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Title: Elements of agricultural chemistry and geology
Author: Johnston, Jas. F. W.
Language: English
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_Wiley & Putnam’s New Publications._
LECTURES
ON
AGRICULTURAL
CHEMISTRY AND GEOLOGY;
TO WHICH ARE ADDED,
SUGGESTIONS FOR EXPERIMENTS
IN PRACTICAL AGRICULTURE.
BY
JAS. F. W. JOHNSTON, M.A., F.R.SS. L. & E.
Fellow of the Geological Society, Honorary Member of the Royal
Agricultural Society, &c. &c.; Reader in Chemistry and
Mineralogy in the University of Durham, &c.
These Lectures will be divided into four Parts, of which the First is
now ready; the others are in course of publication, and the whole will
be completed in two volumes.
OUTLINE OF PART I.—“_On the Organic Constituents of Plants._”—Lecture
I. Elementary substances of which plants subsist. II. and III. Compound
substances which minister to the growth of plants. IV. Sources from
which plants immediately derive their elementary constituents. V. How
the food enters into the circulation of plants—general structure of
plants. VI. Into what substances the food is changed in the interior
of plants—substances of which plants chiefly consist. VII. Chemical
changes by which the substances of which plants chiefly consist are
formed from those on which they live. VIII. How the supply of food for
plants is kept up in the general vegetation of the globe.
OUTLINE OF PART II.—“_On the Inorganic Constituents of Plants—the
Origin, Classification, and Chemical Constitution of Soils—General
and Special Relations of Geology to Agriculture—Origin, Constitution,
Analyses, and Methods of Improving Soils in different Districts and
under unlike conditions._—Lecture IX. Kind and proportion of inorganic
matter contained in plants. X. Properties of the inorganic compounds
which exist in vegetable substances, or which promote their growth. XI.
Of the nature, origin, and classification of soils—Structure of the
earth’s crust—Classification and general characters of the stratified
rocks—Agricultural capabilities of the soils derived from them. XII.
Granite and trap rocks, and the soils derived from them—Superficial
accumulations. XIII. On the exact chemical constitution, the analysis,
and the physical properties of soils.
PART III.—Methods of improving the soil by mechanical and by chemical
means—Manures, their nature, composition, and mode of action—theory of
their application in different localities.
PART IV.—The results of vegetation—the nature, constitution, and
nutritive properties of different kinds of produce, and by different
modes of cultivation—the feeding of cattle, the making of cheese, &c.
&c. The constitution and differences of various kinds of wood, and the
circumstances which favour their growth.
CRITICAL NOTICES.
“A valuable and interesting course of lectures.”—_Quarterly Review._
“But it is unnecessary to make large extracts from a book which we
hope and trust will soon be in the hands of nearly all our readers.
Considering it as unquestionably the most important contribution that
has recently been made to popular science, and as destined to exert an
extensively beneficial influence in this country, we shall not fail
to notice the forthcoming portions as soon as they appear from the
press.”—_Silliman’s American Journal of Science. Notice of Part I of
the American reprint._
“We think it no compliment to Professor Johnston to say, that among
our own writers of the present day who have recently been endeavouring
to improve our agriculture by the aid of science, there is probably no
other who has been more eminently successful, or whose efforts have
been more highly appreciated.”—_County Herald._
“Prof. Johnston is one who has himself done so much already for English
agriculture, that to behold him still in hot pursuit of the inquiry
into what _can_ be done, supplies of itself a stimulus to further
exertion on the part of others.”—_Berwick Warder._
ELEMENTS
OF
AGRICULTURAL
CHEMISTRY AND GEOLOGY.
BY
JAS. F. W. JOHNSTON, M.A., F.R.S.,
HONORARY MEMBER OF THE ROYAL ENGLISH AGRICULTURAL
SOCIETY, AND AUTHOR OF “LECTURES ON AGRICULTURAL
CHEMISTRY AND GEOLOGY.”
NEW-YORK:
WILEY AND PUTNAM.
MDCCCXLII.
J. P. WRIGHT, Printer,
18 New Street, N. Y.
INTRODUCTION.
The scientific principles upon which the art of culture depends, have
not hitherto been sufficiently understood or appreciated by practical
men. Into the causes of this I shall not here inquire. I may remark,
however, that if Agriculture is ever to be brought to that comparative
state of perfection to which many other arts have already attained, it
will only be by availing itself, as they have done, of the many aids
which Science offers to it; and that, if the practical man is ever to
realize upon his farm all the advantages which Science is capable of
placing within his reach, it will only be when he has become so far
acquainted with the connection that exists between the art by which he
lives and the sciences, especially of Chemistry and Geology, as to be
prepared to listen with candour to the suggestions they are ready to
make to him, and to attach their proper value to the explanations of
his various processes which they are capable of affording.
The following little Treatise is intended to present a familiar outline
of the subjects of _Agricultural Chemistry and Geology_, as treated of
more at large in my LECTURES, of which the first Part is now before the
public. What in this work has necessarily been taken for granted, or
briefly noticed, is in the LECTURES examined, discussed, or more fully
detailed.
_Durham, 8th April, 1842._
CONTENTS.
CHAPTER I.
PAGE
Distinction between Organic and Inorganic Substances
—The Ash of Plants—Constitution of the Organic
Parts of Plants—Preparation and Properties of
Carbon, Oxygen, Hydrogen, and Nitrogen—Meaning
of Chemical Combination. 13
CHAPTER II.
Form in which these different Substances enter into
Plants—Properties of the Carbonic, Humic, and
Ulmic Acids; of Water, of Ammonia, and of Nitric
Acid—Constitution of the Atmosphere. 25
CHAPTER III.
Structure of Plants—Mode in which their Nourishment
is obtained—Growth and Substance of Plants—
Production of their Substance from the Food they
imbibe—Mutual Transformations of Starch, Sugar,
and Woody Fibre. 38
CHAPTER IV.
Of the Inorganic Constituents of Plants—Their
immediate Source—Their Nature—Quantity of each
in certain common Crops. 49
CHAPTER V.
Of Soils—Their Organic and Inorganic Portions—Saline
Matter in Soils—Examination and Classification of
Soils—Diversities of Soils and Subsoils. 67
CHAPTER VI.
Direct Relations of Geology to Agriculture—Origin
of Soils—Causes of their Diversity—Relation to
the Rocks on which they rest—Constancy in the
Relative Position and Character of the Stratified
Rocks—Relation of this Fact to Practical
Agriculture—General Characters of the Soils upon
these Rocks. 78
CHAPTER VII.
Soils of the Granitic and Trap Rocks—Accumulations
of Transported Sands, Gravels, and Clays—Use
of Geological Maps in reference to
Agriculture—Physical Characters and Chemical
Constitution of Soils—Relation between the Nature
of the Soil and the Kind of Plants that naturally
grow upon it. 103
CHAPTER VIII.
Of the Improvement of the Soil—Mechanical and Chemical
Methods—Draining—Subsoiling—Ploughing, and Mixing
of Soils—Use of Lime, Marl, and Shell-sand—
Manures—Vegetable, Animal, and Mineral Manures. 133
CHAPTER IX.
Animal Manures—Their Relative Value and Mode of
Action—Difference between Animal and Vegetable
Manures—Cause of this Difference—Mineral Manures—
Nitrates of Potash and Soda—Sulphate of Soda,
Gypsum, Chalk, and Quicklime—Chemical Action of
these Manures—Artificial Manures—Burning and
Irrigation of the Soil—Planting and Laying Down
to Grass. 165
CHAPTER X.
The Products of Vegetation—Importance of _Chemical_
quality as well as quantity of Produce—Influence
of different Manures on the quantity and quality
of the Crop—Influence of the Time of Cutting—
Absolute quantity of Food yielded by different Crops
—Principles on which the Feeding of Animals depends
—Theoretical and Experimental Value of different kinds
of Food for Feeding Stock—Concluding Observations. 216
ELEMENTS
OF
AGRICULTURAL CHEMISTRY, &c.
CHAPTER I.
Distinction between Organic and Inorganic
Substances.—The Ash of Plants.—Constitution
of the Organic Parts of Plants.—Preparation
and Properties of Carbon, Hydrogen, and
Nitrogen.—Meaning of Chemical Combination.
The object of the practical farmer is to raise from a given extent of
land the largest quantity of the most valuable produce at the least
cost, and with the least permanent injury to the soil. The sciences
either of chemistry or geology throw light on every step he takes or
ought to take, in order to effect this main object.
SECTION I.—OF THE VEGETABLE AND EARTHY OR THE ORGANIC AND INORGANIC
PARTS OF PLANTS.
In the prosecution of his art, two distinct classes of substances
engage his attention—the _living_ crops he raises, and the _dead_
earth from which they are gathered. If he examine any fragment of
an animal or vegetable, either living or dead, he will observe that
it exhibits pores of various kinds arranged in a certain order—that
it has a species of internal structure—that it has various parts or
_organs_—in short, that it is what physiologists term _organized_. If
he examine, in like manner, a lump of earth or rock, he will perceive
no such structure. To mark this distinction, the parts of animals and
vegetables, either living or dead—whether entire or in a state of
decay, are called _organic_ bodies, while earthy and stony substances
are called _inorganic_ bodies.
Organic substances are also more or less readily burned and dissipated
by heat in the open air; inorganic substances are generally fixed and
permanent in the fire.
But the crops which grow upon it, and the soil in which they are
rooted, contain a portion of both of these classes of substances. In
all fertile soils, there exists from 3 to 10 per cent. of vegetable
or other matter of _organic_ origin; while, on the other hand, all
vegetables, as they are collected for food, leave, when burned, from
one-half to twenty per cent. of _inorganic_ ash.
If we heat a portion of soil to redness in the open air, the organic
matter will burn away, and, in general, the soil, if previously
_dry_, will not be materially diminished in bulk. But if a handful of
wheat, or of wheat straw, or of hay, be burned in the same manner,
the proportion that disappears is so great, that in most cases a
comparatively minute quantity only remains behind. Every one is
familiar with this fact who has seen the small bulk of ash that is left
when weeds, or thorns, or trees, are burned in the field, or when a
hay or corn-stack is accidentally consumed. Yet the ash thus left is a
very appreciable quantity, and the study of its true nature throws much
light, as we shall hereafter see, on the practical management of the
land on which any given crop is to be made to grow.
Thus the quantity of ash left by a ton of wheat straw is sometimes as
much as 360 lbs.; by a ton of oat straw as much as 200 lbs.; while a
ton of the grain of wheat leaves only about 40 lbs.; of the grain of
oats about 90 lbs.; and of oak wood only 4 or 5 lbs. The quantities of
_inorganic_ matter, therefore, though comparatively small, yet, in some
cases, amount to a considerable weight in an entire crop. The nature,
source and uses of this earthy matter will be explained in a subsequent
chapter.
SECTION II.—CONSTITUTION OF THE ORGANIC PART OF PLANTS AND ANIMALS.
The organic part of plants, when in a perfectly dry state, constitutes
therefore from 85 to 99 per cent. of their whole weight. Of those parts
of plants which are cultivated for food, it is only hay and straw, and
a very few others, that contain as much as 10 per cent. of inorganic
matter.
This organic part consists of four substances, known to chemists by the
names of carbon, hydrogen, oxygen, and nitrogen. The first of these,
carbon, is a solid substance, the other three are gases or peculiar
kinds of air.
1. CARBON. When wood is burned in a covered heap, as is done by the
charcoal burners, or is distilled in iron retorts, as in making
wood-vinegar, it is charred and converted into common wood charcoal.
This charcoal is the most usual and best known variety of carbon. It
is black, soils the fingers, and is more or less porous according
to the kind of wood from which it has been formed. Coke obtained by
charring or distilling coal is another variety. It is generally denser
or heavier than the former, though less pure. Black lead is a third
variety, still heavier and more impure. The diamond is the only form in
which carbon occurs in nature in a state of perfect purity.
This latter fact, that the diamond is pure carbon—that it is
essentially the same substance with the finest and purest lamp-black—is
very remarkable; but it is only one of many striking circumstances that
every now and then present themselves before the inquiring chemist.
Charcoal, the diamond, lamp-black, and all the other forms of carbon,
burn away more or less slowly when heated in the air, and are converted
into a kind of gas known by the name of _carbonic acid_. The impure
varieties leave behind them a greater or less proportion of ash.
2. HYDROGEN.—If oil of vitriol (sulphuric acid) be mixed with twice
its bulk of water, and then poured upon iron filings, the mixture will
speedily begin to boil up, and bubbles of gas will rise to the surface
of the liquid in great abundance. These are bubbles of hydrogen gas.
If the experiment be performed in a bottle, the hydrogen which is
produced will gradually drive out the atmospheric air it contained,
and will itself take its place. If a bit of wax taper be tied to the
end of a wire, and when lighted be introduced into the bottle, it will
be instantly extinguished; while the hydrogen will take fire, and burn
at the mouth of the bottle with a pale yellow flame. If the taper be
inserted before the common air is all expelled, the mixture of hydrogen
and common air will burn with an explosion more or less violent,
and may even shatter the bottle and produce serious accidents. This
experiment, therefore, ought to be made with care. It may be safely
made in an open tumbler, covered by a plate or a piece of paper, till a
sufficient quantity of hydrogen is collected, when, on the introduction
of the taper, the light will be extinguished, and the hydrogen will
burn with a less violent explosion.
This gas is also an exceedingly light substance, rising through common
air as wood does through water. Hence, when confined in a bag made
of silk, or other light tissue, it is capable of sustaining heavy
substances in the air, and even of transporting them to great heights.
For this reason it is employed for filling and elevating balloons.
Hydrogen gas is not known to occur anywhere in nature in any sensible
quantity. It is very abundant, as we shall hereafter see, in what by
chemists is called a _state of combination_.
3. OXYGEN.—When strong oil of vitriol is poured upon black oxide of
manganese, and heated in a glass retort: or when red oxide of mercury,
or chlorate of potash, is so heated alone; or when saltpetre, or the
same oxide of manganese, is heated alone in an iron bottle;—in all
these cases a kind of air is given off, which, when collected and
examined by plunging a taper into it, is found to be neither common
air nor hydrogen gas. The taper, when introduced, burns with great
rapidity, and with exceeding brilliancy, and continues to burn till
either the whole of the gas disappears, or the taper is entirely
consumed. If a living animal is introduced, its circulation and its
breathing become quicker—it is speedily thrown into a fever—it lives as
fast as the taper burned—and, after a few hours, dies from excitement
and exhaustion. This gas is not light like hydrogen, but is about
one-ninth part heavier than common air.
In the atmosphere, oxygen exists in the state of gas. It forms about
one-fifth of the bulk of the air we breathe, and is the substance
which, in the air, supports all animal life and the combustion of
all burning bodies. Were it by any cause suddenly removed from the
atmosphere of our globe, every living thing would perish, and all
combustion would become impossible.
4. NITROGEN.—If a saucer be half filled with milk of lime, formed by
mixing slaked quicklime with water, a _very_ small tea-cup containing
a little burning sulphur then placed in the middle, and a common large
tumbler inverted over the whole, the sulphur will burn for a while, and
will then gradually die out. On allowing the whole to remain for some
time, the fumes of the sulphur will be absorbed by the milk of lime,
which will rise a certain way into the tumbler. When the absorption has
ceased, a quantity of air will remain in the upper part of the tumbler.
This air is nitrogen gas.
If the whole be now introduced into a large basin of water, the tumbler
being held in the left hand, the cup and saucer may be removed from
beneath. The saucer may then be inverted and introduced with its under
side into the mouth of the tumbler, which may thus be lifted out of
the water and restored to its upright position, the saucer serving the
purpose of a cover. By carefully removing this cover with the one hand,
a lighted taper may be introduced by the other. It will then be seen
that the taper is extinguished by this air, and that no other effect
follows. Or if a living animal be introduced into it, breathing will
instantly cease, and it will drop without signs of life.
This gas possesses no other remarkable property. It is a very little
lighter than common air, and is known to exist in large quantity in the
atmosphere only. Of the air we breathe it forms nearly four-fifths of
the entire bulk.
These three gases are incapable of being distinguished from common
air, or from each other, by the ordinary senses; but by the aid of the
taper they are readily recognised. Hydrogen extinguishes the taper, but
itself takes fire; nitrogen simply extinguishes it; while in oxygen the
taper burns with extraordinary brilliancy and rapidity.
Of this one solid substance, carbon, and these three gases, hydrogen,
oxygen, and nitrogen, all the organic part of vegetable and animal
substances is made up.
Into these substances, however, they enter in very different
proportions. Nearly one-half the weight of all vegetable productions
which are gathered as food for man or beast—in their dry state—consists
of carbon; the oxygen amounts to rather more than one-third, the
hydrogen to little more than five per cent., while the nitrogen rarely
exceeds two and a half or three per cent. of their weight.
This will appear from the following table, which exhibits the actual
constitution by analysis of some varieties of the more common crops
when perfectly dry.
Carbon. Hydrogen. Oxygen. Nitrogen. Ash.
Hay, 458 50 387 15 90
Potatoes, 441 58 439 12 50
Wheat Straw, 485 52 389½ 3½ 70
Oats, 507 64 367 22 40
These numbers represent the weights of each element in pounds,
contained in 1000 lbs. of the dry hay, potatoes, &c.; but in drying by
a gentle heat, 1000 lbs. of hay from the stack, lost 158 lbs. of water,
of potatoes wiped dry externally 722 lbs.,[1] wheat straw 260 lbs., and
oats 151 lbs.
[1] Both potatoes and turnips contain about four-fifths of their weight
of water, or five tons of either of these roots contain nearly four
tons of water.
SECTION III.—OF THE MEANING OF CHEMICAL COMBINATION.
If the three kinds of air above spoken of be mixed together in a
bottle, no change will take place, and if charcoal in fine powder be
added to them, still no new substance will be produced. If we take the
ash left by a known weight of hay or wheat straw, and mix it with the
proper quantities of the four elementary substances, carbon, hydrogen,
&c., as shewn in the above table, we shall be unable by this means
to form either hay or wheat straw. The elements of which vegetable
substances consist, therefore, are not merely _mixed_ together—they are
united in some closer and more intimate manner. To this more intimate
state of union, the term _chemical combination_ is applied—the elements
are said to be _chemically combined_.
Thus, when charcoal is burned in the air, it slowly disappears, and
forms, as already stated, a kind of air known by the name of carbonic
acid gas, which rises into the atmosphere and disappears. Now, this
carbonic acid is formed by the _union_ of the carbon (charcoal), while
burning, with the oxygen of the atmosphere, and in this new air the two
elements, carbon and oxygen, are _chemically combined_.
Again, if a piece of wood or a bit of straw, in which the elements are
already chemically combined, be burned in the air, these elements are
separated and made to assume new states of combination, in which new
states they escape into the air and become invisible. When a substance
is thus changed by the action of heat, it is said to be _decomposed_,
or if it gradually decay and perish by exposure to the air and
moisture, it undergoes slow _decomposition_.
When, therefore, two or more substances unite together, so as to
form a third possessing properties different from both, they enter
into chemical union—they form a _chemical combination_ or chemical
_compound_. When, on the other hand, one compound body is so changed
as to be converted into two or more substances different from itself,
it is _decomposed_. Carbon, hydrogen, &c., are chemically combined in
the interior of the plant during the formation of wood: wood, again,
is decomposed when by the vinegar-maker it is converted among other
substances into charcoal and wood-vinegar, and the flour of grain when
the brewer or distiller converts it into ardent spirits.
CHAPTER II.
Form in which these different substances enter into
Plants. Properties of the Carbonic, Humic, and
Ulmic Acids—of Water, of Ammonia, and of Nitric
Acid. Constitution of the Atmosphere.
SECTION I.—FORM IN WHICH THE CARBON, ETC. ENTER INTO PLANTS.
It is from their food that plants derive the carbon, hydrogen, oxygen,
and nitrogen, of which their organic part consists. This food enters
partly by the minute pores of their roots, and partly by those which
exist in the green part of the leaf and of the young twig. The roots
bring up food from the soil, the leaves take it in directly from the
air.
Now, as the pores in the roots and leaves are very minute, carbon
(charcoal) cannot enter into either in a _solid_ state; and as it does
not dissolve in water, it cannot, in the state of simple carbon, be any
part of the food of plants. Again, hydrogen gas neither exists in the
air nor usually in the soil—so that, although hydrogen is always found
in the substance of plants, it does not enter them in the state of the
gas above described. Oxygen exists in the air, and is directly absorbed
both by the leaves and by the roots of plants; while nitrogen, though
it forms a large part of the atmosphere, is not supposed to enter
_directly_ into plants in any considerable quantity.
The whole of the carbon and hydrogen, and the greater part of the
oxygen and nitrogen also, enter into plants in a state of _chemical
combination_ with other substances; the carbon chiefly in the state of
_carbonic acid_, and of certain other soluble compounds which exist
in the soil; the hydrogen and oxygen in the form of water: and the
nitrogen in those of ammonia or nitric acid. It will be necessary
therefore briefly to describe these several compounds.
SECTION II.—OF THE CARBONIC, HUMIC, AND ULMIC ACIDS.
1. CARBONIC ACID.—If a few pieces of chalk or limestone be put into
the bottom of a tumbler, and a little spirit of salt (muriatic acid)
be poured upon them, a boiling up or _effervescence_ will take place,
and a gas will be given off, which will gradually collect and fill the
tumbler; and when produced very rapidly, may even be seen to run over
its edges. This gas is carbonic acid. It cannot be distinguished from
common air by the eye; but if a taper be plunged into it, the flame
will immediately be extinguished, while the gas remains unchanged.
This kind of air is so heavy, that it may be poured from one vessel
into another, and its presence recognised by the taper. It has also a
peculiar odour, and is exceedingly suffocating, so that if a living
animal be introduced into it, life immediately ceases. It is absorbed
by water, a pint of water absorbing or dissolving a pint of the gas.
Carbonic acid exists in the atmosphere; it is given off from the lungs
of all living animals while they breathe; it is also produced largely
during the burning of wood, coal, and all other combustible bodies, so
that an unceasing supply of this gas is poured into the air. Decaying
animal and vegetable substances also give off this gas, and hence it is
always present in greater or less abundance in the soil, and especially
in such soils as are rich in vegetable matter. During the fermentation
of malt liquors, or of the expressed juices of different fruits,—the
apple, the pear, the grape, the gooseberry—it is produced, and the
briskness of such fermented liquors is due to the escape of this gas.
From the dung and compost heap it is also given off; and when put into
the ground in a fermenting state, farm-yard manure affords a rich
supply of carbonic acid to the young plant.
Carbonic acid consists of carbon and oxygen only, combined together in
the proportion of 28 of the former to 72 of the latter, or 100 lbs. of
carbonic acid contain 28 lbs. of carbon and 72 lbs. of oxygen.
2. HUMIC AND ULMIC ACIDS.—The soil always contains a portion of
vegetable matter (called _humus_ by some writers), and such matter is
always added to it when it is manured from the farm-yard or the compost
heap. During the decay of this vegetable matter, carbonic acid, as
above stated, is given off in large quantity, but other substances
are also formed at the same time. Among these are the two to which
the names of humic and ulmic acids are respectively given. They both
contain much carbon, are both capable of entering the roots of plants,
and both, no doubt, in favourable circumstances, help to feed the plant.
If the common soda of the shops be dissolved in water, and a portion
of a rich vegetable soil, or a bit of peat, be put into this solution,
and the whole boiled, a brown liquid is obtained. If to this brown
liquid, spirit of salt (muriatic acid) be added till it is sour to the
taste, a brown flocky powder falls to the bottom. This brown substance
is _humic_ acid. But if in this process we use spirit of hartshorn
(liquid ammonia), instead of the soda, _ulmic_ acid is obtained.
These acids exist along with other substances in the rich brown liquor
of the farm-yard, which is so often allowed to run to waste; they are
also produced in greater or less quantity during the decay of the
manure after it is mixed with the soil, and no doubt yield to the plant
a portion of that supply of food which it must necessarily receive from
the soil.
SECTION III.—OF WATER, AMMONIA, AND NITRIC ACID.
1. WATER.—If hydrogen be prepared in a bottle, in the way already
described, and a gas-burner be fixed into its mouth, the hydrogen may
be lighted, and will burn as it escapes into the air. Held over this
flame a cold tumbler will become covered with dew, or with little drops
of water. This water is _produced_ during the burning of the hydrogen;
and as it takes place in pure oxygen gas as well as in the open air,
this water must contain the hydrogen and oxygen which disappear, or
_must consist of hydrogen and oxygen only_.
This is a very interesting fact; and were it not that chemists are now
familiar with many such, it could not fail to appear truly wonderful
that the two gases, oxygen and hydrogen, by their union, should form so
very different a substance as water is from either. It consists of 1
of hydrogen to 8 of oxygen, or every 9 lbs. of water contain 8 lbs. of
oxygen and 1 lb. of hydrogen.
Water is so familiar a substance, that it is unnecessary to dwell upon
its properties. When pure, it has neither colour, taste, nor smell. At
32° of Fahrenheit’s[2] scale (the freezing point), it solidifies into
ice, and at 212° it boils, and is converted into steam. There are two
others of its properties which are especially interesting in connection
with the growth of plants.
[2] This is the scale of the common thermometer used in this country.
_1st_, If sugar or salt be put into water, they disappear or are
_dissolved_. Water has the power of thus dissolving numerous other
substances in greater or less quantity. Hence, when the rain falls
and sinks into the soil, it dissolves some of the soluble substances
it meets in its way, and rarely reaches the roots of plants in a pure
state. So waters that rise up in springs are rarely pure. They always
contain earthy and saline substances in solution, and these they carry
with them, when they are sucked in by the roots of plants.
It has been above stated, that water absorbs (dissolves) its own bulk
of carbonic acid; it dissolves also smaller quantities of the oxygen
and nitrogen of the atmosphere; and hence, when it meets any of these
gases in the soil, it becomes impregnated with them, and conveys them
into the plant, there to serve as a portion of its food.
_2d_, Water is composed of oxygen and hydrogen; by certain chemical
processes it can readily be resolved or decomposed _artificially_ into
these two gases. The same thing takes place _naturally_ in the interior
of the living plant. The roots absorb the water, but if in any part
of the plant hydrogen be required, to make up the substance which it
is the function of that part to produce, a portion of the water is
decomposed and worked up, while the oxygen is set free, or converted to
some other use. So, also, in any case where oxygen is required water is
decomposed, the oxygen made use of, and the hydrogen liberated. Water,
therefore, which abounds in the vessels of all growing plants, if not
directly converted into the substance of the plant, is yet a ready and
ample source from which a supply of either of the elements of which it
consists may at any time be obtained.
It is a beautiful adaptation of the properties of this all-pervading
compound (water), that its elements should be so fixedly bound together
as rarely to separate in external nature, and yet to be at the command
and easy disposal of the vital powers of the humblest order of living
plants.
2. AMMONIA.—If the sal ammoniac of the shops be mixed with quicklime,
a powerful odour is immediately perceived, and an invisible gas is
given off which strongly affects the eyes. This gas is ammonia. Water
dissolves or absorbs it in very large quantity, and this solution forms
the common hartshorn of the shops. The white solid smelling-salts of
the shops are a compound of ammonia with carbonic acid,—a solid formed
by the union of two gases.
The gaseous ammonia consists of nitrogen and hydrogen only, in the
proportion of 14 of the former to 3 of the latter, or 17 lbs. of
ammonia contain 3 lbs. of hydrogen.
The chief natural source of this compound is, in the decay of animal
substances. During the putrefaction of dead animal bodies ammonia is
invariably given off. From the animal substances of the farm-yard it is
evolved, and from all solid and liquid manures of animal origin. It is
also formed in lesser quantity during the decay of vegetable substances
in the soil; and in volcanic countries, it escapes from many of the hot
lavas, and from the crevices in the heated rocks.
It is produced artificially by the distillation of animal substances
(hoofs, horns, &c.), or of coal. Thousands of tons of the ammonia
present in the ammoniacal liquors of the gas-works, which might be
beneficially applied as a manure, are annually carried down by the
rivers, and lost in the sea.
The ammonia which is given off during the putrefaction of animal
substances rises partially into the air, and floats in the atmosphere,
till it is either decomposed by natural causes, or is washed down by
the rains. In our climate, cultivated plants derive a considerable
portion of their nitrogen from ammonia. It is supposed to be one of the
most valuable fertilizing substances contained in farm-yard manure; and
as it is present in greater proportion by far in the liquid than in the
solid contents of the farm-yard, there can be no doubt that much real
wealth is lost, and the means of raising increased crops thrown away in
the quantities of liquid manure which are almost everywhere permitted
to run to waste.
3. NITRIC ACID—is a powerfully corrosive liquid known in the shops by
the familiar name of _aquafortis_. It is prepared by pouring oil of
vitriol (sulphuric acid) upon saltpetre, and distilling the mixture.
The aquafortis of the shops is a mixture of the pure acid with water.
Pure nitric acid consists of nitrogen and oxygen only; the union of
these two gases, so harmless in the air, producing the burning and
corrosive compound which this is known to be.
It never reaches the roots of plants in this free and corrosive state.
It exists in many soils, and is naturally formed in compost heaps,
and in most situations where vegetable matter is undergoing decay
in contact with the air; but it is always in a state of chemical
combination in these cases. With potash, it forms _nitrate of potash_
(saltpetre); with soda, _nitrate of soda_; and with lime, _nitrate
of lime_; and it is generally in one or other of these states of
combination that it reaches the roots of plants.
Nitric acid is also naturally formed, and in some countries probably in
large quantities, by the passage of electricity through the atmosphere.
The air, as has been already stated, contains much oxygen and nitrogen
_mixed_ together, but when an electric spark is passed through a
quantity of air, a certain quantity of the two _unite_ together
chemically, so that every spark that passes forms a small portion of
nitric acid. A flash of lightning is only a large electric spark;
and hence every flash that crosses the air produces along its path a
quantity of this acid. Where thunder-storms are frequent, much nitric
acid must be produced in this way in the air. It is washed down by the
rains, in which it has frequently been detected, and thus reaches the
soil, where it produces one or other of the _nitrates_ above mentioned.
