of, 153;
   source of nitrogen, 153.

 Oil-seeds, source of nitrogen, 153.

 Oligoclase felspars, 103, 214;
   composition of, 103;
   potash in, 220.

 Organic matter, in bones, 363;
   in dung, 228, 260;
   in manures, 543;
   not necessary for nitrifying organism, 169.

 Orthoclase felspars, 103, 214;
   composition of, 103;
   potash in, 220.

 Ox-dung, fertilising ingredients in, for food consumed, 228.

 Ox-urine, fertilising ingredients in, 232.

 Oxalic acid in guano, action of, 330.

 Oxen, excrements of, 280;
   food aided by, 280;
   solid excreta voided by, 280;
   urine voided by, 280.

 Oxidation, 79;
   products of, 79, 80.

 Oxygen, absorbed by plant-roots, 81;
   absorbed by soil, 81;
   evolved by plants, 11;
   necessary for fertility, 81;
   necessary for nitrification, 52, 173;
   percentage of, in plants, 39;
   source of, in plants, 39.


 Pabellon de Pica, guano from, 298, 302, 327;
   nitrogen in, 330;
   phosphoric acid in, 330.

 Pacific Islands, guano from, 298.

 Pacific Ocean, sea-weed in, 339.

 Palagonite as potash manure, 213.

 Palm-kernel meal, manurial constituents in, 282.

 Pasteur, on fermentation in urine, 255;
   on nitrification, 166.

 Pastures, accumulation of nitrogen in, 134;
   benefited by basic slag, 414;
   deficient in lime, 451;
   effect of manure on herbage of, 505;
   nitrogen in, 158;
   permanent, 138, 194
     --manuring of, 509;
   season influences, 507;
   soil influences, 507.

 Patagonian guano, 308, 327;
   nitrogen in, 330;
   phosphoric acid in, 330.

 Patent phosphate meal, 405.

 Patillos, guano deposits at, 327.

 Patos Island, guano deposits at, 328;
   phosphoric acid in, 330.

 Patterson on superphosphate, 399.

 Payen and Boussingault on composition of dried flesh, 425.

 Peas, manurial constituents in, 282;
   manuring of, 527;
   phosphorus in, 205;
   source of nitrogen, 153.

 Peat, absorbing properties of, 239;
   adulterant of guano, 317;
   analysis of stable-manure from, 281;
   litter, 239;
   nitrogen in, 240;
   retaining properties of, 240;
   soils, 123.

 Pelicans, guano from, 297.

 Penguin Island guano, 330;
   nitrogen in, 330;
   phosphoric acid in, 330.

 Penguins, guano from, 297.

 Percival on carbonic acid in plants, 12.

 Peru, guano deposits in, 327;
   guano first used in, 297;
   nitrate of soda from, 161, 162.

 Peruvian guano, 300-306;
   appearance of, 303;
   composition of, 304-306;
   deposits of, 301;
   imports of, 151, 297;
   source of nitrogen, 151.

 Peters and Eichhorn on solvent power of salt, 471.

 Petzholdt on sources of plant's nitrogen, 42.

 Pfeffer on action of light on plant-growth, 38.

 Phoenix Island guano, 309.

 Phosphate of iron in Chincha guano, 305.

 Phosphate of lime, in Algerian phosphate, 379;
   in apatite, 374;
   in Belgian phosphate, 377;
   in bones, 364;
   in Cambridge coprolites, 374;
   in Carolina phosphates, 376;
   in crust guanos, 379;
   in Estremadura phosphate, 375;
   in Florida phosphate, 378;
   in French phosphates, 379;
   in Lahn phosphates, 379;
   in Somme phosphate, 378;
   reverted in mixing manures, 537.

 Phosphates of lime, 385-388, 398;
   importance of mechanical condition of, 542.

 Phosphates, mineral, 373-381;
   imports of, 381;
   value as a manure, 380.

 Phosphatic guano, 308, 330.

 Phosphatic manures, application of, 480.

 Phosphoretted hydrogen in farmyard manure, 258.

