perfectly-developed leaves absorb continually carbonic acid and
ammonia from the atmosphere, which are converted into elements of
new leaves, buds, and shoots; but this metamorphosis cannot be
effected without the aid of the alkalies, and other mineral
substances. If the soil is moist, the latter are continually
supplied to an adequate amount, and the plant retains its lively
green colour; but if this supply ceases from a want of moisture to
dissolve the mineral elements, a separation takes place in the plant
itself. The mineral constituents of the juice are withdrawn from the
leaves already formed, and are used for the formation of the young
shoots; and as soon as the seeds are developed, the vitality of the
leaves completely ceases. These withered leaves contain only minute
traces of soluble salts, while the buds and shoots are very rich in
them.
On the other hand, it has been observed, that where a soil is too
highly impregnated with soluble saline materials, these are
separated upon the surface of the leaves. This happens to culinary
vegetables especially, whose leaves become covered with a white
crust. In consequence of these exudations the plant sickens, its
organic activity decreases, its growth is disturbed; and if this
state continues long, the plant dies. This is most frequently seen
in foliaceous plants, the large surfaces of which evaporate
considerable quantities of water. Carrots, pumpkins, peas, &c., are
frequently thus diseased, when, after dry weather, the plant being
near its full growth, the soil is moistened by short showers,
followed again by dry weather. The rapid evaporation carries off the
water absorbed by the root, and this leaves the salts in the plant
in a far greater quantity than it can assimilate. These salts
effloresce upon the surface of the leaves, and if they are
herbaceous and juicy, produce an effect upon them as if they had
been watered with a solution containing a greater quantity of salts
than their organism can bear.
Of two plants of the same species, this disease befalls that which
is nearest its perfection; if one should have been planted later, or
be more backward in its development, the same external cause which
destroys the one will contribute to the growth of the other.
LETTER XII
My dear Sir,
Having now occupied several letters with the attempt to unravel, by
means of chemistry, some of the most curious functions of the animal
body, and, as I hope, made clear to you the distinctions between the
two kinds of constituent elements in food, and the purposes they
severally subserve in sustaining life, let me now direct your
attention to a scarcely less interesting and equally important
subject--the means of obtaining from a given surface of the earth
the largest amount of produce adapted to the food of man and
animals.
Agriculture is both a science and an art. The knowledge of all the
conditions of the life of vegetables, the origin of their elements,
and the sources of their nourishment, forms its scientific basis.
From this knowledge we derive certain rules for the exercise of the
ART, the principles upon which the mechanical operations of farming
depend, the usefulness or necessity of these for preparing the soil
to support the growth of plants, and for removing every obnoxious
influence. No experience, drawn from the exercise of the art, can be
opposed to true scientific principles, because the latter should
include all the results of practical operations, and are in some
instances solely derived therefrom. Theory must correspond with
experience, because it is nothing more than the reduction of a
series of phenomena to their last causes.
A field in which we cultivate the same plant for several successive
years becomes barren for that plant in a period varying with the
nature of the soil: in one field it will be in three, in another in
seven, in a third in twenty, in a fourth in a hundred years. One
field bears wheat, and no peas; another beans or turnips, but no
tobacco; a third gives a plentiful crop of turnips, but will not
bear clover. What is the reason that a field loses its fertility for
one plant, the same which at first flourished there? What is the
reason one kind of plant succeeds in a field where another fails?
These questions belong to Science.
What means are necessary to preserve to a field its fertility for
one and the same plant?--what to render one field fertile for two,
for three, for all plants?
These last questions are put by Art, but they cannot be answered by
Art.
If a farmer, without the guidance of just scientific principles, is
trying experiments to render a field fertile for a plant which it
otherwise will not bear, his prospect of success is very small.