It has been long observed that those parts of India are the most
fertile in which saltpetre exists in the soil in the greatest
abundance. Nitrate of soda, also, in this country, has been found
wonderfully to promote vegetation in many localities; and it is a
matter of frequent remark, that vegetation seems to be refreshed and
invigorated by the fall of a thunder-shower. There is, therefore, no
reason to doubt that nitric acid is really beneficial to the general
vegetation of the globe. And since vegetation is most luxuriant
in those parts of the globe where thunder _or_ lightning are most
abundant, it would appear as if the natural production of this compound
body in the air, to be afterwards brought to the earth by the rains,
were a wise and beneficent contrivance by which the health and vigour
of universal vegetation is intended to be promoted.
It is from this nitric acid, thus universally produced and existing,
that plants appear to derive a large—probably, taking vegetation in
general, the largest—portion of their nitrogen. In all climates they
also derive a portion of this element from ammonia; but less from this
source in tropical than in temperate climates.[3]
[3] For fuller information on this point, see the Author’s “LECTURES
_on Agricultural Chemistry and Geology_,” Part I.
SECTION IV.—OF THE CONSTITUTION OF THE ATMOSPHERE.
The air we breathe, and from which plants also derive a portion of
their nourishment, consists of a mixture of oxygen and nitrogen gases,
with a minute quantity of carbonic acid, and a variable proportion
of watery vapour. Every hundred gallons of dry air contain about 21
gallons of oxygen and 79 of nitrogen. The carbonic acid amounts only to
one gallon in 2500, while the watery vapour in the atmosphere varies
from 1 to 2½ gallons (of steam) in 100 gallons of common air.
The oxygen in the air is necessary to the respiration of animals, and
to the support of combustion (burning of bodies). The nitrogen serves
principally to dilute the strength, so to speak, of the pure oxygen,
in which gas, if unmixed, animals would live and combustibles burn
with too great rapidity. The small quantity of carbonic acid affords
an important part of their food to plants, and the watery vapour in
the air aids in keeping the surfaces of animals and plants in a moist
and pliant state; while, in due season, it descends also in refreshing
showers, or studs the evening leaf with sparkling dew.
There is a beautiful adjustment in the constitution of the atmosphere
to the nature and necessities of living beings. The energy of the pure
oxygen is tempered, yet not too much weakened, by the admixture of
nitrogen. The carbonic acid, which alone is noxious to life, is mixed
in so minute a proportion as to be harmless to animals, while it is
still beneficial to plants; and when the air is overloaded with watery
vapour, it is provided that it shall descend in rain. These rains at
the same time serve another purpose. From the surface of the earth
there are continually ascending vapours and exhalations of a more or
less noxious kind; these the rains wash out from the air, and bring
back to the soil, at once purifying the atmosphere through which they
descend, and refreshing and fertilizing the land on which they fall.
CHAPTER III.
Structure of plants—Mode in which their nourishment
is obtained—Growth and substance of plants—
Production of their substance from the food
they imbibe—Mutual transformations of starch,
sugar, and woody fibre.
From the compound substances, described in the preceding chapter,
plants derive the greater portion of the carbon, hydrogen, oxygen,
and nitrogen, of which their organic part consists. The living
plant possesses the power of absorbing these compound bodies, of
_decomposing_ them in the interior of its several vessels, and of
_recompounding_ their elements in a different way, so as to produce new
substances,—the ordinary products of vegetable life. Let us briefly
consider the general structure of plants, and their mode of growth.
SECTION I.—OF THE STRUCTURE OF PLANTS, AND THE MODE IN WHICH THEIR
NOURISHMENT IS OBTAINED.
A perfect plant consists of three several parts,—a root which throws
out arms and fibres in every direction into the soil,—a trunk which
branches into the air on every side,—and leaves which, from the ends of
the branches and twigs, spread out a more or less extended surface into
the surrounding air. Each of these parts has a peculiar structure and a
special function assigned to it.
The _stem_ of any of our common trees consists of three parts,—the
pith in the centre, the wood surrounding the pith, and the bark which
covers the whole. The pith consists of bundles of minute hollow tubes,
laid horizontally one over the other; the wood and inner bark, of long
tubes bound together in a vertical position, so as to be capable of
carrying liquids up and down between the roots and the leaves. When a
piece of wood is sawn across, the ends of these tubes may be distinctly
seen. The branch is only a prolongation of the stem, and has a similar
structure.
The _root_, immediately on leaving the trunk or stem, has also a
similar structure; but as the root tapers away, the pith gradually
disappears, the bark also thins out, the wood softens, till the white
tendrils, of which its extremities are composed, consist only of a
colourless spongy mass, full of pores, but in which no distinction of
parts can be perceived. In this spongy mass the vessels or tubes which
descend through the stem and root lose themselves, and by them these
spongy extremities are connected with the leaves.
The _leaf_ is an expansion of the twig. The fibres which are seen
to branch out from the base over the inner surface of the leaf are
prolongations of the vessels of the wood. The green exterior portion
of the leaf is, in like manner, a continuation of the bark in a very
thin and porous form. The green of the leaf, though full of pores,
especially on the under part, yet also consists of, or contains, a
collection of tubes or vessels, which stretch along its surface, and
communicate with those of the bark.
Most of these vessels in the living plant are full of sap, and this sap
is in almost continual motion. In spring and autumn the motion is more
rapid, and in winter it is sometimes scarcely perceptible; yet the sap
is supposed to be rarely quite stationary in every part of the tree.
From the spongy part of the root the sap ascends through the vessels
of the _wood_, till it is diffused over the inner surface of the leaf
by the fibres which the wood contains. Hence, by the vessels in the
green of the leaf, it is returned to the bark, and through the vessels
of the inner bark it descends to the root.
Every one understands why the roots send out fibres in every direction
through the soil,—it is in search of water and of _liquid_ food, which
the spongy fibres suck in and send forward with the sap to the upper
parts of the tree. It is to aid these roots in procuring food that, in
the art of culture, such substances are mixed with the soil where these
roots are, as are supposed to be necessary, or at least favourable, to
the growth of the plant.
It is not so obvious that the leaves spread out their broad surfaces
into the air for the same purpose precisely as that for which the roots
diffuse their fibres through the soil. The only difference is, that
while the roots suck in chiefly _liquid_, the leaves inhale almost
solely _gaseous_ food. _In the sunshine, the leaves are continually
absorbing carbonic acid from the air and giving off oxygen gas._ That
is to say, they are continually appropriating carbon from the air.[4]
_When night comes, this process ceases, and they begin to absorb
oxygen and to give off carbonic acid._ But this latter process does
not go on so rapidly as the former, so that, on the whole, plants
when growing gain a large portion of carbon from the air. The actual
quantity, however, varies with the season, with the climate, and with
the kind of tree. The proportion of the whole carbon contained by a
plant, which has been derived from the air, is greatly modified also
by the quality of the soil in which it grows, and by the comparative
abundance of liquid food which happens to be within reach of its roots.
It has been ascertained, however, that in our climate, on an average,
not less than from one-third to three-fourths of the entire quantity of
carbon contained in the crops we reap from land of average fertility,
is really obtained from the air.
[4] Since carbonic acid, as shewn in the previous chapter, consists
only of carbon and oxygen, they retain the carbon and reject the oxygen.
We see then why, in arctic climates, where the sun once risen never
sets again during the entire summer, vegetation should almost rush up
from the frozen soil—the green leaf is ever gaining from the air and
never losing, ever taking in and never giving off carbonic acid, since
no darkness ever interrupts or suspends its labours.
How beautiful, too, does the contrivance of the expanded leaf appear!
The air contains only one gallon of carbonic acid in 2500, and this
proportion has been adjusted to the health and comfort of animals to
whom this gas is hurtful. But to catch this minute quantity, the tree
hangs out thousands of square feet of leaf in perpetual motion, through
an ever-moving air; and thus, by the conjoined labours of millions of
pores, the substance of whole forests of solid wood is slowly extracted
from the fleeting winds. The green stem of the young shoot, and the
green stalks of the grasses, also absorb carbonic acid as the green of
the leaf does, and thus a larger supply is afforded when the growth is
most rapid, or when the short life of the annual plant demands much
nourishment within a limited time.
SECTION II.—OF THE GROWTH AND SUBSTANCE OF PLANTS.
In this way the perfect plant derives its food from the soil and
from the air; but perfect plants arise from seeds; and the study of
the entire life—the career, so to speak—of a plant, presents many
interesting and instructive subjects of consideration.
When a portion of flour is made into dough, and this dough is kneaded
with the hand under a stream of water upon a fine sieve, as long as the
water passes through milky, there will remain on the sieve a glutinous
sticky substance resembling birdlime, while the milky water will
gradually deposit a pure white powder. This powder is _starch_, that
which remains on the sieve is _gluten_. Both of these substances exist,
therefore, in the flour; they both also exist in the grain. The starch
consists of carbon, hydrogen, and oxygen only; the gluten, in addition
to these, contains also nitrogen.
When ground into flour, these substances serve for food to man; in
the unbruised grain they are intended to feed the future plant in its
earliest infancy.
When a seed is committed to the earth, if the warmth and moisture are
favourable, it begins to sprout. It pushes a shoot upwards, it thrusts
a root downwards, but, until the leaf expand, and the root has fairly
entered the soil, the young plant derives no nourishment other than
water, either from the earth or from the air. It lives on the starch
and gluten contained in the seed. But these substances, though capable
of being separated from each other by means of water, as above stated,
yet are neither of them soluble in water. Hence, they cannot, without
undergoing a previous change, be taken up by the sap, and conveyed
along the pores of the young shoot they are destined to feed. But it is
so arranged that, when the seed first shoots, there is produced at the
base of the germ, from a portion of the gluten, a small quantity of a
substance (diastase) which has so powerful an effect upon the starch
as immediately to render it soluble in the sap, which is thus enabled
to take it up and convey it by degrees, just as it is wanted, to the
shoot or to the root.[5] As the sap ascends, it becomes sweet,—the
starch thus dissolved changes into sugar. When the shoot first becomes
tipped with green, the sugar is again changed into the woody fibre, of
which the stem of perfect plants chiefly consists. By the time that the
food contained in the seed is exhausted,—often, as in the potato, long
before,—the plant is able to live by its own exertions, at the expense
of the air and the soil.
[5] In malting barley, it is made to sprout a certain length, and the
growth is then arrested by heating and drying it. Mashed barley, before
sprouting, will not dissolve in water, but when sprouted, the whole of
the starch (the flour) it contains dissolves readily by a gentle heat.
The _diastase_ formed during the germination effects this. By further
heating in the brewer’s wort, this starch is converted into sugar as it
is in the growing plant.
This change of the sugar of the sap into woody fibre is observable
more or less in all plants. When they are shooting fastest the sugar
is most abundant; not, however, in those parts which are growing, but
in those which convey the sap to the growing parts. Thus the sugar of
the ascending sap of the maple and the alder disappears in the leaf
and in the extremities of the twig; thus the sugar-cane _sweetens_
only a certain distance above the ground, up to where the new growth
is proceeding; and thus also the young beet and turnip abound most in
sugar, while in all these plants the sweet principle diminishes as the
year’s growth draws nearer to a close.
In the ripening of the ear also, the sweet taste, at first so
perceptible, gradually diminishes and finally disappears; the sugar of
the sap is here changed into the _starch_ of the grain, which, as above
described, is afterwards destined, when the grain begins to sprout, to
be reconverted into sugar for the nourishment of the rising germ.
In the ripening of fruits a different series of changes presents
itself. The fruit is first tasteless, then becomes sour, and at last
sweet. In this case the acid of the unripe is changed into the sugar of
the ripened fruit.
The substance of plants,—their solid parts that is—consist chiefly of
_woody fibre_, the name given to the fibrous substance, of which wood
evidently consists. It is interesting to inquire how this substance
can be formed from the compounds, carbonic acid and water, of which the
food of plants in great measure consists. Nor is it difficult to find
an answer.
It will be recollected that the leaf drinks in carbonic acid from the
air, and delivers back its oxygen, retaining only its carbon. It is
also known that water abounds in the sap. Hence carbon and water are
thus abundantly present in the pores or vessels of the green leaf. Now,
woody fibre _consists only of carbon and water_ chemically combined
together,—100 lbs. of dry woody fibre consisting of 50 lbs. of carbon
and 50 lbs. of water. It is easy, therefore, to see how, when the
carbon and water meet in the leaf, woody fibre may be produced by their
mutual combination.
If, again, we inquire how this important principle of plants may be
formed from the other substances, which enter by their roots, from the
ulmic acid, for example, the answer is equally ready. This acid also
consists of carbon and water only, 50 lbs. of carbon with 37½ of water
forming ulmic acid, so that when it is introduced into the sap of the
plant, all the materials are present from which the woody fibre may be
produced.
Nor is it more difficult to see how starch may be converted into sugar,
and this again into woody fibre; or how, again, sugar may be converted
into starch in the ear of corn, or woody fibre into sugar during the
ripening of the winter pear after its removal from the tree. _Any one
of these substances may be represented by carbon and water only._ Thus,—
50 lbs. of carbon with 50 of water, make 100 of woody fibre.
50 lbs. 37½ 87½ of ulmic acid.
{ of cane sugar,
50 lbs. 72½ 122½ { of starch, or
{ of gum.
50 lbs. 56 106 of vinegar.
In the interior of the plant, therefore, it is obvious that, whichever
of these substances be present in the sap, the elements are at hand out
of which any of the others may be produced. In what way they really
are produced, the one from the other, and by what circumstances these
transformations are favoured, it would lead into too great detail to
attempt here to explain.[6]
We cannot help admiring to what varied purposes in nature the
same elements are applied, and from how few and simple materials,
substances, the most varied in their properties, are in the living
vegetable daily produced.
[6] For fuller and more precise explanations on these interesting
topics, see the Author’s LECTURES _on Agricultural Chemistry and
Geology_, Part I.
CHAPTER IV.
Of the Inorganic Constitution of Plants—Their
immediate Source—Their Nature—Quantity of each in
certain common Crops.
SECTION I.—SOURCE OF THE EARTHY MATTER OF PLANTS—SUBSTANCES OF WHICH IT
CONSISTS.
When plants are burned, they always leave more or less of ash behind.
This ash varies in quantity in different plants, in different parts of
the same plant, and sometimes in different specimens of the same kind
of plant, especially if grown upon different soils; yet it is never
wholly absent. It seems as necessary to their existence in a state of
perfect health as any of the elements which constitute the organic or
combustible part of their substance. They must obtain it therefore
along with the food on which they live: it is in fact a part of their
natural food, since without it they become unhealthy. We shall speak of
it therefore as the _inorganic food_ of plants.
We have seen that all the elements which are necessary to the
production of the woody fibre, and of the other organic parts of the
plant, may be derived either from the air, from the carbonic acid
and watery vapour taken in by the leaves, or from the soil, through
the medium of the roots. In the air, however, only rare particles of
inorganic or earthy matter are known to float, and these in a solid
form, so as to be unable to enter by the leaves; the earthy matter
which constitutes the ash, therefore, must be all derived from the soil.
The earthy part of the soil, therefore, serves a double use. It is not
merely, as some have supposed, a substratum in which the plant may so
fix and root itself, as to be able to maintain its upright position
against the force of winds and tempests; but it is a storehouse of
food also, from which the roots of the plant may select such earthy
substances as are necessary to, or are fitted to promote, its growth.
The ash of plants consists of a mixture of several, sometimes of as
many as eleven, different earthy substances. These substances are the
following:—
1. _Potash._—The common pearl-ash of the shops is a compound of potash
with carbonic acid; it is a _carbonate of potash_. By dissolving the
pearl-ash in water, and boiling it with quicklime, the carbonic acid is
separated, and potash alone, or caustic potash, as it is often called,
is obtained.
2. _Soda._—The common soda of the shops is a carbonate of soda, and by
boiling it with quicklime, the carbonic acid is separated, as in the
case of pearl-ash.
3. _Lime._—This is familiar to every one as the lime-shells, or
unslaked lime of the limekilns. The unburned limestone is a _carbonate
of lime_; the carbonic acid in this case being separated by the
roasting in the kiln.
4. _Magnesia._—This is the calcined magnesia of the shops. The
uncalcined is a _carbonate of magnesia_, from which heat drives off the
carbonic acid.
5. _Silica._—This is the name given by chemists to the substance of
flint, quartz, and of siliceous sands and sandstones.
6. _Alumina_ is the pure earth of alum, obtained by dissolving alum in
water, and adding liquid ammonia (hartshorn) to the solution. It forms
about two-fifths of the weight of porcelain and pipe-clays, and of some
other very stiff kinds of clay.
7. _Oxide of Iron._—The most familiar form of this substance is the
rust that forms on metallic iron in damp places. It is a compound of
iron with oxygen, hence the name _oxide_.
8. _Oxide of Manganese_ is a brown powder, which consists of oxygen
in combination with a metal resembling iron, to which the name of
manganese is given. It exists in plants, and in soils only in very
small quantity.
9. _Sulphur._—This substance is well known. It generally exists in
the ash in the state of _sulphuric acid_ (oil of vitriol), which is a
compound of sulphur with oxygen. It does not always exist in living
plants, however, in this state.
_Sulphuric acid_ forms with potash a _sulphate of potash_,—with soda,
_sulphate of soda_ (or Glauber’s salts),—with lime, _sulphate of lime_
(gypsum),—with magnesia, _sulphate of magnesia_ (Epsom salts),—with
alumina, _sulphate of alumina_,—and with oxide of iron, _sulphate of
iron_ or green vitriol. When the sulphate of potash is combined with
sulphate of alumina, it forms common alum.
10. _Phosphorus_ is a soft pale yellow substance which readily takes
fire in the air, and gives off, while burning, a dense white smoke. The
white fumes which form this smoke are a compound of phosphorus with
oxygen obtained from the air, and are called _phosphoric acid_. In the
ash of plants the phosphorus is found in the state of phosphoric acid,
though it probably does not all exist in the living plant in that state.
_Phosphoric acid_ forms _phosphates_ with potash, soda, lime, and
magnesia. When bones are burned, a large quantity of a white earth
remains (bone-earth), which is a _phosphate of lime_, consisting of
lime and phosphoric acid. Phosphate of lime is generally present in the
ash of plants; phosphate of magnesia is contained most abundantly in
the ash of wheat and other varieties of grain.
11. _Chlorine._—This is a very suffocating gas, which gives its
peculiar smell to chloride of lime, and is used for bleaching and
disinfecting. It is readily obtained by pouring muriatic acid (spirit
of salt) on the black oxide of manganese of the shops. In combination
with the metallic bases of potash, soda, lime, and magnesia, it forms
the _chlorides_ of potassium, sodium (common salt), calcium and
magnesium,[7] and in one or other of these states it generally enters
into the roots of plants, and exists in their ash.
[7] Potash, soda, lime, and magnesia, are compounds of the metals here
named with oxygen. It is a very striking fact, that the suffocating
gas chlorine, when combined with sodium, a metal which takes fire when
placed upon water, should form the agreeable and necessary condiment,
_common salt_.
Such are the inorganic substances usually found mixed or combined
together in the ash of plants. It has already been observed, that the
quantity of ash left by a given weight of vegetable matter varies
with a great many conditions. This fact deserves a more attentive
consideration.
SECTION II.—OF THE DIFFERENCE IN THE QUANTITY OF ASH.
1. The quantity of ash yielded by _different plants_ is unlike. Thus
1000 lbs. of
Wheat leave 12 lbs.
Oats 26 lbs.
Turnips 8 lbs.
Red Clover 16 lbs.
Rye Grass 17 lbs.
Barley 25 lbs.
Potatoes 8 lbs.
Carrots 7 lbs.
White Clover 17 lbs.
So that the quantity of inorganic food required by different vegetables
is greater or less according to their nature; and if a soil be of such
a kind that it can yield only a small quantity of this inorganic food,
then only those plants will grow well upon it which require the least.
Hence, trees may often grow where arable crops fail to thrive, because
many of them require and contain very little inorganic matter. Thus
while 1000 lbs. of elm wood leave 19 lbs. and of poplar 20 lbs. of
ash, the same weight of the willow leaves only 4½ lbs., of the beech 4
lbs., of the birch 3½ lbs., of different pines less than 3 lbs., and of
the oak only 2 lbs. of ash when burned.
2. The quantity of inorganic matter varies in _different parts of the
same plant_. Thus while 1000 lbs. of the turnip root sliced and dried
in the air leave 70 lbs. of ash, the dried leaves give 130 lbs.; and
while the grain of wheat yields only 12 lbs., wheat straw will yield
60 lbs. of earthy matter. So, though the willow and other _woods_
leave little ash, as above stated, yet the willow leaf leaves 82 lbs.,
the beech leaf 42 lbs., the birch 50 lbs., the different pine leaves
20 lbs. to 30 lbs., and the leaves of the elm as much as 120 lbs. of
incombustible matter when burned in the air.
Most of the inorganic matter, therefore, which is withdrawn from
the soil in a crop of corn is returned to it again, by the skilful
husbandman, in the fermented straw,—in the same way as nature, in
causing the trees periodically to shed their leaves, returns with them
to the soil a very large portion of the soluble inorganic substances
which had been drawn from it by the roots during the season of growth.
Thus an annual top-dressing is given to the land where forests grow;
and that which the roots from spring to autumn are continually sucking
up, and carefully collecting from considerable depths, winter strews
again on the surface, so as, in the lapse of time, to form a soil
which cannot fail to prove fertile,—because it is made up of those
very materials of which the inorganic substance of former races of
vegetables has been entirely composed.
2. The quantity of inorganic matter often differs in _different
specimens of the same plant_. Thus, 1000 lbs. of wheat straw, grown at
different places, gave to four different experimenters 43, 44, 35, and
155 lbs. of ash respectively. Wheat straw, therefore, does not always
leave the same quantity of ash.
To what is this difference owing? Is it to the nature of the soil, or
does it depend upon the _variety_ of wheat experimented upon? It seems
to depend partly upon both. Thus, on the same field, in Ravensworth
dale, Yorkshire, on a rich clay soil abounding in lime, the _Golden
Kent_ and _Flanders Red_ wheats were sown in the spring of 1841. The
former gave an excellent crop, while the latter was a total failure,
the ear containing 20 or 30 grains only of poor wheat. The straw of the
former left 165 lbs. of ash from 1000 lbs., that of the latter only 120
lbs. Something, therefore, depends upon the variety. But as from the
straw of a good wheat crop grown near Durham this last summer on a clay
loam I obtained only 66 lbs. of ash, I am persuaded that the very wide
variations in the quantity of ash left, by different wheat straws, must
be dependent in some considerable degree upon the soil.
The truth, so far as it can as yet be made out, seems to be this—that
every plant must have a certain quantity of inorganic matter to make it
grow in the most healthy manner;—that it is capable of living, growing,
and even ripening seed with very much less than this quantity;—but
that those soils will produce the most perfect plants which can best
supply all their wants,—and that the best seed will be raised in those
districts where the soil, without being too rich or rank, yet can yield
both organic and inorganic food in such proportions as to maintain the
corn plants in their most healthy condition.
SECTION III.—OF THE QUALITY OF THE ASH OF PLANTS.
But much also depends upon the _quality_ as well as upon the _quantity_
of the ash. Plants may leave the same weight of ash when burned, and
yet the nature of the two specimens of ash, the kind of matter of
which they respectively consist, may be very different. The ash of one
may contain much lime, of another much potash, of a third much soda,
while in a fourth much silica may be present. Thus 100 lbs. of the ash
of _bean_ straw contain 53½ lbs. of potash, while that of _barley_
straw contains only 3½ lbs. in the hundred; and, on the other hand, the
same weight of the ash of the latter contains 73½ lbs. of silica, while
in that of the former there are only 7½ lbs.
The quality of the ash seems to vary with the same conditions by which
its quantity is affected. Thus—
1. It varies with the kind of plant. 100 lbs. of the ash of wheat,
barley, and oats, for example, contain, respectively,
Wheat. Barley. Oats.
Potash, 19 12 6
Soda, 20½ 12 5
Lime, 8 4½ 3
Magnesia, 8 8 2½
Alumina, 2 1 ½
Oxide of Iron, 0 trace. 1½
Silica, 34 50 76½
Sulphuric acid, 4 2½ 1½
Phosphoric acid, 3½ 9 3
Chlorine, 1 1 ½
--------- --------- ---------
100 100 100
A comparison of the several numbers opposite to each other in these
three columns, shews how unlike the quantities of the different
substances are, which are contained in an equal weight of the ash of
these three varieties of grain. The ash of wheat contains 19 lbs. of
potash in the 100 lbs., while that of oats contains only 6 lbs. In
wheat are 20½ per cent. of soda, in oats only 5 per cent. Wheat also
contains more sulphuric acid than either of the other grains, while
barley contains a still greater predominance of phosphoric acid.
It is thus evident that a crop of wheat will carry off from the
soil—even suppose the whole _quantity_ of ash left by each the same in
weight—very different quantities of potash, soda, &c. from a crop of
oats. It will take more of these, of sulphuric acid, and of certain
other substances, from the soil. It will, therefore, exhaust the soil
more of _these_ substances—as barley and oats will of others—hence
_one_ reason why a piece of land may suit one of these crops and not
suit the others. That which cannot grow wheat may yet grow oats.
Hence, also, two successive crops of _different_ kinds of grain may
grow where it would greatly injure the soil to take two in succession
of the _same_ kind, especially of either wheat or barley; and hence
we likewise deduce one natural reason for a rotation of crops. The
surface soil may be so far exhausted of one inorganic substance, that
it cannot afford it in sufficient quantity during the present season
to bring a given crop to healthy maturity, and yet may, by natural
processes, be so far supplied again, during the intermediate growth
of certain other crops, as to be prepared in a future season fully to
supply all the wants of the same crop, and to yield a plentiful harvest.
2. The kind of inorganic matter varies with the _part_ of the plant.
Thus the grain and the straw of the corn plants contain very unlike
quantities of the several inorganic constituents, as will appear by
comparing the following with the preceding table:—
Wheat Barley Oat
Straw. Straw. Straw.
Potash, ½ 3½ 15
Soda, ¾ 1 trace.
Lime, 7 10½ 2¾
Magnesia, 1 1½ ½
Alumina, 2¾ 3 trace.
Oxide of Iron, }
Oxide of manganese, } 0 ½ trace.
Silica, 81 73½ 80
Sulphuric acid, 1 2 1½
Phosphoric acid, 5 3 ¼
Chlorine, 1 1½ trace.
------- ------- --------
100 100 100
Not only are the quantities of the several inorganic substances kinds
of straw very unlike—especially the proportions of potash, lime, and
phosphoric acid in each—but these quantities are also very different
from those exhibited by the numbers in the preceding table as contained
in the three varieties of grain. In this difference we see, further,
_one_ reason why the same soil which may be favourable to the growth
of straw may not be equally propitious to the growth of the ear. Wheat
straw contains little either of potash or of soda; the ash of the
grain contains a large proportion; while the ash of the oat-straw,
on the other hand, contains a much larger proportion of potash than
that of its own ear does. It is clear, therefore, that the roots may,
in certain plants and in certain soils, succeed in fully nourishing
the straw while they cannot fully ripen the ear; or contrariwise,
where they feed but a scanty straw, may yet _be able_ to give ample
sustenance to the filling ear.[8]
[8] And occasionally _do_ give; for a plump grain, and even a
well-filled ear, are not unfrequently found where the straw is
unusually deficient.
3. The quality of the ash varies also with the soil in which it grows.
This will be understood from what is stated above. Where the soil is
favourable, the roots can send up into the straw every thing which the
healthy plant requires; when it is poorly supplied with some of those
inorganic constituents which the plant desires, life may be prolonged,
a stunted or unhealthy crop may be raised, in which the kind, and
perhaps the quantity, of ash left in burning will necessarily be
different from that left by the same species of plant grown under more
favouring circumstances. Of this fact there can be no doubt, though
the extent to which such variations may take place without absolutely
killing the plant, has not yet been by any means made out.
4. It varies also with the period of a plant’s growth, or the season at
which it is reaped. Thus, in the young leaf of the turnip and potato,
a greater proportion of the inorganic matter they contain consists of
potash than in the old leaf. The same is true of the stalk of wheat;
and similar differences prevail in almost every kind of plant at
different stages of its growth.
The enlightened agriculturist will perceive that all the facts above
stated have a perceptible connection with the ordinary processes of
practical agriculture, and tend to throw considerable light on some of
the principles by which they ought to be regulated. One illustration of
this is exhibited in the following section.
SECTION IV.—QUANTITY OF INORGANIC MATTER CONTAINED IN AN ORDINARY CROP
OR SERIES OF CROPS.
The importance of the inorganic matter contained in living vegetables,
or in vegetable substances when reaped and dry, will appear more
distinctly if we consider the actual quantity carried off from the soil
in a series of crops.
In a four-years’ course of cropping, in which the crops gathered amount
per acre to—
1st year, _Turnips_, 25 tons of bulbs, and 7 tons of tops.
2d year, _Barley_, 38 bushels of 63 lbs. each,
and 1 ton of straw.
3d year, _Clover and Rye Grass_, 1 ton of each in hay.
4th year, _Wheat_, 25 bushels of 60 lbs., and 1¾ tons of straw.
The quantity of inorganic matter carried off in the four crops,
supposing none of them to be eaten on the land, amounts to—
Potash, 281 lbs.
Soda, 130 ”
Lime, 242 ”
Magnesia, 42 ”
Alumina, 11 ”
Silica, 318 ”
Sulphuric acid, 111 ”
Phosphoric acid, 61 ”
Chlorine, 39 ”
--------
Total, 1240
or, in all, about 11 cwt.—of which gross weight the different
substances form very unlike proportions.