 Phosphoric acid, 199-211;
   in ash of plants, 54;
   in basic slag, 404;
   in bat guano, 325;
   in bones, 363;
   condition of, in soil, 203;
   in cow-dung, 226-228;
   in cow excrements, 280;
   in cow-urine, 230;
   in farmyard manure, 260;
   in fish-guano, 321;
   fixed by soils, 58;
   gain of, 208;
   in guano, percentage of, 329, 330;
   guano a source of, 202;
   in hoofs and horns, 426;
   in horse-dung, 226-228;
   in horse-urine, 230;
   importance of, 88;
   loss of, artificial sources of, 206
     --by drainage, 206
     --in farmyard manure, 208
     --in milk, 207
     --in sewage, 208
     --sources of, in agriculture, 205;
   in meat-guano, 324;
   mineral sources of, 200;
   necessary for plant-growth, 55;
   occurrence of, in animals, 205
     --in nature, 199
     --in plants, 204
     --in soil, 203;
   in oxen excrements, 280;
   in pig-dung, 226, 227;
   in pig excrements, 281;
   in pig-urine, 230;
   position of, in agriculture, 199-211;
   relative trade values of, in manures, 400;
   in rocks, 202, 211;
   in sewage-sludge, 441;
   in sheep-dung, 226-228;
   in sheep excrements, 280;
   in sheep-urine, 230;
   statement of, in analyses of manures, 541.

 Phosphorite, 201, 374.

 Phosphorus, in albuminoids, 205;
   in animals, 205;
   in beans, 205;
   in peas, 205;
   in plants, 204;
   in pig-iron, 401.

 Physical properties of soils, 66-87.

 Pichard on action of gypsum on nitrification, 173.

 Pig-dung, composition of, 226;
   in dry state, 227.

 Pig excrements, 281;
   composition of, 281.

 Pig-manure, 250;
   amount produced per day, 251;
   mineral matter in, 251;
   nitrogen in, 251;
   poor in nitrogen, 251.

 Pig-urine, composition of, 230
     --in dry state, 231.

 Pigeon-dung, 320, 325;
   analysis of, 331.

 Pigs, excrements of, 281;
   food consumed by, 281.

 Pisagua, nitrate-fields at, 340.

 Plant, action of light on, 38;
   amount of hydrogen in, 40
     --nitrogen in, 40
     --oxygen in, 40;
   ash constituents of, 53-55;
   carbon fixed by, 37, 38;
   food, absorption of, by, 55;
   phosphoric acid in, 204;
   potash in, 216;
   proximate composition of, 36;
   relation of ammonia to, 48-50;
   source of hydrogen in, 40
     --nitrogen in, 40-52
     --oxygen in, 39, 40.

 Plant-food, absorption of, 490;
   amount of soluble, in soil, 100;
   early theories on source of, 4;
   retained by soil, 57.

 Plant-roots, grow downwards, 84;
   nitrification promoted by, 181;
   openness required by, 83;
   room required by, 85;
   soil in relation to, 84.

 Pliny, on lime as a manure, 449;
   on salt as a manure, 465.

 Pockets a source of phosphoric acid, 202.

 Poisons, effect of, on nitrifying organisms, 176.

 Polstorff on ash constituents of plants, 53.

 Polyhallite, potash in, 220, 420.

 Porphyry, in guano, 303;
   phosphoric acid in, 202, 211.

 Potash, 212-220, 418-423;
   in ash of plants, 54;
   in barilla, 420;
   chloride of, 218;
   condition of, in soil, 216;
   in cows' excrements, 280;
   in drainage-waters, 217;
   in farmyard manure, 260;
   in felspars, 220;
   in fleece, 217;
   fixed by soils, 58;
   importance of, in soil, 88;
   in kelp, 420;
   less important than phosphoric acid, 212;
   manures, 218, 418-423;
   muriate of, 218, 421;
   necessary for nitrification, 171;
   necessary for plant-growth, 55;
   occurrence of, 213;
   in ocean, 213;
   in oxen excrements, 280;
   in pig excrements, 280;
   in plants, 216;
   position of, in agriculture, 212-220;
   relative manurial value of, 556;
   Scottish soils supplied with, 419;
   in sheep excrements, 280;
   soda replaces, 466;
   sources of loss of, 217;
   in Stassfurt salts, 214;
   statement of, in analyses of manures, 542;
   in sugar-beet refuse, 219;
   sulphate of, 218, 421;
   in wood-ashes, 218, 220, 419.

