Familiar Letters on Chemistry
by
Justus Liebig
Part 2 out of 3
nitrogen, because that element is essential to the composition of
the above-named organs; because the organs cannot create it from the
other elements presented to them; and, finally, because no nitrogen
is absorbed from the atmosphere in the vital process.
The substance of the brain and nerves contains a large quantity of
albumen, and, in addition to this, two peculiar fatty acids,
distinguished from other fats by containing phosphorus (phosphoric
acid?). One of these contains nitrogen (Fremy).
Finally, water and common fat are those ingredients of the body
which are destitute of nitrogen. Both are amorphous or unorganised,
and only so far take part in the vital process as that their
presence is required for the due performance of the vital functions.
The inorganic constituents of the body are, iron, lime, magnesia,
common salt, and the alkalies.
The nutritive process is seen in its simplest form in carnivorous
animals. This class of animals lives on the blood and flesh of the
graminivora; but this blood and flesh are, in all their properties,
identical with their own. Neither chemical nor physiological
differences can be discovered.
The nutriment of carnivorous animals is derived originally from
blood; in their stomach it becomes dissolved, and capable of
reaching all other parts of the body; in its passage it is again
converted into blood, and from this blood are reproduced all those
parts of their organisation which have undergone change or
metamorphosis.
With the exception of hoofs, hair, feathers, and the earth of bones,
every part of the food of carnivorous animals is capable of
assimilation.
In a chemical sense, therefore, it may be said that a carnivorous
animal, in supporting the vital process, consumes itself. That which
serves for its nutrition is identical with those parts of its
organisation which are to be renewed.
The process of nutrition in graminivorous animals appears at first
sight altogether different. Their digestive organs are less simple,
and their food consists of vegetables, the great mass of which
contains but little nitrogen.
From what substances, it may be asked, is the blood formed, by means
of which of their organs are developed? This question may be
answered with certainty.
Chemical researches have shown, that all such parts of vegetables as
can afford nutriment to animals contain certain constituents which
are rich in nitrogen; and the most ordinary experience proves that
animals require for their support and nutrition less of these parts
of plants in proportion as they abound in the nitrogenised
constituents. Animals cannot be fed on matters destitute of these
nitrogenised constituents.
These important products of vegetation are especially abundant in
the seeds of the different kinds of grain, and of peas, beans, and
lentils; in the roots and the juices of what are commonly called
vegetables. They exist, however, in all plants, without exception,
and in every part of plants in larger or smaller quantity.
These nitrogenised forms of nutriment in the vegetable kingdom may
be reduced to three substances, which are easily distinguished by
their external characters. Two of them are soluble in water, the
third is insoluble.
When the newly-expressed juices of vegetables are allowed to stand,
a separation takes place in a few minutes. A gelatinous precipitate,
commonly of a green tinge, is deposited, and this, when acted on by
liquids which remove the colouring matter, leaves a grayish white
substance, well known to druggists as the deposite from vegetable
juices. This is one of the nitrogenised compounds which serves for
the nutrition of animals, and has been named vegetable fibrine. The
juice of grapes is especially rich in this constituent, but it is
most abundant in the seeds of wheat, and of the cerealia generally.
It may be obtained from wheat flour by a mechanical operation, and
in a state of tolerable purity; it is then called gluten, but the
glutinous property belongs, not to vegetable fibrine, but to a
foreign substance, present in small quantity, which is not found in
the other cerealia.
The method by which it is obtained sufficiently proves that it is
insoluble in water; although we cannot doubt that it was originally
dissolved in the vegetable juice, from which it afterwards
separated, exactly as fibrine does from blood.
The second nitrogenised compound remains dissolved in the juice
after the separation of the fibrine. It does not separate from the
juice at the ordinary temperature, but is instantly coagulated when
the liquid containing it is heated to the boiling point.
When the clarified juice of nutritious vegetables, such as
cauliflower, asparagus, mangelwurzel, or turnips, is made to boil, a
coagulum is formed, which it is absolutely impossible to distinguish
from the substance which separates as a coagulum, when the serum of
blood, or the white of an egg, diluted with water, are heated to the
boiling point. This is vegetable albumen. It is found in the
greatest abundance in certain seeds, in nuts, almonds, and others,
in which the starch of the gramineae is replaced by oil.
The third nitrogenised constituent of the vegetable food of animals
is vegetable caseine. It is chiefly found in the seeds of peas,
beans, lentils, and similar leguminous seeds. Like vegetable
albumen, it is soluble in water, but differs from it in this, that
its solution is not coagulated by heat. When the solution is heated
or evaporated, a skin forms on its surface, and the addition of an
acid causes a coagulum, just as in animal milk.
These three nitrogenised compounds, vegetable fibrine, albumen, and
caseine, are the true nitrogenised constituents of the food of
graminivorous animals; all other nitrogenised compounds occurring in
plants, are either rejected by animals, as in the case of the
characteristic principles of poisonous and medicinal plants, or else
they occur in the food in such very small proportion, that they
cannot possibly contribute to the increase of mass in the animal
body.
The chemical analysis of these three substances has led to the very
interesting result that they contain the same organic elements,
united in the same proportion by weight; and, what is still more
remarkable, that they are identical in composition with the chief
constituents of blood, animal fibrine, and albumen. They all three
dissolve in concentrated muriatic acid with the same deep purple
colour, and even in their physical characters, animal fibrine and
albumen are in no respect different from vegetable fibrine and
albumen. It is especially to be noticed, that by the phrase,
identity of composition, we do not here intend mere similarity, but
that even in regard to the presence and relative amount of sulphur,
phosphorus, and phosphate of lime, no difference can be observed.
How beautifully and admirably simple, with the aid of these
discoveries, appears the process of nutrition in animals, the
formation of their organs, in which vitality chiefly resides! Those
vegetable principles, which in animals are used to form blood,
contain the chief constituents of blood, fibrine and albumen, ready
formed, as far as regards their composition. All plants, besides,
contain a certain quantity of iron, which reappears in the colouring
matter of the blood. Vegetable fibrine and animal fibrine, vegetable
albumen and animal albumen, hardly differ, even in form; if these
principles be wanting in the food, the nutrition of the animal is
arrested; and when they are present, the graminivorous animal
obtains in its food the very same principles on the presence of
which the nutrition of the carnivora entirely depends.
Vegetables produce in their organism the blood of all animals, for
the carnivora, in consuming the blood and flesh of the graminivora,
consume, strictly speaking, only the vegetable principles which have
served for the nutrition of the latter. Vegetable fibrine and
albumen take the form in the stomach of the graminivorous animal as
animal fibrine and albumen do in that of the carnivorous animal.
From what has been said, it follows that the development of the
animal organism and its growth are dependent on the reception of
certain principles identical with the chief constituents of blood.
In this sense we may say that the animal organism gives to the blood
only its form; that it is incapable of creating blood out of other
substances which do not already contain the chief constituents of
that fluid. We cannot, indeed, maintain that the animal organism has
no power to form other compounds, for we know that it is capable of
producing an extensive series of compounds, differing in composition
from the chief constituents of blood; but these last, which form the
starting-point of the series, it cannot produce.
The animal organism is a higher kind of vegetable, the development
of which begins with those substances with the production of which
the life of an ordinary vegetable ends. As soon as the latter has
borne seed, it dies, or a period of its life comes to a termination.
In that endless series of compounds, which begins with carbonic
acid, ammonia, and water, the sources of the nutrition of
vegetables, and includes the most complex constituents of the animal
brain, there is no blank, no interruption. The first substance
capable of affording nutriment to animals is the last product of the
creative energy of vegetables.
The substance of cellular tissue and of membranes, of the brain and
nerves, these the vegetable cannot produce.
The seemingly miraculous in the productive agency of vegetables
disappears in a great degree, when we reflect that the production of
the constituents of blood cannot appear more surprising than the
occurrence of the fat of beef and mutton in cocoa beans, of human
fat in olive-oil, of the principal ingredient of butter in palm-oil,
and of horse fat and train-oil in certain oily seeds.
