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?