 Potash manures, 218, 418-423;
   application of, 422, 480
     --rate of, 423;
   barilla as, 420;
   crops suited for, 423;
   relative importance of, 418;
   soils suited for, 423;
   sources of, 419;
   Stassfurt salts as, 420;
   wood-ashes a source of, 419.

 Potassium phosphate in concretionary nodules, 328.

 Potassium sulphate, in Chincha guano, 305;
   in concretionary nodules, 328.

 Potatoes, effect of farmyard manure on, 520;
   fertilising ingredients removed from soil by, 485;
   grown with covered manure, 289;
   Highland Society's experiments on, 518;
   manurial constituents in, 282;
   manuring of, 517-522
     --in Jersey, 529
     --influences composition of, 521;
   potash removed in, 217;
   Rothamsted experiments on, 519, 571.

 Precipitated ammonium phosphate in concretionary nodules, 328.

 Precipitated phosphate, 330, 387.

 Precipitation, treatment of sewage by, 436.

 Priestley, discovery of evolution of oxygen by plants, 11;
   on nitrogen in plants, 40.

 Prussiate of potash, manufacture of, 353.

 Pugh on sources of plant-nitrogen, 42.

 Punta de Lobos guano, 302;
   nitrogen in, 303;
   phosphoric acid in, 303.

 Punta de Patillos, guano deposits at, 327.

 Pyroxene, potash in, 220.


 Quartz, evaporation of water from, 99.

 Queensland, meat-meal guano from, 324.

 Quercitan, experiments of, with roses, 8.


 Rape-cake, capable of nitrification, 182;
   manurial constituents in, 282.

 Rape-seeds, imports of, 153.

 Raza Island guano, 328;
   phosphoric acid, 330.

 Rectified guano, 311.

 Relative trade values of phosphoric acid, 400.

 Resin in guano, 305.

 Retentive power of soils for water, 70-73.

 Retrogression, nitrogen lost by, 142.

 Reverted phosphates, 389-391;
   determination of amount of, 391;
   formation of, 387;
   value of, 391.

 Rhine, nitrates in waters of, 158.

 Rice-meal, an adulterant of guano, 319;
   manurial constituents of, 282.

 Rocks, phosphoric acid in, 202.

 Roots, influence of manures on composition of, 512;
   manuring of, 510-522;
   Norfolk experiments on, 513;
   potash removed in, 217.

 Rotation of crops, bearing of, on nitrification, 195.

 Rotations, phosphoric acid in, 290;
   potash removed in, 290.

 Rothamsted, alternate wheat and bean rotation at, 524;
   ammonia in rain at, 49;
   barley experiments at, 566;
   Broadbalk Field, alteration in composition of, 159
     --manuring of, 159
     --produce of wheat on, 159;
   early experiments at, 33-36;
   experiments, 560-572;
   experiments with nitrate of soda at, 347;
   experiments on nitrogen question at, 115
     --mangel-wurzel, 568
     --oats, 567
     --potatoes at, 519
     --value of nitrogen in farmyard manure, 271;
   increase of nitrogen with manures at, 137, 513;
   nitrates in barley-soils of, 158;
   nitrates in cropped soils of, 130, 157;
   nitrates in drainage of, 189;
   nitrates in wheat-soils of, 131, 157;
   nitrogen as nitrates in soils of, 129, 198;
   nitrogen, decrease of, in soils, 159;
   nitrogen in pasture at, 126;
   pasture, increase of nitrogen in, 158;
   retrogression of nitrogen at, 142;
   soil, nature of, 561
     --nitrogen in, at various depths, 156;
   total amount of nitrogen lost at, 142;
   turnip experiments at, 568;
   unmanured fallow-land loses nitrogen by drainage at, 141;
   wheat experiments at, 500, 562-565.

 Roy on sources of plant-nitrogen, 42.

 Rubidia in ash of plants, 55.

 Ruffle, John, on superphosphate, 388.

 Rye, manurial constituents in, 282.

 Rye-grass suited for sewage, 435.

 Rye-straw, summer, composition of, 238;
   winter, composition-of, 238.


 St Helena, experiments at, with Peruvian guano, 301.

 Saldanha Bay guano, 328;
   nitrogen in, 329;
   phosphoric acid in, 329.

 _Salinas_, 335.