A still clearer idea of these quantities will be obtained by a
consideration of the fact, that if we carry off the entire produce, and
return none of it again in the shape of manure, we must or ought in its
stead, if the land is to be restored to its original condition, add to
each acre every four years:—
Pearl or Potash, 390 lbs. at a cost of _l._3 10 0
Crystallized carbonate of soda, 440 2 5 0
Common salt, 65 0 2 0
Quick (burned) lime, 240 0 1 0
Epsom salts, 250 1 5 0
Alum, 84 0 8 0
Bone-dust, 260 0 16 0
------ ---------
Total, 1729 _l._8 7 0
Several observations suggest themselves from a consideration of the
above statements: _first_, that if this inorganic matter be really
necessary to the plant, the gradual and constant removal of it from the
land ought by-and-by to impoverish the soil of this inorganic food;
_second_, that the more of what grows upon the land we can again return
to it in manure, the less will this deterioration be perceptible;
_third_, that as many of these inorganic substances are readily soluble
in water, the liquid manure of the farm-yard, so often allowed to run
to waste, carries with it to the rivers much of the saline matter
that ought to be returned to the land; and, _lastly_, that the utility
and often indispensable necessity of certain artificial manures is
owing, it may be, in some districts, to the natural poverty of the
land in certain inorganic substances,—but more frequently to a want
of acquaintance with the facts above stated, among practical men, and
to the long continued neglect and waste which has been the natural
consequence.
In certain districts, the soil and subsoil contain within themselves
an almost unfailing supply of some of these inorganic substances, so
that the waste is long in being felt; in others they become sooner
exhausted, and hence call for more care, and, when exhausted, for a
more expensive cultivation, in order to replace them.
One thing is of essential importance to be remembered by the practical
farmer—that the deterioration of land is often an exceedingly slow
process. In the hands of successive generations a field may so
imperceptibly become less valuable, that a century even may elapse
before the change prove such as to make a sensible diminution in the
valued rental. Such slow changes, however, have been seldom recorded;
and hence the practical man is occasionally led to despise the clearest
theoretical principles, because he has not happened to see them
verified in his own limited experience, and to neglect therefore the
suggestions and the wise precautions which these principles lay before
him.
The agricultural history of tracts of land of different qualities,
shewing how they had been cropped and tilled, and the average produce
in grain, hay, straw, and other crops, every five years, during an
entire century, would be invaluable materials both to theoretical and
to practical agriculture.
CHAPTER V.
Of Soils—their Organic and Inorganic Portions—Saline
Matter in Soils—Examination and Classification of
Soils—Diversities of Soils and Subsoils.
Soils consist of two parts,—of an _organic_ part, which can readily be
burnt away when the soil is heated to redness; and of an _inorganic_
part, which is fixed in the fire, and which consists entirely of earthy
and saline substances.
SECTION I.—OF THE ORGANIC PART OF SOILS.
The organic part of soils is derived chiefly from the remains of
vegetables and animals which have lived and died in or upon the soil,
which have been spread over it by rivers and rains, or which have been
added by the hand of man for the purpose of increasing its natural
fertility.
This organic part varies very much in quantity in different soils. In
some, as in peaty soils, it forms from 50 to 70 per cent. of their
whole weight, and even in some rich long cultivated lands it has been
found, in a few rare cases, to amount to as much as 25 per cent. In
general, however, it is present in much smaller proportion, even in our
best arable lands. Oats and rye will grow upon a soil containing only
1½ per cent., barley when 2 to 3 are present, while good wheat soils
generally contain from 4 to 8 per cent. In stiff and very clayey soils
10 to 12 per cent. may occasionally be detected. In very old pasture
lands and in gardens, vegetable matter occasionally accumulates, so as
to overload the upper soil.
To this organic matter in the soil the name of _humus_ has been given
by some writers. It contains or yields to the plant the ulmic and
humic acids described in a previous chapter. It supplies also, by
its decay, in contact with the air which penetrates the soil, much
carbonic acid, which is supposed to enter the roots and minister to the
growth of living vegetables. During the same decay ammonia is likewise
produced,—and in larger quantity, if animal matter be present in
considerable abundance,—which ammonia is found to promote vegetation
in a remarkable manner. Other substances, more or less nutritious,
are also formed from it in the soil. These enter by the roots, and
contribute to nourish the growing plant, though the extent to which
it is fed from this source is dependent, both upon the abundance with
which these substances are supplied, and upon the nature of the plant
itself, and of the climate in which it grows.
Another influence of this organic portion of the soil, whether
naturally formed in it, or added to it as manure, is not to be
neglected. It contains,—as we have seen that all vegetable substances
do,—a considerable quantity of inorganic, that is, of saline and earthy
matter, which is liberated as the organic part decays. Thus living
plants derive from the remains of former races buried beneath the
surface, a portion of that inorganic food which can only be obtained
in the soil,—and which, if not thus directly supplied, must be sought
for by the slow extension of their roots through a greater depth and
breadth of the earth in which they grow. The addition of manure to the
soil, therefore, places within the easy reach of the roots not only
organic but inorganic food also.
SECTION II.—OF THE INORGANIC PART OF SOILS.
The inorganic part of soils,—that which remains behind, when every
thing combustible is burned away by heating it to redness in the open
air,—consists of two portions, one of which is _soluble_ in water, the
other _insoluble_. The soluble consists of _saline_ substances, the
insoluble of _earthy_ substances.
1. _The saline or soluble portion._—In this country the surface soil
of our fields, in general, contains very little soluble matter. If a
quantity of soil be dried in an oven, a pound weight of it taken, and
a pint and a half of pure boiling rain-water poured over it, the whole
well stirred and allowed to settle,—the clear liquid, when poured off
and boiled to dryness, may leave from 2 to 20 grains of saline matter.
This saline matter will consist of common salt, gypsum, sulphate of
soda (Glauber’s salts), sulphate of magnesia (Epsom salts), with
traces of the chlorides of calcium, magnesium, and potassium, and of
the nitrates of potash, soda, and lime.[9] It is from these soluble
substances that the plants derive the greater portion of the saline
ingredients contained in the ash they leave when burned.
[9] See pages 51 and 52, where these substances are described.
Nor must the quantity thus obtained from a soil be considered too small
to yield the whole supply which a crop requires. A single grain of
saline matter in every pound of a soil a foot deep, is equal to 500
lbs. in an acre, which is more than is carried off from the soil in
10 rotations (40 years), where only the wheat and barley are sent to
market, and the straw and green crops are regularly returned to the
land in the manure.[10]
[10] A further portion, it will be recollected, is carried off in the
cattle that are sent to market,—this is here neglected.
In some countries, indeed in some districts of our own country, the
quantity of saline matter in the soil is so great, as in hot seasons to
form a distinct incrustation on the surface. This may often be seen in
the neighbourhood of Durham; and is more especially to be looked for
in districts where the subsoil is sandy and porous, and more or less
full of water. In hot weather the evaporation on the surface causes
the water to ascend from the porous subsoil: and as this water always
brings with it a quantity of saline matter,—which it leaves behind when
it rises in vapour,—it is evident that the longer the dry weather and
the consequent evaporation from the surface continue, the thicker the
incrustations will be, or the greater the accumulation of saline matter
on the surface. Hence, where such a moist and porous subsoil exists in
countries rarely visited by rain, as in the plains of Peru, of Egypt,
or of India, the country is whitened over in the dry season with an
unbroken covering of the different saline substances above mentioned.
When rain falls, the saline matter is dissolved, and descends again to
the subsoil,—in dry weather it reascends. Thus the surface soil of any
field will contain a larger proportion of soluble inorganic matter in
the middle of a hot season than in one of even ordinary rain; and hence
the fine dry weather which, in early summer, hastens the growth of
corn, and later in the season favours its ripening, does so, among its
other modes of action, by bringing up to the roots from beneath a more
ready supply of those saline compounds which the crop requires for its
healthful growth.
2. _The earthy or insoluble portion._—The earthy or insoluble portion
of soils rarely constitutes less than 95 lbs. in a hundred of their
whole weight. It consists chiefly of _silica_ in the form of _sand_,
of _alumina_ in the form of _clay_, and of _lime_ in the form of
_carbonate of lime_. It is rarely free, however, from one or two per
cent. of oxide of iron; and where the soil is of a red colour, this
oxide is present in a still larger quantity. A trace of magnesia also
may be almost always detected, and a minute quantity of phosphate of
lime. The principal ingredients, however, of the earthy part of all
soils are sand, clay, and lime; and soils are named or classified
according to the quantities of each of these three they may happen to
contain.
If an ounce of soil be boiled in a pint of water till it is perfectly
softened and diffused through it, and, after shaking, the heavy parts
be allowed to settle for a few minutes, the sand will subside, while
the clay—which is in finer particles, and is less heavy—will still
remain floating. If the water and clay be now poured into another
vessel, and be allowed to stand till the water has become clear, the
sandy part of the soil will be on the bottom of the one vessel, the
clayey part on that of the other, and they may be dried and weighed
separately.
If 100 grains of dry soil leave no more than 10 of clay, it is called
a _sandy soil_; if from 10 to 40, a _sandy loam_; if from 40 to 70, a
_loamy soil_; if from 70 to 85, a _clay loam_; from 85 to 95, a _strong
clay soil_; and when no sand is separated at all by this process, it is
a pure _agricultural clay_.
The _strong clay soils_ are such as are used for making tiles and
bricks; the pure _agricultural clay_ is such as is commonly employed
for the manufacture of pipes (pipe-clay).
Soils consist of these three substances _mixed_ together. The pure
_clay_ is a chemical _compound_ of silica and alumina, in the
proportion of about 60 of the former to 40 of the latter. Pure clay
soils rarely occur—it being well known to all practical men, that the
strong clays (tile clays) which contain from 5 to 15 per cent. of
sand, are brought into arable cultivation with the greatest possible
difficulty. It will rarely happen, therefore, that arable land will
contain more than 30 to 35 of alumina.
If a soil contain more than 5 per cent. of carbonate of lime, it is
called a _marl_; if more than 20 per cent., it is a _calcareous_ soil.
_Peaty soils_, of course, are those in which the vegetable matter
predominates very much.
To estimate the lime, a quantity of the soil should be burned in the
air, and a weighed portion, 100 or 200 grains, diffused through half a
pint of cold water mixed with half a wine glassful of spirit of salt
(muriatic acid), and allowed to stand for a couple of hours, with
occasional stirring. The water is then poured off, the soil dried,
heated to redness as before, and weighed: the loss is nearly all
lime.[11]
[11] Unless the soil happen to contain a large quantity of magnesia,
which is rarely the case.
The quantity of vegetable or other organic matter is determined by
drying the soil _well_ upon paper in an oven, and then burning a
weighed quantity in the air: the loss is _nearly all_ organic matter.
In stiff clays this loss will comprise a portion of water, which is
not wholly driven off from such soils by drying upon paper in the way
described.
SECTION III.—OF THE DIVERSITIES OF SOILS AND SUBSOILS.
Though the substances of which soils _chiefly_ consist are so few in
number, yet every practical man knows how very diversified they are
in character—how very different in agricultural value. Thus, in some
of our southern counties, we have a white soil, consisting apparently
of nothing else but chalk; in the centre of England a wide plain of
dark red land; in the border counties of Wales, and on many of our
coal-fields, tracts of country almost perfectly black; while yellow,
white, and brown sands give the prevailing character to the soils of
other districts. Such differences as these arise from the different
proportions in which the sand, lime, clay, and the oxide of iron which
colours the soils, have been mixed together.
But how have they been so mixed—differently in different parts of the
country. By what natural agency?—for what end?
Again, the soil on the surface rests on what is usually denominated the
_subsoil_. This, also, is very various in its character and quality.
Sometimes it is a porous sand or gravel, through which water readily
ascends from beneath or sinks in from above; sometimes it is light and
loamy like the soil that rests upon it; sometimes stiff and impervious
to water.
The most ignorant farmer knows how much the value of a piece of land
depends upon the characters of the surface soil,—the intelligent
improver understands best the importance of a favourable subsoil. “When
I came to look at this farm,” said an excellent agriculturist to me,
“it was spring, and damp growing weather: the grass was beautifully
green, the clover shooting up strong and healthy, and the whole farm
had the appearance of being very good land. Had I come in June, when
the heat had drunk up nearly all the moisture which the _sandy subsoil_
had left in the surface, I should not have offered so much rent for
it by ten shillings an acre.” He might have said also, “Had I taken a
spade, and dug down 18 inches in various parts of the farm, I should
have known what to expect in seasons of drought.”
But how come subsoils thus to differ—one from the other—and from the
surface soil that rests upon them? Are there any principles by which
such differences can be accounted for—by which they can be foreseen—by
the aid of which we can tell what kind of soil may be expected in this
or that district—even without visiting the spot—and on what kind of
subsoil it is likely to rest?
Geology explains the cause of all such differences, and supplies us
with principles by which we can predict the general quality of the soil
and subsoil in the several parts of entire kingdoms;—and where the soil
is of inferior quality and yet susceptible of improvement, the same
principles indicate whether the means of improving it are likely, in
any given locality, to be attainable at a reasonable cost.
It will be proper shortly to illustrate these direct relations of
geology to agriculture.
CHAPTER VI.
Direct relations of Geology to Agriculture—Origin
of Soils—Causes of their Diversity—Relation to
the Rocks on which they rest—Constancy in the
relative Position and Character of the Stratified
Rocks—Relation of this fact to Practical
Agriculture—General Character of the Soils upon
these Rocks.
Geology is that branch of knowledge which embodies all ascertained
facts in regard to the nature and internal structure, both physical and
chemical, of the solid parts of our globe. This science has many close
relations with practical agriculture, and especially throws much light
on the nature and origin of soils,—on the cause of their diversity,—on
the kind of materials by the admixture of which they may be permanently
improved,—and on the sources from which these materials may be derived.
SECTION I.—OF THE ORIGIN OF SOILS.
If we dig down through the soil and subsoil to a sufficient depth, we
always come sooner or later to the solid rock. In many places the rock
actually reaches the surface, or rises in cliffs, hills, or ridges,
far above it. The surface (or crust) of our globe, therefore, consists
everywhere of a solid mass of rock, overlaid with a covering, generally
thin, of loose materials. The upper or outer part of these loose
materials forms the soil.
The geologist has travelled over great part of the earth’s surface,
has examined the nature of the rocks, which everywhere repose beneath
the soil, and has found them to be very unlike in character, in
composition, and in hardness—in different countries and districts. In
some places he has met with a sandstone, in other places a limestone,
in others a slate or hardened rock of clay. But a careful comparison
of all the kinds of rock he has observed, has led him to the general
conclusion, _that they are all either sandstones, limestones, or clays
of different degrees of hardness, or a mixture in different proportions
of two or more of these kinds of matter_.
When the loose covering of earth is removed from the surface of any of
these rocks, and it is left exposed, summer and winter, to the action
of the winds and rains and frosts, it may be seen gradually to crumble
away. Such is the case even with many of those which, on account of
their greater hardness, are employed as building-stones, and are kept
generally dry; how much more with such as are less hard, and, beneath
a covering of moist earth, are continually exposed to the action of
water. The natural crumbling of a naked rock thus gradually covers
it with loose materials, in which seeds fix themselves and vegetate,
and which eventually forms a soil. The soil thus produced partakes
necessarily of the character of the rock on which it rests, and to the
crumbling of which it owes its origin. If the rock be a sandstone the
soil is sandy; if a claystone, it is a more or less stiff clay; if a
limestone, it is more or less calcareous; and if the rock consist of
any peculiar mixture of those three substances, a similar mixture is
observed in the earthy matter into which it has crumbled.
Led by this observation, the geologist, after comparing the rocks
of different countries with one another, compared next the soils of
various districts with the rocks on which they immediately rest. The
_general_ result of this comparison has been, that in almost every
country the soils have as close a resemblance to the rocks beneath
them—as the loose earth derived from the crumbling of a rock before
our eyes, bears to the rock of which it lately formed a part. The
conclusion therefore is irresistible, that soils, generally speaking,
have been formed by the crumbling or decay of the solid rocks,—that
there was a time when these rocks were uncovered by any loose
materials,—and that the accumulation of soil has been the slow result
of the natural degradation (wearing away) of the solid crust of the
globe.
SECTION II.—CAUSE OF THE DIVERSITY OF SOILS.
The cause of the diversity of soils in different districts, therefore,
is no longer obscure. If the subjacent rocks in two localities differ,
the soils met with there must differ also, and in an equal degree.
But why, it may be asked, do we find the soil in some countries
uniform, in mineral[12] character and general fertility, over hundreds
or thousands of square miles, while in others it varies from field to
field,—the same farm often presenting many well marked differences both
in mineral character and in agricultural value? The cause of this is to
be found in the mode in which the different rocks are observed to lie,
one upon or by the side of the other.
[12] That is, containing the same general proportions of sand, clay,
lime, &c., or coloured red by similar quantities of oxide of iron.
Geologists distinguish rocks into two classes, the _stratified_ and the
_unstratified_. The former are found lying over each other in separate
beds or _strata_, like the leaves of a book, when laid on its side,
or like the layers of stones in the wall of a building; the latter
form hills, mountains, or sometimes ridges of mountains, consisting
of one more or less solid mass of the same material, in which no
layers or strata are any where distinctly perceptible. Thus, in the
following diagram, (No. 1), A and B represent _unstratified_ masses,
in connection with a series of _stratified_ deposits, 1, 2, 3, lying
over each other in a horizontal position. On A one kind of soil will be
formed, on C another, on B a third, and on D a fourth,—the rocks being
all different from each other.
[Illustration: NO. 1.]
If from A to D be a wide valley of many miles in extent, the undulating
plain at the bottom of the valley, resting in great part on the same
rock (2), will be covered by a similar soil. On B the soil will be
different for a short space; and again at C, and on the first ascent to
A, where the rock (3) rises to the surface. In this case the stratified
rocks lie horizontally; and it is the undulating nature of the country
which, bringing different kinds of rock to the surface, causes a
necessary diversity of soil.
But the degree of _inclination_, which the beds possess, is a more
frequent cause of variation in the characters of the soil in the same
district, and even at shorter distances. This is shewn in the annexed
diagram (No. 2), where A, B, C, D, E, represent the mode in which the
stratified rocks of a district of country not unfrequently occur in
connection with each other.
[Illustration: NO. 2.]
Proceeding from E in the plain, the soil would change when we came upon
the rock D, but would then continue uniform till we reached the layer
C. Each of these layers may stretch over a comparatively level tract of
perhaps hundreds of miles in extent. Again, on climbing the hill-side,
another soil would present itself, which would not change till we
arrived at B. Then, however, we begin to walk over the edges of the
beds, and the soil may vary with every new _stratum_ (or bed) we pass
over, till we gain the ascent to A, where the beds are much thinner,
and where, therefore, still more frequent variations may present
themselves.
Everywhere over the British islands valleys are hollowed out, as in
the former of these diagrams (No. 1), by which the rocks beneath are
exposed, and differences of soil produced,—or the beds are more or less
inclined, as in the latter diagram (No. 2), causing still more frequent
variations of the land to appear. By a reference to these facts, nearly
all the _great_ diversities which the soils of the country present may
be satisfactorily accounted for.
SECTION III.—OF THE CONSTANCY IN THE CHARACTER AND ORDER OF SUCCESSION
OF THE STRATIFIED ROCKS.
Another fact alike important to agriculture and to geology, is the
natural order or mode of arrangement in which the stratified rocks
are observed to occur in the crust of the globe. Thus, if 1, 2, 3, in
diagram No. 1 represent three different kinds of rock, a limestone, for
example, a sandstone, and a hard clay rock (a shale or slate), lying
over each other, in the order here represented; then, in whatever part
of the country nay, in whatever part of the world, these same rocks are
met with, they will always be found in the same relative position. The
bed 2 or 3 will never be observed to lie over the bed 1.
This fact is important to geology, because it enables this science to
arrange all the stratified rocks in a certain invariable order,—which
order indicates their relative age or antiquity,—since that which is
lowest, like the lowest layer of stones in the wall of a building, must
generally have been the first deposited, or must be the oldest. It also
enables the geologist, on observing the kind of rock which forms the
surface in any country, to predict at once, whether certain other rocks
are likely to be met with in that country or not. Thus at C (diagram,
No. 1), where the rock (3) comes to the surface, he knows it would be
in vain, either by sinking or otherwise, to seek for the rock (1), the
natural place of which is far above it; while at D he knows that by
sinking he is likely to find either 2 or 3, if it be worth his while to
seek for them.
To the agriculturist this fact is important, among other reasons,—
1. Because it enables him to predict whether certain kinds of rock,
which might be used with advantage in improving his soil, are likely
to be met with within a reasonable distance or at an accessible depth.
Thus if the bed D (diagram No. 2) be a limestone, the instructed farmer
at E knows that it is not to be found by sinking into his own land,
and, therefore, brings it from D; while, to the farmer upon C, it may
be less expensive to dig down to the bed D in one of his own fields,
than to cart it from a distant spot where it occurs on the surface. Or
if the farmer requires clay, or marl, or sand, to ameliorate his soil,
this knowledge of the constant relative position of beds enables him to
say where these materials are to be got, or where they are to be looked
for, and whether the advantage to be derived is likely to repay the
cost of procuring them.
2. It is observed, that when the soil on the surface of each of a
series of rocks, such as C, or D, or E, in the same diagram, is
uniformly bad, it is almost invariably of better quality at the point
where the two rocks meet. Thus C may be dry, sandy, and barren; D may
be cold, unproductive clay; and E a more or less unfruitful limestone
soil: yet at either extremity of the tract D, where the soil is made up
of an admixture of the decayed portions of the two adjacent rocks, the
land may be of average fertility—the sand of C may adapt the adjacent
clay to the growth of turnips, while the lime of E may cause it to
yield large returns of wheat.[13] Thus, to the tenant in looking out
for a farm, or to the capitalist in seeking an eligible investment, a
knowledge of the mutual relations of geology and agriculture will often
prove of the greatest assistance. Yet how little is such really useful
knowledge diffused among either class of men—how little are either
tenants or proprietors guided by it in their choice of the localities
in which they desire to live!
[13] See page 94.
And yet here and there the agricultural practice of more or less
extended districts, if not really founded upon or directed by, is
yet to be explained only by principles such as those I have above
illustrated. I shall mention only one example. The chalk in Yorkshire,
in Suffolk, and in other southern counties, consists of a vast number
of beds, which, taken all together, form a deposit of very great
thickness. Now, the upper beds of the chalk form poor, thin, dry soils,
producing a scanty herbage, and only under the most skilful culture
yielding profitable crops of corn. The lower beds, on the contrary,
are marly; produce a more stiff, tenacious, and even fertile soil;
and are found in a remarkable degree to enrich the soils of the upper
chalk, when laid on as a top-dressing in autumn, and allowed to crumble
under the action of the winter’s frost. Hence in Yorkshire, Wiltshire,
Hampshire, and Kent, where the lower chalk covers the surface, or is
found at no great depth beneath it, it is dug out of the sides of the
hills, or pits are sunk for it, and it is immediately laid upon the
land with great benefit to the soil. But in parts of Suffolk, where the
soil equally rests upon the upper chalk, there is no other chalk in the
neighbourhood, or to be met with at any reasonable depth, which will
materially improve the land. The farmers find it, from long experience,
to be more economical to bring chalk by sea from Kent to lay on their
lands in Suffolk, than to cover them with any portion of the same
material from their own farms. The following imaginary section will
fully explain the fact here mentioned:—
[Illustration: NO. 3.
Suffolk. Mouth of the Thames. Kent.
]
In this diagram 1 represents the London clay; 2, the plastic clay which
is below it; 3, the upper chalk with flints, rising to the surface in
Suffolk; and 4, the lower chalk, without flints, which is too deep to
be reached in Suffolk, but which rises to the surface in Kent,—where it
is abundant, is easily accessible, and whence it is transmitted across
the estuary of the Thames into Suffolk.
3. The further fact that the several stratified rocks are remarkably
constant in their mineral character, renders this knowledge of
the order of relative superposition still more valuable to the
agriculturist. Thousands of different beds are known to geologists
to occur on various parts of the earth’s surface—each occupying its
own unvarying place in the series. Most of these beds also, when they
crumble or are worn down, produce soils possessed of some peculiarity
by which their general agricultural capabilities are more or less
affected,—and these peculiarities may generally be observed in soils
formed from rocks of the same age—that is, occupying the same place in
the series—in whatever part of the world we find them. Hence if the
agricultural geologist be informed that his friend has bought, or is
in treaty for a farm or an estate, and that it is situated upon such
and such a rock, or geological formation, he can immediately give a
very probable opinion in regard to the agricultural value of the soil,
whether the property be in England, in Australia, or in New Zealand. If
he knows the nature of the climate also, he will be able to estimate
with tolerable correctness how far the soil is likely to repay the
labours of the practical farmer,—nay, even whether it is likely to suit
better for arable land or for pasture, and if for arable, what species
of white crops it may be expected to produce most abundantly.
These facts are so very curious, and illustrate so beautifully
the value of geological knowledge—if not to A and B, the holders
or proprietors of this and that small farm, yet to enlightened
agriculturists,—to scientific agriculture in general,—that I shall
explain this part of the subject more fully in a separate section.
To those who are now embarking in such numbers in quest of new
homes in our numerous colonies, who hope to find, if not a more
willing, at least a more attainable soil in new countries, no kind
of agricultural knowledge can at the outset,—I may say, even through
life,—be so valuable as that to which the rudiments of geology will
lead them. Those who prepare themselves the best for becoming farmers
or proprietors in Canada, in New Zealand, or in wide Australia,
yet leave their native land in general without a particle of that
preliminary _practical_ knowledge, which would qualify them to say,
when they reach the land of their adoption, “On this spot, rather than
that,—in this district, rather than that,—will I purchase my allotment,
because, though both appear equally inviting, yet I know from the
geological structure of the country, that here I shall have the more
permanently productive soil; here I am more within reach of the means
of agricultural improvement; here, in addition to the riches of the
surface, my descendants may hope to derive the means of wealth from
mineral riches beneath.” And this oversight has arisen chiefly from
the value of such knowledge not being understood—often from the very
nature of it being unknown, even to otherwise well instructed practical
men. It is not to men well skilled merely in the details of local
farming, and who are therefore deservedly considered as authorities
and good teachers in regard to local or district practice, that we are
to look for an exposition, often not even for a correct appreciation,
of those general principles on which a universal system of agriculture
must be based—without which principles, indeed, it must ever remain
a mere collection of empirical rules, to be studied and laboriously
mastered in every new district we go to—as the traveller in foreign
lands must acquire a new language every successive frontier he passes.
England, the mistress of so many wide and unpeopled lands, over which
the dwellings of her adventurous sons are hereafter to be scattered, on
which their toil is to be expended, and the glory of their motherland
by their exertions to be perpetuated—England should especially
encourage all such learning, and the sons of English farmers willingly
avail themselves of every opportunity of acquiring it.
SECTION IV.—OF GEOLOGICAL FORMATIONS, AND THE GENERAL CHARACTERS OF THE
SOILS THAT REST UPON THEM.
The thousands of beds or strata of which I have spoken as lying one
over the other in the crust of the globe, have, partly for convenience,
and partly in consequence of certain remarkably distinctive characters
observed among them, been separated by geologists into three great
divisions—the _primary_, which are the lowest and the oldest; the
_secondary_, which lie over them; and the _tertiary_, which are
uppermost, and have been most recently formed. The strata, in these
several divisions, have again been subdivided into groups, called
_formations_. The following table exhibits the names and thicknesses
of these formations, and the mineralogical characters of the rocks of
which they severally consist.
I. TERTIARY STRATA.
1. The _London and Plastic clays_, 500 to 900 feet thick, consist of
stiff, almost impervious, dark-coloured clays,—chiefly in pasture.
The lower beds are mixed with sand, and produce an arable soil, but
extensive heaths and wastes rest upon them in Berkshire, Hampshire, and
Dorset.
II. SECONDARY STRATA.
2. _The Chalk_, about 600 feet in thickness, consists in the upper part
(see diagram, No. 3, p. 88) of a purer chalk with layers of flint; in
the lower, of a marly chalk without flints. The soil of the upper chalk
is chiefly in sheep-walks, that of the lower chalk is very productive
of corn.
3. The _Green Sand_, 500 feet thick, consists of 150 feet of clay, with
about 100 feet of sand above, and 250 feet below it. The upper sand
forms a very productive arable soil, and the clay impervious, wet and
cold lands chiefly in pasture. The lower sand is generally unproductive.
It is an important agricultural remark, that where the clay (plastic
clay) comes in contact with the top of the chalk, an improved soil is
produced, and that where the chalk and the green sand mix, extremely
fertile patches of country present themselves. (See pages 86 and 87.)
4. The _Wealden formation_, nearly 1000 feet thick, consists of 400
feet of sand, covered by 300 of clay, and resting upon 250 of marls and
limestones. The clay forms the poor wet pastures of Sussex and Kent. On
the sands below the clay rest heaths and brushwood; but where the marls
and limestones come to the surface, the land is of better quality, and
is susceptible of profitable arable culture.
5. In the _Upper Oolite_, of 600 feet in thickness, we have a bed of
clay (Kimmeridge clay) 500 feet thick, covered by 100 feet of sandy
limestones. The clay lands are difficult and expensive to work, and are
chiefly in old pasture. The sandy limestone soils above the clay are
also poor, but where they rest immediately upon, and are intermixed
with the clay, excellent arable land is produced.
6. The _Middle Oolite_ of 500 feet consists also of a clay (Oxford
clay) dark-blue, adhesive, and nearly 1000 feet thick, covered by 100
feet of limestones and sandstones. These latter produce good arable
land where the lime happens to abound; the clays form close heavy
compact soils, most difficult and expensive to work. The extensive
pasture lands of Bedford, Huntingdon, Northampton, Lincoln, Wilts,
Oxford, and Gloucester, rest chiefly upon this clay, as do also the
fenny tracts of Lincoln and Cambridge.
7. The _Lower or Bath Oolite_, of 500 feet in thickness, consists of
many beds of limestone and sandstone, with about 200 feet of clay in
the centre of the formation. The soils are very various in quality,
according as the sandstone or limestone predominates. The clays are
chiefly in pasture,—the rest is more or less productive, easily worked,
arable land. In Gloucester, Northampton, Oxford, the east of Leicester,
and in Yorkshire, this formation is found to lie immediately beneath
the surface, and a little patch of it occurs also on the south-eastern
coast of Sutherland.
