xi

lations which exist between plants and their soil, and in what man-
ner these relations influence the productiveness of a soil for plants
which may be cultivated upon it. These considerations may lead
to the sure foundation of a useful, and thoroughly practical classifi-
cation of soils, a knowledge, from which we are yet distant, which,
however, it may be confidently hoped, will one day be reached by
the united co-operation of practical farmers and chemists.           

The soil consists substantially of two parts, of which the one
constitutes almost its entire bulk, and through which we find the
other, in small quantities, uniformly divided and diffused. The
soil can be defined as the medium of the processes of vegetable
life, and both of its parts, unequal as they are in their quantity, take
nevertheless an equally important share in this function.             

We have before observed that the bulk of plants consists of sub-
stances of organic form, composed of Carbon, Hydrogen, Nitro-
gen, and Oxygen, whilst only a small part of their constituents,
belongs to the mineral kingdom. This fact, compared with the
composition of soils, will enable us to throw a light on the pro-
found and wise manner in which nature has transferred the medi-
ation of the processes of vegetable life to the soil.                       

We find that the constituents of the bulk of the soil do not
directly participate in the nutrition of plants; that none of them is
carried away by a crop, and their quantity therefore not diminished ;
but that the small quantity of substances which form the other part
of fertile soils, and which are diffused through the bulk of the for-
mer, serve as nourishment for plants, and consequently diminish in
soils by being cropped and carried away from them.                     

The functions which this latter part exercise on the mediation
of the processes of life, consist, therefore, in supplying the plants
with their necessary mineral constituents, and are perfectly and
plainly understood.                                                                         

The functions of the main part of soils therefore remain only, to
be considered, and we may judge a priori, that this par!, if we ab-
stract from it, its utility as a mass easily penetrated by the roots of
plants, and thus capable of rendering them the necessary mechani-
cal support,—is that which we have especially to assign as the
medium of communication between the atmosphere and plants, and
which therefore supplies them with Water, Carbonic Acid and
Ammonia; for though the plants are in direct contact with the at-
mosphere, and, to a large extent, directly nourished from it by
means of their leaves; they nevertheless depend, to the same ex-
tent, on the constituents of the atmosphere which, through their
roots, enter the system. In the first period of their growth, e. g.
they are solely referred to this, latter manner of nutrition.- These
nourishments are therefore supplied to the roots, either by the air,
(in its gaseous form as contained in the soil, or dissolved in rain-
water, which falls on the earth and penetrates the soil,) or supplied
by Humus in a manner as already explained.