 Salm-Horstmar, Prince, on water-culture, 54.

 Salt, 465-473;
   action of, on crops, 472;
   adulterant of guano, 319;
   amount applied, 473;
   antiquity of use of, 465;
   an antiseptic, 468;
   application of, 472;
   clarifies water, 470;
   coagulates clay, 470;
   decomposes minerals, 470;
   a germicide, 468;
   indirect action of, 468;
   mechanical action of, 470;
   nature of action of, 465;
   not a necessary plant-food, 466;
   occurrence of, 467;
   prevents rapid fermentation, 471;
   quantity to apply, 473;
   solvent action of, 470;
   sources of, 468.

 Saltpetre, formation of, 164;
   occurrence of, 215;
   plantations, 163.

 Sand, absorptive power of, 68;
   an adulterant of guano, 319;
   calcareous, absorptive power of, 98;
   siliceous, absorptive power of, 98.

 Sandy soils deficient in lime, 451.

 Sandwich Islands, guano deposits at, 328.

 Saragossa Sea, sea-weed in, 339.

 Saussure, De, on absorption of gases by soil, 81;
   on nitrogen in plants, 41;
   researches on plant-food by, 15.

 Sawdust an adulterant of guano, 319.

 Scheibler, Professor, on basic slag, 404.

 Schloesing and Müntz, on nitrification, 51, 166;
   experiments on rate of nitrification by, 185;
   on denitrification, 179;
   on ferments effecting nitrification, 167;
   on fixation of free nitrogen, 42;
   on ammonia in air, 119, 132;
   on nitrogen absorbed by soil from air, 132;
   on temperature favourable for nitrification, 175.

 Schoenite, potash in, 220.

 Schübler, on absorptive power of soils, 98;
   on retentive power of soils, 98.

 Schulze on fixers, 246.

 Scutch, 427;
   manufacture of, 427;
   nitrogen in, 427.

 Sea-weed, nitric acid in, 339.

 Seals, guano from, 297.

 Seed, fertilising ingredients lodge in, 491.

 Seine, nitrates in waters of, 158.

 Sénébier, Jean, on carbon in plants, 12;
   on nitrogen in plants, 41.

 Sewage, 430-441;
   charcoal a filter for, 437;
   crops suited for, 434;
   denitrification in, 179;
   dry matter in, 431;
   effects of continued applications of, 433;
   filters for, 437;
   irrigation with, 431-433;
   nitrification in, 166;
   nitrogen lost in, 149;
   phosphoric acid lost in, 149;
   purified by soils, 435;
   treatment of, by precipitation, 436;
   value of, as a manure, 430.

 Sewage-sick land, 433.

 Sewage-sludge, 438-441;
   as a manure, experiments with, 438;
   nitrogen in, 439;
   phosphoric acid in, 439;
   profitable treatment of, 441;
   value of, 439;
   water in, 438.

 Shale-works, sulphate of ammonia, from, 358.

 Shark's Bay guano, 309, 328.

 Sheep, excrements of, 280, 281;
   solid excreta voided by, 280;
   urine voided by, 280.

 Sheep-dung, alkalies in, 226;
   composition of, in dry state, 227;
   most valuable excrement, 227;
   nitrogen in, 226;
   phosphoric acid in, 226;
   water in, 226.

 Sheep-manure, 251;
   amount produced per day, 251
     --per year, 252;
   dry matter in, 252;
   mineral matter in, 252;
   nitrogen in, 252.

 Sheep-urine, alkalies in, 230;
   composition of, in dry state, 231;
   most valuable urine, 231;
   nitrogen in, 230;
   phosphoric acid in, 230;
   water in, 230.

 Shoddy, 427;
   production of, 152, 425;
   nitrogen in, 152, 427.

 Sicily, bones from, 360.

 Sidney Island guano, phosphoric acid in, 330.

 Siemens, Dr, experiments by, with light on plants, 38.

 Silica, in ash of plants, 55;
   in Chincha guano, 305;
   jelly, 169;
   necessary for plant-growth, 55.

 Silicates, 102;
   absorbed by cereals, 494.

 Silicic acid fixed by soils, 58.

 Simon on humus in soil, 47.

 Slaked lime, 454.

 Slugs killed by lime, 461.

 Smut prevented by lime, 461.