6. The _Lias_ is an immense deposit of blue clay from 500 to 1000 feet
in thickness, which produces cold, blue, unproductive, clay soils. It
forms a long stripe of land from the mouth of the Tees, in Yorkshire,
to Lyme Regis in Dorset. It is chiefly in old, and often very valuable
pasture.
9. The _New Red Sandstone_, though only 500 feet in thickness, forms
the surface of nearly the whole central plain of England, and stretches
north through Cheshire to Carlisle and Dumfries. It consists of red
sandstones and marls,—the soils on which are easily and cheaply worked,
and form some of the richest and most productive arable lands in the
island. In whatever part of the world the red soils of this formation
have been met with, they have been found to possess in general the same
agricultural capabilities.
10. The _Magnesian Limestone_, from 100 to 500 feet in thickness, forms
a stripe of generally poor thin soil from Durham to Nottingham, capable
of improvement as arable land by high farming, but bearing naturally a
poor pasture, intermingled with sometimes magnificent furze.
11. The _Coal Measures_, from 300 to 3000 feet thick, consist of
beds of sandstones and dark-blue shales (hard clays), intermingled
(_interstratified_) with beds of coal. Where the sands come to the
surface, the soil is thin, poor, hungry, sometimes almost worthless.
The shales, on the other hand, produce stiff, wet, almost unmanageable
clays;—not unworkable, yet expensive to work, and requiring draining,
lime, skill, capital, and a zeal for improvement, to be applied to
them, before they can be made to yield the remunerating crops of corn
they are capable of producing.
12. To the _Millstone Grits_ of 600 feet or upwards in thickness the
same remarks apply. They are often only a repetition of the sandstones
and shales of the coal measures, forming in many cases soils still more
worthless. When the sandstones prevail, large tracts lie naked, or
bear a thin and stunted heath; where the shales abound, the naturally
difficult soils of the coal shales again recur. These rocks are
generally found on the outskirts of our coal-fields.
13. The _Mountain Limestone_, 800 to 1000 feet thick, is a hard blue
limestone rock, separated here and there into distinct beds by layers
of sandstones, of sandy slates, or of blue shales like those of the
coal measures. The soil upon the limestone is generally thin, but
produces a naturally sweet herbage. When the limestone and clay (shale)
adjoin each other, arable land occurs, which is naturally productive of
oats, yet, when the climate is favourable, capable of being converted
into good wheat land. In the north of England a considerable tract of
country is covered by these rocks, but in Ireland they form nearly the
whole of the interior of the island.
14. The _Old Red Sandstone_ varies in thickness from 500 to 10,000
feet. It possesses many of the valuable agricultural qualities of the
_new red_, consisting, like it, of red sandstones and marls, which
crumble down into rich red soils. Such are the soils of Brecknock,
Hereford, and part of Monmouth; of part of Berwick and Roxburgh; of
Haddington and Lanark; of southern Perth; of either shore of the
Moray Firth; and of the county of Sutherland. In Ireland, also, these
rocks abound in Tyrone, Fermanagh, and Monaghan; in Waterford, in
Mayo, and in Tipperary. In all these places, the soils they form are
generally the best in their several neighbourhoods, though here and
there,—where the sandstones are harder, more siliceous and impervious
to water,—tracts, sometimes extensive, of heath and bog occur.
III.—PRIMARY STRATA.
15. _The Upper Silurian system_ is nearly 4000 feet in thickness, and
forms the soils over the lower border counties of Wales. It consists
of sandstones and shales, with occasional limestones; but the soils
formed from these beds take their character from the general abundance
of clay. They are cold, usually unmanageable, _muddy_ clays, with
the remarkably inferior agricultural value of which the traveller is
immediately struck, as he passes westward off the red sandstones of
Hereford on to the upper silurian rocks of Radnor.
16. The _Lower Silurian_ rocks are also nearly 4000 feet in thickness,
and in Wales lie to the west of the upper silurian rocks. They consist
of about 2500 feet of sandstone, on which, when the surface is not
naked, barren heaths alone rest.
Beneath these sandstones lie 1200 feet of sandy and earthy limestones,
from the decay of which, as may be seen on the southern edge of
Caermarthen, fertile arable lands are produced.
17. The _Cambrian System_, of many thousand yards in thickness,
consists in great part of clay slates, more or less hard, which often
weather slowly, and almost always produce either poor and thin soils,
or cold, difficultly manageable clays, expensive to work, and requiring
_high farming_ to bring them into profitable arable cultivation.
Cornwall, western Wales, and the mountains of Cumberland, in England;
the high country which stretches from the Lammermuir hills to
Portpatrick, in Scotland; the mountains of Tipperary, and a large tract
on the extreme south of Ireland,—on its east coast, and far inland
from the bay of Dundalk,—are covered by these slate rocks. Patches of
rich, well cultivated land occur here and there on this formation, with
much also that is improvable; but the greater part of it is usurped by
worthless heath and extensive bogs.
18. The _Mica Slate and Gneiss systems_ are of unknown thickness, and
consist chiefly of hard and slaty rocks, crumbling slowly, forming
poor, thin soils, which rest on an impervious rock, and which, from
the height to which this formation generally rises, are rendered more
unproductive by an unpropitious climate. They form extensive heathy
tracts in Perth and Argyle, and on the north and west of Ireland. Here
and there only, in the valleys or sheltered slopes, and by the margins
of the lakes, spots of bright green meet the eye, and patches of a
willing soil, fertile in corn.
A careful perusal of the preceding sketch of the general agricultural
capabilities of the soils formed from the several classes of
stratified rocks, will have presented to the reader many illustrations
of the facts stated in the preceding section; he will have drawn
for himself—to specify a few examples—the following among other
conclusions.
1. That some formations, like the new red sandstone, yield a soil
almost always productive; others, as the coal measures and millstone
grits, a soil almost always _naturally_ unproductive.
2. That good, or better land at least, than generally prevails in a
district, may be expected where two formations or two different kinds
of rock meet,—as when a limestone and a clay mingle their mutual ruins
for the formation of a common soil.
3. That in almost every country extensive tracts of land on certain
formations will be found laid down to natural grass, _in consequence of
the original difficulty and expense of working_. Such are the Lias, the
Oxford, the Kimmeridge, and the London clays. In raising corn, it is
natural that the lands which are easiest and cheapest worked should be
first subjected to the plough; it is not till implements are improved,
skill increased, capital accumulated, and population presses, that the
heavier lands will be rescued from perennial grass, and made to produce
that greatly increased amount of food for both man and beast, which
they are easily capable of yielding.
The turnip soils of Great Britain are in many districts, it may be,
but indifferently farmed; and the state has reason to complain of
much individual neglect of known and certain methods of increasing
their productiveness; but the next _great_ achievement which British
agriculture has to effect, is to subdue the stubborn clays, and to
convert them into what many of them are yet destined to become, the
richest corn-bearing lands in the kingdom.
CHAPTER VII.
Soils of the Granitic and Trap Rocks.—Accumulations
of transported Sands, Gravels, and Clays.—Use
of Geological Maps in reference to Agriculture.
—Physical characters and Chemical constitution
of Soils.—Relation between the nature of the Soil
and the kind of Plants that naturally grow upon it.
It was stated, in the preceding lecture, (see p. 82,) that rocks are
divided by geologists into the stratified and the _unstratified_.[14]
The stratified rocks cover by far the largest portion of the globe, and
thus form a variety of soils, of which a general description has just
been given. The unstratified rocks are of two kinds—the _granites_
and the _trap_ rocks; and as a considerable portion of the area of our
island is covered by them, it will be proper shortly to consider the
peculiar characters of each, and the differences of the soils produced
from them.
[14] The unstratified are often called _crystalline_ rocks, because
they frequently have a glassy appearance, or contain regular crystals
of certain mineral substances; often also _igneous_ rocks, because they
appear all to have been originally in a melted state, or to have been
produced by fire.
SECTION I.—SOILS OF THE GRANITES AND TRAP ROCKS.
1. The _granites_ consist of a mixture, in different proportions, of
three minerals, known by the names of _quartz_, _felspar_, and _mica_.
The latter, however, is generally present in such small quantity, that
in our general description it may be safely left out of view. Granites,
therefore, consist chiefly of quartz and felspar, in proportions which
vary very much, but the former, on an average, constitutes perhaps from
one-third to one-half of the whole.
_Quartz_ has already been described—(see p. 51)—as the substance of
flint, the silica of the chemist. When the granite decays, this portion
of it forms a more or less coarse siliceous sand.
_Felspar_ is a white, greenish, or flesh-coloured mineral, often more
or less earthy in its appearance, but generally hard and brittle, and
sometimes glassy. It is scratched by, and thus is readily distinguished
from, quartz. When it decays, it forms an exceedingly fine clay.
A remarkable difference appears thus to exist, in chemical
constitution, between these two minerals—a difference which must affect
also the soils produced from them. A granite soil, in addition to the
siliceous sand, will consist chiefly of silica, alumina, and potash;
a hornblende soil, in addition to silica and alumina, of much lime,
magnesia, and oxide of iron—of nearly 2½ cwt. of each of these latter
for every ton of decayed rock. A hornblende soil, therefore, contains
more of those inorganic constituents which the plants require for their
healthy sustenance, and therefore will prove more generally productive
than a soil of decayed felspar. But when the two are mixed, as in the
greenstones, the soil must be still more favourable to vegetable life.
The potash and soda, of which the hornblende is nearly destitute, the
felspar is able abundantly to supply; while, by the hornblende are
yielded lime and magnesia, which are known to exercise a remarkable
influence on the progress of vegetation.
2. The _trap_ rocks, comprising the greenstones and basalts, consist
essentially[15] of felspar and _hornblende_ or _augite_. In contrasting
the trap rocks with the granites, it may be stated _generally_, that
while the granites consist of felspar and _quartz_, the traps consist
of felspar and hornblende (or augite). In the traps, both the felspar
and the hornblende are reduced, by the action of the weather to a more
or less fine powder, affording materials for a soil; in the granites
the felspar is the principal source of all the earthy matter they are
capable of yielding. If we compare together, therefore, the chemical
composition of the two minerals (hornblende and felspar), we shall see
in what respect these two varieties of soil ought to differ. Thus they
consist of
Felspar. Hornblende.
Silica, 65 42
Alumina, 18 14
Potash and soda, 17 trace.
Lime, trace. 12
Magnesia, ” 14
Oxide of iron, ” 14½
Oxide of manganese, ” ½
----- -------
100 97
[15] The reader is referred for more _precise_ information to the
author’s “LECTURES,” pp. 377 to 390.
Thus theory shews, that while granite soils may be eminently
unfruitful, trap soils may be eminently fertile. And such is actually
the result of observation and experience in every part of the globe.
Unproductive granite soils cover nearly the whole of Scotland north
of the Grampians, and large tracts of land in Devon and Cornwall, and
on the east and west of Ireland; while fertile trap soils extend over
thousands of square miles in the lowlands of Scotland, and in the
north of Ireland; and where in Cornwall they occasionally mix with
the granite soils, they are found to redeem them from their natural
barrenness.
While such is the general rule in regard to these two classes of soils,
it happens on some spots that the presence of other minerals in the
granites, or of hornblende or mica in larger quantity than usual, give
rise to a granitic soil of average fertility, as is the case in the
Scilly isles; while, in like manner, the trap rocks are sometimes, as
in parts of the isle of Skye, so peculiar in constitution as to condemn
the land to almost hopeless infertility.
In some districts the decayed traps are dug up, and applied with
advantage, as a top-dressing, to other kinds of land; and as by
admixture with the decayed trap, the granitic soils are known to be
improved in quality, so an admixture of decayed granite with many trap
soils, were it readily accessible, might add to their fertility also.
SECTION II.—OF THE SUPERFICIAL ACCUMULATIONS OF TRANSPORTED MATERIALS
ON DIFFERENT PARTS OF THE EARTH’S SURFACE.
It is necessary to guard the reader against disappointment, when he
proceeds to examine the existing relation between the soils and the
rocks on which they lie, or to infer the quality of the soil from the
known nature of the rock in conformity with what has been above laid
down,—by explaining another class of geological appearances which
present themselves not only in our own country but in almost every
other part of the globe.
The unlearned reader of the preceding section and chapter may say—I
know excellent land resting upon the granites, fine turnip soils on the
Oxford or London clays, tracts of fertile fields on the coal measures,
and poor, gravelly farms on the boasted new red sandstone: I have no
faith in theory—I can have none in theories which are so obviously
contradicted by natural appearances. Such, it is to be feared, is
the hasty mode of reasoning among too many _locally_[16] excellent
practical men, familiar, it may be, with many useful and important
facts, but untaught to look through and beyond isolated facts to the
principles on which they depend.
[16] By _locally_ excellent, I mean those who are the best possible
farmers of their own district and after their own way, but who would
fail in other districts requiring other methods. To the possessor of
agricultural principles the modifications required by difference of
crop, soil, and climate, readily suggest themselves, where the mere
practical man is bewildered, disheartened, and in despair.
Every one who has lived long, on the more exposed shores of our island,
has seen, that when the weather is dry, and the sea winds blow strong,
the sands of the beach are carried inland and spread over the soil,
sometimes to a considerable distance from the coast. In some countries
this sand-drift takes place to a very great extent, and gradually
swallows up large tracts of fertile land.
Again, most people are familiar with the fact, that during periods of
long continued rain, when the rivers are flooded and overflow their
banks, they not unfrequently bear with them loads of sand and gravel,
which they carry far and wide, and strew at intervals over the surface
soil.
So the annual overflowings of the Nile, the Ganges, and the river of
Amazons, gradually deposit accumulations of soil over surfaces of great
extent;—and so also the bottoms of most lakes are covered with thick
beds of sand, gravel, and clay, which have been conveyed into them from
the higher grounds by the rivers through which they are fed.
To these and similar agencies, a large portion of the existing dry land
of the globe has been, and is still exposed. Hence in many places,
the rocks, and the soils naturally derived from them, are buried
beneath accumulated heaps or layers of sand, gravel, and clay, which
have been brought from a greater or less distance, and which have
not unfrequently been derived from rocks of a totally different kind
from those of the district in which they are now found. On these
accumulations of transported materials, a soil is produced which often
has no relation in its characters to the rocks which cover the country,
and the nature of which a familiar acquaintance with these rocks would
not enable us to predict.
To this cause is due that discordance between the first indications
of geology, as to the origin of soils from the rocks on which they
rest, and the actually observed character of those soils in certain
districts—of which discordance mention has been made as likely to
awaken doubt and distrust in the mind of the less instructed student in
regard to the predictions of agricultural geology. There are several
circumstances, however, by which the careful observer is materially
aided in endeavouring to understand what the nature of the soils is
likely to be, and how they ought to be treated, even when the subjacent
rocks are thus overlaid by masses of drifted materials. Thus—
1. It not unfrequently happens, that the materials brought from a
distance are more or less mixed up with the fragments and decayed
matter of the rocks which are native to the spot, so that though
modified in quality, the soil, nevertheless, retains the general
characters of that which is formed on other spots from the decay of
these rocks alone.
2. Where the formation is extensive, or covers a large area, as the
new red sandstones and coal measures do in this country,—the mountain
limestones in Ireland, and the granites in the north of Scotland—the
transported sand, gravel, or clay, strewed over one part of the
formation, has not unfrequently been derived from the rocks of another
part of the _same_ formation, so that, after all, the soils may be said
to be produced from the rocks on which they rest, and may be judged of
from the known constitution of these rocks.
3. Or if not from the rocks of the same formation, they have most
frequently been derived from those of a neighbouring formation—from
rocks which are to be found at _no great distance_, and generally on
higher ground. Thus the ruins of the millstone grit rocks are often
spread over the surface of the coal measures—of these, again, over the
magnesian limestone,—of the latter, over the new red sandstone, and so
on. The effect of this kind of transport upon the soils, is merely to
overlap, as it were, the edges of one formation with the proper soils
of the formations that adjoin it in the particular direction from which
the drifted materials are known to have come.
It appears, therefore, that the occurrence on certain spots, or tracts
of country, of soils that have no apparent relation to the rocks on
which they immediately rest, tends in no way to throw doubt upon, to
discredit or to disprove, the conclusions drawn from the more general
facts and principles of geology. It is still generally true that soils
_are_ derived from the rocks on which they rest. The exceptions are
local, and the difficulties which these local exceptions present,
require only from the agricultural geologist a more careful study of
the structure of each district, before he pronounces a decided opinion
as to the degree of fertility it either naturally possesses, or by
skilful cultivation may be made to attain.
Geological _maps_ point out with more or less precision the extent of
country over which the chalk, the red sandstone, the granites, &c.,
are found immediately beneath the loose materials on the surface; and
these maps are of great value in indicating also the general quality of
the soils over the same districts. It may be true, that here and there
the _natural_ soils are masked or buried by transported materials,
yet the _political economist_ may, nevertheless, with safety estimate
the general agricultural capabilities and resources of a country by
the study of its geological structure—the _capitalist_ judge in what
part of it he is likely to meet with an agreeable investment—and the
_practical farmer_ in what country he may expect to find land that will
best reward his labours—that will admit of the kind of culture to which
he is most accustomed, or, by the application of better methods, will
manifest the greatest agricultural improvement.
SECTION III.—OF THE PHYSICAL CHARACTERS OF SOILS.
The influence of climate on the fertility of a soil is often very
great. This influence depends very much upon what are called the
_physical_ properties of soils.
1. Some soils are heavier and denser than others, sand and marls being
the heaviest, and peaty soils the lightest. In reclaiming peat lands,
it is found to be highly beneficial to increase their density by a
covering of clay, sand, or limestone gravel.
2. Again, some soils absorb the rains that fall, and retain them in
larger quantity and for a longer period than others. Strong clays
absorb and retain nearly three times as much water as sandy soils do,
while peaty soils absorb a still larger proportion. Hence the more
frequent necessity for draining clayey than sandy soils; hence also the
reason why, in peaty lands, the drains must be kept carefully open, in
order that the access of springs and of other water from beneath, may
be as much as possible prevented.
3. When dry weather comes, soils lose water by evaporation with
different degrees of rapidity. In this way a siliceous sand will give
off the same weight of water in the form of vapour, in one-third of the
time necessary to evaporate it from a stiff clay, a peat, or a rich
garden mould, when all are equally exposed to the air. Hence the reason
why plants are so soon burned up in a sandy soil. Not only do such
soils _retain_ less of the rain that falls, but that which is retained
is also more speedily dissipated by evaporation. When rains abound,
however, or in very moist seasons, these same properties of sandy soils
enable them to sustain a luxuriant vegetation, when plants will perish
on clay lands from excess of moisture.
4. In drying under the influence of the sun, soils contract and
diminish in bulk in proportion to the quantity of clay or of peaty
matter they contain. Sand does not at all diminish in bulk in drying,
but peat shrinks in one-fifth, and agricultural clay nearly as much.
The roots are thus compressed, and air is excluded, especially from
the hardened clays, and thus the plant is placed in a condition
unfavourable to its growth. Hence the value of proper admixtures of
sand and clay. By the latter (the clay), a sufficient quantity of
moisture is retained, and for a sufficient length of time; while, by
the former, the roots are preserved from compression, and a free access
of air is permitted.
5. In the hottest and most drying weather, the soil has seasons of
respite from the scorching influence of the sun. During the cooler
season of the night, even when no perceptible dew falls, it has the
power of again extracting from the air a portion of the moisture it
had lost during the day. Perfectly pure sand possesses this power in
the least degree; it absorbs little or no moisture from the air. A
stiff clay, on the other hand, will in a single night absorb sometimes
as much as a 30th part of its own weight, and a dry peat as much as a
12th of its weight; and, generally, the quantity thus drunk in by soils
of various qualities, is dependent upon the proportions of clay and
vegetable matter they severally contain. We cannot fail to perceive
from these facts, how much of the productive capabilities of a soil
is dependent upon the proportions in which its different earthy and
vegetable constituents are mixed together.
6. The temperature of a soil, or the degree of warmth it is capable
of attaining under the influence of the sun’s rays, materially affects
the progress of vegetation. Every gardener knows how much _bottom_
heat forces the growth, especially of young plants; and wherever a
natural warmth exists in the soil, independent of the sun, as in the
neighbourhood of volcanoes, there it exhibits the most exuberant
fertility. One main influence of the sun in spring and summer is
dependent upon its power of thus warming the soil around the young
roots, and thus rendering it propitious to their rapid growth. But the
sun does not warm all soils alike: some become much hotter than others,
though exposed to the same sunshine. When the temperature of the air
in the shade is no higher than 60° to 70°, a _dry_ soil may become so
warm as to raise the thermometer to 90° or 100°. Mrs. Ellis states,
that among the Pyrenees the rocks actually smoke after rain under the
influence of the summer sun, and become so hot, that you cannot sit
down upon them. In _wet_ soils the temperature rises more slowly, and
never attains the same height as in a dry soil by 10° or 15°. Hence it
is strictly correct to say, that wet soils are _cold_; and it is easy
to understand how this coldness is removed by perfect drainage. Dry
sands and clays, and blackish garden mould, become warmed to nearly
an equal degree under the same sun; brownish red soils are heated
somewhat more, and dark-coloured peat the most of all. It is probable,
therefore, that the presence of dark-coloured vegetable matter renders
the soil more absorbent of heat from the sun, while the colour of the
dark-red marls of the new and old red sandstones may, in some degree,
aid the other causes of fertility in the soils which they produce.
Granite generally forms hills and sometimes entire ridges of mountains.
When it decays, the rains and streams wash out and carry down the fine
felspar clay, and leave the (quartz) sand on the sides of the hills.
Hence the soil in the bottoms and flats of granite countries consists
of a cold, stiff, wet, more or less impervious clay, which often bears
only heath, bog, or a poor and unnutritive pasture. The hill sides are
either bare or covered with a thin, sandy, and ungrateful soil, of
which little can be made by the aid even of skill and industry. Yet
the opposite sides of the same mountains often present a remarkable
difference in this respect, those which are most beaten by the rains
having the light clay most thoroughly washed from their surfaces, and
being therefore the most barren.
In reading the above observations, the practical reader can hardly
fail to have been struck with the remarkable similarity in physical
properties between stiff clay and peaty soils. Both retain much of the
water that falls in rain, and both part with it slowly by evaporation.
Both contract much in drying; and both absorb moisture readily from the
air in the absence of the sun. In this similarity of properties, we
see not only why the first steps in improving both kinds of soil must
be very nearly the same; but why, also, a mixture either of clay or of
vegetable matter will equally impart to a sandy soil many of those aids
to, or elements of, fertility—of which they are alike possessed.
SECTION IV.—OF THE CHEMICAL CONSTITUTION OF SOILS.
Soils perform at least three functions, in reference to vegetation.
They serve as a basis in which plants may fix their roots and sustain
themselves in their erect position,—they supply inorganic food to
vegetables at every period of their growth,—and they are the medium in
which many chemical changes take place, that are essential to a right
preparation of the various kinds of food which the soil is destined to
yield to the growing plant.
We have spoken of soils as consisting chiefly of sand, lime, and clay,
with certain saline and organic substances in smaller and variable
proportions. But the study of the ash of plants (see chap. iv.) shews
us, that a fertile soil must of necessity contain an appreciable
quantity of at least eleven different substances, which in most cases
exist in greater or less relative abundance in the ash both of wild and
of cultivated plants.
Two well known geological facts lead to precisely the same conclusion.
We have seen that the soils formed from the unstratified rocks,—the
granites and the traps,—while they each contain certain earthy
substances in proportions peculiar to themselves, yet contain also in
general a _trace_ of most of those different kinds of matter which are
found in the ash of plants. And when to this fact is added the other,
that the stratified rocks appear to be only the long accumulating
fragments and ruins of more ancient unstratified masses—which, under
various agencies, have gradually crumbled to dust, been strewed over
the surface in alternate layers, and afterwards again consolidated,—the
reader will readily grant, that in all rocks, and consequently in all
soils, _traces_ of every one of these substances may generally be
presumed to exist.
Actual _chemical analysis_ confirms these deductions in regard
to the constitution of soils. It shews that, in most soils, the
presence of the several constituents of the ash of plants may be
detected, though in very variable proportions. And following up its
investigations, in regard to the effect of this difference in the
proportion of the generally less abundant constituents of the soil, it
establishes certain other points of the greatest possible importance to
agricultural practice. Thus, it has found, for example,
1. That as a proper adjustment of the proportions of clay and sand
is necessary, in order that a soil may possess the most favourable
_physical_ properties—so that the mere presence of the various kinds
of inorganic food in a soil is not sufficient to make it productive
of a given crop, but that they must be so adjusted in quantity that
the plant shall be able readily and at the proper time to obtain an
adequate supply of each.
2. That when a soil is particularly poor in certain of these
substances, the valuable, cultivated corn crops, grasses, and trees,
refuse to grow upon them in a healthy manner, and to yield remunerating
returns. And,
3. That when certain other substances are present in too great
abundance, the soil is rendered equally unpropitious to the most
important crops.
In these facts the intelligent reader will perceive the foundation of
the varied applications to the soil which are everywhere made under
the direction of a skilful practice, and of the difficulties which, in
so many localities, lie in the way of bringing the land into such a
state as shall fit it readily to supply all the wants of those kinds of
vegetables which it is the special object of artificial culture easily
and abundantly to raise.
Chemical analysis is a difficult art,—one which demands much chemical
knowledge, and skill in chemical practice (manipulation, as it is
called), and calls for both time and perseverance—if valuable,
trustworthy, and _minutely correct_ results are to be obtained. I
believe it is only by aiming after such minutely correct results that
chemical analysis is likely to throw light on the peculiar properties
of those soils which, while they possess much general similarity in
composition and in physical properties, are yet found in practice to
possess very different agricultural capabilities. Many such cases
occur in every country, and they are the kind of difficulties in
regard to which agriculture has a right to say to chemistry—“These are
matters which I hope and expect you will satisfactorily clear up.” But
while agriculture has a right to use such language, she has herself
preliminary duties to perform. She has no right in one breath to deny
the value of chemical theory to agricultural practice, and in another
to ask the sacrifice of time and labour in doing her chemical work.
Chemistry is a wide field, and many zealous lives may be spent in the
prosecution of it without at all entering upon the domain of practical
agriculture. It may be that here and there it may fall in with the
humour or natural bias of some one chemist to apply his knowledge to
this most important art; but hitherto the appreciation of such efforts
has, in general, been so small—the reception of scientific results and
suggestions by the agricultural body so ungracious—that little wonder
can exist that so many have quitted the field in disgust—that the
majority of capable men should studiously avoid it.
Hence it has happened that, in England, the analysis of soils has
rarely been undertaken, except as a matter of professional business,
where so much time was, by a fair calculation, given for so much money,
and an analysis made, of that degree of accuracy only which the time
allotted to it permitted the analyst to attain.
In order, therefore, to illustrate the deductions which, as above
stated, may be drawn from an accurate chemical analysis, I shall
exhibit the constitution of three different soils as determined
by Sprengel, a German chemist, now at the head of the Prussian
Agricultural school, and whose own taste, as well as his professional
function, have long directed his attention, and with much success, to
scientific agriculture.
No. 1 is a very fertile alluvial soil from East Friesland, formerly
overflowed by the sea, but for 60 years cultivated with corn and pulse
crops _without manure_.
No. 2 is a fertile soil near Göttingen, which produces excellent crops
of clover, pulse, rape, potatoes, and turnips, the two last more
especially _when manured with gypsum_.
No. 3 is a very barren soil from Lunenburg.
When washed with water in the manner described in pages 70 to 73, they
gave, respectively, from 1000 parts of soil—
No. 1. No. 2. No. 3.
Soluble saline matter, 18 1 1
Fine earthy and organic matter (clay), 937 839 599
Siliceous sand, 45 160 400
---- ---- ----
1000 1000 1000
The most striking distinction presented by these numbers is the large
quantity of saline matter in No. 1. This soluble matter consisted of
common salt, chloride of potassium, sulphate of potash and gypsum,
with a trace of sulphate of magnesia, sulphate of iron, and phosphate
of soda. The presence of this comparatively large quantity of these
different saline substances,—originally derived, no doubt, in great
part from the sea,—was probably one reason why it could be so long
cropped without manure.
The unfruitful soil is much the lightest of the three, containing
40 per cent. of sand; but this is not enough to account for its
barrenness—many light soils containing a larger proportion of sand, and
yet being sufficiently fertile.
The finer portions separated from the sand, and soluble matter,
consisted in 1000 parts of
No. 1. No. 2. No. 3.
Organic matter, 97 50 40
Silica, 648 833 778
Alumina, 57 51 91
Lime, 59 18 4
Magnesia, 8½ 8 1
Oxide of iron, 61 30 81
Oxide of manganese, 1 3 ½
Potash, 2 trace. trace.
Soda, 4 ” ”
Ammonia, trace. ” ”
Chlorine, 2 ” ”
Sulphuric acid, 2 ¾ ”
Phosphoric acid, 4½ 1¾ ”
Carbonic acid, 40 4½ ”
Loss, 14 — 4½
------ ------ ------
1000 1000 1000
1. The composition of No. 1 illustrates the first of those general
deductions above stated, that a considerable supply of _all_ the
species of inorganic food is necessary to render a soil eminently
fertile. Not only does this soil contain a comparatively large quantity
of soluble saline matter, but it contains also nearly 10 per cent.
of organic matter, and, what in connection with this is of great
importance, 6 per cent. of lime. The potash and soda, and the several
acids, are also present in sufficient abundance.
2. In the second,—a fertile soil, but one which _cannot dispense with
manure_,—there is little soluble saline matter, and in the insoluble
portion we see that there are mere _traces_ of potash, soda, and the
important acids. It contains also 5 per cent. only of organic matter,
and about 2 per cent. of lime, which smaller proportions, together
with the deficiencies above stated, remove this soil from the most
_naturally_ fertile class to that class which is susceptible, in
hands of ordinary skill, of being _brought to_, and _kept in_, a very
productive condition.
