How may it affect excrementitious matter of plants?

What effect has it on the mechanical condition of the soil?]

_Oxygen_, though not taken up by plants in its pure form, may justly be
classed among manures, if we consider its effects both chemical and
mechanical in the soil.

1. By oxidizing or _rusting_ some of the constituents of the soil, it
prepares them for the uses of plants.

2. It unites with the _prot_oxide of iron, and changes it to the
_per_oxide.

3. If there are _acids_ in the soil, which make it sour and unfertile,
it may be opened to the circulation of the air, and the oxygen will
prepare some of the mineral matters contained in the soil to unite with
the acids and neutralize them.

4. Oxygen combines with the carbon of organic matters in the soil, and
causes them to decay. The combination produces carbonic acid.

5. It combines with the nitrogen of decaying substances and forms
_nitric acid_, which is serviceable as food for plants.

6. It undoubtedly affects in some way the matter which is thrown out
from the roots of plants. This, if allowed to accumulate, and remain
unchanged, is often very injurious to plants; but, probably, the oxygen
and carbonic acid of the air in the soil change it to a form to be
inoffensive, or even make it again useful to the plant.

7. It may also improve the _mechanical_ condition of the soil, as it
causes its particles to crumble, thus making it finer; and it roughens
the surfaces of particles, making them less easy to move among each
other.

These properties of oxygen claim for it a high place among the
atmospheric fertilizers.

WATER.

[Why may water be considered an atmospheric manure?

What classes of action have manures?

What are chemical manures? Mechanical?]

_Water_ may be considered an atmospheric manure, as its chief supply to
vegetation is received from the air in the form of rain or dew. Its many
effects are already too well known to need farther comment.

The means of supplying water to the soil by the deposit of _dew_ will be
fully explained in Section IV.

CHAPTER XI.

RECAPITULATION.

Manures have two distinct classes of action in the soil, namely,
_chemical_ and _mechanical_.

_Chemical_ manures are those which enter into the construction of
plants, or produce such chemical effects on matters in the soil as shall
prepare them for use.

_Mechanical_ manures are those which improve the mechanical condition
of the so