Thousands of farmers try such experiments in various directions, the
result of which is a mass of practical experience forming a method
of cultivation which accomplishes the desired end for certain
places; but the same method frequently does not succeed, it indeed
ceases to be applicable to a second or third place in the immediate
neighbourhood. How large a capital, and how much power, are wasted
in these experiments! Very different, and far more secure, is the
path indicated by SCIENCE; it exposes us to no danger of failing,
but, on the contrary, it furnishes us with every guarantee of
success. If the cause of failure--of barrenness in the soil for one
or two plants--has been discovered, means to remedy it may readily
be found.
The most exact observations prove that the method of cultivation
must vary with the geognostical condition of the subsoil. In basalt,
graywacke, porphyry, sandstone, limestone, &c., are certain elements
indispensable to the growth of plants, and the presence of which
renders them fertile. This fully explains the difference in the
necessary methods of culture for different places; since it is
obvious that the essential elements of the soil must vary with the
varieties of composition of the rocks, from the disintegration of
which they originated.
Wheat, clover, turnips, for example, each require certain elements
from the soil; they will not flourish where the appropriate elements
are absent. Science teaches us what elements are essential to every
species of plants by an analysis of their ashes. If therefore a soil
is found wanting in any of those elements, we discover at once the
cause of its barrenness, and its removal may now be readily
accomplished.
The empiric attributes all his success to the mechanical operations
of agriculture; he experiences and recognises their value, without
inquiring what are the causes of their utility, their mode of
action: and yet this scientific knowledge is of the highest
importance for regulating the application of power and the
expenditure of capital,--for insuring its economical expenditure and
the prevention of waste. Can it be imagined that the mere passing of
the ploughshare or the harrow through the soil--the mere contact of
the iron--can impart fertility miraculously? Nobody, perhaps,
seriously entertains such an opinion. Nevertheless, the modus
operandi of these mechanical operations is by no means generally
understood. The fact is quite certain, that careful ploughing exerts
the most favourable influence: the surface is thus mechanically
divided, changed, increased, and renovated; but the ploughing is
only auxiliary to the end sought.
In the effects of time, in what in Agriculture are technically
called fallows--the repose of the fields--we recognise by science
certain chemical actions, which are continually exercised by the
elements of the atmosphere upon the whole surface of our globe. By
the action of its oxygen and its carbonic acid, aided by water,
rain, changes of temperature, &c., certain elementary constituents
of rocks, or of their ruins, which form the soil capable of
cultivation, are rendered soluble in water, and consequently become
separable from all their insoluble parts.
These chemical actions, poetically denominates the "tooth of time,"
destroy all the works of man, and gradually reduce the hardest rocks
to the condition of dust. By their influence the necessary elements
of the soil become fitted for assimilation by plants; and it is
precisely the end which is obtained by the mechanical operations of
farming. They accelerate the decomposition of the soil, in order to
provide a new generation of plants with the necessary elements in a
condition favourable to their assimilation. It is obvious that the
rapidity of the decomposition of a solid body must increase with the
extension of its surface; the more points of contact we offer in a
given time to the external chemical agent, the more rapid will be
its action.
The chemist, in order to prepare a mineral for analysis, to
decompose it, or to increase the solubility of its elements,
proceeds in the same way as the farmer deals with his fields--he
spares no labour in order to reduce it to the finest powder; he
separates the impalpable from the coarser parts by washing, and
repeats his mechanical bruising and trituration, being assured his
whole process will fail if he is inattentive to this essential and
preliminary part of it.
The influence which the increase of surface exercises upon the
disintegration of rocks, and upon the chemical action of air and
moisture, is strikingly illustrated upon a large scale in the
operations pursued in the gold-mines of Yaquil, in Chili. These are
described in a very interesting manner by Darwin. The rock
containing the gold ore is pounded by mills into the finest powder;
this is subjected to washing, which separates the lighter particles
from the metallic; the gold sinks to the bottom, while a stream of
water carries away the lighter earthy parts into ponds, where it
subsides to the bottom as mud. When this deposit has gradually
filled up the pond, this mud is taken out and piled in heaps, and
left exposed to the action of the atmosphere and moisture. The
washing completely removes all the soluble part of the disintegrated
rock; the insoluble part, moreover, cannot undergo any further
change while it is covered with water, and so excluded from the
influence of the atmosphere at the bottom of the pond. But being
exposed at once to the air and moisture, a powerful chemical action
takes place in the whole mass, which becomes indicated by an
efflorescence of salts covering the whole surface of the heaps in
considerable quantity. After being exposed for two or three years,
the mud is again subjected to the same process of washing, and a
considerable quantity of gold is obtained, this having been
separated by the chemical process of decomposition in the mass. The
exposure and washing of the same mud is repeated six or seven times,
and at every washing it furnishes a new quantity of gold, although
its amount diminishes every time.