3. In the fine part of the third soil, we observe that there are many
more substances deficient than in No. 2. The organic matter amounts
apparently to 4 per cent., and there seems to be nearly half a per
cent. of lime. But it will be recollected, that this soil contains 40
per cent. of sand, so that in every hundred of soil there are only 60
of the fine matter, of which the composition is presented in the table,
or 100 lbs. of the native soil contain only 2½ lbs. of organic matter
and ¼ lb. of lime.
But all these _wants_ would not condemn the soil to hopeless
barrenness, because in favourable circumstances, and where it was worth
the cost, they might all be supplied. But the oxide of iron amounts to
8 per cent. of this fine matter, a proportion of this substance which,
in a soil containing so little organic matter, appears, from practical
experience, to be incompatible with the healthy growth of cultivated
crops. To this soil, therefore, there requires to be added not only
those substances of which it is destitute, but such other substances
also as shall prevent the injurious effect of the large proportion of
oxide of iron.
In these three soils, then, we have examples, _first_, of one which
contains within itself all the elements of fertility; _second_, of a
soil which is destitute, or nearly so, of certain substances,—which,
however, can be readily added by the ordinary manures in general
use,—and to which the elements of gypsum are especially useful, in
aiding it to feed the potato and the turnip; and, _third_, of a soil
not only poor in many of the necessary species of the inorganic food
of plants, but too rich in one which, when present in excess, is
prejudicial to vegetable life.
This illustration, therefore, will aid the general reader in
comprehending how far rigid chemical analysis is fitted to throw light
upon the capabilities of soils, and to _direct_ agricultural practice.
SECTION V.—OF THE RELATION THAT EXISTS BETWEEN THE CHARACTER OF THE
SOIL AND THE KIND OF PLANTS THAT GROW UPON IT.
The importance of this study of the chemical constitution of soils
will, perhaps, be most readily appreciated by a glance at the very
different kinds of vegetables which, under the same circumstances,
different soils naturally produce.
There are none so little skilled in regard to the capabilities of the
soil, as not to be aware that some lands naturally produce abundant
herbage or rich crops, while others refuse to yield a nourishing
pasture, and are deaf to the often repeated solicitations of the
diligent husbandman. There exists, therefore, a universally understood
connection between the kind of soil and the kind of plants that
naturally grow upon it. It is interesting to observe how close this
relation in many cases is.
1. The sands of the sea-shore, and the margins of salt-lakes, are
distinguished by their peculiar tribes of salt-loving plants;—the
drifted sands more remote from the beach produce their own long waving
coarser grass,—while further inland again, other vegetable races appear.
2. Peaty soils laid down to grass, or existing as natural meadows,
produce one woolly soft grass almost exclusively (the _Holcus
lanatus_); when limed, again, these same soils become propitious to
green crops and produce much straw, but refuse to fill the ear.
3. On the margins of water-courses, in which silica abounds, the
mare’s-tail (_Equisetum_) springs up in abundance; while, if the stream
contain much carbonate of lime, the water-cress appears and lines its
sides, and the bottom of its shallow bed, sometimes for many miles from
its source.
4. The Cornish heath (_Erica vagans_) shews itself only above the
serpentine rocks; the red clover and the vetch delight in the presence
of gypsum; and white clover, of alkaline matter in the soil.
5. Then, again, plants seem to alternate with each other on the same
soil. Burn down a forest of pines in Sweden, and one of birch takes
its place _for a while_. The pines after a time again spring up and
ultimately supersede the birch. The same takes place naturally. On the
shores of the Rhine are seen ancient forests of oak from two to four
centuries old,—gradually giving place to a natural growth of beech; and
others where the pine is succeeding to both. In the Palatinate, the
ancient oak woods are followed by natural pines; and in the Jura, the
Tyrol, and Bohemia, the pine alternates with the beech.
These and other similar differences depend upon the chemical
constitution of the soil. The slug may live well, and therefore infest
a field almost deficient in lime; the common land snail will abound at
the roots of the hedges only where lime is plentiful, and can easily
be obtained for the construction of its shell. So it is with plants.
Each grows spontaneously where its wants can be most fully and most
easily supplied. If they cannot move from place to place like the
living animal, yet their seeds can lie dormant, until either the hand
of man or the operation of natural causes produces such a change in the
constitution of the soil as to fit it for ministering to their most
important wants.
And such changes do naturally come over the soil. The oak, after
thriving for long generations on a particular spot, gradually sickens;
its entire race dies out,—and other races succeed it. The operation
of natural causes has gradually removed from the soil that which
favoured the oak, and has introduced or given the predominance to those
substances which favour the beech or the pine.
In the hands of the farmer the land grows sick of this crop,—it becomes
tired of that. These facts are generally indications of a change in
the chemical constitution of the soil. This alteration may proceed
slowly and for many years, and the same crops may still grow upon it
for a succession of rotations. At length the change is too great for
the plant to bear; it sickens, yields an unhealthy crop, and becomes
ultimately extinct.
The plants we raise for food have similar likes and dislikes with those
that are naturally produced. On some kinds of food they thrive,—fed
with others, they sicken or die. The soil must therefore be prepared
for their special growth.
In an artificial rotation of crops, we only follow nature. One crop
extracts from the soil a certain quantity of all the inorganic
constituents of plants; but some of these in much larger proportions
than others. A second crop carries off in preference a larger quantity
of those substances which the former had left; and thus it is clearly
seen, both why an abundant manuring may so alter the constitution of
the soil, as to enable it to grow almost any crop; and why, at the same
time, this soil may in succession yield more abundant crops and in
greater number, if the kinds of plant sown and reaped be so varied as
to extract from the soil, one after the other, the several different
substances which the manure we have originally added is known to
contain.
The management and tilling of the soil, in fact, is a branch of
practical chemistry, which, like the art of dyeing or of lead smelting,
may advance to a certain degree of perfection, without the aid of pure
science; but which can only have its processes explained, and be led on
to shorter,—more simple,—more economical,—and more perfect processes,
by the aid of scientific principles.
CHAPTER VIII.
Of the Improvement of the Soil—Mechanical and Chemical
Methods—Draining—Subsoiling—Ploughing, and Mixing
of Soils—Use of Lime, Marl, and Shell-sand—Manures
—Vegetable, Animal, and Mineral Manures.
The soil is possessed of certain existing and obvious qualities, and
of certain other dormant capabilities; how are these qualities to be
improved,—these dormant capabilities to be awakened?
There are two distinct methods by which these ends may be, in some
measure, attained,—by the use of _mechanical_, and by the application
of _chemical_, means. Mechanical operations produce changes _chiefly_
in the physical properties of the soil,—chemical means alter its
elementary constitution. Ploughing, draining, mixing, &c. belong to
the former class of operations; manuring and irrigation belong to the
latter. It will be proper to consider these methods separately.
SECTION I.—OF MECHANICAL METHODS OF IMPROVING THE SOIL.
1. _Draining._—The first step to be taken, in order to increase the
fertility of nearly all the improveable lands of Great Britain, is
to drain them. So long as they remain wet, they will continue to be
cold. The heat of the sun’s rays, which is intended by nature to warm
the soil, will be expended in evaporating the water from its surface;
and thus the plants will never receive that genial warmth about their
roots which so much favours their rapid growth. Where too much water
is present in the soil also, that food of the plant which the soil
supplies is so much diluted, that either a much greater quantity of
fluid must be taken in by the roots,—much more work done,—or the plant
will be scantily nourished. The presence of so much water in the stem
and leaf keeps down _their_ temperature likewise, when the sunshine
appears; an increased evaporation takes place from their surfaces, a
lower natural heat, in consequence, prevails in the interior of the
plant, and the chemical changes on which its growth depends proceed
with less rapidity.
By the removal of the water, the physical properties of the soil also
are in a remarkable degree improved. Dry pipe-clay can be easily
reduced to a fine powder, but it naturally, and of its own accord,
runs together when water is poured upon it. So it is with clays in
the field. The soil expands, becomes close and adhesive, and excludes
the air from the roots of the growing plant,—the access of which air
appears to be almost an essential element in the healthy growth of the
most important vegetable productions.
Open an outlet for the water below, and as it trickles away, the air
from above will follow it and take its place among the pores of the
soil, carrying to every root the salutary influences it is appointed
to bear with it wherever it penetrates. When freed from water also,
the stiff soil becomes more mellow; and when once stirred up to a
considerable depth, more universally porous,—so that air can make
its way everywhere, and the roots can find their easy way in every
direction. The presence of vegetable matter,—whether existing naturally
in a soil thus physically altered, or artificially added to it,—becomes
of double value. When drenched with water, this vegetable matter
either decomposes very slowly, or produces acid compounds more or less
unwholesome to the plant, and even exerts injurious chemical reactions
upon the earthy and saline constituents of the soil. In the presence
of air, on the contrary, this vegetable matter decomposes rapidly,
produces carbonic acid in large quantity, as well as other compounds
fit for food, and even renders the inorganic constituents of the soil
more fitted to enter the roots, and thus to supply more rapidly what
the several parts of the plant require.
Nor is it only stiff and clayey soils to which draining can with
advantage be applied. It will be obvious to every one, that when
springs rise to the surface in sandy soils, a drain must be made to
carry off the water,—it will also readily occur, that where a sandy
soil rests upon a hard or clayey bottom, drains may also be necessary;
but it is not unfrequently supposed, that when the subsoil is sand or
gravel, that drains can only in special cases be necessary.
Every one, however, is familiar with the fact, that when water is
applied to the bottom of a flower-pot full of soil, it will gradually
find its way to the surface, however light the soil may be. So it is
in sandy soils or subsoils in the open field. If water abound at the
depth of a few feet, or if it so abound at certain seasons of the year,
_that_ water will rise to the surface; and as the sun’s heat dries it
off by evaporation, more water will follow to supply its place. This
attraction from beneath will always go on when the air is dry and warm,
and thus a double evil will ensue—the soil will be kept moist and cold,
and instead of a constant circulation of air downwards, there will be a
constant current of water upwards. Thus will the roots, the under soil,
and the organic matter it contains, be all deprived of the benefits
which the access of the air is fitted to confer. The remedy for these
evils is to be found in an efficient system of drainage.
On this subject I shall add one important practical remark, which will
readily suggest itself to the geologist who has studied the action of
air and water on the various clay beds that occur here and there as
members of the series of stratified rocks. _There are no clays which do
not gradually soften under the united influence of air and of running
water. It is false economy, therefore, to lay down tiles without
soles_—however hard and stiff the clay subsoil may appear to be. In
the course of ten or fifteen years the stiffest clays will soften, so
as to allow the tile to sink; and many very much sooner. The passage
for the water is thus gradually narrowed; and when the tile has sunk a
couple of inches, the whole must be taken up. Thousands of miles of
drains have been thus laid down, both in the low country of Scotland
and in the southern counties of England, which have now become nearly
useless; and yet the system still goes on. It would appear even as if
the farmers and proprietors of each district—unwilling to believe in or
to be benefitted by the experience of others—were determined to prove
the matter in their own case also, before they will consent to adopt
that surer system which, though demanding a slightly greater outlay
at first, will return upon the drainer with no after-calls for either
time or capital. If my reader live in a district where this practice
is now exploded, and if he be inclined to doubt if other counties be
farther behind the advance of knowledge than his own, I would invite
him to spend a week in crossing the county of Durham, where he may find
opportunities not only of satisfying his own doubts, but of scattering
here and there a few words of useful advice among the more intelligent
of our practical farmers.
2. _Subsoiling._—The subsoil plough is an auxiliary to the drain.
Though there are few subsoils through which the water will not at
length make its way, yet there are some so stiff either naturally
or from long consolidation, that the good effect of a well-arranged
line of drains is lessened by the slowness with which they allow the
superfluous rains to pass through them. In such cases, the use of the
subsoil plough is most advantageous in loosening the under layers of
clay, and allowing the water to find a ready escape downwards and to
either side until it reach the drains.
It is well known that if a piece of stiff clay be cut into the shape of
a brick, and then allowed to dry, it will contract and harden—it will
form an air-dried brick, almost impervious to any kind of gas—wet it
again, it will swell and become still more impervious. Cut up _while
wet_, it will only be divided into so many pieces, each of which will
harden when dry, or the whole of which will again attach themselves
and stick together if exposed to pressure. But tear it asunder _when
dry_, and it will fall into many pieces, will more or less crumble, and
will readily admit the air into its inner parts. So it is with a clay
subsoil.
After the land is provided with drains, the subsoil being very
retentive, the subsoil plough is used to open it up—to let out the
water and to let in the air. If this is not done, the stiff under-clay
will contract and bake as it dries, but it will neither sufficiently
admit the air nor open a free passage for the roots. But let this
operation be performed when the clay is still too wet, a good effect
will follow, in the first instance; but after a while, the cut clay
will again cohere, and the former will pronounce subsoiling to be a
useless expense _on his land_. Defer the use of the subsoil plough till
the clay is dry—it will then _tear_ and _break_ instead of _cutting_,
and its openness will remain. Once give the air free access, and it,
after a time, so modifies the drained clay, that it no longer has an
equal tendency to cohere.
Mr. Smith of Deanston very judiciously recommends that the subsoil
plough should never be used till at least a year after the land has
been thoroughly drained. This in many cases will be a sufficient
safeguard—will allow a sufficient time for the clay to dry; in other
cases two years may not be too much. But this precaution has by some
been neglected, and subsoiling being with them a failure, they have
sought, in some supposed chemical or other quality _of their soil_,
for the cause of a want of success which is to be found in their own
neglect of a most necessary precaution. Let not the practical man be
too _hasty_ in desiring to attain those benefits which attend the
adoption of improved modes of culture; let him give every method a fair
trial; _and above all, let him make his trial in the way and with
the precautions recommended by the author of the method_, before he
pronounce its condemnation.
3. _Deep-ploughing_, like subsoiling, aids the effect of the drains,
and so far, and where it goes nearly as deep, more completely effects
the same object. But independent of this, it has other uses and merits,
and where it has been successfully applied, has improved the land by
the operation of other causes.
Subsoiling only lets out the water, and allows access to the air and a
free passage to the roots. Deep-ploughing, in addition to these, brings
new earth to the surface, forms thus a deeper soil, and more or less
alters both its physical qualities and its chemical constitution.
If the plough be made to bring up two inches of clay or sand, it will
stiffen or loosen the soil, as the case may be, or it may affect its
colour or density. It is clear and simple enough, therefore, that by
deep-ploughing the physical properties of the soil may be altered.
But there are certain substances contained in every soil, whether
in pasture or under the plough, which gradually make their way down
towards the subsoil. They sink till they reach at last that point
beyond which the plough does not usually penetrate. Every farmer knows
that lime thus sinks. In peat-soils top-dressed with clay, the clay
thus sinks. In sandy soils also which have been clayed, the clay sinks;
and in all these cases, I believe, the sinking takes place more rapidly
when the land is laid down to grass. Where soils are marled, the marl
sinks; and the rains, in like manner, gradually wash out that which
gives their fertilizing virtue to the under chalk-soils (see page 88),
and render necessary a new application from beneath, to renovate its
productive powers.
If this be the case with earthy substances such as those now mentioned,
which are insoluble in water, it will be readily believed that those
saline ingredients of the soil which are readily soluble will be still
sooner washed out of the upper and conveyed to the under soil. Thus
the subsoil may gradually become rich in those substances of which the
surface-soil has been robbed. Bring up a portion of this subsoil by
deep-ploughing, and you restore to the land a portion of what it has
lost—substances, perhaps, which may render it much more fruitful than
before. Such is an outline of the theory of deep-ploughing, and it is
entirely unexceptionable.
But suppose the land to have originally contained something noxious
to vegetation, which in process of time has been washed down into the
subsoil, then to bring this again to the surface would be materially
to injure the land. This also is true, and a sound discretion must no
doubt be employed, in judging when and where such evil effects are
likely to follow.
Such cases, however, are more rare than many suppose. There are few
subsoils which a full and fair exposure to a winter’s frost will not in
a great degree deprive of all their noxious qualities, and render fit
to ameliorate the general surface of the poorer lands. If the reader
doubt this fact, let him visit Yester, and give a calm consideration to
the efforts produced by the use of deep-ploughing on the home-farm of
the Marquis of Tweeddale.
In many cases the farmer fears, as he does in the county of Durham, to
bring up a single inch of the yellow clay that lies beneath his soil.
In the first inch lodges, among other substances, the iron worn from
his plough, which in some soils, and after a lapse of years, amounts to
a considerable quantity. Till it is exposed to the air, this iron is
hurtful to vegetation, and one of the benefits of a winter’s exposure
of such subsoils to the air, is the effect produced upon the iron it
contains.
It is the want of drainage, however, and of the free access of
air, that most frequently renders subsoils for a time injurious to
vegetation. Let the lands be well drained—let the subsoils be washed
for a few years by the rain-water passing through them,—and there are
few of those which are clayey in their nature that may not ultimately
be brought to the surface, not only with safety, but with advantage to
the soil.
4. _Ploughing._—Other benefits, again, attend upon the ordinary
ploughings, hoeings, and workings of the land. Its parts are more
minutely divided—the air gets access to every particle—it is rendered
lighter, more open, more permeable to the roots. The vegetable matter
it contains decomposes more rapidly by a constant turning of the soil,
so that wherever the fibres of the roots penetrate, they find organic
food provided for them, and an abundant supply of the oxygen of the
atmosphere to aid in preparing it. The production of ammonia and of
nitric acid also (see pages 33 to 36), and the absorption of one or
both from the air, take place to a greater extent, the finer the soil
is pulverised, and the more it has been exposed to the action of the
atmosphere. The general advantage, indeed, to be derived from the
constant working of the soil, may be inferred from the fact, that Tull
reaped twelve successive crops of wheat from the same land by the
repeated use of the plough and the horse-hoe. There are few soils so
stubborn as not to shew themselves grateful in proportion to the amount
of this kind of labour that may be bestowed upon them.
5. _Mixing._—It has been shewn (page 114), that the physical properties
of the soil have an important influence upon its average fertility. The
admixture of pure sand with clay soils produces an alteration which is
often beneficial, and which is wholly physical. The sand merely opens
the pores of the clay, and makes it more permeable to the air.
The admixture of clay with sandy or peaty soils, however, produces both
a physical and a chemical alteration. The clay not only consolidates
and gives body to the sand or peat, but it also mixes with them certain
earthy and saline substances useful or necessary to the plant, which
neither the sand nor peat might originally contain in sufficient
abundance. It thus alters its chemical constitution, and fits it for
nourishing new races of plants.
Such is the case also with admixtures of marl, of shell-sand, and of
lime. They slightly consolidate the sands and open the clays, and
thus improve the mechanical texture of both kinds of soil, but their
main operation is chemical; and the almost universal benefit they
produce depends upon the new chemical element they introduce into the
constitution of the soil.
It is a matter of almost universal remark, that in our climate soils
are fertile—clayey or loamy soils, that is—only when they contain an
appreciable quantity of lime. In whatever way it acts, therefore, the
mixing of lime in any of the forms above mentioned, with a soil in
which little or no lime exists, is one of the surest practical methods
of bringing it nearer in composition to those soils from which the
largest returns of agriculture produce are usually obtained. Some of
the chemical effects of the lime upon the soil will be explained in a
subsequent section. (See page 195.)
SECTION II.—OF THE CHEMICAL METHOD OF IMPROVING THE SOIL BY THE USE OF
MANURES.
None of the above methods of improving the soil are mechanical
only—they all involve some chemical alterations also, which are readily
to be explained by a knowledge of elementary chemical principles.
But the manuring of the land is more strictly a chemical operation,
and may therefore with propriety be separated from those methods of
improving its quality which involve at the same time important and
expensive mechanical operations.
In commencing the tillage of a piece of land, the conscientious farmer
may have three objects in view in regard to it.
1. He may wish to reclaim a waste, or to restore a neglected farm to an
average condition of fertility.
2. Finding the land in this average state, his utmost ambition may be
to keep it in its present condition; or,
3. By _high_ farming he may wish to develope all its capabilities,
and to increase its permanent productiveness in the greatest possible
degree.
The man who aims at the last of these objects is not only the best
tenant and the best citizen, but he is also his own best friend. The
highest farming, skilfully and prudently conducted, is also the most
remunerating.
But whichever of these three ends he aims at, he will be unable to
attain it without a due knowledge of the various manures it may be in
his power to apply to his land—what these manures are, or of what they
consist—the general and special purposes they are each intended to
serve—which are the most effective for this or that crop—how they are
to be obtained in the greatest abundance, and at the least cost—how
their strength may be economized,—and in what state and at what seasons
they may be most beneficially applied to the land. Such are a few of
the questions which the skilful farmer should be ready to ask himself,
and should be able to answer.
By a _manure_ is to be understood whatever is capable of feeding or of
supplying food to the plant. And us plants require earthy and saline
as well as vegetable food, gypsum and nitrate of soda are as properly
called manures as farm-yard dung, bone-dust, or night-soil.
Manures naturally divide themselves into such as are of _vegetable_, of
_animal_, and of _mineral_ origin.
I. OF VEGETABLE MANURES.
There are two purposes which vegetable manure is generally supposed to
serve when added to the soil. It loosens the land, opens its pores,
and makes it lighter; and it also serves to supply organic food to the
roots of the growing plant. It serves, however, a third purpose: it
yields to the roots those saline and earthy matters which it is their
duty to find in the soil, and which exist in decaying plants in a state
more peculiarly fitted to enter readily into the circulating system of
new races.
Decayed vegetable matters, therefore, are in reality mixed manures,
and their value in enriching the land must vary considerably with the
_kind_ of plants and with the _parts_ of those plants of which they
are chiefly made up. This depends upon the remarkable difference which
exists in the _quantity_ and _kind_ of inorganic matter present in
different vegetable substances, as indicated by the ash they leave (see
pages 52 to 62). Thus if 1000 lbs. of the saw-dust of the willow be
fermented, and added to the soil, they will enrich it by the addition
of only 4½ lb. of saline and earthy matter, while 1000 lbs. of the
dry leaves of the same tree fermented, and laid on, will add 82 lbs.
of inorganic matter. Thus, independent of the effect of the vegetable
matter in each, the one will produce a very much greater effect upon
the soil than the other.[17]
[17] It is owing to this large quantity of saline and other inorganic
matter that fermented leaves form too strong a dressing for flower
borders, and that gardeners therefore generally mix them up into a
compost.
There are three states in which vegetable matter is collected by the
husbandman for the purpose of being applied to the land—the _green_
state; the _dry_ state; and that state of imperfect decay in which it
forms _peat_.
1. _Green Manuring._—When grass is mown in the field, and laid in
heaps, it speedily heats, ferments, and rots. But, if turned over
frequently and dried into hay, it may be kept for a great length of
time without undergoing any material alteration. The same is true of
all other vegetable substances—they all rot more readily in the green
state. The reason of this is, that the sap or juice of the green plant
begins very soon to ferment in the interior of the stem and leaves,
and speedily communicates the same condition to the moist fibre of
the plant itself. When once it has been dried, the vegetable matter
of the sap loses this easy tendency to decay, and thus admits of long
preservation.
The same rapid decay of green vegetable matter takes place when it is
buried in the soil. Thus the cleanings and scourings of the ditches and
hedge-sides form a compost of mixed earth and fresh vegetable matter,
which soon becomes capable of enriching the ground. When a green crop
is ploughed into a field, the whole of its surface is converted into
such a compost—the vegetable matter in a short time decays into a
light, black mould, and enriches in a remarkable degree and fertilizes
the soil.
Hence the practice of green manuring has been in use from very early
periods. The second or third crop of _lucerne_ was ploughed in by the
ancient Romans—as it still is by the modern Italians. In Tuscany, the
_white lupin_ is ploughed in, in preference—in Germany, _borage_.
In French Flanders, two crops of _clover_ are cut, and the third is
ploughed in. In Sussex, _turnip_ seed has been sown at the end of
harvest, and after two months again ploughed in, with great benefit to
the land. Turnip leaves and potato tops decay more readily, and more
perfectly, and are more enriching when buried in the green state. It is
a prudent economy, therefore, where circumstances admit of it, to bury
the potato tops on the spot from which the potatoes are raised. Since
the time of the Romans, it has been the custom to bury the cuttings
of the vine stocks at the roots of the vines themselves; and many
vineyards flourish for a succession of years without any other manuring.
Buckwheat, winter tares, clover, and rape, are all occasionally sown
for the purpose of being ploughed in. This should be done _when the
flower has just begun to open_, and if possible at a season when the
warmth of the air and the dryness of the soil are such as to facilitate
decomposition.
That the soil should become richer in vegetable matter by this burial
of a crop than it was before the seed of that crop was sown, and should
also be otherwise benefitted, will be understood by recollecting (see
page 42) that perhaps three-fourths of the whole organic matter we bury
has been derived from the air—that by this process of ploughing in,
the vegetable matter is more equally diffused through the whole soil
than it could ever be by any merely mechanical means;—and that by the
natural decay of this vegetable matter, ammonia and nitric acid are,
to a greater extent (pages 33 and 34), produced in the soil, and its
agricultural capabilities in consequence materially increased.
These considerations, while they explain the effect and illustrate
the value of green manuring, will also satisfy the intelligent
agriculturist that there are methods of improving his land without
the aid either of town or of foreign manures—and that he overlooks
an important natural means of wealth who neglects the green sods
and crops of weeds that flourish by his hedgerows and ditches. Left
to themselves, they ripen their seeds and sow them annually in his
fields—collected in compost heaps they would materially add to his
yearly crops of corn.
_Sea-weeds._—Among green manures, the use of fresh sea-ware deserves
especial mention, from the remarkably fertilizing properties it is
known to possess, as well as from the great extent to which it is
employed on all our coasts. The produce of the isle of Thanet in Kent
is said to have been doubled or tripled by the use of this manure; the
farms on the Lothian coasts are said to be let for 20s. or 30s. more
rent when they have a right of way to the sea, where the weed is thrown
on shore; and in the Western Isles the sea-ware, the shell-marl, and
the peat-ash, are the three great natural fertilizers to which the
agriculture of the district is indebted for the comparative prosperity
to which it has in some of the islands already attained.
Sea-weeds decompose with great ease when collected in heaps or spread
upon the land. During their decay, they yield not only organic food to
the plant but saline matters also, to which much of their efficacy both
on the grass and the corn crops is no doubt to be ascribed.
2. _Manuring with dry Vegetable Matter._—Almost every one knows that
the saw-dust of most common woods decays very slowly—so slowly, that
it is rare to meet with a practical farmer who considers it worth the
trouble of mixing with his composts. This property of slow decay is
possessed in a certain degree by all _dry_ vegetable matter. Heaps
of dry straw alone, or even mixed with earth, will ferment with
comparative difficulty and with great slowness. It is necessary,
therefore, to mix it, as is usually done, with some substance that
ferments more readily, and which will impart its own condition to the
straw. Animal matters of any kind, such as the urine and droppings of
cattle, are of this character; and it is by admixture with these that
the straw which is trodden down in the farm-yard is made to undergo a
more or less rapid fermentation.
The object of this fermentation is twofold—first, to reduce the
particles of the straw to such a minute state of division, that they
may admit of being diffused through the soil; and, second, that the dry
vegetable matter may be so changed by exposure to the air, and other
agencies, as to be fitted to yield both organic and inorganic food to
the roots of the plants it is intended to nourish.
We have seen that this decomposition takes place very speedily, and of
its own accord, when the vegetable matter is green, but that it can
be _induced_ or brought on in the case of dry straw by the agency
of animal matter. The same means will cause the fermentation of any
other vegetable substance which is in a minute state of division.
Even saw-dust made into a compost heap with soil or sods, and watered
regularly and copiously with the liquid manure of the farm-yards, may
be thus converted into a fertilizing vegetable mould.
Differences of opinion have prevailed, and discussions have taken
place, as to the relative efficacy of long and short—or of half
fermented and of fully rotten dung. But if it be added _solely_ for the
purpose of yielding food to the plant, or of preparing food for it, the
case is very simple. The more complete the state of fermentation—if not
carried too far—the more immediate will be the agency of the manure;
hence the propriety of the application of short dung to turnips and
other plants it is desirable to bring rapidly forward; but if the
manure be only half decayed, it will require time in the soil to
complete the decomposition, so that its action will be more gradual and
prolonged.
Though in the latter case the immediate action is not so perceptible,
yet the ultimate benefit to the soil, and to the crops, may be even
greater, supposing them to be such as require no special forcing
at one period of the year. With a view to this slow amelioration,
vegetable matter of any kind may be added with benefit, if in a
sufficient state of division, to the soil. Even saw-dust applied
largely to the land, has been found to improve it, though little at
first, yet more during the second year after it was applied, still more
during the third, and most of all in the fourth season after it was
mixed with the soil. That any dry vegetable matter, therefore, does not
produce an immediate effect, ought not to induce the practical farmer
to despise the application to his land—either alone, or in the form
of a compost—of every thing of the kind he can readily obtain. If his
fields are not already very rich in vegetable matter, both he and they
are likely to be ultimately benefitted by such additions to the soil.
_Rape Dust._—It is from the straw of the corn-bearing plants, or from
the stems and leaves of the grasses, that the largest portion of the
strictly vegetable manures applied to the soil is generally obtained
or prepared. But the seeds of all plants are much more enriching
than the substance of their leaves and stems. These seeds, however,
are in general too valuable for food to admit of their application
as a manure. Still the refuse of some, as that of different kinds
of rape-seed after the oil is expressed, and which is unpalatable to
cattle, is applied with great benefit to the land. Drilled in with
spring wheat, or scattered as a top-dressing in spring at the rate of 5
cwt. to an acre, it gives a largely increased and remunerating return.
It is applied with equal success to the cultivation of potatoes, and
generally it may be substituted for farm-yard manure at the rate of
about 1 cwt. of rape dust for each ton of manure.
_Malt Dust_ consists of the dried sprouts of barley, which, when the
sprouted seed is dried in the process of malting, break off and form
a coarse powder. This is found to be almost equal to rape dust in
fertilizing power.
_Charcoal Powder_ possesses the remarkable property of absorbing
noxious vapours from the air and soil, and unpleasant impurities from
water. It also sucks into its pores much oxygen from the air. Owing
to these and other properties, it is a valuable substance for mixing
with liquid manure, night-soil, farm-yard manure, ammoniacal liquor, or
other rich applications to the soil. It is even capable by itself of
yielding slow supplies of nourishment to living plants, and is said, in
many cases, without any admixture, to have been used with advantage in
practical agriculture. In moist charcoal the seeds of the gardener are
found to sprout with remarkable quickness and certainty.