LETTER IX
My dear Sir,
The facts detailed in my last letter will satisfy you as to the
manner in which the increase of mass in an animal, that is, its
growth, is accomplished; we have still to consider a most important
question, namely, the function performed in the animal system by
substances destitute of nitrogen; such as sugar, starch, gum,
pectine, &c.
The most extensive class of animals, the graminivora, cannot live
without these substances; their food must contain a certain amount
of one or more of them, and if these compounds are not supplied,
death quickly ensues.
This important inquiry extends also to the constituents of the food
of carnivorous animals in the earliest periods of life; for this
food also contains substances, which are not necessary for their
support in the adult state. The nutrition of the young of carnivora
is obviously accomplished by means similar to those by which the
graminivora are nourished; their development is dependent on the
supply of a fluid, which the body of the mother secretes in the
shape of milk.
Milk contains only one nitrogenised constituent, known under the
name of caseine; besides this, its chief ingredients are butter
(fat), and sugar of milk. The blood of the young animal, its
muscular fibre, cellular tissue, nervous matter, and bones, must
have derived their origin from the nitrogenised constituent of
milk--the caseine; for butter and sugar of milk contain no nitrogen.
Now, the analysis of caseine has led to the result, which, after the
details I have given, can hardly excite your surprise, that this
substance also is identical in composition with the chief
constituents of blood, fibrine and albumen. Nay more--a comparison
of its properties with those of vegetable caseine has shown--that
these two substances are identical in all their properties;
insomuch, that certain plants, such as peas, beans, and lentils, are
capable of producing the same substance which is formed from the
blood of the mother, and employed in yielding the blood of the young
animal.
The young animal, therefore, receives in the form of caseine,--which
is distinguished from fibrine and albumen by its great solubility,
and by not coagulating when heated,--the chief constituent of the
mother's blood. To convert caseine into blood no foreign substance
is required, and in the conversion of the mother's blood into
caseine, no elements of the constituents of the blood have been
separated. When chemically examined, caseine is found to contain a
much larger proportion of the earth of bones than blood does, and
that in a very soluble form, capable of reaching every part of the
body. Thus, even in the earliest period of its life, the development
of the organs, in which vitality resides, is, in the carnivorous
animal, dependent on the supply of a substance, identical in organic
composition with the chief constituents of its blood.
What, then, is the use of the butter and the sugar of milk? How does
it happen that these substances are indispensable to life?
Butter and sugar of milk contain no fixed bases, no soda nor potash.
Sugar of milk has a composition closely allied to that of the other
kinds of sugar, of starch, and of gum; all of them contain carbon
and the elements of water, the latter precisely in the proportion to
form water.
There is added, therefore, by means of these compounds, to the
nitrogenised constituents of food, a certain amount of carbon; or,
as in the case of butter, of carbon and hydrogen; that is, an excess
of elements, which cannot possibly be employed in the production of
blood, because the nitrogenised substances contained in the food
already contain exactly the amount of carbon which is required for
the production of fibrine and albumen.
In an adult carnivorous animal, which neither gains nor loses
weight, perceptibly, from day to day, its nourishment, the waste of
organised tissue, and its consumption of oxygen, stand to each other
in a well-defined and fixed relation.
The carbon of the carbonic acid given off, with that of the urine;
the nitrogen of the urine, and the hydrogen given off as ammonia and
water; these elements, taken together, must be exactly equal in
weight to the carbon, nitrogen, and hydrogen of the metamorphosed
tissues, and since these last are exactly replaced by the food, to
the carbon, nitrogen, and hydrogen of the food. Were this not the
case, the weight of the animal could not possibly remain unchanged.
But, in the young of the carnivora, the weight does not remain
unchanged; on the contrary, it increases from day to day by an
appreciable quantity.
This fact presupposes, that the assimilative process in the young
animal is more energetic, more intense, than the process of
transformation in the existing tissues. If both processes were
equally active, the weight of the body could not increase; and were
the waste by transformation greater, the weight of the body would
decrease.
Now, the circulation in the young animal is not weaker, but, on the
contrary, more rapid; the respirations are more frequent; and, for
equal bulks, the consumption of oxygen must be greater rather than
smaller in the young than in the adult animal. But, since the
metamorphosis of organised parts goes on more slowly, there would
ensue a deficiency of those substances, the carbon and hydrogen of
which are adapted for combination with oxygen; because, in the
carnivora, nature has destined the new compounds, produced by the
metamorphosis of organised parts, to furnish the necessary
resistance to the action of the oxygen, and to produce animal heat.
What is wanting for these purposes an Infinite Wisdom has supplied
to the young in its natural food.
The carbon and hydrogen of butter, and the carbon of the sugar of
milk, no part of either of which can yield blood, fibrine, or
albumen, are destined for the support of the respiratory process, at
an age when a greater resistance is opposed to the metamorphosis of
existing organisms; or, in other words, to the production of
compounds, which, in the adult state, are produced in quantity amply
sufficient for the purpose of respiration.
The young animal receives the constituents of its blood in the
caseine of the milk. A metamorphosis of existing organs goes on, for
bile and urine are secreted; the materials of the metamorphosed
parts are given off in the form of urine, of carbonic acid, and of
water; but the butter and sugar of milk also disappear; they cannot
be detected in the faeces.
The butter and sugar of milk are given out in the form of carbonic
acid and water, and their conversion into oxidised products
furnishes the clearest proof that far more oxygen is absorbed than
is required to convert the carbon and hydrogen of the metamorphosed
tissues into carbonic acid and water.
The change and metamorphosis of organised tissues going on in the
vital process in the young animal, consequently yield, in a given
time, much less carbon and hydrogen in the form adapted for the
respiratory process than correspond to the oxygen taken up in the
lungs. The substance of its organised parts would undergo a more
rapid consumption, and would necessarily yield to the action of the
oxygen, were not the deficiency of carbon and hydrogen supplied from
another source.
The continued increase of mass, or growth, and the free and
unimpeded development of the organs in the young animal, are
dependent on the presence of foreign substances, which, in the
nutritive process, have no other function than to protect the
newly-formed organs from the action of the oxygen. The elements of
these substances unite with the oxygen; the organs themselves could
not do so without being consumed; that is, growth, or increase of
mass in the body,--the consumption of oxygen remaining the
same,--would be utterly impossible.
The preceding considerations leave no doubt as to the purpose for
which Nature has added to the food of the young of carnivorous
mammalia substances devoid of nitrogen, which their organism cannot
employ for nutrition, strictly so called, that is, for the
production of blood; substances which may be entirely dispensed with
in their nourishment in the adult state. In the young of carnivorous
birds, the want of all motion is an obvious cause of diminished
waste in the organised parts; hence, milk is not provided for them.
The nutritive process in the carnivora thus presents itself under
two distinct forms; one of which we again meet with in the
graminivora.
In graminivorous animals. we observe, that during their whole life,
their existence depends on a supply of substances having a
composition identical with that of sugar of milk, or closely
resembling it. Everything that they consume as food contains a
certain quantity of starch, gum, or sugar, mixed with other matters.
The function performed in the vital process of the graminivora by
these substances is indicated in a very clear and convincing manner,
when we take into consideration the very small relative amount of
the carbon which these animals consume in the nitrogenised
constituents of their food, which bears no proportion whatever to
the oxygen absorbed through the skin and lungs.
A horse, for example, can be kept in perfectly good condition, if he
obtain as food 15 lbs. of hay and 4 1/2 lbs. of oats daily. If we
now calculate the whole amount of nitrogen in these matters, as
ascertained by analysis (1 1/2 per cent. in the hay, 2.2 per cent.
in the oats), in the form of blood, that is, as fibrine and albumen,
with the due proportion of water in blood (80 per cent.), the horse
receives daily no more than 4 1/2 oz. of nitrogen, corresponding to
about 8 lbs. of blood. But along with this nitrogen, that is,
combined with it in the form of fibrine or albumen, the animal
receives only about 14 1/2 oz. of carbon.
Without going further into the calculation, it will readily be
admitted, that the volume of air inspired and expired by a horse,
the quantity of oxygen consumed, and, as a necessary consequence,
the amount of carbonic acid given out by the animal, are much
greater than in the respiratory process in man. But an adult man
consumes daily abut 14 oz. of carbon, and the determination of
Boussingault, according to which a horse expires 79 oz. daily,
cannot be very far from the truth.