Precisely similar is the chemical action which takes place in the
soil of our fields; and we accelerate and increase it by the
mechanical operations of our agriculture. By these we sever and
extend the surface, and endeavour to make every atom of the soil
accessible to the action of the carbonic acid and oxygen of the
atmosphere. We thus produce a stock of soluble mineral substances,
which serves as nourishment to a new generation of plants, materials
which are indispensable to their growth and prosperity.
LETTER XIII
My dear Sir,
Having in my last letter spoken of the general principles upon which
the science and art of agriculture must be based, let me now direct
your attention to some of those particulars between chemistry and
agriculture, and demonstrate the impossibility of perfecting the
important art of rearing food for man and animals, without a
profound knowledge of our science.
All plants cultivated as food require for their healthy sustenance
the alkalies and alkaline earths, each in a certain proportion; and
in addition to these, the cerealia do not succeed in a soil
destitute of silica in a soluble condition. The combinations of this
substance found as natural productions, namely, the silicates,
differ greatly in the degree of facility with which they undergo
decomposition, in consequence of the unequal resistance opposed by
their integral parts to the dissolving power of the atmospheric
agencies. Thus the granite of Corsica degenerates into a powder in a
time which scarcely suffices to deprive the polished granite of
Heidelberg of its lustre.
Some soils abound in silicates so readily decomposable, that in
every one or two years, as much silicate of potash becomes soluble
and fitted for assimilation as is required by the leaves and straw
of a crop of wheat. In Hungary, extensive districts are not uncommon
where wheat and tobacco have been grown alternately upon the same
soil for centuries, the land never receiving back any of those
mineral elements which were withdrawn in the grain and straw. On the
other hand, there are fields in which the necessary amount of
soluble silicate of potash for a single crop of wheat is not
separated from the insoluble masses in the soil in less than two,
three, or even more years.
The term fallow, in Agriculture, designates that period in which the
soil, left to the influence of the atmosphere, becomes enriched with
those soluble mineral constituents. Fallow, however, does not
generally imply an entire cessation of cultivation, but only an
interval in the growth of the cerealia. That store of silicates and
alkalies which is the principal condition of their success is
obtained, if potatoes or turnips are grown upon the same fields in
the intermediate periods, since these crops do not abstract a
particle of silica, and therefore leave the field equally fertile
for the following crop of wheat.
The preceding remarks will render it obvious to you, that the
mechanical working of the soil is the simplest and cheapest method
of rendering the elements of nutrition contained in it accessible to
plants.
But it may be asked, Are there not other means of decomposing the
soil besides its mechanical subdivision?--are there not substances,
which by their chemical operation will equally well or better render
its constituents suitable for entering into vegetable organisms?
Yes: we certainly possess such substances, and one of them, namely,
quick-lime, has been employed for the last century past in England
for this purpose; and it would be difficult to find a substance
better adapted to this service, as it is simple, and in almost all
localities cheap and easily accessible.
