nesia,
sulphuric acid, phosphoric acid, chlorine, silica, oxide of iron, and
oxide of manganese.

Plants obtain their organic food as follows:--Oxygen and hydrogen from
water, nitrogen from some compound containing nitrogen (chiefly from
ammonia), and carbon from the atmosphere where it exists as carbonic
acid--a gas.

They obtain their inorganic food from the soil.

The water which supplies oxygen and hydrogen to plants is readily
obtained without the assistance of manures.

Ammonia is obtained from the atmosphere, by being absorbed by rain and
carried into the soil, and it enters plants through their roots. It may
be artificially supplied in the form of animal manure with profit.

Carbonic acid is absorbed from the atmosphere by leaves, and decomposed
in the green parts of plants under the influence of daylight; the carbon
is retained, and the oxygen is returned to the atmosphere.

[When plants are destroyed by combustion or decay, what
becomes of their constituents?

How does the inorganic matter enter the plant?

Are the alkalies soluble in their pure forms?

Which one of them is injurious when too largely present?

How may sulphuric acid be supplied?

Is phosphoric acid important?

How must silica be treated?

From what source may we obtain chlorine?]

When plants are destroyed by decay, or burning, their organic
constituents pass away as water, ammonia, carbonic acid, etc., ready
again to be taken up by other plants.

The inorganic matters in the soil can enter the plant only when
dissolved in water. _Potash_, _soda_, _lime_, and _magnesia_, are
soluble in their pure forms. Magnesia is injurious when present in too
large quantities.

_Sulphuric_ acid is often necessary as a manure, and is usually most
available in the form of sulphate of lime or plaster. It is also
valuable in its pure form to prevent the escape of ammonia from
composts.

_Phosphoric_ acid is highly important, from its frequent deficiency in
worn-out soils. It is available only under certain conditions which will
be described in the section on manures.

_Silica_ is the base of common sand, and must be united to an alkali
before it can be used by the plant, because it is insoluble except when
so united.

_Chlorine_ is a constituent of common salt (chloride of sodium), and
from this source may be obtained in sufficient quantities for manurial
purposes.

[What is the difference between _per_oxide and _prot_oxide of
iron?