In the nitrogenised constituents of his food, therefore, the horse
receives rather less than the fifth part of the carbon which his
organism requires for the support of the respiratory process; and we
see that the wisdom of the Creator has added to his food the
four-fifths which are wanting, in various forms, as starch, sugar,
&c. with which the animal must be supplied, or his organism will be
destroyed by the action of the oxygen.
It is obvious, that in the system of the graminivora, whose food
contains so small a portion, relatively, of the constituents of the
blood, the process of metamorphosis in existing tissues, and
consequently their restoration or reproduction, must go on far less
rapidly than in the carnivora. Were this not the case, a vegetation
a thousand times more luxuriant than the actual one would not
suffice for their nourishment. Sugar, gum, and starch, would no
longer be necessary to support life in these animals, because, in
that case, the products of the waste, or metamorphosis of the
organised tissues, would contain enough carbon to support the
respiratory process.
LETTER X
My dear Sir,
Let me now apply the principles announced in the preceding letters
to the circumstances of our own species. Man, when confined to
animal food, requires for his support and nourishment extensive
sources of food, even more widely extended than the lion and tiger,
because, when he has the opportunity, he kills without eating.
A nation of hunters, on a limited space, is utterly incapable of
increasing its numbers beyond a certain point, which is soon
attained. The carbon necessary for respiration must be obtained from
the animals, of which only a limited number can live on the space
supposed. These animals collect from plants the constituents of
their organs and of their blood, and yield them, in turn, to the
savages who live by the chase alone. They, again, receive this food
unaccompanied by those compounds, destitute of nitrogen, which,
during the life of the animals, served to support the respiratory
process. In such men, confined to an animal diet, it is the carbon
of the flesh and of the blood which must take the place of starch
and sugar.
But 15 lbs. of flesh contain no more carbon than 4 lbs. of starch,
and while the savage with one animal and an equal weight of starch
should maintain life and health for a certain number of days, he
would be compelled, if confined to flesh alone, in order to procure
the carbon necessary for respiration, during the same time, to
consume five such animals.
It is easy to see, from these considerations, how close the
connection is between agriculture and the multiplication of the
human species. The cultivation of our crops has ultimately no other
object than the production of a maximum of those substances which
are adapted for assimilation and respiration, in the smallest
possible space. Grain and other nutritious vegetables yield us, not
only in starch, sugar, and gum, the carbon which protects our organs
from the action of oxygen, and produces in the organism the heat
which is essential to life, but also in the form of vegetable
fibrine, albumen, and caseine, our blood, from which the other parts
of our body are developed.
Man, when confined to animal food, respires, like the carnivora, at
the expense of the matters produced by the metamorphosis of
organised tissues; and, just as the lion, tiger, hyaena, in the
cages of a menagerie, are compelled to accelerate the waste of the
organised tissues by incessant motion, in order to furnish the
matter necessary for respiration, so, the savage, for the very same
object, is forced to make the most laborious exertions, and go
through a vast amount of muscular exercise. He is compelled to
consume force merely in order to supply matter for respiration.
Cultivation is the economy of force. Science teaches us the simplest
means of obtaining the greatest effect with the smallest expenditure
of power, and with given means to produce a maximum of force. The
unprofitable exertion of power, the waste of force in agriculture,
in other branches of industry, in science, or in social economy, is
characteristic of the savage state, or of the want of knowledge.
In accordance with what I have already stated, you will perceive
that the substances of which the food of man is composed may be
divided into two classes; into nitrogenised and non-nitrogenised.
The former are capable of conversion into blood; the latter are
incapable of this transformation.
Out of those substances which are adapted to the formation of blood,
are formed all the organised tissues. The other class of substances,
in the normal state of health, serve to support the process of
respiration. The former may be called the plastic elements of
nutrition; the latter, elements of respiration.
Among the former we reckon--
Vegetable fibrine.
Vegetable albumen.
Vegetable caseine.
Animal flesh.
Animal blood.
Among the elements of respiration in our food, are--
Fat. Pectine.
Starch. Bassorine.
Gum. Wine.
Cane sugar. Beer.
Grape sugar. Spirits.
Sugar of milk.
The most recent and exact researches have established as a universal
fact, to which nothing yet known is opposed, that the nitrogenised
constituents of vegetable food have a composition identical with
that of the constituents of the blood.
No nitrogenised compound, the composition of which differs from that
of fibrine, albumen, and caseine, is capable of supporting the vital
process in animals.
The animal organism unquestionably possesses the power of forming,
from the constituents of its blood, the substance of its membranes
and cellular tissue, of the nerves and brain, and of the organic
part of cartilages and bones. But the blood must be supplied to it
perfect in everything but its form--that is, in its chemical
composition. If this be not done, a period is rapidly put to the
formation of blood, and consequently to life.
This consideration enables us easily to explain how it happens that
the tissues yielding gelatine or chondrine, as, for example, the
gelatine of skin or of bones, are not adapted for the support of the
vital process; for their composition is different from that of
fibrine or albumen. It is obvious that this means nothing more than
that those parts of the animal organism which form the blood do not
possess the power of effecting a transformation in the arrangement
of the elements of gelatine, or of those tissues which contain it.
The gelatinous tissues, the gelatine of the bones, the membranes,
the cells and the skin suffer, in the animal body, under the
influence of oxygen and moisture, a progressive alteration; a part
of these tissues is separated, and must be restored from the blood;
but this alteration and restoration are obviously confined within
very narrow limits.
While, in the body of a starving or sick individual, the fat
disappears and the muscular tissue takes once more the form of
blood, we find that the tendons and membranes retain their natural
condition, and the limbs of the dead body their connections, which
depend on the gelatinous tissues.
On the other hand, we see that the gelatine of bones devoured by a
dog entirely disappears, while only the bone earth is found in his
excrements. The same is true of man, when fed on food rich in
gelatine, as, for example, strong soup. The gelatine is not to be
found either in the urine or in the faeces, and consequently must
have undergone a change, and must have served some purpose in the
animal economy. It is clear that the gelatine must be expelled from
the body in a form different from that in which it was introduced as
food.
When we consider the transformation of the albumen of the blood into
a part of an organ composed of fibrine, the identity in composition
of the two substances renders the change easily conceivable. Indeed
we find the change of a dissolved substance into an insoluble organ
of vitality, chemically speaking, natural and easily explained, on
account of this very identity of composition. Hence the opinion is
not unworthy of a closer investigation, that gelatine, when taken in
the dissolved state, is again converted, in the body, into cellular
tissue, membrane and cartilage; that it may serve for the
reproduction of such parts of these tissues as have been wasted, and
for their growth.
And when the powers of nutrition in the whole body are affected by a
change of the health, then, even should the power of forming blood
remain the same, the organic force by which the constituents of the
blood are transformed into cellular tissue and membranes must
necessarily be enfeebled by sickness. In the sick man, the intensity
of the vital force, its power to produce metamorphoses, must be
diminished as well in the stomach as in all other parts of the body.
In this condition, the uniform experience of practical physicians
shows that gelatinous matters in a dissolved state exercise a most
decided influence on the state of the health. Given in a form
adapted for assimilation, they serve to husband the vital force,
just as may be done, in the case of the stomach, by due preparation
of the food in general.
Brittleness in the bones of graminivorous animals is clearly owing
to a weakness in those parts of the organism whose function it is to
convert the constituents of the blood into cellular tissue and
membrane; and if we can trust to the reports of physicians who have
resided in the East, the Turkish women, in their diet of rice, and
in the frequent use of enemata of strong soup, have united the
conditions necessary for the formation both of cellular tissue and
of fat.
LETTER XI
My dear Sir,
In the immense, yet limited expanse of the ocean, the animal and
vegetable kingdoms are mutually dependent upon, and successive to
each other. The animals obtain their constituent elements from the
plants, and restore them to the water in their original form, when
they again serve as nourishment to a new generation of plants.
The oxygen which marine animals withdraw in their respiration from
the air, dissolved in sea water, is returned to the water by the
vital processes of sea plants; that air is richer in oxygen than
atmospheric air, containing 32 to 33 per cent. Oxygen, also,
combines with the products of the putrefaction of dead animal
bodies, changes their carbon into carbonic acid, their hydrogen into
water, and their nitrogen assumes again the form of ammonia.
