olid 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 car