n compared to that removed by crops, only a very
small proportion is in a condition _available_ to the plant. This leads
us to consider the different forms in which nitrogen is present in the
soil, and their respective quantities.

_Organic Nitrogen in the Soil._

Nitrogen occurs in the soil as organic nitrogen, nitric acid, nitrous
acid, and ammonia. By far the largest proportion is present in the first
of these forms. This is a wise provision, for otherwise the soil would
be apt to become very speedily impoverished in nitrogen; for that
present as nitrates it has scarcely any power to retain, while that
present as ammonia is soon converted into nitrates by the process of
_nitrification_.

The organic nitrogen of the soil, although we are apt to think of it as
such, is by no means of a homogeneous character, or of equal value as a
source of plant-food. Some of it, it would seem from recent
investigations, is in a condition more susceptible of being converted
into an available form than the rest. Thus in the process of
nitrification, a process which we shall consider at length immediately,
there seems to be generally a certain small proportion more ready to
undergo this change than the rest; so that when this small amount is
used up nitrification proceeds more slowly. In short, although we as yet
know very little of the nature of the organic nitrogen of soils, we
cannot doubt but that there is a constant series of changes in its
composition taking place, resulting in the gradual elaboration of more
available forms, until ultimately these are converted into ammonia and
nitrates.

The great bulk of the organic nitrogen, however, in the soil must be
regarded as in an _inert_ condition, and by no means available for the
crop. What the exact chemical form of this nitrogen is it is extremely
difficult to say. Mulder was of the opinion that a considerable
proportion was in the form of humate of ammonia. This opinion, as we
shall have occasion to see immediately, was based on false grounds. It
is highly probable that it may be in some form approximating to amide
nitrogen. Its inert character is against the belief that it long remains
as albuminoid nitrogen.

_Different Character of Surface and Subsoil Nitrogen._

A point of very considerable importance to notice is, that the
nitrogenous organic matter of the surface-soil is very different from
that found in the subsoil. This difference is shown by the variation in
t