Thus we observe in the ocean a circulation takes place without the
addition or subtraction of any element, unlimited in duration,
although limited in extent, inasmuch as in a confined space the
nourishment of plants exists in a limited quantity.
We well know that marine plants cannot derive a supply of humus for
their nourishment through their roots. Look at the great sea-tang,
the Fucus giganteus: this plant, according to Cook, reaches a height
of 360 feet, and a single specimen, with its immense ramifications,
nourishes thousands of marine animals, yet its root is a small body,
no larger than the fist. What nourishment can this draw from a naked
rock, upon the surface of which there is no perceptible change? It
is quite obvious that these plants require only a hold,--a fastening
to prevent a change of place,--as a counterpoise to their specific
gravity, which is less than that of the medium in which they float.
That medium provides the necessary nourishment, and presents it to
the surface of every part of the plant. Sea-water contains not only
carbonic acid and ammonia, but the alkaline and earthy phosphates
and carbonates required by these plants for their growth, and which
we always find as constant constituents of their ashes.
All experience demonstrates that the conditions of the existence of
marine plants are the same which are essential to terrestrial
plants. But the latter do not live like sea-plants, in a medium
which contains all their elements and surrounds with appropriate
nourishment every part of their organs; on the contrary, they
require two media, of which one, namely the soil, contains those
essential elements which are absent from the medium surrounding
them, i.e. the atmosphere.
Is it possible that we could ever be in doubt respecting the office
which the soil and its component parts subserve in the existence and
growth of vegetables?--that there should have been a time when the
mineral elements of plants were not regarded as absolutely essential
to their vitality? Has not the same circulation been observed on the
surface of the earth which we have just contemplated in the
ocean,--the same incessant change, disturbance and restitution of
equilibrium?
Experience in agriculture shows that the production of vegetables on
a given surface increases with the supply of certain matters,
originally parts of the soil which had been taken up from it by
plants--the excrements of man and animals. These are nothing more
than matters derived from vegetable food, which in the vital
processes of animals, or after their death, assume again the form
under which they originally existed, as parts of the soil. Now, we
know that the atmosphere contains none of these substances, and
therefore can replace none; and we know that their removal from a
soil destroys its fertility, which may be restored and increased by
a new supply.
Is it possible, after so many decisive investigations into the
origin of the elements of animals and vegetables, the use of the
alkalies, of lime and the phosphates, any doubt can exist as to the
principles upon which a rational agriculture depends? Can the art of
agriculture be based upon anything but the restitution of a
disturbed equilibrium? Can it be imagined that any country, however
rich and fertile, with a flourishing commerce, which for centuries
exports its produce in the shape of grain and cattle, will maintain
its fertility, if the same commerce does not restore, in some form
of manure, those elements which have been removed from the soil, and
which cannot be replaced by the atmosphere? Must not the same fate
await every such country which has actually befallen the once
prolific soil of Virginia, now in many parts no longer able to grow
its former staple productions--wheat and tobacco?
In the large towns of England the produce both of English and
foreign agriculture is largely consumed; elements of the soil
indispensable to plants do not return to the fields,--contrivances
resulting from the manners and customs of English people, and
peculiar to them, render it difficult, perhaps impossible, to
collect the enormous quantity of the phosphates which are daily, as
solid and liquid excrements, carried into the rivers. These
phosphates, although present in the soil in the smallest quantity,
are its most important mineral constituents. It was observed that
many English fields exhausted in that manner immediately doubled
their produce, as if by a miracle, when dressed with bone earth
imported from the Continent. But if the export of bones from Germany
is continued to the extent it has hitherto reached, our soil must be
gradually exhausted, and the extent of our loss may be estimated, by
considering that one pound of bones contains as much phosphoric acid
as a hundred-weight of grain.
The imperfect knowledge of Nature and the properties and relations
of matter possessed by the alchemists gave rise, in their time, to
an opinion that metals as well as plants could be produced from a
seed. The regular forms and ramifications seen in crystals, they
imagined to be the leaves and branches of metal plants; and as they
saw the seed of plants grow, producing root, stem and leaves, and
again blossoms, fruit and seeds, apparently without receiving any
supply of appropriate material, they deemed it worthy of zealous
inquiry to discover the seed of gold, and the earth necessary for
its development. If the metal seeds were once obtained, might they
not entertain hopes of their growth?
Such ideas could only be entertained when nothing was known of the
atmosphere, and its participation with the earth, in administering
to the vital processes of plants and animals. Modern chemistry
indeed produces the elements of water, and, combining them, forms
water anew; but it does not create those elements--it derives them
from water; the new-formed artificial water has been water before.
Many of our farmers are like the alchemists of old,--they are
searching for the miraculous seed,--the means, which, without any
further supply of nourishment to a soil scarcely rich enough to be
sprinkled with indigenous plants, shall produce crops of grain a
hundred-fold.
The experience of centuries, nay, of thousands of years, is
insufficient to guard men against these fallacies; our only security
from these and similar absurdities must be derived from a correct
knowledge of scientific principles.
In the first period of natural philosophy, organic life was supposed
to be derived from water only; afterwards, it was admitted that
certain elements derived from the air must be superadded to the
water; but we now know that other elements must be supplied by the
earth, if plants are to thrive and multiply.
The amount of materials contained in the atmosphere, suited to the
nourishment of plants, is limited; but it must be abundantly
sufficient to cover the whole surface of the earth with a rich
vegetation. Under the tropics, and in those parts of our globe where
the most genial conditions of fertility exist,--a suitable soil, a
moist atmosphere, and a high temperature,--vegetation is scarcely
limited by space; and, where the soil is wanting, it is gradually
supplied by the decaying leaves, bark and branches of plants. It is
obvious there is no deficiency of atmospheric nourishment for plants
in those regions, nor are these wanting in our own cultivated
fields: all that plants require for their development is conveyed to
them by the incessant motions of the atmosphere. The air between the
tropics contains no more than that of the arctic zones; and yet how
different is the amount of produce of an equal surface of land in
the two situations!
This is easily explicable. All the plants of tropical climates, the
oil and wax palms, the sugar cane, &c., contain only a small
quantity of the elements of the blood necessary to the nutrition of
animals, as compared with our cultivated plants. The tubers of the
potato in Chili, its native country, where the plant resembles a
shrub, if collected from an acre of land, would scarcely suffice to
maintain an Irish family for a single day (Darwin). The result of
cultivation in those plants which serve as food, is to produce in
them those constituents of the blood. In the absence of the elements
essential to these in the soil, starch, sugar and woody fibre, are
perhaps formed; but no vegetable fibrine, albumen, or caseine. If we
intend to produce on a given surface of soil more of these latter
matters than the plants can obtain from the atmosphere or receive
from the soil of the same surface in its uncultivated and normal
state, we must create an artificial atmosphere, and add the needed
elements to the soil.
The nourishment which must be supplied in a given time to different
plants, in order to admit a free and unimpeded growth, is very
unequal.
On pure sand, on calcareous soil, on naked rocks, only a few genera
of plants prosper, and these are, for the most part, perennial
plants. They require, for their slow growth, only such minute
quantities of mineral substances as the soil can furnish, which may
be totally barren for other species. Annual, and especially summer
plants, grow and attain their perfection in a comparatively short
time; they therefore do not prosper on a soil which is poor in those
mineral substances necessary to their development. To attain a
maximum in height in the short period of their existence, the
nourishment contained in the atmosphere is not sufficient. If the
end of cultivation is to be obtained, we must create in the soil an
artificial atmosphere of carbonic acid and ammonia; and this surplus
of nourishment, which the leaves cannot appropriate from the air,
must be taken up by the corresponding organs, i.e. the roots, from
the soil. But the ammonia, together with the carbonic acid, are
alone insufficient to become part of a plant destined to the
nourishment of animals. In the absence of the alkalies, the
phosphates and other earthy salts, no vegetable fibrine, no
vegetable caseine, can be formed. The phosphoric acid of the
phosphate of lime, indispensable to the cerealia and other
vegetables in the formation of their seeds, is separated as an
excrement, in great quantities, by the rind and barks of ligneous
plants.