_Soot_, whether from the burning of wood or of coal, is of vegetable
origin, and consists chiefly of a finely-divided charcoal, possessing
the properties above mentioned. It contains, however, ammonia and
certain other substances in small quantity, to which its well known,
and especially its _immediate_, effects upon vegetation are in part to
be ascribed.
3. _The use of Peat._—In many parts of the world, and in none more
abundantly, perhaps, than in Gt. Britain, is vegetable matter collected
in the form of peat. This ought to supply an inexhaustible store of
organic matter for the amelioration of the adjacent soils. We know that
by draining off the sour and unwholesome water, and afterwards applying
lime and clay, the surface of peat bogs may be gradually converted into
rich corn-bearing lands. It must, therefore, be possible to convert
peat itself by a similar process into a compost fitted to improve the
condition of other soils.
The late Lord Meadowbank, who made many important experiments on
this subject, found, that after being partially dried by exposure to
the air, peat might be readily fermented, and brought into the state
of a rich fertilizing compost by the same means which are adopted
in the ordinary fermenting of straw. He mixed with it a portion of
animal matter, which soon communicated its own fermenting quality to
the surrounding peat, and brought it readily in to a proper heat. He
found that one ton of hot fermenting manure, mixed in alternate layers
with two of half dry peat, and covered by the same, was sufficient to
ferment the whole; and subsequently that the vapours which rise from
naturally fermenting farm-yard manure or animal matters, would alone
produce the same effect upon peat, placed so as readily to receive and
absorb them.
As ammonia is one of the compounds specially given off by putrifying
animal substances, it is not unlikely that a watering with _ammoniacal
liquor_ would materially prepare the peat for undergoing fermentation.
At all events it seems possible to prepare any quantity of valuable
peat compost by mixing the peat with a still less quantity of fermented
manure than was employed by Lord Meadowbank, provided the liquid manure
of the farm-yard be collected in a cistern, and be thrown at intervals
by means of a pump over the prepared heaps.
One important use also to which I think peat may be applied is, after
it is partly dried, to build it into covered heaps, and half burn or
char it till it become readily reducible into a fine powder. In this
state it would be of great value as a mixture to preserve the virtues
of liquid manures of all kinds, of night-soil, and of ammoniacal liquor.
SECTION III.—RELATIVE VALUE OF DIFFERENT VEGETABLE MANURES.
There are two principles on which the relative value of different
vegetable substances, as manures, may be stated to depend—_first_, on
the relative quantity and kind of _inorganic matter_ they contain; and
_second_, on the relative proportions of _nitrogen_ present in each.
1. Valued according to the _quantity_ of inorganic matter they
contain—the worth of the several kinds of straw and hay would be
represented by the following numbers:—
Wheat straw, 70 to 360
Oat straw, 100 to 180
Hay, 100 to 200
Barley straw, 100 to 120
Pea straw, 100
Bean straw, 60 to 80
Rye straw, 50 to 70
Dry potato tops, 100
Dry turnip tops, 260
Rape, cake, 120
that is, a _ton weight_ of each of these substances, when made into
manure—provided nothing is washed out by the rains—will return to the
soil the above quantities of inorganic matter _in pounds_. Generally,
perhaps, these numbers will give the reader an idea of the relative
_permanent_ effect of these different kinds of vegetable matter when
laid upon the soil. But, by a reference to the facts stated in pp. 58
to 64, in regard to the _quality_ of the inorganic matter contained in
plants, he will satisfy himself, that the effect of these manures on
particular crops is not to be judged of solely by the absolute quantity
of earthy and saline matter they contain;—that which the turnip-top,
for example, or the bean-stalk, returns to the soil, may not be exactly
what will best promote the growth of wheat.
2. On the other hand, if the fertilizing value of vegetable substances
is to be calculated by the relative quantities of nitrogen they
severally contain, we should place them in the following order:—the
number opposite to each substance representing that weight of it in
pounds, which would produce the same effect as 100 pounds of farm-yard
manure, consisting of the mixed droppings and litter of cattle.
Equivalent
quantities
in pounds.
Farm-yard manure, 100
Wheat straw, 80 to 170
Oat straw, 150
Barley straw, 180
Buckwheat, 85
Pea straw, 45
Wheat chaff, 50
Green grass, 80
Potato tops, 75
Fresh sea-weed, 80
Rape dust, 8
Fir saw-dust, 250
Oak saw-dust, 180
Coal soot, 30
This table again presents the same substances in a somewhat different
order of value; shewing, for example, not only that such substances
as rape dust and soot should produce a much more remarkable effect
upon vegetation, than the same weight even of farm-yard manure, but
also that certain dry vegetables, such as chaff and pea straw, will
yield, when not unduly fermented, a more enriching manure than barley,
oat, or wheat straw. It agrees, also, with the known effect of green
manuring upon the land, since 80 pounds of meadow-grass ploughed in,
will be equal in virtue to 100 of farm-yard manure.
Some writers ascribe the _entire_ action of these measures to the
nitrogen they contain. This, however, is taking a one-side view of
their real natural operation. The nitrogen, during their decay, is
liberated chiefly in the form of ammonia—an evanescent substance,
producing an immediate effect in hastening or carrying further forward
the growth of the plant, but not remaining permanently in the soil. The
reader, therefore, will form an opinion consistent alike with theory
and with practice, if he conclude—
1. That the _immediate_ effect of a vegetable manure, in hastening the
growth of plants, is dependent, in a great degree, upon the quantity of
nitrogen it contains and gives off during its decay in the soil.
2. That the _permanent_ effect and value of manures is to be estimated
chiefly by the quantity and quality of the inorganic matter they
contain—of the ash they leave when burned.
The effect of the nitrogen may be nearly expended in a single
season—that of the earthy and saline matter may not be exhausted for
several years.
Nor is the carbon of vegetable substances without its important uses
to vegetation. From the statements contained in the earlier chapters
of the present work, it may be inferred that, however much influence
we may allow to the nitrogen and to the earthy matter of plants in
aiding the growth of future races—the soundest view of these important
natural operations is that which considers each element present in
decaying plants to be capable of ministering food to such as are still
alive,—though we may not be able as yet, either to estimate the precise
importance of each element to any particular kind of crop, or exactly
to adjust their relative quantities in our manures, so as to promote
the growth of such a crop in the greatest possible degree.
CHAPTER IX.
Animal Manures—Their relative value and mode of
Action—Difference between Animal and Vegetable
Manures—Cause of this difference—Mineral Manures
—Nitrates of Potash and Soda—Sulphate of Soda,
Gypsum, Chalk, and Quicklime—Chemical action of
these Manures—Artificial Manures—Burning and
Irrigation of the Soil—Planting and laying down
to grass.
The animal substances employed as manure consist chiefly of the flesh,
blood, bones, horns, and hair of animals, of fish—which in some places
are found in sufficient quantity to be laid upon the land—and of the
solid and liquid excrements of animals and birds.
SECTION I.—OF UNDIGESTED ANIMAL MANURES.
Animal substances, in general, act more powerfully as manures than
vegetable substances—it is only the seeds of plants which can at all
compare with them in efficacy.
The _flesh_ of animals is rarely used as a manure, except in the case
of dead horses, or cattle which cannot be used for food. Fish is
chiefly applied in the form of the refuse of the herring and pilchard
fisheries, though occasionally such shoals of sprats, herrings, and
even mackerel, have been caught on our shores, as to make it necessary
to employ them as manure. These recent animal substances are found to
be too _strong_ when applied directly to the land; they are generally,
therefore, made into a compost, with a large quantity of soil. Five
barrels of fish, or fish refuse, made into twenty loads of compost,
will be sufficient for an acre. The refuse of fish oils,—of the fat of
animals that has been melted for the extraction of the tallow—of skins
that have been boiled for the manufacture of glue—horns, hair, wool
(woollen rags), and all similar substances, when made into composts,
exercise, in proportion to their weight, a much greater influence upon
vegetation than any of the more abundant forms of vegetable matter.
Even the bodies of insects are in many parts of the world important
manures of the soil. In warm climates, a handful of soil sometimes
seems almost half made up of the wings and skeletons of dead
insects—the peasant in Hungary and Carinthia occasionally collects as
many as thirty cart-loads of dead marsh flies in a single year;—and in
the richer soils of France and England, where worms and other insects
abound, the presence of their remains in the soil must also aid its
natural productiveness.
_Blood_ is rarely applied to the land directly—though, like the other
parts of animals, it makes an excellent compost. As it comes from
the sugar refineries, however, in which, with lime water and animal
charcoal, it is employed for the refining of sugar, it has obtained
a very extensive employment, especially in the south of France. This
animal black, or _animalized charcoal_, as it is sometimes called,
contains about twenty per cent. of blood, and has risen to such a price
in France, that the sugar refiners actually sell it for more than the
unmixed blood and animal charcoal originally cost them. This has given
rise to the manufacture of artificial mixtures of charcoal, fecal
matters, and blood, which are also sold under the name of animalized
charcoal. The only disadvantage attending these artificial preparations
is, that they are liable to be adulterated, or, for cheapness, prepared
in a less efficient manner.
_Horn_, _hair_, and _wool_, depend for their efficacy precisely on the
same principles as the blood and flesh of animals. They differ chiefly
in this, that they are _dry_, while blood and flesh contain 80 to 90
per cent. of their weight of water. Hence, a ton of horn shavings, of
hair, or of dry woollen rags, ought to enrich the soil as much as ten
tons of blood. In consequence, however, of their dryness, the horn and
wool decompose much more slowly than the blood. Hence, the effect of
soft animal matters is more immediate and apparent, that of hard and
dry substances less visible, but continuing for a much longer period of
time.
_Bones_, again, while they resemble horn in being dry, differ from it
in containing, besides the animal matter, a large quantity of earthy
matter also, and hence they introduce a new agent to aid their effect
upon the soil. Thus, the bones of the cow consist of 100 lbs. of
Phosphate of lime, 55½
Phosphate of magnesia, 3
Soda and common salt, 3½
Carbonate of lime, 3¾
Fluoride of calcium, 1
Gelatine (the substance of _horn_), 33¼
-------
100
While 100 lbs. of bone-dust, therefore, add to the soil as much
_organic_ animal matter as 33 lbs. of horn, or as 300 or 400 lbs.
of blood or flesh, they add, at the same time, much _inorganic_
matter—lime, magnesia, soda, common salt, and phosphoric acid (in
the phosphates),—all of which, as we have seen, must be present in a
fertile soil, since the plants require a certain supply of them all
at every period of their growth. These substances, like the inorganic
matter of plants, may remain in the soil, and may exert a beneficial
action upon vegetation after all the organic or gelatinous matter has
decayed and disappeared.
From what is above stated, therefore, the reader will gather these
general conclusions:
1. That animal substances which, like flesh and blood, contain much
water, decay rapidly, and are fitted to operate _immediately_ and
powerfully upon vegetation, but are only temporary or evanescent in
their action.
2. That when dry, as in horn, hair, and wool, they decompose, and
consequently act more slowly, and continue to manifest an influence, it
may be, for several seasons.
3. That bones, acting like horn, in so far as their animal matter
is concerned, and, like it, for a number of seasons, more or less,
according as they have been more or less finely crushed—may ameliorate
the soil by their earthy matter for a still longer period—permanently
improving the condition and adding to the natural capabilities of the
land.
SECTION II.—OF DIGESTED ANIMAL MANURES.
Practical men have long been of opinion that the digestion of food,
either animal or vegetable,—the passing of it through the bodies of
animals,—enriches its fertilizing power, weight for weight, when
added to the land. Hence, in causing animals to eat up as much of the
vegetable productions of the farm as possible, it is supposed that not
only is so much food saved, but that the value of the remainder in
fertilizing the land is greatly increased. In a subsequent section we
shall see how far theory serves to throw light upon these opinions.
(See Section IV., p. 182 to 186.)
I. LIQUID EXCRETIONS.
The digested animal substances usually employed as manures are, the
urine of the cow and the sheep, the solid excrements of the horse, the
cow, the sheep, and the pig, the droppings of pigeons and other birds,
and night-soil. The liquid manures act chiefly through the saline
substances they hold in solution, while the solid manures contain also
insoluble matters, which decay slowly in the soil, and there become
useful only after a time. The former, therefore, will influence
vegetation more powerfully at first; the action of the latter will be
less evident, but will continue to operate for a much longer period.
_Urine._—Human urine consists, in 1000 parts, of
Water, 932
_Urea_, and other organic matters containing nitrogen, 49
_Phosphates_ of ammonia, lime, soda, and magnesia, 6
_Sulphates_ of soda and ammonia, 7
Sal ammoniac and common salt, 6
----
1000
A thousand pounds of urine therefore contain 68 lbs. of dry fertilizing
matter of the richest quality, worth, _at the present rate of selling
artificial manures in this country_, at least 20s. a cwt. As each
person voids almost 1000 lb. of urine in a year, the national waste
incurred in this form amounts, at the above valuation, to 12s. a head.
And if five tons of farm-yard manure per acre, added year by year, will
keep a farm in good heart, four cwt. of the solid matter of urine would
probably have an equal effect; or the urine alone discharged into the
rivers by a population of 10,000 inhabitants would supply manure to a
farm of 1500 acres, yielding a return of 4500 quarters of corn or an
equivalent produce of other crops.
The urine of the cow is said to contain less water than that of man,
though of course much must depend upon the kind of food with which it
is fed. Reckoning, then, the large quantity of liquid manure that
is yielded by the cow (2000 or 3000 gallons a year), we may safely
estimate the solid matter given off by a healthy animal in this form in
twelve months at 1200 to 1500 pounds weight, worth, _if it were in the
dry state_, from £10 to £12 sterling. In the _liquid_ state, the urine
of one cow collected and preserved as it is in Flanders, is valued in
that country at about £2 a year. Any practical farmer may calculate for
himself, therefore, how much real wealth, taking it even at the Flemish
value, is lost in his own farm-yard—how much of the natural means of
reproductive industry passes into his drains or evaporates into the air.
This liquid manure is invaluable, when collected in tanks, for watering
the manure and compost heaps, and thus hastening their decomposition;
but great part of it may also be sprinkled directly upon the fields of
grass and upon the young corn, with the best effects. It must, however,
be permitted to stand till fermentation commences, and afterwards
diluted with a considerable quantity of water, before it will be in the
best condition for laying on the land.
_Urate._—In order to obtain the virtues of animal urine in a
concentrated form, the custom has been adopted of mixing burnt gypsum
with it, in the proportion of 10 lbs. to every 7 gallons, allowing the
mixture, occasionally stirred, to stand some time, pouring off the
liquid, and drying and crushing the gypsum. This is sold by manure
manufacturers under the name of _urate_. It never can possess, however,
the virtues of the urine, since it does not contain the soluble saline
substances, which the gypsum does not carry down with it. Except the
gypsum, indeed, 100 lbs. of urate contain no greater weight of saline
and organic matter than 10 gallons of urine. If it be true, then, as
the manufacturers state, that 3 or 4 cwt. of urate are sufficient
manure for an acre, the practical farmer will, I hope, draw the
conclusion,—not that it is well worth his while to venture his money
in trying a portion of it upon a piece of his land,—but that a far
more promising adventure will be to go to some expense in saving his
own liquid manure, and, after mixing it with burned gypsum, to lay it
abundantly upon all his fields.
II. SOLID EXCRETIONS.
_Cow and Horse Dung._—So much of the saline, nutritive, and soluble
organic matters from the cow pass off in the liquid form, that cow
dung is correctly called cold, since it does not readily heat and run
into fermentation. Mixed with other manures, however, or well diffused
through the soil, it aids materially in promoting vegetation. The horse
being fed generally on less liquid food, and discharging less urine,
yields a hotter and richer dung, which, however, answers best also when
mixed with other varieties. The dung of the swine is soft and _cold_,
like that of the cow, containing, like it, at least 75 per cent. of
water. As this animal lives on more varied food than any other reared
for the use of man, the manure obtained from it is also very variable
in quality. Applied alone, as a manure to roots, it is said to give
them an unpleasant taste, and even to injure the flavour of tobacco. It
answers best for hemp, and, it is said, also for hops; but, mixed with
other manures, it may be applied to any crop.
_Night-soil_ is probably the most valuable, and yet, in Europe at
least, the most disliked and neglected of all the solid animal manures.
It varies no doubt in richness with the food of the inhabitants of each
district,—chiefly with the quantity of animal food they consume,—but
when dry, no other solid manure, weight for weight, can probably be
compared with it in general efficacy. It contains much soluble and
saline matter, and as it is made up from the constituents of the food
we eat, of course it contains most of those elementary substances
which are necessary to the growth of the plants on which we principally
live.
Attempts have been made to dry this manure also, so as to render it
more portable,—to destroy its unpleasant smell, so as to reconcile
practical men to a more general use of it,—and by certain chemical
additions, to prevent the waste of ammonia and other volatile
substances, which are apt to escape and be lost when this and other
powerful animal manures begin to putrify through decay. In Paris,
Berlin, and other large cities, the night-soil, dried first in the air
with or without a mixture of gypsum or lime, then upon drying plates,
and finally in stoves, is sold under the name of _poudrette_, and is
extensively exported in casks to various parts of the country. In
London also it is dried with various mixtures, while in others of our
large towns an _animalized charcoal_ is prepared by mixing and drying
night-soil with gypsum and ordinary wood charcoal in fine powder.
The half-burned peat above described (p. 80,) would answer well for
such a purpose, while few simple and easily attainable substances would
make a better compost with night-soil, and more thoroughly preserve its
virtues, than half-dry peat or rich vegetable soil, mixed with more or
less marl or gypsum. It is impossible to estimate the proportion of
waste which this valuable manure undergoes by being allowed to ferment,
without mixture, in the open air.
_Taffo._—In China it is kneaded into cakes with clay, which are dried
in the air, and, under the name of taffo, form an important article of
export from all the large cities of the empire.
_Pigeons’ Dung._—The dung of all birds is found to possess eminent
fertilizing virtues. Some varieties are stronger than others, or more
immediate in their action, and all are improved for the use of the
farmer by being some time kept, either alone or in compost. In Flanders
the manure of one hundred pigeons is considered worth 20s. a year for
agricultural purposes.
_Guano_ is the name given by the natives of Peru to the dung of
sea-fowl, which in former periods used to be deposited in vast
quantities on the rocky shores and isles of the Peruvian coast. The
numerous shipping of modern times has disturbed and driven away many of
the sea-fowl, so that comparatively little of their recent droppings
is now preserved or collected. Ancient heaps of it, however, still
exist in many places, more or less covered up with drifted sand, and
also more or less decomposed. These are now largely excavated for
exportation, not only to different parts of the coast of Peru, as seems
to have been the case from the most remote periods, but also to Europe,
and especially to England. It is at present sold at 20s. a cwt. in this
country, and is capable of entirely replacing farm-yard dung,—that is
to say, turnips may be manured successfully with guano alone;—but it
has not yet been satisfactorily determined that the English farmer can
afford to use it in this way to any extent, at the price now asked for
it.
The dung of birds possesses the united virtues of both the liquid and
solid excretions of other animals. It contains every part of the food
of the bird, with the exception of what is absolutely necessary for
the support and for the right discharge of the functions of its own
body. It is thus fitted, therefore, to return to the plant a greater
number of those substances on which plants live, than either the solid
or the fluid excrements of other animals; in other words, to be more
nourishing to vegetable growth.
SECTION III.—OF THE RELATIVE GROWTH OF THE DIFFERENT ANIMAL MANURES.
The fertilizing power of animal manures, in general, is dependent,
like that of the soil itself, upon the happy admixture they contain
of a great number, if not of all, those substances which are required
by plants in the universal vegetation of the globe. Nothing they
contain, therefore, is without its share of influence upon their
general effects, yet the amount of nitrogen present in each affords the
readiest and most simple criterion by which their agricultural value,
compared with that of vegetable matters and with that of each other,
can be pretty nearly estimated.
In reference to their relative quantities of nitrogen, therefore, they
have been arranged in the following order, the number opposite to each
representing the weight in pounds which is equivalent to or would
produce the same sensible effect upon the soil as 100 lbs. of farm-yard
manure.
Farm-yard manure, 100
Solid excrements of the cow, 125
” ” ” horse, 73
Liquid ” ” cow, 91
” ” ” horse, 16
Mixed ” ” cow, 98
” ” ” horse, 54
” ” ” sheep, 36
” ” ” pig, 64
Dry flesh, 3
Pigeons’ dung, 5
Flemish liquid manure, 200
Liquid blood, 15
Dry blood, 4
Feathers, 3
Cow hair, 3
Horn shavings, 3
Dry woollen rags, 2½
It is probable that the numbers in this table do not err very widely
from the true relative value of these different manures, in so far as
the _organic_ matter they severally contain is concerned. The reader
will bear in mind, however,
1. That the most powerful substances in this table, woollen rags, for
example,—2½ lbs. of which are equal in virtue to 100 lbs. of farm-yard
manure,—may yet shew less immediate sensible effect upon the crop than
an equal weight of sheep’s dung, or even of urine. Such dry substances
are long in dissolving and decomposing, and continue to evolve
fertilizing matter, after the softer and more fluid manures have spent
their force. Thus, while farm-yard manure or rape dust will immediately
hasten the growth of turnips, woollen rags will come into operation at
a later period, and prolong their growth into the autumn.
2. That besides their general relative value, as represented in the
above table, each of these substances has a further special value not
here exhibited, dependent upon the kind and quantity of saline and
other inorganic matter which they severally contain. Thus three of dry
flesh are equal to five of pigeons’ dung, in so far as the _organic_
part is concerned; but the latter contains also a considerable quantity
of bone-earth and of saline matter scarcely present at all in the
former. Hence pigeons’ dung will benefit vegetation in circumstances
where dry flesh would in some degree fail. So the liquid excretions
contain much important saline matter not present in the solid
excretions,—not present either in such substances as horn, wool, and
hair,—and, therefore, each must be capable of exercising an influence
upon vegetation peculiar to itself.
Hence the practical farmer sees the reason why no one _simple_ manure
can long answer on the same land; and why in all ages and countries the
habit of employing _mixed_ manures and artificial composts has been
universally diffused.
SECTION IV.—NATURAL DISTINCTION OR DIFFERENCE BETWEEN ANIMAL AND
VEGETABLE MANURES, AND THE CAUSE OF THIS DIFFERENCE.
In what do animal manures differ from vegetable manures,—what is the
cause of this difference,—how does the digestion of vegetable matter
improve its value as a manure?
1. The characteristic distinction between animal and vegetable manures
is this,—that the former contain a much larger proportion of nitrogen
than the latter. This will be seen at once, by comparing together
the tables given in the two preceding sections, in which the numbers
represent the relative agricultural values of certain animal and
vegetable substances compared with farm-yard manure. The lowest numbers
represent the highest value, and the largest amount of nitrogen, and
these low numbers are always opposite to the purest animal substances.
2. In consequence of containing so much nitrogen, animal substances
are further distinguished by the rapidity with which, when moist, they
putrify or run to decay. During this decay the nitrogen they contain
gradually assumes the form of ammonia, which is perceptible by the
smell, and which, when proper precautions are not taken, is apt in
great part to escape into the air. Hence the loss by fermenting manure
too completely,—or without proper precautions to prevent the escape of
volatile substances. And as animal manure, when thus over-fermented, or
permitted to lose its ammonia into the air, is found much less active
upon vegetation than before; it is reasonably concluded, that to this
ammonia, chiefly, their peculiar virtue, when rightly prepared, is in a
great measure to be ascribed.
Vegetable substances do not decay so rapidly,—do not emit the odour of
ammonia when fermenting,—nor, when prepared in the most careful way,
does vegetable manure exhibit the same remarkable action upon vegetable
life as is displayed by almost every substance of animal origin.
3. Whence do animal substances derive all this nitrogen? Animals live
only upon vegetable productions containing little nitrogen; can they
then procure all they require from this source alone? Again, does the
act of digestion produce any chemical alteration upon the food of
animals, that their excretions should be a better manure,—should be
richer in nitrogen than the substances on which they feed? Does theory
throw any light upon the opinion generally entertained among practical
men upon this point?
These two apparently distinct questions will be explained by a brief
reference to one common natural principle.
Animals have two necessary vital functions to perform,—to breathe and
to digest. Both are of equal importance to the health and general
welfare of the animal. The digester (the stomach) receives the food,
melts it down, extracts from it what is best suited to its purposes,
and conveys it into the blood. The breathers (the lungs) sift the blood
thus mixed up with the newly digested food, combine oxygen with it,
and extract carbon,—which carbon, in the form of carbonic acid, they
discharge by the mouth and nostrils into the air.
Such is a general description of these two great processes,—their
effect upon the food that remains in the body and has to be rejected
from it, is not difficult to perceive.
Suppose an animal to be full grown. Take a full grown man. All that he
eats as food is intended merely to renovate or replenish his system,
to restore that which is daily removed from every part of his body by
natural causes. _In the full grown state, every thing that enters the
body must come out of the body_ in one form or another. The first part
of the food that escapes is that portion of its carbon that passes off
from the lungs during respiration. This quantity varies in different
individuals—chiefly according to the quantity of exercise they take.
From 5 to 9 ounces a day is the average quantity, though in periods of
violent bodily exertion 13 to 15 ounces of carbon are breathed out in
the form of carbonic acid.
Suppose a man to eat a pound and a half of bread and a pound of beef in
24 hours, and that he gives off by respiration 8 ounces of carbon (3500
grains) during the same time. Then he has
Carbon. Nitrogen.
Taken, in his food, about 4500 grains, and 500 grs. while
He has given off in }
respiration, } 3500 and little or no nitrogen,
Leaving to be converted }
into food, or to } 1000 grs. and 500 grs.
be rejected, }
Our two conclusions, therefore, are clear. The vegetable food, by
respiration, is freed from a large portion of its carbon, which is
discharged into the air,—nearly the whole of the nitrogen remaining
behind. In the food consumed the carbon was to the nitrogen as 9 to 1;
in that which remains, after respiration has done its work, the carbon
is to the nitrogen in the proportion of only 2 to 1.
It is out of this residue, rich in nitrogen, that the several parts of
animal bodies are built up. Hence the reason why they can be formed
from food poor in nitrogen, and yet be themselves rich in the same
element.
It is this same residue also which, after it has performed its
functions within the body, is discharged again in the form of solid and
liquid excretions. Hence the greater richness in nitrogen,—the greater
fertilizing power of the dung of animals than of the food on which they
live.
Two other remarks I shall add for the benefit of the practical man.
1. The manure of the cow, taking it mixed, is not so rich in nitrogen
as that of man,—because the cow in the stall, large though it be, and
great the bulk of food it consumes, does not give off much more carbon
by respiration than an active full grown man. Hence the proportion of
carbon in the excretions of this animal is greater than in those of
man. The dry manure is richer than the dry food, weight for weight, but
not in the same proportion as if the cow respired a quantity of carbon
more nearly corresponding to its bulk, when compared with the weight of
carbon thrown off from the lungs of man.
2. Since the parts of animals—their blood, muscles, tendons, and
the gelatinous portion of the bones—contain much nitrogen, young
beasts which are growing, must appropriate to their own use, and
work up into flesh and bone, a portion of the nitrogen contained
in the _non-respired_ part of their food. But the more they thus
appropriate, the less will pass off into the fold-yard; and hence it
is natural to suppose that the manure, either liquid or solid, which
is prepared where many growing cattle are fed, will not be so rich
as that which is yielded by full grown animals. I am not aware how
far this deterioration has been observed in practice, but it may with
some degree of certainty be expected to take place,—unless by giving
a richer food to the young cattle, the difference to the farm-yard be
made up.[18]
[18] Though I have dwelt as long upon these interesting and, I believe,
novel considerations, as the limits of this little work will permit,
yet I must refer the reader for fuller details, and to perhaps a
clearer exposition of the principles above advanced than I have here
been able to give, to my “LECTURES _on Agricultural Chemistry and
Geology_.”
SECTION V.—OF MINERAL WATERS.
The general nature and mode of operation of such mineral substances as
are capable of acting as manures, will be in some measure understood
from what has already been so fully stated in regard to the necessity
of inorganic food to living plants, and to the kinds of such food
which they specially require. A slight notice, therefore, of the more
important of these manures now in use will here be sufficient.
1. _Nitrates of Potash and Soda._—Saltpetre and nitrate of soda have
been deservedly commended for their beneficial action, especially
upon _young_ vegetation. They are distinguished by imparting to the
leaves a beautiful dark green colour, and are applied with advantage
to grass and young corn, at the rate of 1 cwt. to ½ cwt. per acre. The
nitric acid they contain yields nitrogen to the plant, while potash and
soda are also put within reach of its roots, and no doubt serve many
beneficial purposes.
_Sulphate of Soda_, or Glauber’s salt, has lately been recommended in
this country for clovers, grasses, and green crops. Mixed with nitrate
of soda it produces remarkable crops of potatoes.[19]
[19] See the author’s “_Suggestions for Experiments in Practical
Agriculture_,” Nos. 1 & 2.
_Sulphate of Magnesia_, or Epsom salts, might also be beneficially
applied in agriculture, probably to clovers and corn crops. As it can
be had in pure crystals at 10s. a cwt., and in an impure state at a
much less price, from the alum works, it might readily be submitted to
trial.
_Sulphate of Lime_, or Gypsum, is in Germany applied to grass lands
with great success, over large tracts of country. In the United States
it is used for every kind of crop. It is especially adapted to clovers
and legumes.
These three substances all afford sulphur to the growing plant,
while the lime, soda, and magnesia are themselves in part directly
appropriated by it, and in part employed in preparing other kinds of
food, and in conveying them into the ascending sap.
