20

silica, iron, alumina, lime, magnesia, potash, soda; also phos-
phoric and sulphuric acids and chlorine: they are necessary in a
soil because they are necessary to animals who have to derive their
food from the soil.

The most abundant supply of all the four organic elements
first mentioned, which exist in the air, could not produce the
smallest plant or the veriest insect that crawls, unless it was in
union with the other bodies last named, which are derived from
the soil. The soil thus furnishes a material class of bodies
which cannot exist in the air, necessary to the animal and ne-
cessary to the plant, and with which we must supply the soil, if
they be absent, before vegetable life can be produced. In my
first report I gave a table showing that all of the mineral con-
stituents named above were necessary; that the absence or de-
ficiency of one could not be compensated for by an excess of the
others. Thus—no amount of lime would compensate for the ab-
sence of magnesia; no abundance of chlorine be a substitute for
sulphuric acid; no quantity of sulphuric acid could counterbalance
a deficiency of phosphoric acid or phosphates. Let those who
deny this doctrine and say that all these constituents are not ne-
cessary, show a single grain of wheat, a single blade of grass, or
any soil capable of producing either, which does not contain them.
But not only this, a part of the substances existing in the air are
first taken in by the soil and then furnished to the plant—the soil
being a medium, a kind of stomach for the proper digestion of the
food obtained from the air, so that it may be used by the plant.
This digestive power, or ability to produce nutritive matter, such
as ammonia, from crude substances, (hydrogen and nitrogen,) de-
pends partly on texture, partly on chemical constitution. Ammo-
nia, as I have before said, is a compound of hydrogen and nitrogen,
which is supplied abundantly to crops by the air, provided the
soil has the capacity of absorbing it, or of forming it from its ele-
ments; but many soils have not this power, and it then must be
supplied from other sources, such as Peruvian guano, stable ma-
nure, &c. So, then, the capacity of production in soils depends
upon the presence in them of the last ten substances named, and
upon their ability to form ammonia from its elements, to absorb
and retain it when already formed, and to absorb and retain wa-
tery vapor—products of the four first named. This power of ab-
sorption is due partly to their chemical, partly to their mechanical
nature. We must look to their chemical constitution to determine
the presence, absence, or deficiency of those first named, to the
chemical constituents and to their mechanical texture, to the fine-
ness of the several particles which make up the mass of soils, to
determine their absorbent quality.

The organic elements form, by different modes of combination,