How different are the evergreen plants, the cacti, the mosses, the
ferns, and the pines, from our annual grasses, the cerealia and
leguminous vegetables! The former, at every time of the day during
winter and summer, obtain carbon through their leaves by absorbing
carbonic acid which is not furnished by the barren soil on which
they grow; water is also absorbed and retained by their coriaceous
or fleshy leaves with great force. They lose very little by
evaporation, compared with other plants. On the other hand, how very
small is the quantity of mineral substances which they withdraw from
the soil during their almost constant growth in one year, in
comparison with the quantity which one crop of wheat of an equal
weight receives in three months!
It is by means of moisture that plants receive the necessary
alkalies and salts from the soil. In dry summers a phenomenon is
observed, which, when the importance of mineral elements to the life
of a plant was unknown, could not be explained. The leaves of plants
first developed and perfected, and therefore nearer the surface of
the soil, shrivel up and become yellow, lose their vitality, and
fall off while the plant is in an active state of growth, without
any visible cause. This phenomenon is not seen in moist years, nor
in evergreen plants, and but rarely in plants which have long and
deep roots, nor is it seen in perennials in autumn and winter.
The cause of this premature decay is now obvious. The
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 conseqently 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 necesary 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.
These changes take place in the healthy animal body during every
moment of life; a waste and loss of substance proceeds continually;
and if this loss is to be restored, and the original weight and
substance repaired, an adequate supply of materials must be
furnished, from whence the blood and wasted tissues may be
regenerated. This supply is obtained from the food.
In an adult person in a normal or healthy condition, no sensible
increase or decrease of weight occurs from day to day. In youth the
weight of the body increases, whilst in old age it decreases. There
can be no doubt that in the adult, the food has exactly replaced the
loss of substance: it has supplied just so much carbon, hydrogen,
nitrogen, and other elements, as have passed through the skin,
lungs, and urinary organs. In youth the supply is greater than the
waste. Part of the elements of the food remain to augment the bulk
of the body. In old age the waste is greater than the supply, and
the body diminishes. It is unquestionable, that, with the exception
of a certain quantity of carbon and hydrogen, which are secreted
through the skin and lungs, we obtain, in the solid and fluid
excrements of man and animals, all the elements of their food.
We obtain daily, in the form of urea, all the nitrogen taken in the
food both of the young and the adult; and further, in the urine, the
whole amount of the alkalies, soluble phosphates and sulphates,
contained in all the various aliments. In the solid excrements are
found all those substances taken in the food which have undergone no
alteration in the digestive organs, all indigestible matters, such
as woody fibre, the green colouring matter of leaves ( chlorophyle),
wax, &c.
Physiology teaches us, that the process of nutrition in animals,
that is, their increase of bulk, or the restoration of wasted parts,
proceeds from the blood. The purpose of digestion and assimilation
is to convert the food into blood. In the stomach and intestines,
therefore, all those substances in the food capable of conversion
into blood are separated from its other constituents; in other
words, during the passage of the food through the intestinal canal
there is a constant absorption of its nitrogen, since only azotised
substances are capable of conversion into blood; and therefore the
solid excrements are destitute of that element, except only a small
portion, in the constitution of that secretion which is formed to
facilitate their passage. With the solid excrements, the phosphates
of lime and magnesia, which were contained in the food and not
assimilated, are carried off, these salts being insoluble in water,
and therefore not entering the urine.
We may obtain a clear insight into the chemical constitution of the
solid excrements without further investigation, by comparing the
faeces of a dog with his food. We give that animal flesh and
bones--substances rich in azotised matter--and we obtain, as the
last product of its digestion, a perfectly white excrement, solid
while moist, but becoming in dry air a powder. This is the phosphate
of lime of the bones, with scarcely one per cent. of foreign organic
matter.
Thus we see that in the solid and fluid excrements of man and
animals, all the nitrogen--in short, all the constituent ingredients
of the consumed food, soluble and insoluble, are returned; and as
food is primarily derived from the fields, we possess in those
excrements all the ingredients which we have taken from it in the
form of seeds, roots, or herbs.
One part of the crops employed for fattening sheep and cattle is
consumed by man as animal food; another part is taken directly--as
flour, potatoes, green vegetables, &c.; a third portion consists of
vegetable refuse, and straw employed as litter. None of the
materials of the soil need be lost. We can, it is obvious, get back
all its constituent parts which have been withdrawn therefrom, as
fruits, grain and animals, in the fluid and solid excrements of man,
and the bones, blood and skins of the slaughtered animals. It
depends upon ourselves to collect carefully all these scattered
elements, and to restore the disturbed equilibrium of composition in
the soil. We can calculate exactly how much and which of the
component parts of the soil we export in a sheep or an ox, in a
quarter of barley, wheat or potatoes, and we can discover, from the
known composition of the excrements of man and animals, how much we
have to supply to restore what is lost to our fields.
If, however, we could procure from other sources the substances
which give to the exuviae of man and animals their value in
agriculture, we should not need the latter. It is quite indifferent
for our purpose whether we supply the ammonia (the source of
nitrogen) in the form of urine, or in that of a salt derived from
coal-tar; whether we derive the phosphate of lime from bones,
apatite, or fossil excrements (the coprolithes).
The principal problem for agriculture is, how to replace those
substances which have been taken from the soil, and which cannot be
furnished by the atmosphere. If the manure supplies an imperfect
compensation for this loss, the fertility of a field or of a country
decreases; if, on the contrary, more are given to the fields, their
fertility increases.
An importation of urine, or of solid excrements, from a foreign
country, is equivalent to an importation of grain and cattle. In a
certain time, the elements of those substances assume the form of
grain, or of fodder, then become flesh and bones, enter into the
human body, and return again day by day to the form they originally
possessed.
The only real loss of elements we are unable to prevent is of the
phosphates, and these, in accordance with the customs of all modern
nations, are deposited in the grave. For the rest, every part of
that enormous quantity of food which a man consumes during his
lifetime ( say in sixty or seventy years), which was derived from
the fields, can be obtained and returned to them. We know with
absolute certainty, that in the blood of a young or growing animal
there remains a certain quantity of phosphate of lime and of the
alkaline phosphates, to be stored up and to minister to the growth
of the bones and general bulk of the body, and that, with the
exception of this very small quantity, we receive back, in the solid
and fluid excrements, all the salts and alkaline bases, all the
phosphate of lime and magnesia, and consequently all the inorganic
elements which the animal consumes in its food.
We can thus ascertain precisely the quantity, quality, and
composition of animal excrements, without the trouble of analysing
them. If we give a horse daily 4 1/2 pounds' weight of oats, and 15
pounds of hay, and knowing that oats give 4 per cent. and hay 9 per
cent. of ashes, we can calculate that the daily excrements of the
horse will contain 21 ounces of inorganic matter which was drawn
from the fields. By analysis we can determine the exact relative
amount of silica, of phosphates, and of alkalies, contained in the
ashes of the oats and of the hay.
You will now understand that the constituents of the solid parts of
animal excrements, and therefore their qualities as manure, must
vary with the nature of the creature's food. If we feed a cow upon
beetroot, or potatoes, without hay, straw or grain, there will be no
silica in her solid excrements, but there will be phosphate of lime
and magnesia. Her fluid excrements will contain carbonate of potash
and soda, together with compounds of the same bases with inorganic
acids. In one word, we have, in the fluid excrements, all the
soluble parts of the ashes of the consumed food; and in the solid
excrements, all those parts of the ashes which are insoluble in
water.
If the food, after burning, leaves behind ashes containing soluble
alkaline phosphates, as is the case with bread, seeds of all kinds,
and flesh, we obtain from the animal by which they are consumed a
urine holding in solution these phosphates. If, however, the ashes
of food contain no alkaline phosphates, but abound in insoluble
earthy phosphates, as hay, carrots, and potatoes, the urine will be
free from alkaline phosphates, but the earthy phosphates will be
found in the faeces. The urine of man, of carnivorous and
graminivorous animals, contains alkaline phosphates; that of
herbivorous animals is free from these salts.