Though there can be no question that these and similar substances are
really useful to vegetation, yet the intelligent reader will not be
surprised to find, or to hear, that this or that mineral substance has
not succeeded in benefitting the land in this or that district. If he
has already bricks enough at hand, you must carry the builder mortar,
or he will be unable to go on with his work: so, if the soil contain
gypsum or sulphate of magnesia in sufficient natural abundance, it is
at once a needless and a foolish waste to attempt to improve the land
by adding more; it is still more foolish to conclude that these same
saline compounds are unlikely to reward the patient experimenter in
other localities.
_Common Salt_ has undoubtedly, in very many districts, a fertilizing
influence upon the soil. The theoretical agriculturist knows that a
small quantity of it is absolutely necessary to the healthy growth
of all our cultivated crops, and he will therefore, early try by a
preliminary experiment upon one of his fields, whether or not they
require the addition of this species of vegetable food. It is in inland
and sheltered situations, and on high lands often washed by the rains,
that the effect of common salt is likely to be most appreciable. The
spray of the sea, borne to great distances by the winds, is in many
districts, where prevailing sea winds are known, sufficient to supply
an ample annual dressing of common salt to the land.
_Kelp._—Among mineral substances kelp ought not properly to be
included, since it is the ash left by the burning of sea-weed. It,
however, partakes of the nature of mineral substances, and may,
therefore, be properly considered in this place. It contains potash,
soda, silica, sulphur, chlorine, and several other of the inorganic
constituents of plants required by them for food. It is nearly the same
also—with the exception of the organic matter which is burned away—with
the sea-weed which produces such remarkably beneficial effects upon the
soil. In the Western Isles a method is practised of half burning or
charring sea-weed, by which it is prevented from melting together, and
is readily obtained in the form of a fine black powder. The use of this
variety ought to combine the beneficial action of the ordinary saline
constituents of kelp, with the remarkable properties observed in animal
and vegetable charcoals.
_Wood-ash_, among other compounds, contains a portion of common
_pearl-ash_ in an impure form, with sulphate also, and _silicate_ of
potash. These are all valuable in feeding and in preparing the food of
plants, and hence the extensive use of wood-ash as a manure in every
country where it can readily be procured.
_Dutch ashes_ are the ashes of peat burned for the purpose of being
applied to the land. They vary in constitution with the kind of peat
from which they have been prepared. They often contain traces of potash
and soda, and generally a quantity of gypsum and carbonate of lime, a
trace of phosphate of lime, and much siliceous matter. In almost every
country where peat abounds, the value of peat ashes as a manure has
been more or less generally recognised.
SECTION VI.—USE OF LIME, SHELL-SAND, AND MARL.
The use of lime is of the greatest importance in practical
agriculture. It has been employed, in Europe at least, in one or
other of its forms of shells, shell-sand, marl, chalk, limestone, and
quicklime, from the most remote periods.
Native limestone, and all the unburned varieties of chalk, shells, &c.
consist of _carbonate of lime_ (p. 51), more or less pure. When burned
in the kiln, the carbonic acid is driven off, and lime, burned lime, or
quicklime remains.
Quicklime, when exposed to the air, gradually falls into the state
of an exceedingly fine white powder. It will do so more rapidly if
water be thrown upon it, when it also heats much, swells, and becomes
about one-third heavier than before. After being exposed to the air
for some time in this white powdery state, it is found to have again
absorbed from the air a portion of carbonic acid, though a very long
period generally elapses before it is all reconverted into carbonate.
In compost heaps, where much carbonic acid is formed during the
fermentation, the conversion of any quicklime that may be mixed with
them into carbonate of lime, is much more rapid and complete than in
the open air.
Lime, therefore, is laid on the land in two states.
_1st_, In the _mild_ state—that of carbonate—in marls, in chalk, in
shell-sand, &c.
_2d_, In the _caustic_, or quick state, as it comes hot from the kiln,
or after it is simply slaked.
Limes are laid on also in a more or less pure form. Marl contains only
from 5 to 20 per cent. of carbonate of lime, generally in the state
of a very fine powder. Shell-sand consists of a mixture of minute
fragments of shells with from 20 to 50 per cent. of siliceous sand. The
limestones which are burned are also more or less impure, though, when
the impurity is very great, they do not burn well, and are therefore
usually rejected.
Some limestones contain much magnesia, by which their agricultural
qualities are materially affected. These are known by the name of
_magnesian_ limestones. There are few limestones in which a small
quantity of magnesia may not be detected, and this minute proportion is
likely to be beneficial rather than otherwise; but when it is present
to the amount of 10 per cent. or upwards, it appears to have for some
time a poisonous influence upon vegetation, if added in the same large
doses in which other lime may be safely spread upon the land.
The quantity of lime laid on at a single dressing, and the frequency
with which it may be repeated, must depend upon the kind of land, upon
the depth of the soil, and upon the species of culture to which it is
subjected. If land be wet, or badly drained, a larger application is
necessary to produce the same effect, and it must be more frequently
repeated. When the soil is thin, again, a smaller addition will
thoroughly impregnate the whole, than where the plough usually descends
to the depth of 8 or 10 inches. On old pasture lands, where the tender
grasses live in two or three inches of soil only, a feebler dressing,
more frequently repeated, appears to be the more reasonable practice,
though in reclaiming and laying down lands to grass, a heavy first
liming is often indispensable.
In arable culture larger doses are admissible, both because the soil
through which the roots penetrate must necessarily be deeper, and
because the tendency to sink beyond the reach of the roots is generally
counteracted by the frequent turning up of the earth by the plough.
Where vegetable matter abounds, much lime may be usefully added, and on
stiff clay lands after draining, its good effect is most remarkable.
On light land, chiefly because there is neither moisture nor vegetable
matter present in equal quantity, very large applications of lime are
not so usual, and some prefer adding it to such lands in the shape of
composts only.
The largest doses, however, which are applied in practice, alter in
a very immaterial degree the chemical constitution of the soil. We
have seen that the best soils generally contain a natural proportion
of lime, not fixed in quantity, yet scarcely ever wholly wanting. But
an ordinary liming, when well mixed up with a deep soil, will rarely
amount to _one per cent._ of its entire weight. It requires about 300
bushels of burned lime per acre to add one per cent. of lime to a soil
of twelve inches in depth; if only mixed to a depth of six inches, this
quantity would add about two per cent. to the soil.
The most remarkable visible alterations produced by liming are—upon
_pastures_, the greater fineness, closeness, and nutritive character
of the grasses—on _arable lands_, the improvement in the texture and
mellowness of stiff clays, the more productive crops and the earlier
period at which they ripen.
But these effects gradually diminish year by year, till the land
returns again nearly to its original condition. On analyzing the
soil, the lime originally added is found to be in great measure, or
altogether, gone. In this condition the land must either be limed
again, or must be left to produce sickly and un-remunerating crops.
This removal arises from two causes. The rain-water that descends upon
the land holds in solution carbonic acid which it has absorbed from
the air. But water charged with carbonic acid is capable of dissolving
carbonate of lime, and thus year after year the rains slowly remove
as they sink to the drains, or run over the surface, a portion of the
lime which the soil contains. Acid substances are also formed naturally
in the land, by which another portion of the lime is rendered easily
soluble in water, and, therefore, readily removable by every shower
that falls.
The _chemical_ effects of lime upon the soil are chiefly the following:—
1. When laid upon the land in the _caustic_ state, the first action of
the lime is to combine immediately with every portion of acid matter
it may contain, and thus to sweeten the soil. Some of the compounds it
thus forms being soluble in water, either enter into the roots and feed
the plant,—supplying it at once with lime and with organic matter,—or
are washed out by the springs and rains, while other compounds, which
are insoluble, remain more permanently in the soil.
2. Another portion decomposes certain saline compounds of iron,
manganese, and alumina, which naturally form themselves in the soil,
and thus renders them unhurtful to vegetation. A similar action is
exerted upon certain compounds of potash and soda, and of ammonia,—if
any such are present,—by which these substances are set and placed
within the reach of the plant.
3. Its presence in the caustic state further disposes the organic
matter of the soil to undergo more rapid decomposition—it being
observed, that where lime is present in readiness to combine with the
substances produced during the decay of organic matter, that decay,
if other circumstances be favourable, will proceed with much greater
rapidity. The reader will not fail to recollect, that during this
decay many compounds are formed which are of importance in promoting
vegetation.
4. Further, quicklime has the advantage of being soluble in cold water,
and thus the complete diffusion of it through the soil is aided by the
power of water to carry it in solution in every direction.
5. When it has absorbed carbonic acid, and become reconverted into
carbonate, the original caustic lime has no _chemical_ virtue over
chalk, rich shell-sand or marl, or crushed limestone. It has, however,
the important _mechanical_ advantage of being in the form of a far
finer powder, than any to which we could reduce the limestone by
art—in consequence of which it can be more uniformly diffused through
the soil, and placed within the reach of every root, and of almost
every particle, of vegetable matter that is undergoing decay. I shall
mention only three of the important purposes which, in this state of
_carbonate_, lime serves upon the land.
1. It directly affords food to the plant, which, as we have seen,
languishes where lime is not attainable. It serves also to convey other
food to the roots in a state in which it can be made available to
vegetable growth.
2. It neutralizes (removes the _sourness_) of all acid substances as
they are formed in the soil, and thus keeps the land in a condition to
nourish the tenderest plants. This is one of the important agencies
of shell-sand when laid on undrained grass lands—and this effect it
produces in common with wood-ashes, and many similar substances.
3. During the decay of organic matter in the soil, it aids and
promotes the production of nitric acid,—so influential, as I believe,
in the general vegetation of the globe (see page 35). With this acid
it combines and forms _nitrate of lime_—a substance very soluble in
water—entering readily, therefore, into the roots of plants, and
producing upon their growth effects precisely similar to those of the
now well known _nitrate of soda_. The success of frequent ploughings,
harrowings, hoeings, and other modes of stirring the land, is partly
owing to the facilities which these operations afford to the production
of this and other natural nitrates.
SECTION VII.—OF THE IRRIGATION OF THE LAND.
The irrigation of the land is, in general, only a more refined method
of manuring it. The nature of the process itself, however, is different
in different countries, as are also the kind and degree of effect it
produces, and the theory by which these effects are to be explained.
In dry and arid climates, where rain rarely falls, the soil may contain
all the elements of fertility, and require only water to call them
into operation. In such cases, as in the irrigations practised so
extensively in eastern countries, and without which, whole provinces
in Africa and Southern America would lie waste, it is unnecessary to
suppose any other virtue in irrigation than the mere supply of water it
affords to the parched and cracking soil.
But in climates such as our own, there are two other beneficial
purposes in reference to the soil, which irrigation may, and one at
least of which it always does, serve.
2. The occasional flow of _pure_ water over the surface, as in
our irrigated meadows, and its descent into the drains, where the
drainage is perfect, washes out acid and other noxious substances
naturally generated in the soil, and thus purifies and sweetens it.
The beneficial effect of such washing will be readily understood in
the case of peat lands laid down to water meadow, since, as every one
knows, peat soils abound in matters unfavourable to general vegetation,
and which are usually in part drawn off by drainage, and in part
destroyed by lime and by exposure to the air, before boggy lands can be
brought into profitable cultivation.
3. But it seldom happens that _pure_ water is employed for the purposes
of irrigation. The water of rivers, more generally, is diverted from
its course, more or less loaded with mud and other finer particles of
matter, which are either gradually filtered from it as it passes over
and through the soil, or in the case of floods subside naturally when
the waters come to rest. Or in less frequent cases, the drainings of
towns, and the waters from common sewers, or from the little streams
enriched by them, are turned with benefit upon the favoured fields.
These are evidently cases of gradual and uniform manuring. And even
where the water employed is clear and apparently undisturbed by mud,
it always contains saline substances grateful to the plant in its
search for food, and which it always contrives to extract more or less
copiously as the water passes over its leaves or along its roots. Every
fresh access of water affords the grass in reality another liquid
manuring.
In the refreshment continually afforded to the plant by a plentiful
supply of water, in the removal of noxious substances from the soil, or
in the frequent additions of enriching food to the land—the efficiency
of irrigation, therefore, seems entirely to consist.
SECTION VIII.—OF PARING AND BURNING, AND OF BURNED CLAY.
A mode of improvement often resorted to is the paring and burning of
poor land. The efficacy of burned clay, also, even in superseding
manure on good lands, has been highly extolled by some practical men.
1. The effect of paring and burning is easily understood. The matted
sods consist of a mixture of much vegetable with a comparatively small
quantity of earthy matter. When these are burned the ash of the plants
only is left, intimately mixed with the calcined earth. To strew this
mixture over the soil is much the same as to dress it with peat or
wood ashes, the beneficial effect of which upon vegetation is almost
universally recognised. And the beneficial influence of the ash itself
is chiefly due to the ready supply of inorganic food it yields to the
seed, and to the effect which the potash and soda it contains exercise
either in preparing organic food in the soil, or in assisting its
digestion and assimilation in the interior of the plant.
Another part of this process is, that the roots of the weeds and poorer
grasses are materially injured by the paring, and that the subsequent
dressing of ashes is unfavourable to their further growth.
2. Much greater uncertainty hangs over the alleged virtues of burned
clay. That benefits are supposed to have been derived from its use
there can be no doubt, though in many cases the better tillage of the
land generally prescribed along with the use of burned clay, may have
had some share in producing the good results actually experienced
during its use.
By the burning, in kilns or otherwise, any organic matter the clay may
contain will be consumed, and the texture of the clay itself will be
mechanically altered. It will crumble down like a burned brick into a
hard friable powder, and will never again cohere into a paste as before
the burning. It will, therefore, render clay soils more open, and may
thus, when mixed in large quantity, produce a permanent amelioration
in the mechanical texture of many stiff wheat soils. It cannot itself
undergo any chemical change that is likely so to alter its constitution
as to make it a more useful chemical constituent of the soil than
before. Any saline matter we may suppose to be set free could be far
more cheaply added in the form of a top-dressing to the soil.
Bricks, however, are generally more porous than the clay from which
they are formed; burned clay is so also. And all porous substances suck
in and _condense_ much air and many vapours in large quantities into
their pores. In consequence of this property, porous substances, like
charcoal and burned clay, are supposed, when mixed with the soil, to be
continually yielding air to decaying vegetable matter on the one hand,
and as continually re-absorbing it from the atmosphere on the other,
and by this means to be of singular service in supplying the wants of
plants in the earlier seasons of their growth. The vapours of nitric
acid and of ammonia, which float in the air, they are also supposed to
imbibe, and by the beneficial action of the substances believed to be
thus conveyed by burned clay into the soil, the fertilizing virtues
ascribed to it are attempted to be explained.
It must be confessed, however, that on this point considerable
obscurity still rests. It is in some measure doubtful what the true
action of charcoal and of burned clay is, both in _kind_ and in
_quantity_. It is the part of science, therefore, to decline offering
more than a mere conjecture till the facts to be explained are more
fully and satisfactorily demonstrated.
SECTION IX.—PLANTING AND LAYING DOWN TO GRASS.
1. _Planting._—It has been observed that lands which are unfit for
arable culture, and which yield only a trifling rent as natural
pasture, are yet in many cases capable of growing profitable
plantations, and of being greatly increased in permanent value by the
prolonged growth of wood. Not only, however, do all trees not thrive
alike on the same soil, but all do not improve the soil on which they
grow in an equal degree.
Under the Scotch fir, for example, the pasture is not worth 6d. more
per acre than before it was planted—under the beech and spruce, it
is worth even less than before, though the spruce affords excellent
shelter;—under ash, it gradually acquires an increased value of 2s.
or 3s. per acre. In oak copses, it becomes worth 5s. or 6s., but only
during the last eight years (of the twenty-four), before it is cut
down. But under the larch, after the first thirty years, when the
thinnings are all cut, land not worth originally more than 1s. per
acre, becomes worth 8s. to 10s. per acre for permanent pasture.[20]
[20] The result of trials made on the _mica slate_ and _gneiss soils_
(see page 100) of the Duke of Atholl.
The cause of this improvement is to be found in the nature of the soil,
which gradually accumulates beneath the trees by the shedding of their
leaves. The shelter from the sun and rain which the foliage affords,
prevents the vegetable matter which falls from being so speedily
decomposed, or from being so much washed away, and thus permits it to
collect in larger quantities in a given time, than where no such cover
exists. The more complete the shelter, therefore, the more rapid will
the accumulation of soil be in so far as it depends upon this cause.
But the quantity of leaves which annually falls has also much influence
upon the extent to which the soil is capable of being improved by any
given species of tree, as well as the degree of rapidity with which
those leaves, under ordinary circumstances, undergo decay. The broad
membranous leaf of the beech and oak decay more quickly than the
needle-shaped leaves of the pine tribes, and this circumstance may
assist in rendering the larch more valuable as a permanent improver.
We should expect likewise that the quantity and quality of the
inorganic matter contained in the leaves,—brought up year by year from
the roots, and strewed afterwards uniformly over the surface where the
leaves are shed,—would materially affect the value of the soil they
form. The leaves of the oak contain about 5 per cent. of saline and
earthy matter, and those of the Scotch fir less than 2 per cent.; so
that, supposing the actual weight of leaves which falls from each kind
of tree to be equal, we should expect a greater depth of soil to be
formed in the same time by the oak than by the Scotch fir. I am not
aware of any experiments on the quantity of ash left by the leaves of
the larch.
The improvement of the land, therefore, by the planting of trees,
depends in part upon the quantity of _organic_ food which the trees
can extract from the air, and afterwards drop in the form of leaves
upon the soil, and in part upon the kind and quantity of _inorganic_
matter which the roots can bring up from beneath, and in like manner
strew upon the surface. The quantity and quality of the latter will, in
a great measure, determine the kind of grasses which will spring up,
and the consequent value of the pasture in the feeding of stock. In the
larch districts of the Duke of Atholl, the most abundant grasses that
spring up are said to be the holcus mollis and the holcus lanatus, (the
_creeping_ and the _meadow_ soft-grasses.)
2. _Laying down to grass._—On this point two facts seem to be pretty
generally acknowledged:
_First_, that land laid down to artificial grasses for one, two, three,
or more years, is in some degree rested or recruited, and is fitted for
the better production of after-corn crops. Letting it lie a year or two
longer in grass, therefore, is one of the received modes of bringing
back to a sound condition a soil that has been exhausted by injudicious
cropping.
_Second_, that land thus laid down with artificial grasses deteriorates
more or less after two or three years, and only by slow degrees
acquires a thick sward of rich and nourishing herbage. Hence the
opinion, that grass-land improves in quality the longer it is
permitted to lie,—the unwillingness to plough up old pasture,—and the
comparatively high rents which, in some parts of the country, old grass
lands are known to yield.
Granting that grass lands do thus _generally_ increase in value,
_three_ important facts must be borne in mind before we attempt to
assign the cause of this improvement, or the circumstances under which
it is likely to take place for the longest time and to the greatest
extent.
1. The value of the grass in any given spot may increase for an
indefinite period—but it will never improve beyond a certain extent—it
will necessarily be limited, as all other crops are, by the quality
of the land. Hence the mere laying down to grass will not make _all_
land _good_, however long it may lie. The extensive commons, heaths,
and wastes, which have been in grass from the most remote times, are
evidence of this. They have in most cases yielded so poor a herbage
as to have been considered unworthy of being enclosed as a permanent
pasture.
2. Some grass lands will retain the good condition they thus slowly
acquire for a very long period, and _without manuring_, in the same
way, and upon the same principle, that some rich corn lands have
yielded successive crops for 100 years without manure. The rich grass
lands of England, and especially of Ireland, many of which have been in
pasture from time immemorial, without, it is said, receiving any return
for all they have yielded, are illustrations of this fact.
3. But that others, if grazed, cropped with sheep or meadowed, will
gradually deteriorate, unless some proper supply of manure be given to
them,—which required supply must vary with the nature of the soil, and
with the kind of treatment to which it has been subjected.
In regard to the acknowledged benefit of laying down to grass, then,
two points require consideration,—what form does it assume?—and how is
it effected?
1. The improvement takes place by the gradual accumulation of a
dark-brown soil on the surface, rich in vegetable matter: and which
soil thickens or deepens in proportion to the time which elapses from
its being first laid down to grass.
If the soil be very light and sandy, the thickening is sooner arrested;
if it be moderately heavy land, the improvement continues for a longer
period; and some of the heaviest clays in England are known to bear
the richest permanent pastures. On analyzing the soils of the richest
of these pastures, whatever be the degree of tenacity of the clays
or loams (the subsoils) on which they rest, or their deficiency in
vegetable matter,—they are found to be generally characterized by
containing from 8 to 12 per cent. of organic, chiefly vegetable matter,
from 5 to 10 only of alumina, and from 1 to 6 per cent. of lime.
Thus the soil formed on the surface of all rich old pasture lands
is possessed of a remarkable degree of uniformity,—both in physical
character and in chemical composition. This uniformity they gradually
_acquire_, even upon the stiff clays of the Lias and of the Oxford
clay, which originally, no doubt, have been,—as many clay lands still
are,—left to natural pasture from the difficulty and expense of
submitting them to arable culture.
2. But how do they acquire this new character, and why is it the work
of so much time? When the young grass throws up its leaves into the
air, from which it derives so much of its nourishment, it throws down
its roots into the soil in quest of food of another kind. The leaves
may be mown or cropped by animals, and carried off the field, but the
roots remain in the soil, and, as they die, gradually fill its upper
part with vegetable matter. It is not known what average proportion the
roots of the natural grasses bear to the leaves; no doubt it varies
much, both with the kind of grass and with the kind of soil. When wheat
is cut down, the quantity of straw left in the field, in the form of
stubble and roots, is sometimes greater than the quantity carried off
in the sheaf. Upon a grass field two or three tons of hay may be reaped
from an acre; and if we suppose only a tenth part of this quantity to
die every year in the form of roots or parts of roots, or of excretions
from roots, we can easily understand how the vegetable matter in
the soil thus gradually accumulating, should at length become very
considerable in quantity. In arable land this accumulation is prevented
by the constant turning up of the soil, by which the vegetable fibres
being exposed to the free access of air and moisture, are made to
undergo a more rapid decomposition.
But the roots and leaves of the grasses contain inorganic earthy and
saline matter also. Dry hay leaves from an eighth to a tenth part of
its weight of ash when burned. Along with the dead vegetable matter of
the soil, this inorganic matter accumulates also on the surface, in the
form of an exceedingly fine earthy powder; hence _one_ cause of the
universal fineness of the surface mould of old grass fields. And the
earthy portion of this inorganic matter consists chiefly of silica and
lime, with scarcely a trace of alumina, so that, even on the stiffest
clays, a surface soil may be ultimately formed, in which the quantity
of alumina will be comparatively small.
But there are still other agencies at work by which the surface of
stiff soils is made to undergo a change. As the roots penetrate into
the clay, they more or less open up a way into it for the rains. Now
the rains in nearly all lands, when they have a passage downwards,
have a tendency to carry down the clay along with them. They do so,
it has been observed, on sandy and peaty soils, and more quickly when
these soils are laid down to grass. Hence the mechanical action of the
rains,—slowly in many localities, yet surely,—has a tendency to lighten
the soil, by removing a portion of its clay. They constitute one of
those natural agencies by which, as elsewhere explained, important
differences are ultimately established, almost everywhere, between the
surface crop-bearing soil and the subsoil on which it rests.
But further, the heats of summer and the frosts of winter aid this slow
alteration. In the extremes of heat and of cold, the soil contracts
more than the roots of the grasses do; and similar though less striking
differences take place during the changes of temperature experienced
in our climate in a single day. When the rain falls on the parched
field, or when a thaw comes on, the earth expands, while the roots of
the grasses remain nearly fixed; hence the soil rises up among the
leaves, mixes with the vegetable matter, and thus assists in the slow
accumulation of a rich vegetable mould.
The reader has witnessed in winter how, on a field or a by-way side,
the earth rises above the stones, and appears inclined to cover them;
he may even have seen in a deserted and undisturbed highway, the stones
gradually sinking and disappearing altogether, when the repetition of
this alternate contraction and expansion of the soil for a succession
of winters has increased in a great degree the effects which follow
from a single accession of frosty weather.
So it is in the fields. And if a person skilled in the soils of a given
district can make a guess at the time when a given field was laid
down to grass, by the depth at which the stones are found beneath the
surface, it is because this loosening and expansion of the soil, while
the stones remain fixed, tends to throw the latter down by an almost
imperceptible quantity every year that passes.
Such movements as these act in opening up the surface-soil, in mixing
it with the decaying vegetable matter, and in allowing the slow action
of the rains gradually to give its earthy portion a lighter character.
But with these, among other causes, conspire also the action of living
animals. Few persons have followed the plough without occasionally
observing the vast quantities of earthworms with which some fields seem
to be filled. On a close shaven lawn many have noticed the frequent
little heaps of earth which these worms during the night have thrown
out upon the grass. These and other minute animals are continually
at work, especially beneath an undisturbed and grassy sward—and they
nightly bring up from a considerable depth, and discharge on the
surface, their burden of fine fertilizing loamy earth. Each of these
burdens is an actual gain to the rich surface soil, and who can doubt
that in the lapse of years, the unseen and unappreciated labours of
these insect tribes must both materially improve its quality and
increase its depth?
There are natural causes, then, which we _know_ to be at work, that are
sufficient to account for nearly all the facts that have been observed,
in regard to the effect of laying lands down to grass. Stiff clays will
gradually become lighter on the surface, and if the subsoil be rich in
all the kinds of inorganic food which the grasses require, will go
on improving for an indefinite period without the aid of manure. Let
them, however, be deficient, or let them gradually become exhausted of
any one kind of this food, and the grass lands will either gradually
deteriorate after they have reached a certain degree of excellence—or
they must be supplied with that ingredient—that manure of which they
stand in need. It is doubtful if any pasture lands are so naturally
rich as to bear to be cropped for centuries without the addition of
manure, and at the same time without deterioration.[21]
[21] See pages 240 and 241.
On soils that are light, again, which naturally contain little clay,
the grasses will thrive more rapidly, a thick sward will be sooner
formed, but the tendency of the rains to wash out the clay may prevent
them from ever attaining that luxuriance which is observed upon the old
pastures of the clay lands.
On undrained heaths and commons, and generally on any soil which is
deficient in some fertilizing element, neither abundant herbage, nor
good crops of any other kind, can be expected to flourish. Laying such
lands down, or permitting them to remain in grass, may prepare them
for by-and-by yielding one or two average crops of corn, but cannot be
expected _alone_ to convert them into valuable pasture.
Finally, plough up the old pastures, on the surface of which this light
and most favourable soil has been long accumulating—and the heavy soil
from beneath will be again mixed up with it—the vegetable matter will
disappear rapidly by exposure to the air,—and if again laid down to
grass, the slow changes of many years must again be begun through the
agency of the same natural causes, before it become capable of again
bearing the same rich herbage it was known to nourish while it lay
undisturbed.
Many have supposed that by sowing down with the _natural_ grasses, a
thick sward may at once be obtained—and on light loamy lands, rich in
vegetable matter, this method may, to a certain extent, succeed—but on
heavy lands, in which vegetable matter is defective, disappointment
will often follow the sowing of the most carefully selected seeds.
By the agency of the causes above adverted to—_the soil gradually
changes_, so that it is unfit, when first laid down, to bear those
grasses which, ten or twenty years afterwards, will naturally and
luxuriantly grow upon it.
CHAPTER X.
The Products of Vegetation—Importance of
_Chemical_ quality as well as quantity of
Produce—Influence of different Manures on the
quantity and quality of the Crop—Influence of the
time of Cutting—Absolute quantity of Food yielded
by different Crops—Principles on which the Feeding
of Animals depends—Theoretical and experimental
value of different kinds of Food for Feeding
Stock—Concluding Observations.
The first object of the practical farmer is, to reap from his land the
largest possible return of the most valuable crops, without permanently
exhausting the soil. With this view he adopts one or other of the
methods of treatment above adverted to, by which either the physical
condition or the chemical constitution of the soil is altered for
the better. It may be useful to shew how very much both the quantity
and the quality of a crop is dependent upon the mode in which it is
cultivated and reaped, and how much control, therefore, the skilful
agriculturist really possesses over the ordinary productions of nature.
SECTION I.—OF THE INFLUENCE OF MANURE ON THE QUANTITY OF THE WHEAT AND
OTHER CORN CROPS.
Every one knows that some soils naturally produce much larger returns
of wheat, oats, and barley than others do, and that the same soil will
produce more or less according to the mode in which the land has been
prepared by manure, or otherwise, for the reception of the seed. The
following table shews the effect produced upon the quantity of the crop
by _equal quantities_ of different manures applied to the _same soil_,
sown with an equal quantity of the same seed.
Return in bushels from each
Manure applied. bushel of seed.
Wheat. Barley. Oats. Rye.
Blood, 14 16 12½ 14
Night-soil, — 13 14½ 13½
Sheep’s dung, 12 16 14 13
Horse dung, 10 13 14 11
Pigeon’s dung, — 10 12 9
Cow dung, 7 11 16 9
Vegetable matter, 3 7 13 6
Without manure, — 4 5 4
It is probable that on different soils the returns obtained by the use
of these several manures may not be always in the same order, yet,
generally speaking, it will always be found that blood, night-soil, and
sheep, horse, and pigeon’s dung, are among the most enriching manures
that can be employed.
We have already seen a theoretical reason for believing that night-soil
should be among the most enriching manures, and the result of actual
trial here shews that it is one of the most practically valuable which
the farmer can employ.
Two other facts will strike the practical man on looking at the above
table.
1. That exclusive of blood, sheep’s dung gave the greatest increase in
the barley crop. The favourite Norfolk system of eating off turnips
with sheep previous to barley, besides other benefits which are known
to attend the practice, may owe part of its acknowledged utility to
this powerful action of sheep’s dung upon the barley crop.
2. The action of cow dung upon oats is equally striking, and the
large return obtained by the use of vegetable manure alone—thirteen
fold—may perhaps explain why in poorly farmed districts oats should be
a favourite and comparatively profitable crop, and why they may be
cultivated with a certain degree of success on lands to which no rich
manure is ever added.
SECTION II.—INFLUENCE OF THE KIND OF MANURE ON THE CHEMICAL QUALITY OF
THE GRAIN.