 Soda, in ash of plants, 54;
   fixed by soils, 58;
   necessary for plant-growth, 55;
   nitrate of, 332-351;
   in _salinas_, 335;
   replaces potash, 466.

 Sodium chloride in Chincha Island guano, 305.

 Sodium phosphate in concretionary nodules, 328.

 Sodium sulphate in concretionary nodules, 328.

 Soil, 65-108;
   absorptive power of, for water, 67, 98;
   acids fixed by, 58-60;
   action of lime on, 453;
   ammonia absorbed by, 81;
   amount of soluble plant-food in, 100;
   artificial, 54;
   barley, nitrates in, 158;
   bases fixed by, 58-60;
   best suited for nitrification, 192;
   biological properties of, 92-96;
   capacity for heat, 76-78;
   carbonic acid absorbed by, 81;
   chemical composition of, 87-92, 101-107;
   colour of, 80;
   cropped, nitrates in, 157;
   denitrification in, 177;
   evaporation from, 71, 72;
   farmyard manure, action of, on, 272;
   fertilising ingredients in, 87;
   fertility of, 65-108;
   fineness of, 69-70;
   gases in, 100;
   hygroscopic power of, 75-76, 99;
   improved by humus, 272;
   influence of farmyard manure on, 475;
     on nitrification, 180;
   manures increase fertility of, 474;
   nitrates in, amount of, 128-131;
   nitrifying organisms in, 179;
     distribution of, 179;
   nitrogen absorbed by, 81, 82, 131;
   nitrogen accumulates, 133;
   nitrogen in, amount of, 120-128;
   nitrogen least abundant of manurial ingredients in, 270;
   nitrogen at various depths in, 156;
   oxygen absorbed by, 81;
   phosphoric acid in, 203
     --condition of, in, 203
     --occurrence of, in, 203;
 peat, 123;
   possesses power of fixing ammonia, 57;
   potash in, 215
     --condition of, in, 216;
   potential fertility of, 549;
   power of, for absorbing gases, 81;
   relation of, to plant-roots, 84;
   retention of plant-food by, 57;
   retentive power of, for water, 70-73;
   sewage purified by, 435;
   shrinkage of, 74;
   variation in absorbing powers of, 82;
   varieties of, 67;
   virgin, 133;
   water in, most favourable amount of, 75;
   water-logged, 179;
   wheat, nitrates in, 157.

 Soluble phosphate, 386.

 Sombrero phosphate, 308, 328, 330, 379;
   phosphoric acid in, 330.

 Somme phosphate, 378.

 Soot, 428;
   application of, rate of, 429;
   crops suited by, 429;
   nitrogen in, 428.

 South America, guano deposits in, 327;
   meat-meal guano from, 324.

 Starbuck Island guano, 309, 328;
   phosphoric acid in, 330.

 Stassfurt salts, 214;
   potash in, 215, 420.

 Stead and Ribsdale on formation of basic slag, 407.

 Stoeckhardt, on composition of solid excreta, 226;
   on composition of urine, 229.

 Storer, Professor, on composition of birds' dung, 331;
   on composition of leaves, 242;
   on fish-guano, 323;
   on nitrogen removed in milk, 147.

 Straw, composition of, 238;
   imports of, 153;
   as litter, 236, 248;
   mineral matter in, 238, 243;
   nitrogen in, 237, 243;
   variation in composition of, 237.

 Subsoil, conditions favourable for nitrification in, 181.

 Suffolk coprolites, 374.

 Sugar-beet refuse, potash in, 219.

 Sulphate of alumina, a precipitant of sewage, 437.

 Sulphate of ammonia, 352-358;
   ammonia in, 355;
   application of, 356;
   composition of, 355;
   a concentrated nitrogenous manure, 356;
   converted into nitrates, 356;
   from gas-works, 353;
   from iron-works, 355;
   from shale-works, 354;
   manure for cereals, 356;
   most easily nitrifiable manure, 191;
   production of, 151, 358;
   properties of, 355;
   source of nitrogen, 149;
   sources of, 353, 354, 358;
   sulphocyanate of ammonia in, 355.

 Sulphate of lime a fixer, 246.

 Sulphate of magnesia, an adulterant of guano, 319;
   as a fixer, 246.