The analysis of the excrements of man, of the piscivorous birds (as
the guano), of the horse, and of cattle, furnishes us with the
precise knowledge of the salts they contain, and demonstrates, that
in those excrements, we return to the fields the ashes of the plants
which have served as food,--the soluble and insoluble salts and
earths indispensable to the development of cultivated plants, and
which must be furnished to them by a fertile soil.
There can be no doubt that, in supplying these excrements to the
soil, we return to it those constituents which the crops have
removed from it, and we renew its capability of nourishing new
crops: in one word, we restore the disturbed equilibrium; and
consequently, knowing that the elements of the food derived from the
soil enter into the urine and solid excrements of the animals it
nourishes, we can with the greatest facility determine the exact
value of the different kinds of manure. Thus the excrements of pigs
which we have fed with peas and potatoes are principally suited for
manuring crops of potatoes and peas. In feeding a cow upon hay and
turnips, we obtain a manure containing the inorganic elements of
grasses and turnips, and which is therefore preferable for manuring
turnips. The excrement of pigeons contains the mineral elements of
grain; that of rabbits, the elements of herbs and kitchen
vegetables. The fluid and solid excrements of man, however, contain
the mineral elements of grain and seeds in the greatest quantity.
LETTER XV
My dear Sir,
You are now acquainted with my opinions respecting the effects of
the application of mineral agents to our cultivated fields, and also
the rationale of the influence of the various kinds of manures; you
will, therefore, now readily understand what I have to say of the
sources whence the carbon and nitrogen, indispensable to the growth
of plants, are derived.
The growth of forests, and the produce of meadows, demonstrate that
an inexhaustible quantity of carbon is furnished for vegetation by
the carbonic acid of the atmosphere.
We obtain from an equal surface of forest, or meadow-land, where the
necessary mineral elements of the soil are present in a suitable
state, and to which no carbonaceous matter whatever is furnished in
manures, an amount of carbon, in the shape of wood and hay, quite
equal, and oftimes more than is produced by our fields, in grain,
roots, and straw, upon which abundance of manure has been heaped.
It is perfectly obvious that the atmosphere must furnish to our
cultivated fields as much carbonic acid, as it does to an equal
surface of forest or meadow, and that the carbon of this carbonic
acid is assimilated, or may be assimilated by the plants growing
there, provided the conditions essential to its assimilation, and
becoming a constituent element of vegetables, exist in the soil of
these fields.
In many tropical countries the produce of the land in grain or
roots, during the whole year, depends upon one rain in the spring.
If this rain is deficient in quantity, or altogether wanting, the
expectation of an abundant harvest is diminished or destroyed.
Now it cannot be the water merely which produces this enlivening and
fertilising effect observed, and which lasts for weeks and months.
The plant receives, by means of this water, at the time of its first
development, the alkalies, alkaline earths, and phosphates,
necessary to its organization. If these elements, which are
necessary previous to its assimilation of atmospheric nourishment,
be absent, its growth is retarded. In fact, the development of a
plant is in a direct ratio to the amount of the matters it takes up
from the soil. If, therefore, a soil is deficient in these mineral
constituents required by plants, they will not flourish even with an
abundant supply of water.
The produce of carbon on a meadow, or an equal surface of forest
land, is independent of a supply of carbonaceous manure, but it
depends upon the presence of certain elements of the soil which in
themselves contain no carbon, together with the existence of
conditions under which their assimilation by plants can be effected.
We increase the produce of our cultivated fields, in carbon, by a
supply of lime, ashes, and marl, substances which cannot furnish
carbon to the plants, and yet it is indisputable,--being founded
upon abundant experience,--that in these substances we furnish to
the fields elements which greatly increase the bulk of their
produce, and consequently the amount of carbon.
If we admit these facts to be established, we can no longer doubt
that a deficient produce of carbon, or in other words, the
barrenness of a field does not depend upon carbonic acid, because we
are able to increase the produce, to a certain degree, by a supply
of substances which do not contain any carbon. The same source
whence the meadow and the forest are furnished with carbon, is also
open to our cultivated plants. The great object of agriculture,
therefore, is to discover the means best adapted to enable these
plants to assimilate the carbon of the atmosphere which exists in it
as carbonic acid. In furnishing plants, therefore, with mineral
elements, we give them the power to appropriate carbon from a source
which is inexhaustible; whilst in the absence of these elements the
most abundant supply of carbonic acid, or of decaying vegetable
matter, would not increase the produce of a field.
With an adequate and equal supply of these essential mineral
constituents in the soil, the amount of carbonic acid absorbed by a
plant from the atmosphere in a given time is limited by the quantity
which is brought into contact with its organs of absorption.
The withdrawal of carbonic acid from the atmosphere by the vegetable
organism takes place chiefly through its leaves; this absorption
requires the contact of the carbonic acid with their surface, or
with the part of the plant by which it is absorbed.
The quantity of carbonic acid absorbed in a given time is in direct
proportion to the surface of the leaves and the amount of carbonic
acid contained in the air; that is, two plants of the same kind and
the same extent of surface of absorption, in equal times and under
equal conditions, absorb one and the same amount of carbon.
In an atmosphere containing a double proportion of carbonic acid, a
plant absorbs, under the same condition, twice the quantity of
carbon. Boussingault observed, that the leaves of the vine, inclosed
in a vessel, withdrew all the carbonic acid from a current of air
which was passed through it, however great its velocity. (Dumas
Lecon, p.23.) If, therefore, we supply double the quantity of
carbonic acid to one plant, the extent of the surface of which is
only half that of another living in ordinary atmospheric air, the
former will obtain and appropriate as much carbon as the latter.
Hence results the effects of humus, and all decaying organic
substances, upon vegetation. If we suppose all the conditions for
the absorption of carbonic acid present, a young plant will increase
in mass, in a limited time, only in proportion to its absorbing
surface; but if we create in the soil a new source of carbonic acid,
by decaying vegetable substances, and the roots absorb in the same
time three times as much carbonic acid from the soil as the leaves
derive from the atmosphere, the plant will increase in weight
fourfold. This fourfold increase extends to the leaves, buds,
stalks, &c., and in the increased extent of the surface, the plant
acquires an increased power of absorbing nourishment from the air,
which continues in action far beyond the time when its derivation of
carbonic acid through the roots ceases. Humus, as a source of
carbonic acid in cultivated lands, is not only useful as a means of
increasing the quantity of carbon--an effect which in most cases may
be very indifferent for agricultural purposes--but the mass of the
plant having increased rapidly in a short time, space is obtained
for the assimilation of the elements of the soil necessary for the
formation of new leaves and branches.
Water evaporates incessantly from the surface of the young plant;
its quantity is in direct proportion to the temperature and the
extent of the surface. The numerous radical fibrillae replace, like
so many pumps, the evaporated water; and so long as the soil is
moist, or penetrated with water, the indispensable elements of the
soil, dissolved in the water, are supplied to the plant. The water
absorbed by the plant evaporating in an aeriform state leaves the
saline and other mineral constituents within it. The relative
proportion of these elements taken up by a plant, is greater, the
more extensive the surface and more abundant the supply of water;
where these are limited, the plant soon reaches its full growth,
while if their supply is continued, a greater amount of elements
necessary to enable it to appropriate atmospheric nourishment being
obtained, its development proceeds much further. The quantity, or
mass of seed produced, will correspond to the quantity of mineral
constituents present in the plant. That plant, therefore, containing
the most alkaline phosphates and earthy salts will produce more or a
greater weight of seeds than another which, in an equal time has
absorbed less of them. We consequently observe, in a hot summer,
when a further supply of mineral ingredients from the soil ceases
through want of water, that the height and strength of plants, as
well as the development of their seeds, are in direct proportion to
its absorption of the elementary parts of the soil in the preceding
epochs of its growth.
The fertility of the year depends in general upon the temperature,
and the moisture or dryness of the spring, if all the conditions
necessary to the assimilation of the atmospheric nourishment be
secured to our cultivated plants. The action of humus, then, as we
have explained it above, is chiefly of value in gaining time. In
agriculture, this must ever be taken into account and in this
respect humus is of importance in favouring the growth of
vegetables, cabbages, &c.