But the quality of the grain also, as well as its quantity, is
materially affected by the kind of manure by which its growth is
assisted. The apparent quality of wheat and oats is very various; but
in samples apparently equal in quality, important chemical differences
may exist, by which it is believed that the nourishing properties of
the grain are materially affected.
It has been stated in a previous chapter (p. 43), that when flour is
made into dough, and this dough is washed upon a linen cloth with water
as long as the latter passes through milky,—the flour is separated into
_starch_, which subsides from the water, and _gluten_, which remains
behind. The quantity of gluten thus left varies more or less with
almost every sample of flour, and the nutritive properties of each
sample are supposed to depend very much upon the quantity of gluten it
contains. So far it seems to be pretty well ascertained, that those
varieties of grain which contain the largest amount of gluten yield
also the greatest return of flour, and the heaviest weight of bread.
The weight of gluten contained in 100 lbs. of dry wheat has been found
to vary from 8 to 34 lbs., and this proportion is affected in a very
remarkable manner by the kind of manure which has been applied to the
land. Thus the proportions of starch and gluten in 100 lbs. of the
grain of the same wheat, grown on the same land, differently manured,
was as follows:—
Manure. Starch. Gluten.
Blood, 41 lbs. 34 lbs.
Sheep’s dung, 42 ” 33 ”
Horse dung, 62 ” 14 ”
Cow dung, 62 ” 12 ”
Vegetable manure, 66 ” 10 ”
Potato-flour, which consists entirely of starch, makes a fine light
bread, easily raised. Wheaten-flour, which contains little gluten,
approaches in this respect to potato-flour. When the quantity of gluten
is large, greater care is required to make a good light bread; but
the bread from such flour is generally found to be more nutritive in
its quality. A dough peculiarly rich in gluten is required for the
manufacture of macaroni and vermicelli; such is said to be the flour
naturally produced in southern Italy. By the above table it appears,
that the use of richer animal, or poorer vegetable manures, would
enable the farmer to raise, at his pleasure, either a rich macaroni
wheat, or one poor in gluten suited for the makers of fancy bread.
An equally striking effect is not produced upon other kinds of grain by
varying the manure. Thus the proportions of starch and gluten in the
dry grain of barley and oats, differently manured, were found to be as
follows:—
BARLEY. OATS.
Starch. Gluten. Starch. Gluten.
Blood, 66½ 6½ 60 5½
Night-soil, 66 6½ 60 5
Sheep dung, 66½ 6½ 61 4½
Horse dung, 66 6½ 61½ 4½
Cow dung, 69 3½ 62 3½
Vegetable manure, 69 3 66½ 2¼
Unmanured, 69½ 3 66½ 2¼
Though a variation in the proportion of gluten can be observed in both
of these kinds of grain, according as one or other of the above kinds
of manure was employed, yet neither the average quantity of gluten
present in them, nor the variations to which the quantity is liable,
are at all equal in amount to what are observed in the case of wheat.
The malting of barley is known to be affected by a variety of
circumstances. It should be so uniform in ripeness as to sprout
uniformly, so that no part of it should be beginning to shoot when
the rest has already germinated sufficiently for the maker’s purpose.
On this perfect sprouting of the _whole_ depends in some degree the
swelling of the malt, which is of considerable consequence to the
manufacturer.
But the _melting_ quality of the grain, which is of more consequence
to the brewer and distiller, is modified chiefly by the proportion of
gluten which the barley contains. That which contains the least gluten,
and therefore the most starch, will melt the most easily and the most
completely, and will yield the strongest beer or spirit from the same
quantity of grain. Hence the preference given by the brewer to the
malt of particular districts, even where the sample appears otherwise
inferior. Thus the brewers on the sea-board of the county of Durham
will not purchase the barley of their own neighbourhood, while Norfolk
grain can be had at a moderate increase of price. But that which
refuses to melt well in the hands of the brewer, will cause pigs and
other stock to thrive well in the hands of the feeder, and this is the
chief outlet for the barley which the brewer and distiller reject.
So far as a practical deduction can be drawn from the effects of
different manures on the proportion of gluten in barley, it would
appear that the larger the quantity of cow dung contained in the manure
applied to barley land—in other words, _the greater the numbers of
stock folded about the farm-yard, the more likely is the barley to be
such as will bring a high price from the brewer_.
The folding of _sheep_ produces a larger return (p. 206), from the
barley crop—while the folding of _cattle_ gives grain of a better
malting quality.
SECTION III.—INFLUENCE OF THE TIME OF CUTTING, ON THE QUANTITY AND
QUALITY OF THE PRODUCE.
The period at which hay is cut, or corn reaped, materially affects the
quantity (by weight) and the quality of the produce. It is commonly
known that when radishes are left too long in the ground they become
hard and woody—that the soft turnippy stem of the young cabbage
undergoes a similar change as the plant grows old,—and that the
artichoke becomes tough and uneatable if left too long uncut. The same
natural change goes on in the grasses which are cut for hay.
In the blades and stems of the young grasses there is much sugar,
which, as they grow up, is gradually changed, first into starch, and
then into woody fibre (pages 44 and 45.) The more completely the latter
change is effected—that is, the riper the plant becomes—the less sugar
and starch, both readily soluble substances, they contain. And though
it has been ascertained that woody fibre is not wholly indigestible,
but that the cow, for example, can appropriate a portion of it for food
as it passes through her stomach; yet the reader will readily imagine,
that those parts of the food which dissolve most easily, are also
likely—other things being equal—to be most nourishing to the animal.
It is ascertained, also, that the weight of hay or straw reaped, is
actually less when allowed to become fully ripe; and therefore, by
cutting soon after the plant has attained its greatest height, a larger
quantity, as well as a better quality of hay, will be obtained, while
the land also will be less exhausted.
The same remarks apply to crops of corn,—both to the straw and to the
grain they yield. The _rawer_ the crop is cut, the heavier and more
nourishing the straw. Within three weeks of being fully ripe, the straw
begins to diminish in weight, and the longer it remains uncut after
that time, the lighter it becomes and the less nourishing.
On the other hand, the ear which is sweet and milky a month before it
is ripe, gradually consolidates, the sugar changing into starch, and
the milk thickening into the gluten and the _albumen_[22] of the flour.
As soon as this change is nearly completed, or about a fortnight before
ripening, the grain contains the largest proportion of starch and
gluten; if reaped at this time, the bushel will be heavier, and will
yield the largest quantity of fine flour and the least bran.
[22] _Albumen_ is the name given by chemists to the _white of the egg_.
A small quantity of this substance is present in every kind of grain.
It is closely related to gluten.
At this period the grain has a thin skin, and hence the small quantity
of bran. But if the crop be still left uncut, the next natural step in
the ripening process is, to cover the grain with a better protection,
a thicker skin. A portion of the starch of the grain is changed into
woody fibre,—precisely as in the ripening of hay, of the soft shoots of
the dog-rose, and of the roots of the common radish. By this change,
therefore, the quantity of starch is lessened and the weight of husk
increased; hence the diminished yield of flour, and the increased
produce of bran.
Theory and experience, therefore, indicate about a fortnight before
full ripening as the most proper time for cutting corn. The skin is
then thinner, the grain fuller, the bushel heavier, the yield of flour
greater, the quantity of bran less; while, at the same time, the straw
is heavier, and contains more soluble matter than when it is left uncut
until it is considered to be fully ripe.[23]
[23] On this subject the reader will consult with advantage an
excellent practical paper in the _Quarterly Journal of Agriculture_ for
June 1841, by Mr. Hannam of North Deighton, Yorkshire, to whom I have
to express my obligations for information regarding the results of some
further experiments made by him during the last autumn (1841).
SECTION IV.—ON THE ABSOLUTE QUANTITY OF FOOD YIELDED BY DIFFERENT CROPS.
The quantity of food capable of yielding nourishment to man, which can
be grown from an acre of land of average quality, depends very much
upon the kind of crop we raise.
In seeds, when fully ripe, little sugar or gum is generally present,
and it is chiefly by the amount of starch and gluten they contain, that
their nutritive power is to be estimated. In bulbs, such as the turnip
and potato, sugar and gum are almost always present in considerable
quantity in the state in which these roots are consumed, and this is
especially the case with the turnip. These substances, therefore, must
be included among the nutritive ingredients of such kinds of food.
If we suppose an acre of land to yield the following quantities of the
usually cultivated crops, namely—
Of wheat, 25 bushels, or 1500 lbs.
Of barley, 38 ” or 2000 ”
Of oats, 50 ” or 2250 ”
Of peas, 15 ” or 1000 ”
Of beans, 25 ” or 1600 ”
Of Indian corn, 60 ” or 3120 ”
Of potatoes, 10 tons, or 22400 ”
Of turnips, 25 ” or 56000 ”
The weight of dry starch, gluten, sugar, and gum, reaped in each crop,
will be represented very nearly by the following numbers:—
Starch. Gluten and Sugar and Woody
Albumen. Gum. Fibre.
Wheat, 825 lbs. 315 lbs. 60 —
Barley, 1200 ” 120 ” 160 —
Oats, 1215 ” 100 ” 250 —
Peas, 420 ” 260 ” 20 —
Beans, 670 ” 370 ” — —
Indian corn, 2100 ” 280 ” 90 —
Potatoes, 2688 ” 224 ” — 1253
Turnips, 3090 ” 1400 ” 5000 —
If it be granted that the crops above stated are fair average returns
from the same quality of land—that the acre, for example, which
produces 25 bushels of wheat, will also produce 10 tons of potatoes,
and so on—then it appears that the land which, by cropping with wheat,
would yield a given weight of _starch_, would, when cropped with barley
or oats, yield one-half more, with Indian corn or potatoes about three
times as much, and with turnips five times the same quantity. In other
words, the piece of ground which, when sown with wheat, will maintain
one man, would support one and a half if sown with barley or oats,
three with Indian corn or potatoes, and five with _turnips_—_in so far
as the nutritive power of these crops depends upon the starch and sugar
they contain_.
Again, if we compare the relative quantities of gluten, we see that
wheat, beans, and Indian corn yield, from the same breadth of land,
nearly an equal quantity of this kind of nourishment—potatoes one-third
less, and barley and oats only one-third of the quantity—while turnips
yield four times as much as either wheat, beans, or Indian corn.
On whichever of these two substances, therefore, the starch or the
gluten, we consider the nutritive property of the above kind of food
to depend, it appears that the turnip is by far the most nutritive
crop we can raise. It is by no means the most nutritive weight for
weight, but the largeness of the crop (25 tons) affords us from the
same field a much greater weight of food than can be reaped in the form
of any of the other crops here mentioned.
In this the practical farmer will see the peculiar adaptation of the
turnip husbandry to the rearing and fattening of stock. Could the
turnip be made an agreeable article of general human consumption, the
produce of the land might be made to sustain a much larger population
than under any other of the above kinds of cropping.
The relative nourishing power or value as food of different vegetable
substances, is supposed by some to depend entirely upon the relative
proportions of gluten they contain. According to this view, the pea and
the bean are much more nourishing, weight for weight, even than wheat,
and this latter grain, than any of the other substances mentioned in
the above table. Thus, 56 lbs. of beans would afford as much sustenance
to an animal as 67 of pease, 100 of wheat-flour, or 177 _of rice_.
In order to understand the value of this opinion, it will be proper
to consider the several purposes which the food is destined to serve
in the animal economy—what the animal must derive from its food to
maintain its existing condition, or to admit of a healthy increase of
bulk.
SECTION V.—OF THE FEEDING OF ANIMALS, AND THE PURPOSES SERVED BY THE
FOOD THEY CONSUME.
The food of plants we have seen to consist essentially of two kinds,
the _organic_ and the _inorganic_, both of which we have insisted upon
as equally necessary to the living vegetable—equally indispensable to
its healthy growth. A brief glance at the purposes served by plants in
the feeding of animals, will not only confirm this view, but will also
throw some additional light upon the _kind_ of inorganic food which the
plants must be able to procure, in order that they may be fitted to
fulfil their assigned purpose in the economy of nature.
Man, and all domestic animals, may be supported, may even be fattened,
upon vegetable food alone: vegetables, therefore, must contain all
the substances which are necessary to build up the several parts of
animal bodies, and to supply the waste attendant upon the performance
of the necessary functions of animal life. Let us consider what these
substances are, and in what quantities they must be supplied to the
human body.
1. _The food must supply carbon for respiration._
A man of sedentary habits, or whose occupation requires little bodily
exertion, may respire about 5 ounces of carbon in twenty-four hours—one
who takes moderate exercise, about 8 ounces—and one who has to undergo
violent bodily exertion, from 12 to 15 ounces.
If we take the mean quantity of 8 ounces, then to supply this alone, a
man must eat 18 ounces of starch or sugar every day. If he take it in
the form of wheaten bread, he will require 1¾ lbs. of bread, if in the
form of potatoes, about 7½ lbs. of raw potatoes, to supply the waste
caused by his respiratory organs alone.
When the habits are sedentary, 5 lbs. of potatoes may be sufficient,
when violent and continued exercise is taken, 12 to 15 lbs. may be too
little. At the same time, it must be observed, that where the supply is
less, the quantity of carbonic acid given off will either be less also,
or the deficiency will be supplied at the expense of the body itself.
In either case the strength will be impaired, and fresh food will be
required to recruit the exhausted frame.
2. _The food must repair the daily waste of the muscular parts of the
body._
When the body is full grown, a portion from every part of it is
daily abstracted by natural processes, and rejected either in the
perspiration or in the solid and fluid excrements. This portion must
be supplied by the food, or the strength will diminish—the frame will
gradually waste away.
The muscles of animals, of which lean beef and mutton are examples,
are generally coloured by blood, but when well washed with water, they
become quite white, and, with the exception of a little fat, are found
to consist of a white fibrous substance, to which the name of _fibrin_
has been given by chemists. The clot of the blood consists of the
same substance; while skin, hair, horn, and the organic part of the
bones, are composed of varieties of _gelatine_. This latter substance
is familiarly known in the form of _glue_, and though it differs in
its sensible properties, it is remarkably analogous to _fibrin_ in its
elementary constitution, as both of these substances are to the white
of the egg (_albumen_), to the curd of milk (_casein_), and to the
_gluten_ of flour. They all contain nitrogen, and all consist of the
four elementary bodies (organic elements), very nearly in the following
proportions:—
Carbon, 55
Hydrogen, 7
Nitrogen, 18
Oxygen, 20
----
100
They all contain, likewise, a small proportion of sulphur and of
phosphorus.
The quantity of one or other of these removed from the body in 24
hours, either in the perspiration or in the excretions, amounts to
_about five ounces_, containing 350 grains of nitrogen, and this waste
at least must be made up by the gluten or fibrin of the food.
In the 1¾ lb. of wheaten bread we have supposed to be eaten to supply
carbon for respiration, there will be contained also about 3 ounces
of gluten. Let the other 2 ounces be made up in beef, of which half a
pound contains 2 ounces of dry fibrin, and we have
For respiration. For waste of
muscle, &c.
1¾ lbs. of bread yielding 18 oz. starch and 3 oz. of gluten.
8 oz. of beef yielding 2 oz. of fibrin.
Total consumed by respiration,} } gluten or
and the ordinary waste, } 18 oz. starch and 5 oz.} fibrin.
If, again, the 7½ lbs. of potatoes be eaten, then in these are
contained about 2½ ounces of gluten or albumen, so that there remain 2½
ounces to be supplied by beef, eggs, milk, or cheese.
The reader, therefore, will understand why a diet which will keep up
the human strength is easiest compounded of a mixture of vegetable and
animal food. It is not merely that such a mixture is more agreeable
to the palate, or even that it is absolutely necessary,—for, as
already observed, the strength may be fully maintained by vegetable
food alone;—it is, that without animal food in one form or another,
so large a bulk of vegetable food must be consumed in order to supply
the requisite quantity of nitrogen in the form of gluten. Of ordinary
wheaten bread alone, about 3 lbs. daily must be eaten to supply the
nitrogen,[24] and there would then be a considerable waste of carbon
in the form of starch, by which the stomach would be overloaded, and
which, not being worked up by respiration, would pass off in the
excretions. The wants of the body would be equally supplied, and with
more ease, by 1¾ lbs. of bread and 4 ounces of cheese.
[24] The flour being supposed to contain 15 per cent. of dry gluten, on
which supposition all the above calculations are made.
Of rice, again, no less than 4 lbs. daily would be required to impart
to the system the required proportion of gluten; and it is a familiar
observation of those who have been in India and other countries, where
rice is the usual food of the people, that the degree to which the
natives distend, and apparently overload their stomachs with this
grain, is quite extraordinary.
The stomachs and other digestive apparatus of our domestic animals are
of larger dimensions, and they are able, therefore, to contain with
ease as much vegetable food, of almost any wholesome variety, as will
supply them with the quantity of nitrogen they may require. Yet every
feeder of stock knows that the addition of a small portion of oil-cake,
a substance rich in nitrogen, will not only fatten an animal more
speedily, but will also save a large _bulk_ of other kinds of food.
3. But the blood and other fluids of the body contain much _saline_
matter of various kinds, sulphates, muriates, phosphates, and other
saline compounds of potash, soda, lime, and magnesia. All these have
their special functions to perform in the animal economy, and of each
of them an undetermined quantity daily escapes from the body in the
perspiration, in the urine, or in the solid excretions. This quantity,
therefore, must be daily restored by the food.
No precise experiments have yet been made with the view of determining
how much saline matter is daily excreted from the body of a healthy
man, or in what proportions the different inorganic substances are
present in it; but it is satisfactorily ascertained, that without
a certain _sufficient_ supply, the animal will languish and decay,
even though carbon and nitrogen in the form of starch and gluten be
abundantly given to it. It is a wise and beautiful provision of nature,
therefore, that plants are so organized as to refuse to grow in a soil
from which they cannot readily obtain a supply of soluble inorganic
food, since that saline matter which ministers first to their own wants
is afterwards surrendered by them to the animals they are destined to
feed.
Thus the dead earth and the living animal are but parts of the same
system,—links in the same endless chain of natural existences,—the
plant is the connecting bond by which they are tied together on the one
hand,—the decaying animal matter which returns to the soil, connects
them on the other.
4. The solid bones of the animal are supplied from the same original
source,—the vegetable food on which they live. The bones of the cow
contain 55 per cent. of phosphate of lime, of the sheep 70, of the
horse 67, of the calf 54, and of the pig 52 lbs., in every hundred of
dry bone. All this must come from the vegetable food. Of the bone-earth
also, a portion,—perhaps a variable portion,—is every day rejected from
the animal; the food, therefore, must contain a daily supply, or that
which passes off will be taken from the substance of the bones, and the
animal will become feeble.
It is kindly provided by nature, that a certain proportion of this
ingredient of bones is always associated with the gluten of plants
in its various forms,—with the fibrin of animal muscle and with the
curd of milk. Hence, man, in using any of these latter along with his
vegetable food, obtains from them, with comparative ease, the quantity
of the earth of bones which is necessary to keep his system in repair;
while those animals which live upon vegetables alone, extract all they
require along with the gluten of the plants on which they feed.
The provision is very beautiful by which the young animal,—the muscle
and bones of which are rapidly growing,—is supplied with a larger
portion of nitrogenous food and of bone-earth, than are necessary to
maintain the healthy condition of the full grown animal. The milk of
the mother is the natural food from which its supplies are drawn.
The sugar of the milk supplies the comparatively small quantity of
carbon necessary for the respiration of the young animal; as it gets
older, the calf or young lamb crops green food for itself to supply
an additional portion. The curd of the milk (_casein_) yields the
materials of the growing muscles, and of the animal part of the
bones,—while dissolved along with the curd in the liquid milk is the
phosphate of lime, of which the earthy part of the bones is to be built
up. A glance at the constitution of milk will shew us how copious the
supply of all these substances is,—how beautifully the constitution
of the mother’s milk is adapted to the wants of her infant offspring.
Cow’s milk consists in 1000 parts by weight of—
Butter, 27 to 35
Cheesy matter (casein), 45 to 90
Milk sugar, 36 to 50
_Chloride of potassium_, and
a little chloride of sodium, 1½}
_Phosphates_, chiefly of _lime_, 2¼} to 10
Other salts, 6
Water, 882¼ to 815
----- ----
1000 1000
The quality of the milk, and, consequently, the proportions of the
several constituents above mentioned, vary with the breed of the
cow,—with the food on which it is supported,—with the time that has
elapsed since the period of calving,—with its age, its state of health,
and with the warmth of the weather;[25] but in all cases this fluid
contains the same substances, though in different quantities.
[25] In warm weather the milk contains more butter, in cold weather
more cheese and sugar.
Milk of the quality above analyzed contains, in every ten gallons,
4½ lbs. of casein, equal to the formation of 18 lbs. of ordinary
muscle, and 3½ ounces of phosphate of lime (bone-earth), equal to
the production of 7 ounces of dry bone. But from the casein have to
be formed the skin, the hair, the horn, the hoof, &c. as well as the
muscle, and in all these is contained also a minute portion of the
bone-earth. A portion of all the ingredients of the milk likewise
passes off in the ordinary excretions, and yet every one knows how
rapidly young animals thrive, when allowed to consume the whole of the
milk which nature has provided as their most suitable nourishment.
And whence does the mother derive all this gluten and bone-earth, by
which she can not only repair the natural waste of her own full grown
body, but from which she can spare enough also to yield so large a
supply of nourishing milk? She must extract them from the vegetables on
which she lives, and they again from the soil.
The quantity of solid matter thus yielded by the cow in her milk is
really very large, if we look at the produce of an entire year. If the
average yield of milk be 3000 quarts, or 750 gallons in a year—every 10
gallons of which contain bone-earth enough to form about 7 ounces of
dry bone—then the milking of the cow alone exhausts her of the earthy
ingredients of 33 lbs. of dry bone. And this she draws necessarily from
the soil!
If this milk be consumed on the spot, then all returns again to the
soil in the annual manuring of the land. Let it be carried for sale
to a distance, or let it be converted into cheese and butter, and in
this form exported, there will then be a yearly drain upon the land
of the materials of bones, from this cause alone, equal to 30 lbs. of
bone-dust. After the lapse of centuries, it is conceivable that old
pasture lands in cheese and dairy countries should become poor in the
materials of bones—and that in such districts, as now in Cheshire, the
application of bone-dust should entirely alter the character of the
grasses, and renovate the old pastures.
Thus, as was stated at the commencement of the present section the
study of the nature, and functions of the food of animals throws
additional light upon the nature also and final uses of the food of
plants. It even teaches us what to look for in the soil—what a fertile
soil _must_ contain that it may grow nourishing food—what we must add
to the soil when chemical analysis fails to detect its actual presence,
or when the food it produces is unable to supply all that the animal
requires.
The principles above explained, therefore, shew that the value of any
vegetable production, considered as the _sole_ food of an animal, is
not to be judged of—cannot, in short, be accurately determined—by
the amount it may contain of any _one_ of those substances, _all_ of
which together are necessary to build up the growing body of the young
animal, and to repair the natural waste of such as have attained to
their fullest size.
Hence the failure of the attempts that have been made to support the
lives of animals by feeding them upon pure starch or sugar alone. These
substances would supply carbon for respiration, but all the natural
waste of nitrogen, of saline matter, and of earthy phosphates, must
have been drawn from the existing solids and fluids of their living
bodies. The animals in consequence pined away, and sooner or later died.
Some have expressed surprise that animals have refused to thrive,
and have ultimately died, when fed upon animal jelly or gelatine
(from bones) alone, nourishing though that substance _as part of the
food_ undoubtedly is. When given in sufficient quantity, gelatine
might indeed supply carbon enough for respiration, with a great waste
of nitrogen, but it is deficient in the saline ingredients which a
naturally nourishing food contains.
Even on the natural mixture of starch and gluten in fine white bread,
dogs have been unable to live beyond 50 days, though others fed on
household bread, containing a portion of the bran—in which earthy
matter more largely resides—continued to thrive long after. It is
immaterial whether the general quantity of the _whole_ food be reduced
too low, or whether _one_ of its necessary ingredients only be too much
diminished or entirely withdrawn. In either case, the effect will be
the same—the animal will pine away, and sooner or later die.
SECTION VII.—OF THE PRACTICAL AND THEORETICAL VALUE OF DIFFERENT KINDS
OF FOOD.
From what has been stated in the preceding section, it appears,
that, for various reasons, different kinds of food are not equally
nourishing. This fact is of great importance, not only in the
preparation of human food, but also in the feeding of stock. It has,
therefore, been made the subject of experiment by many practical
agriculturists, with the following general results.
If common hay be taken as the standard of comparison, then to yield
the same amount of nourishment with 10 lbs. of hay, a weight of the
other kinds of food must be given, which is represented by the number
opposite to each in the following table:—
Hay, 10 Carrots, 25 to 30
Clover hay, 8 to 10 Turnips, 50
Green clover, 45 to 50 Cabbage, 20 to 30
Wheat straw, 40 to 50 Pease and Beans, 3 to 5
Barley straw, 20 to 40 Wheat, 5 to 6
Oat straw, 20 to 40 Barley, 5 to 6
Pea straw, 10 to 15 Oats, 4 to 7
Potatoes, 20 Indian corn, 5
Old potatoes, 40? Oil-cake, 2 to 4
It is found in practice, as the above table shews, that twenty stones
of potatoes or three of oil-cake will nourish an animal as much as ten
stones of hay, and five stones of oats as much as either. Something,
however, will depend upon the quality of each kind of food, and upon
the age and constitution of the animal. The skilful feeder of stock
knows also the value of a change of food, or of a mixture of the
different kinds of vegetable food he may have at his command.
The nutritive value of different kinds of food has also been
represented theoretically, by supposing it to be very nearly in
proportion to the quantity of nitrogen, or of gluten, which vegetables
contain. Though this cannot be considered as a correct principle,
yet as the ordinary kind of food on which stock is fed contains in
general an ample supply of carbon for respiration, with a comparatively
small proportion of nitrogen, these theoretical determinations are by
no means without their value, and they approach in many cases very
closely to the practical values above given, as deduced from actual
trial. Thus, assuming that 10 lbs. of hay yield a certain amount
of nourishment, then of the other vegetable substances it will be
necessary, according to theory, to give the following quantities, in
order to produce the same effect:
Hay, 10 Turnips, 60
Clover hay, 8 Carrots, 35
Vetch hay,[26] 4 Cabbage, 30 to 40
Wheat straw, 52 Pease and Beans, 2 to 3
Barley straw, 52 Wheat, 5
Oat straw, 55 Barley, 6
Pea straw, 6 Oats, 5
Potatoes, 28 Indian corn, 6
Old potatoes, 40 Oil-cake, 2
[26] Both cut in flower.
If the feeder be careful to supply his stock with a mixture or
occasional change of food, he may very safely regulate the quantity of
any one he ought to substitute for a given weight of any of the others,
by the numbers in the above tables—since the theoretical and practical
results do not in general very greatly differ.
As has been already stated, it is not strictly correct that this or
that kind of vegetable is more fitted to sustain animal life, simply
because of the larger proportion of nitrogen it contains; but it
is wisely provided, that along with this nitrogen in all plants,
a certain proportion of starch or sugar and of saline and earthy
matter are always associated—so that the quantity of nitrogen may be
considered as a rough practical index of the proportion of some of the
important saline and earthy ingredients also.
An important practical lesson on this subject is taught us by the study
of the wise provisions of Nature. Not only does the milk of the mother
contain all the elements of a nutritive food mixed up together—as the
egg does also for the unhatched bird—but in rich natural pastures the
same mixture uniformly occurs. Hence, in cropping the mixed herbage,
the animal introduces into its stomach portions of various plants—some
abounding more in starch or sugar, some more in gluten or albumen, some
naturally richer in saline, others in earthy constituents; and out of
these varied materials the digestive organs select a due proportion of
each, and reject the rest. Wherever a pasture becomes usurped by one or
two grasses—either animals cease to thrive upon it, or they must crop a
much larger quantity of food to supply the natural waste of _all_ the
parts of their bodies.
It may indeed be assumed as almost a general principle, that whenever
animals are fed on one kind of vegetable only, there is a waste of one
or other of the necessary elements of animal food, and that the great
lesson on this subject taught us by nature is, that, by a judicious
admixture, not only is food economised, but the labour imposed upon the
digestive organs is also materially diminished.
SECTION VIII.—CONCLUDING OBSERVATIONS.
In this little work, now brought to a close, I have presented the
reader with a slight, and I hope plain and familiar, sketch of the
various topics connected with practical agriculture, on which the
sciences of chemistry and geology are fitted to throw the greatest
light.
We have studied the general characters of the organic and inorganic
elements of which the parts of plants are made up, and the several
compounds of these elements which are of the greatest importance in
the vegetable kingdom. We have examined the nature of the seed,—seen
by what beautiful provision it is fed during its early germination—in
what form the elements by which it is nourished are introduced into
the circulation of the young plant when the functions of the seed are
discharged,—and how earth, air, and water are all made to minister
to its after-growth. We have considered the various chemical changes
which take place within the growing plant, during the formation of its
woody stem, the blossoming of its flower, and the ripening of its seed
or fruit,—and have traced the further changes it undergoes, when, the
functions of its short life being discharged, it hastens to serve other
purposes, by mingling with the soil, and supplying food to new races.
The soils themselves in which plants grow, their nature, their origin,
the causes of their diversity in mineral character, and in natural
productiveness, have each occupied a share of our attention—while
the various means of improving their agricultural value by manuring
or otherwise, have been practically considered, and theoretically
explained. Lastly, we have glanced at the comparative worth of the
various products of the land, as food for man or other animals, and
have briefly illustrated the principles upon which the feeding of
animals and the relative nutritive powers of the vegetables on which
they live are known to depend.
In this short and familiar treatise I have not sought so much to
_satisfy_ the demands of the philosophical agriculturist, as to
_awaken_ the curiosity of my less instructed reader, to shew him how
much interesting as well as practically useful information chemistry
and geology are able and willing to impart to him, and thus to allure
him in quest of further knowledge and more accurate details to my
larger work,[27] of which the present exhibits only a brief outline.
[27] LECTURES _on Agricultural Chemistry and Geology_.
J. P. Wright, Printer, 18 New street, N. Y.
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