In order to obtain correct views respecting the effect of quick-lime
upon the soil, let me remind you of the first process employed by
the chemist when he is desirous of analysing a mineral, and for this
purpose wishes to bring its elements into a soluble state. Let the
mineral to be examined be, for instance, feldspar; this substance,
taken alone, even when reduced to the finest powder, requires for
its solution to be treated with an acid for weeks or months; but if
we first mix it with quick-lime, and expose the mixture to a
moderately strong heat, the lime enters into chemical combination
with certain elements of the feldspar, and its alkali (potass) is
set free. And now the acid, even without heat, dissolves not only
the lime, but also so much of the silica of the feldspar as to form
a transparent jelly. The same effect which the lime in this process,
with the aid of heat, exerts upon the feldspar, it produces when it
is mixed with the alkaline argillaceous silicates, and they are for
a long time kept together in a moist state.
Common potters' clay, or pipe-clay, diffused through water, and
added to milk of lime, thickens immediately upon mixing; and if the
mixture is kept for some months, and then treated with acid, the
clay becomes gelatinous, which would not occur without the admixture
with the lime. The lime, in combining with the elements of the clay,
liquifies it; and, what is more remarkable, liberates the greater
part of its alkalies. These interesting facts were first observed by
Fuchs, at Munich: they have not only led to a more intimate
knowledge of the nature and properties of the hydraulic cements,
but, what is far more important, they explain the effects of caustic
lime upon the soil, and guide the agriculturist in the application
of an invaluable means of opening it, and setting free its
alkalies--substances so important, nay, so indispensable to his
crops.
In the month of October the fields of Yorkshire and Oxfordshire look
as it they were covered with snow. Whole square miles are seen
whitened over with quicklime, which during the moist winter months,
exercises its beneficial influence upon the stiff, clayey soil, of
those counties.
According to the humus theory, quick-lime ought to exert the most
noxious influence upon the soil, because all organic matters
contained in it are destroyed by it, and rendered incapable of
yielding their humus to a new vegetation. The facts are indeed
directly contrary to this now abandoned theory: the fertility of the
soil is increased by the lime. The cerealia require the alkalies and
alkaline silicates, which the action of the lime renders fit for
assimilation by the plants. If, in addition to these, there is any
decaying organic matter present in the soil supplying carbonic acid,
it may facilitate their development; but it is not essential to
their growth. If we furnish the soil with ammonia, and the
phosphates, which are indispensable to the cerealia, with the
alkaline silicates, we have all the conditions necessary to ensure
an abundant harvest. The atmosphere is an inexhaustible store of
carbonic acid.
A no less favourable influence than that of lime is exercised upon
the soil of peaty land by the mere act of burning it: this greatly
enhances its fertility. We have not long been acquainted with the
remarkable change which the properties of clay undergo by burning.
The observation was first made in the process of analysing the clay
silicates. Many of these, in their natural state, are not acted on
by acids, but they become perfectly soluble if heated to redness
before the application of the acid. This property belongs to
potters' clay, pipe-clay, loam, and many different modifications of
clay in soils. In their natural state they may be boiled in
concentrated sulphuric acid, without sensible change; but if feebly
burned, as is done with the pipe-clay in many alum manufactories,
they dissolve in the acid with the greatest facility, the contained
silica being separated like jelly in a soluble state. Potters' clay
belongs to the most sterile kinds of soil, and yet it contains
within itself all the constituent elements essential to a most
luxurious growth of plants; but their mere presence is insufficient
to secure this end. The soil must be accessible to the atmosphere,
to its oxygen, to its carbonic acid; these must penetrate it, in
order to secure the conditions necessary to a happy and vigorous
development of the roots. The elements present must be brought into
that peculiar state of combination which will enable them to enter
into plants. Plastic clay is wanting in these properties; but they
are imparted to it by a feeble calcination.
At Hardwicke Court, near Gloucester, I have seen a garden (Mr.
Baker's) consisting of a stiff clay, which was perfectly sterile,
become by mere burning extremely fertile. The operation was extended
to a depth of three feet. This was an expensive process, certainly;
but it was effectual.