 Sulphate of potash, application of, 422
     --rate of, 423;
   compared with muriate, 421;
   sources of, 218, 420.

 Sulphuretted hydrogen from farmyard manure, 258.

 Sulphuric acid, action of, on bones, 382
     --on guano, 311
     --on tricalcic phosphate, 398;
   in ash of plants, 54;
   as a fixer, 245, 285;
   necessary for plant-growth, 55;
   superphosphate manufactured with, 384, 388.

 Superphosphate, 382-400;
   action of, 392-395
     --sometimes unfavourable, 395;
   application of, 395
     --rate of, 397;
   composition of, 391;
   discovery of, 382;
   hastens early growth, 394;
   high-class, 392;
   low-class, 392;
   manufacture of, 383-385
     --phosphates suitable for, 384;
   medium-class, 391;
   production of, 382;
   reversion in, 389, 399, 400
     --causes of, 389, 390;
   reverted in soil, 392.

 Surprise Island guano, 328.

 Swan Island guano, 328.

 Swedes, fertilising ingredients removed from soil by, 485;
   manurial constituents in, 282;
   manuring of, 514.

 Swine-dung, alkalies in, 226;
   composition of, 227;
   nitrogen in, 226;
   phosphoric acid in, 226;
   water in, 226.

 Swine-urine, alkalies in, 230;
   composition of, 231;
   nitrogen in, 230;
   phosphoric acid in, 230;
   water in, 230.

 Sydney Island guano, 309.

 Syenite, 106;
   phosphoric acid in, 202, 211.

 Sylvin, potash in, 220.

 _Symbiosis_, 44.


 Tamarugal, Pampa de, nitrate deposit in, 340.

 Tarapaca, nitrate deposits in, 340.

 Temperature necessary for nitrification, 52, 175.

 Tetracalcic phosphate, 387;
   occurrence of, 387, 405;
   solubility of, 387.

 Thaer on application of farmyard manure, 275.

 Thiocyanates, nitrification in, 182.

 Thomas-Gilchrist process of steel-smelting, 402.

 Thomas-slag. See Basic slag.

 Tillage increases number of plants, 86.

 Timor Island guano, 309.

 Tobacco, potash in, 217.

 Torrefied horn, 426.

 Torrefied leather, 428.

 Tortola guano, 309.

 Trachyte, phosphoric acid in, 202, 211.

 Transpiration, by elm-tree, 71;
   by oak-tree, 71.

 Trees, as pumping-engines, 76;
   water transpired by, 71.

 Tricalcic phosphate, 386, 398.

 Tubercles on roots of plants, 44.

 Tull, Jethro, theory of, on plant-growth, 9-11, 69, 109.

 Turkey, dung produced by, 331.

 Turnips, fertilising ingredients removed from soil by, 485;
   manurial constituents in, 282;
   manuring of, 510, 511, 513-517;
   Rothamsted experiments on growth of, 568.

 Twigs, potash in, 217.

 Tyrosin, assimilated by plants, 47.


 Ulmates in farmyard manure, 259.

 Ulmic acid, in farmyard manure, 258;
   in humus, 47.

 Ulmin in humus, 47.

 Uncovered farmyard manure, 263, 289.

 Unexhausted manures, valuation of, 549-552, 558.

 Unit value of manurial ingredients, 544.

 Units for determining commercial value of manures, 554.

 Urate of ammonium in Chincha Island guano, 305.

 Urea, assimilated by plants, 46;
   in farmyard manure, 257;
   nitrification in, 182.

 Uric acid, experiments with, 46;
   in Chincha Island guano, 305.

 Urine, 228;
   amount voided, 291;
   composition of, varies, 228;
   contains digested manurial ingredients, 228, 232;
   devoid of phosphoric acid, 205;
   and dung, composition of, 234;
   influence of food on, 229;
   nitrification in, 197;
   nitrogen in, 292;
   potash in, 292;
   voided by cows, 280;
   voided by oxen, 280;
   voided by pigs, 281;
   voided by sheep, 280.

 Uruguay, meat-meal guano from, 324.


 Valuation of manures, 539-559.

 Vegetation, desirable to have soil covered with, 194.

 Venezuela, guano deposits at, 327.

 Ville, Georges, on assimilation of ammonia, 50;
   theory of, on source of plant-nitrogen, 41.