But the cerealia, and plants grown for their roots, meet on our
fields, in the remains of the preceding crop, with a quantity of
decaying vegetable substances corresponding to their contents of
mineral nutriment from the soil, and consequently with a quantity of
carbonic acid adequate to their accelerated development in the
spring. A further supply of carbonic acid, therefore, would be quite
useless, without a corresponding increase of mineral ingredients.
From a morgen of good meadow land, 2,500 pounds weight of hay,
according to the best agriculturists, are obtained on an average.
This amount is furnished without any supply of organic substances,
without manure containing carbon or nitrogen. By irrigation, and the
application of ashes or gypsum, double that amount may be grown. But
assuming 2,500 pounds weight of hay to be the maximum, we may
calculate the amount of carbon and nitrogen derived from the
atmosphere by the plants of meadows.
According to elementary analysis, hay, dried at a temperature of 100
deg Reaumur, contains 45.8 per cent. of carbon, and 1 1/2 per cent.
of nitrogen. 14 per cent. of water retained by the hay, dried at
common temperatures, is driven off at 100 deg. 2,500 pounds weight
of hay, therefore, corresponds to 2,150 pounds, dried at 100 deg.
This shows us, that 984 pounds of carbon, and 32.2 pounds weight of
nitrogen, have been obtained in the produce of one morgen of meadow
land. Supposing that this nitrogen has been absorbed by the plants
in the form of ammonia, the atmosphere contains 39.1 pounds weight
of ammonia to every 3640 pounds weight of carbonic acid (=984
carbon, or 27 per cent.), or in other words, to every 1,000 pounds
weight of carbonic acid, 10.7 pounds of ammonia, that is to about
1/100,000, the weight of the air, or 1/60,000 of its volume.
For every 100 parts of carbonic acid absorbed by the surface of the
leaves, the plant receives from the atmosphere somewhat more than
one part of ammonia.
With every 1,000 pounds of carbon, we obtain--
From a meadow . 32 7/10 pounds of nitrogen.
From cultivated fields,
In Wheat . 21 1/2 " "
Oats . 22.3 " "
Rye . 15.2 " "
Potatoes . 34.1 " "
Beetroot . 39.1 " "
Clover . 44 " "
Peas . 62 " "
Boussingault obtained from his farm at Bechelbronn, in Alsace, in
five years, in the shape of potatoes, wheat, clover, turnips, and
oats, 8,383 of carbon, and 250.7 nitrogen. In the following five
years, as beetroot, wheat, clover, turnips, oats, and rye, 8,192 of
carbon, and 284.2 of nitrogen. In a further course of six years,
potatoes, wheat, clover, turnips, peas, and rye, 10,949 of carbon,
356.6 of nitrogen. In 16 years, 27,424 carbon, 858 1/2 nitrogen,
which gives for every 1,000 carbon, 31.3 nitrogen.
From these interesting and unquestionable facts, we may deduce some
conclusions of the highest importance in their application to
agriculture.
1. We observe that the relative proportions of carbon and nitrogen,
stand in a fixed relation to the surface of the leaves. Those
plants, in which all the nitrogen may be said to be concentrated in
the seeds, as the cerealia, contain on the whole less nitrogen than
the leguminous plants, peas, and clover.
2. The produce of nitrogen on a meadow which receives no
nitrogenised manure, is greater than that of a field of wheat which
has been manured.
3. The produce of nitrogen in clover and peas, which agriculturists
will acknowledge require no nitrogenised manure, is far greater than
that of a potato or turnip field, which is abundantly supplied with
such manures.
Lastly. And this is the most curious deduction to be derived from
the above facts,--if we plant potatoes, wheat, turnips, peas, and
clover, (plants containing potash, lime, and silex,) upon the same
land, three times manured, we gain in 16 years, for a given quantity
of carbon, the same proportion of nitrogen which we receive from a
meadow which has received no nitrogenised manure.
On a morgen of meadow-land, we obtain in plants, containing silex,
lime, and potash, 984 carbon, 32.2 nitrogen. On a morgen of
cultivated land, in an average of 16 years, in plants containing the
same mineral elements, silex, lime, and potash, 857 carbon, 26.8
nitrogen.
If we add the carbon and nitrogen of the leaves of the beetroot, and
the stalk and leaves of the potatoes, which have not been taken into
account, it still remains evident that the cultivated fields,
notwithstanding the supply of carbonaceous and nitrogenised manures,
produced no more carbon and nitrogen than an equal surface of
meadow-land supplied only with mineral elements.
What then is the rationale of the effect of manure,--of the solid
and fluid excrements of animals?
This question can now be satisfactorily answered: that effect is the
restoration of the elementary constituents of the soil which have
been gradually drawn from it in the shape of grain and cattle. If
the land I am speaking of had not been manured during those 16
years, not more than one-half, or perhaps than one-third part of the
carbon and nitrogen would have been produced. We owe it to the
animal excrements, that it equalled in production the meadow-land,
and this, because they restored the mineral ingredients of the soil
removed by the crops. All that the supply of manure accomplished,
was to prevent the land from becoming poorer in these, than the
meadow which produces 2,500 pounds of hay. We withdraw from the
meadow in this hay as large an amount of mineral substances as we do
in one harvest of grain, and we know that the fertility of the
meadow is just as dependent upon the restoration of these
ingredients to its soil, as the cultivated land is upon manures. Two
meadows of equal surface, containing unequal quantities of inorganic
elements of nourishment,--other conditions being equal,--are very
unequally fertile; that which possesses most, furnishes most hay. If
we do not restore to a meadow the withdrawn elements, its fertility
decreases. But its fertility remains unimpaired, with a due supply
of animal excrements, fluid and solid, and it not only remains the
same, but may be increased by a supply of mineral substances alone,
such as remain after the combustion of ligneous plants and other
vegetables; namely, ashes. Ashes represent the whole nourishment
which vegetables receive from the soil. By furnishing them in
sufficient quantities to our meadows, we give to the plants growing
on them the power of condensing and absorbing carbon and nitrogen by
their surface. May not the effect of the solid and fluid excrements,
which are the ashes of plants and grains, which have undergone
combustion in the bodies of animals and of man, be dependent upon
the same cause? Should not the fertility, resulting from their
application, be altogether independent of the ammonia they contain?
Would not their effect be precisely the same in promoting the
fertility of cultivated plants, if we had evaporated the urine, and
dried and burned the solid excrements? Surely the cerealia and
leguminous plants which we cultivate must derive their carbon and
nitrogen from the same source whence the graminea and leguminous
plants of the meadows obtain them! No doubt can be entertained of
their capability to do so.
In Virginia, upon the lowest calculation, 22 pounds weight of
nitrogen were taken on the average, yearly, from every morgen of the
wheat-fields. This would amount, in 100 years, to 2,200 pounds
weight. If this were derived from the soil, every morgen of it must
have contained the equivalent of 110,000 pounds weight of animal
excrements (assuming the latter, when dried, at the temperature of
boiling water, to contain 2 per cent.).
In Hungary, as I remarked in a former Letter, tobacco and wheat have
been grown upon the same field for centuries, without any supply of
nitrogenised manure. Is it possible that the nitrogen essential to,
and entering into, the composition of these crops, could have been
drawn from the soil?
Every year renews the foliage and fruits of our forests of beech,
oak, and chesnuts; the leaves, the acorns, the chesnuts, are rich in
nitrogen; so are cocoa-nuts, bread-fruit, and other tropical
productions. This nitrogen is not supplied by man, can it indeed be
derived from any other source than the atmosphere?
In whatever form the nitrogen supplied to plants may be contained in
the atmosphere, in whatever state it may be when absorbed, from the
atmosphere it must have been derived. Did not the fields of Virginia
receive their nitrogen from the same source as wild plants?
Is the supply of nitrogen in the excrements of animals quite a
matter of indifference, or do we receive back from our fields a
quantity of the elements of blood corresponding to this supply?
