41

formation, or be produced in them by their continued culti-
vation. Substances of this class, capable of restoring, in the
one or the other way, the lost fertility of a soil, are generally
known as Manures or Fertilizers, and have been used from
the earliest times, long before science enabled us to account
for their beneficial action. It is my object to devote the fol-
lowing pages of this treatise to a description of the various
kinds of manures employed, and to found upon the resulting
facts a series of conclusions relative to their mode of action,
and of recommendations in behalf of the most proper mode
of action, and of recommendations in behalf of the most pro-
per mode of their management, as best accord with both prac-
tical experience and the present state of science.

We still remember, from our knowledge of the functions
of the soil, that its fertility depends on two factors: mechani-
cal texture and directly nourishing capacity, the product
being the crop. This fact stands in close relation to the wants
of plants. The various single substances which serve as nu-
triments to plants, and which we find as constituents of their
body, are naturally divided into two classes, viz: such as are
capable of assuming the gaseous state and then form a con-
stituent of the atmosphere—water, carbonic acid, and am-
monia—and such as, by their involatility, are bound to the
soil through which they are diffused in minute particles, and
are not liable to loss unless assimilated by the plants and
subsequently removed with the crop—I mean the phosphates,
sulphates, chlorides and silicates of the alkalies and alkaline
earths, or the mineral constituents of plants.

These latter substances, though they form a comparatively
small part of the body of the plant, are nevertheless indis-
pensable for the development of vegetable life; and it is the
absolute quantity of these matters present which we regard as
the totality of the directly nourishing capacity of the soil.
The means by which they may be supplied to soils deficient
in them are given to us in various forms, as bones, mineral
phosphorites, Mexican and Columbian guanos, super-phos-
phates, ashes, common salt, plaster of Paris, &c.

The former class of substances, on the other hand, furnish
the material for the main body of the plant, inasmuch as the
whole of its combustible or organic part is formed from the
constituents of these nutriments. Their supply depends to a
large extent on the mechanical texture of the soil; for it is
the particular proportion in which the fundamental constitu-
ents of soil (sand, clay, lime and humus) are present, which
causes a more or less perfect communication of the soil with
the atmosphere, which furthermore promotes the formation