 Vine, potash removed by, 216.

 Virgin soils, 133.

 Voelcker, Dr, analysis of apatite, 210
     --of farmyard manure, 259;
   on action of superphosphate, 395;
   on fresh and rotted dung, 261, 286;
   on guano, 316;
   on salt as a manure, 473.

 Voss, Hermann, on manures used, 152.


 Wagner, Professor, on, application of basic slag, 416;
   assimilation of organic nitrogen, 46;
   experiments with basic slag, 408-413;
   fineness of basic slag, 409;
   manures, 412;
   relative manurial value of nitrogen compounds, 556;
   solubility of basic slag, 408.

 Wallace, Dr, on sewage purification, 436.

 Walruses, guano from, 297.

 Warington, R,., on ammonia in rain, 49;
   on appearance of nitrous organisms, 168;
   on conditions favourable for nitrification, 181;
   experiments on rate of nitrification, 186;
   on composition of farmyard manure, 260;
   on manufacture of superphosphate, 383;
   on manurial constituents of foods, 282;
   on nitrification in alkaline solutions, 197;
   on nitrogen in excrements, 233;
   on nitrogen in soil, 122;
   on potash in wool, 227;
   researches of, on nitrification, 35, 52, 166-168, 180, 186.

 Water, absorbed by plants, 73;
   amount of, transpired by plant-leaves, 56;
   an adulterant of guano, 319;
   a carrier of plant-food, 55;
   in cow-dung, 226
     --cow-urine, 230;
   from decomposition of farmyard manure, 257;
   in horse-dung, 226
     --horse-urine, 230;
   necessary for plant, 67;
   in pig-dung, 226
     --pig-urine, 230
     --sheep-dung, 226
     --sheep-urine, 230;
   transpired by elm-tree, 71
     --oak-tree, 71.

 Water-culture, 54.

 Water-logged soils, 179.

 Waterloo, bones from, 360.

 Way, Thomas, on retention of plant-food by soil, 57, 59;
   on sewage, 437.

 West Indies, guano from, 298.

 Whales, guano from, 322.

 Wheat, fertilising ingredients removed from soil by, 485;
   Flitcham experiments on, 500;
   manurial constituents in, 282;
   manuring of, 499-501
   nitrogen removed in crop of, 145;
   requires nitrogenous manures, 499;
   Rothamsted experiments on, 500, 562-565;
   a source of nitrogen, 153.

 Wheat soils, nitrates in, 157.

 Wheat-straw, analysis of stable manure made from, 283;
   composition of, 238;
   manurial constituents in, 282.

 White clover, growth of, promoted by lime, 451.

 Wiegmann on ash constituents of plants, 53.

 Wilfarth on nitrogen in plants, 44.

 Wilting, 73.

 Winogradsky, on nitrification, 52, 167, 169, 197;
   on organisms in soil, 94.

 Wolff on, analysis of manure-heap drainings, 290;
   composition of fresh and rotten dung, 288;
   assimilation of organic nitrogen by plants, 47;
   relative manurial value of manurial compounds, 556;
   urine, 232.

 Wood-ashes as potash manure, 218, 419.

 Woodhouse, researches of, on nitrogen in plants, 41.

 Wool, capable of nitrification, 182;
   potash in, 217.

 Wool-waste, 427;
   nitrogen in, 427.

 Woolney, on organisms in soils, 93, 95;
   on water in soils, 75.

 Wrightson, Professor, on application of basic slag, 414.


 Yeast, 94.

 Yorkshire, bones first used in, 359.


 Zeolites, potash in, 220


PRINTED BY WILLIAM BLACKWOOD AND SONS.



+---------------------------------------------------+
|               Transcriber's Note:                 |
|                                                   |
| Carats (^) signify superscript figures.           |
| Underscores (_) followed by a number in curly     |
|     brackets signify subscript figures.           |
|                                                   |
| Typographical errors corrected in the text:       |
|                                                   |
| Page  58 Eichorn changed to Eichhorn              |
| Page 134 diferent changed to different            |
| Page 464 superposphate changed to superphosphate  |
| Page 553 biophosphate changed to biphosphate      |
| Page 579 Gallopagos changed to Galapagos          |
+---------------------------------------------------+





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