The researches of Boussingault have solved this problem in the most
satisfactory manner. If, in his grand experiments, the manure which
he gave to his fields was in the same state, i.e. dried at 110 deg
in a vacuum, as it was when analysed, these fields received, in 16
years, 1,300 pounds of nitrogen. But we know that by drying all the
nitrogen escapes which is contained in solid animal excrements, as
volatile carbonate of ammonia. In this calculation the nitrogen of
the urine, which by decomposition is converted into carbonate of
ammonia, has not been included. If we suppose it amounted to half as
much as that in the dried excrements, this would make the quantity
of nitrogen supplied to the fields 1,950 pounds.
In 16 years, however, as we have seen, only 1,517 pounds of
nitrogen, was contained in their produce of grain, straw, roots, et
cetera--that is, far less than was supplied in the manure; and in
the same period the same extent of surface of good meadow-land (one
hectare = a Hessian morgen), which received no nitrogen in manure,
2,062 pounds of nitrogen.
It is well known that in Egypt, from the deficiency of wood, the
excrement of animals is dried, and forms the principal fuel, and
that the nitrogen from the soot of this excrement was, for many
centuries, imported into Europe in the form of sal ammoniac, until a
method of manufacturing this substance was discovered at the end of
the last century by Gravenhorst of Brunswick. The fields in the
delta of the Nile are supplied with no other animal manures than the
ashes of the burnt excrements, and yet they have been proverbially
fertile from a period earlier than the first dawn of history, and
that fertility continues to the present day as admirable as it was
in the earliest times. These fields receive, every year, from the
inundation of the Nile, a new soil, in its mud deposited over their
surface, rich in those mineral elements which have been withdrawn by
the crops of the previous harvest. The mud of the Nile contains as
little nitrogen as the mud derived from the Alps of Switzerland,
which fertilises our fields after the inundations of the Rhine. If
this fertilising mud owed this property to nitrogenised matters;
what enormous beds of animal and vegetable exuviae and remains ought
to exist in the mountains of Africa, in heights extending beyond the
limits of perpetual snow, where no bird, no animal finds food, from
the absence of all vegetation!
Abundant evidence in support of the important truth we are
discussing, may be derived from other well known facts. Thus, the
trade of Holland in cheese may be adduced in proof and illustration
thereof. We know that cheese is derived from the plants which serve
as food for cows. The meadow-lands of Holland derive the nitrogen of
cheese from the same source as with us; i.e. the atmosphere. The
milch cows of Holland remain day and night on the grazing-grounds,
and therefore, in their fluid and solid excrements return directly
to the soil all the salts and earthy elements of their food: a very
insignificant quantity only is exported in the cheese. The fertility
of these meadows can, therefore, be as little impaired as our own
fields, to which we restore all the elements of the soil, as manure,
which have been withdrawn in the crops. The only difference is, in
Holland they remain on the field, whilst we collect them at home and
carry them, from time to time, to the fields.
The nitrogen of the fluid and solid excrements of cows, is derived
from the meadow-plants, which receive it from the atmosphere; the
nitrogen of the cheese also must be drawn from the same source. The
meadows of Holland have, in the lapse of centuries, produced
millions of hundredweights of cheese. Thousands of hundredweights
are annually exported, and yet the productiveness of the meadows is
in no way diminished, although they never receive more nitrogen than
they originally contained.
Nothing then can be more certain than the fact, that an exportation
of nitrogenised products does not exhaust the fertility of a
country; inasmuch as it is not the soil, but the atmosphere, which
furnishes its vegetation with nitrogen. It follows, consequently,
that we cannot increase the fertility of our fields by a supply of
nitrogenised manure, or by salts of ammonia, but rather that their
produce increases or diminishes, in a direct ratio, with the supply
of mineral elements capable of assimilation. The formation of the
constituent elements of blood, that is, of the nitrogenised
principles in our cultivated plants, depends upon the presence of
inorganic matters in the soil, without which no nitrogen can be
assimilated even when there is a most abundant supply. The ammonia
contained in animal excrements exercises a favourable effect,
inasmuch as it is accompanied by the other substances necessary to
accomplish its transition into the elements of the blood. If we
supply ammonia associated with all the conditions necessary to its
assimilation, it ministers to the nourishment of the plants; but if
this artificial supply is not given they can derive all the needed
nitrogen from the atmosphere--a source, every loss from which is
restored by the decomposition of the bodies of dead animals and the
decay of plants. Ammonia certainly favours, and accelerates, the
growth of plants in all soils, wherein all the conditions of its
assimilation are united; but it is altogether without effect, as
respects the production of the elements of blood where any of these
conditions are wanting. We can suppose that asparagin, the active
constituent of asparagus, the mucilaginous root of the marsh-mallow,
the nitrogenised and sulphurous ingredients of mustard-seed, and of
all cruciferous plants, may originate without the aid of the mineral
elements of the soil. But if the principles of those vegetables,
which serve as food, could be generated without the co-operation of
the mineral elements of blood, without potash, soda, phosphate of
soda, phosphate of lime, they would be useless to us and to
herbivorous animals as food; they would not fulfil the purpose for
which the wisdom of the Creator has destined them. In the absence of
alkalies and the phosphates, no blood, no milk, no muscular fibre
can be formed. Without phosphate of lime our horses, sheep and
cattle, would be without bones.
In the urine and in the solid excrements of animals we carry
ammonia, and, consequently, nitrogen, to our cultivated plants, and
this nitrogen is accompanied by all the mineral elements of food
exactly in the same proportions, in which both are contained in the
plants which served as food to the animals, or what is the same, in
those proportions in which both can serve as nourishment to a new
generation of plants, to which both are essential.
The effect of an artificial supply of ammonia, as a source of
nitrogen, is, therefore, precisely analogous to that of humus as a
source of carbonic acid--it is limited to a gain of time; that is,
it accelerates the development of plants. This is of great
importance, and should always be taken into account in gardening,
especially in the treatment of the kitchen-garden; and as much as
possible, in agriculture on a large scale, where the time occupied
in the growth of the plants cultivated is of importance.
When we have exactly ascertained the quantity of ashes left after
the combustion of cultivated plants which have grown upon all
varieties of soil, and have obtained correct analyses of these
ashes, we shall learn with certainty which of the constituent
elements of the plants are constant and which are changeable, and we
shall arrive at an exact knowledge of the sum of all the ingredients
we withdraw from the soil in the different crops.
With this knowledge the farmer will be able to keep an exact record,
of the produce of his fields in harvest, like the account-book of a
well regulated manufactory; and then by a simple calculation he can
determine precisely the substances he must supply to each field, and
the quantity of these, in order to restore their fertility. He will
be able to express, in pounds weight, how much of this or that
element he must give in order to augment its fertility for any given
kind of plants.
These researches and experiments are the great desideratum of the
present time. TO THE UNITED EFFORTS OF THE CHEMISTS OF ALL COUNTRIES
WE MAY CONFIDENTLY LOOK FOR A SOLUTION OF THESE GREAT QUESTIONS, and
by the aid of ENLIGHTENED AGRICULTURISTS we shall arrive at a
RATIONAL system of GARDENING, HORTICULTURE, and AGRICULTURE,
applicable to every country and all kinds of soil, and which will be
based upon the immutable foundation of OBSERVED FACTS and
PHILOSOPHICAL INDUCTION.
LETTER XVI
My dear Sir,
My recent researches into the constituent ingredients of our
cultivated fields have led me to the conclusion that, of all the
elements furnished to plants by the soil and ministering to their
nourishment, the phosphate of lime--or, rather, the phosphates
generally--must be regarded as the most important.
In order to furnish you with a clear idea of the importance of the
phosphates, it may be sufficient to remind you of the fact, that the
blood of man and animals, besides common salt, always contains
alkaline and earthy phosphates. If we burn blood and examine the
ashes which remain, we find certain parts of them soluble in water,
and others insoluble. The soluble parts are, common salt and
alkaline phosphates; the insoluble consist of phosphate of lime,
phosphate of magnesia, and oxide of iron.
These mineral ingredients of the blood--without the presence of
which in the food the formation of blood is impossible--both man and
animals derive either immediately, or mediately through other
animals, from vegetable substances used as food; they had been
constituents of vegetables, they had been parts of the soil upon
which the vegetable substances were developed.
If we compare the amount of the phosphates in different vegetable
substances with each other, we discover a great variety, whilst
Back to Full Books
|