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
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
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
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
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