dissolved_, it being (so far
as we have been able to ascertain) a fixed rule, as was stated in the
first section, that _no mineral substance can enter into the roots of a
plant except it be dissolved in water_.

The _alkalies_ potash, soda, lime, and magnesia, are in nearly all of
their combinations in the soil sufficiently soluble for the purposes of
growth.

The _acids_ are, as will be recollected, sulphuric and phosphoric. These
exist in the soil in combination with the alkalies, as sulphates and
phosphates, which are more or less soluble under natural circumstances.
Phosphoric acid in combination with lime as phosphate of lime is but
slightly soluble; but, when it exists in the compound known as
_super_-phosphate of lime, it is much more soluble, and consequently
enters into the composition of plants with much greater facility. This
matter will be more fully explained in the section on manures.

[How may silica be rendered soluble?

What is the condition of chlorine in the soil?

Do peroxide and protoxide of iron affect plants in the same way?

How would you treat a soil containing protoxide of iron?

On what does the usefulness of all these matters in the soil depend?]

The _neutrals_, silica, chlorine, oxide of iron, and oxide of manganese,
deserve a careful examination. Silica exists in the soil usually in the
form of _sand,_ in which it is, as is well known, perfectly insoluble;
and, before it can be used by plants, which often require it in large
quantities, it must be made soluble, which is done by combining it with
an alkali.

For instance, if the silica in the soil is insoluble, we must make an
application of an alkali, such as potash, which will unite with the
silica, and form the silicate of potash, which is in the exact condition
to be dissolved and carried into the roots of plants.

Chlorine in the soil is probably always in an available condition.

Oxide of iron exists, as has been previously stated, usually in the form
of the _per_oxide (or red oxide). Sometimes, however, it exists in the
form of the _prot_oxide (or black oxide), which is poisonous to plants,
and renders the soil unfertile. By loosening the soil in such a manner
as to admit air and water, this compound takes up more oxygen, which
renders it a peroxide, and makes it available for plants. The oxide of
manganese is probably of little consequence.

The usefulness of all of these matters in the soil depends on their
_exposure