animals, so the nitrogen
in the soil is indispensable to the growth of cultivated plants. It is
obtained by the soil in the form of ammonia (or nitric acid), from the
atmosphere, or by the application of animal matter. In some cases,
manures called _nitrates_[S] are used; and, in this manner, nitrogen is
given to the soil.

We have now learned that the organic matter in the soil performs the
following offices:--

Organic matter thoroughly decomposed is _carbon_, and has the various
effects ascribed to this substance on p. 79.

Organic matter in process of decay produces carbonic acid, and sometimes
ammonia in the soil; also its decay causes heat.

Organic matter containing _nitrogen_, such as animal substances, etc.,
furnish ammonia, and other nitrogenous substances to the roots of
plants.

FOOTNOTES:

[Q] Produce.

[R] By absorbing and retaining, we mean taking up and holding.

[S] Nitrates are compounds of nitric acid (which consists of nitrogen
and oxygen), and alkaline substances. Thus nitrate of potash
(saltpetre), is composed of nitric acid and potash: nitrate of soda
(cubical nitre), of nitric acid and soda.

CHAPTER III.

USES OF INORGANIC MATTER.

[What effect has clay besides the one already named?

How does it compare with charcoal for this purpose?]

The offices performed by the inorganic constituents of the soil are many
and important.

These, as well as the different conditions in which the bodies exist,
are necessary to be thoroughly studied.

Those parts which constitute the larger proportion of the soil, namely
the clay, sand, and limy portions, are useful for purposes which have
been named in the first part of this section, while the _clay_ has an
additional effect in the absorption of ammonia.

For this purpose, it is as effectual as charcoal, the gases escaping
from manures, as well as those existing in the atmosphere, and in
rain-water, being arrested by clay as well as charcoal.[T]

[What particular condition of inorganic matter is requisite
for fertility?

What is the fixed rule with regard to this?

What is the condition of the alkalies in most of their combinations? Of
the acids?

What is said of phosphate of lime?]

The more minute ingredients of the soil--those which enter into the
construction of plants--exist in conditions which are more or less
favorable or injurious to vegetable growth. The principal condition
necessary to fertility is _capacity to be