The great difference in the properties of burnt and unburnt clay is
illustrated by what is seen in brick houses, built in moist
situations. In the town of Flanders, for instance, where most
buildings are of brick, effloresences of salts cover the surfaces of
the walls, like a white nap, within a few days after they are
erected. If this saline incrustation is washed away by the rain, it
soon re-appears; and this is even observed on walls which, like the
gateway of Lisle, have been erected for centuries. These saline
incrustations consist of carbonates and sulphates, with alkaline
bases; and it is well known these act an important part in
vegetation. The influence of lime in their production is manifested
by their appearing first at the place where the mortar and brick
come into contact.
It will now be obvious to you, that in a mixture of clay with lime,
all the conditions exist for the solution of the silicated clay, and
the solubility of the alkaline silicates. The lime gradually
dissolving in water charged with carbonic acid, acts like milk of
lime upon the clay. This explains also the favourable influence
which marl (by which term all those varieties of clay rich in chalk
are designated) exerts upon most kinds of soil. There are marly
soils which surpass all others in fertility for all kinds of plants;
but I believe marl in a burnt state must be far more effective, as
well as other materials possessing a similar composition; as, for
instance, those species of limestone which are adapted to the
preparation of hydraulic cements,--for these carry to the soil not
only the alkaline bases useful to plants, but also silica in a state
capable of assimilation.
The ashes of coals and lignite are also excellent means of
ameliorating the soil, and they are used in many places for this
purpose. The most suitable may be readily known by their property of
forming a gelatinous mass when treated with acids, or by becoming,
when mixed with cream of lime, like hydraulic cement,--solid and
hard as stone.
I have now, I trust, explained to your satisfaction, that the
mechanical operations of agriculture--the application of lime and
chalk to lands, and the burning of clay--depend upon one and the
same scientific principle: they are means of accelerating the
decomposition of the alkaline clay silicates, in order to provide
plants, at the beginning of a new vegetation, with certain inorganic
matters indispensable for their nutrition.
LETTER XIV
My dear Sir,
I treated, in my last letter, of the means of improving the
condition of the soil for agricultural purposes by mechanical
operations and mineral agents. I have now to speak of the uses and
effects of animal exuviae, and vegetable matters or manures--properly
so called.
In order to understand the nature of these, and the peculiarity of
their influence upon our fields, it is highly important to keep in
mind the source whence they are derived.
It is generally known, that if we deprive an animal of food, the
weight of its body diminishes during every moment of its existence.
If this abstinence is continued for some time, the diminution
becomes apparent to the eye; all the fat of the body disappears, the
muscles decrease in firmness and bulk, and, if the animal is allowed
to die starved, scarcely anything but skin, tendon, and bones,
remain. This emaciation which occurs in a body otherwise healthy,
demonstrates to us, that during the life of an animal every part of
its living substance is undergoing a perpetual change; all its
component parts, assuming the form of lifeless compounds, are thrown
off by the skin, lungs, and urinary system, altered more or less by
the secretory organs. This change in the living body is intimately
connected with the process of respiration; it is, in truth,
occasioned by the oxygen of the atmosphere in breathing, which
combines with all the various matters within the body. At every
inspiration a quantity of oxygen passes into the blood in the lungs,
and unites with its elements; but although the weight of the oxygen
thus daily entering into the body amounts to 32 or more ounces, yet
the weight of the body is not thereby increased. Exactly as much
oxygen as is imbibed in inspiration passes off in expiration, in the
form of carbonic acid and water; so that with every breath the
amount of carbon and hydrogen in the body is diminished. But the
emaciation--the loss of weight by starvation--does not simply depend
upon the separation of the carbon and hydrogen; but all the other
substances which are in combination with these elements in the
living tissues pass off in the secretions. The nitrogen undergoes a
change, and is thrown out of the system by the kidneys. Their
secretion, the urine, contains not only a compound rich in nitrogen,
namely urea, but the sulphur of the tissues in the form of a
sulphate, all the soluble salts of the blood and animal fluids,
common salt, the phosphates, soda and potash. The carbon and
hydrogen of the blood, of the muscular fibre, and of all the animal
tissues which can undergo change, return into the atmosphere. The
nitrogen, and all the soluble inorganic elements are carried to the
earth in the urine.