n elements of plant food and
they must be in such form that the plant roots can use them.

Plant roots can generally get from most soils enough of the magnesium,
calcium, iron, and sulphur to produce well developed plants. But the
nitrogen, phosphorus and potassium, although they exist in sufficient
quantities in the soil, are often in such a form or condition that the
roots cannot get enough of one or more of them to produce profitable
crops. For this reason these three elements are of particular
importance to the farmer for, in order to keep his soil fertile, he
must so treat it that these elements will be made available or he must
add more of them to the soil in the proper form or condition.

_Nitrogen in the soil._--Plant roots use nitrogen in the form of
nitric acid and salts of nitrogen called nitrates. But the nitrogen of
the soil is very largely found in the humus with the roots cannot use.
A chemical change must take place in it and the nitrogen be built into
nitric acid and nitrates. This, we have learned, is done through the
aid of the nitrifying germs.

_Phosphoric acid in the soil._--Phosphorus does not exist pure in the
soil. The plant finds it as a phosphoric acid united with the other
substances forming phosphates. These are often not available to
plants, but can to a certain extent be made available through tillage
and by adding humus to the soil.

_Potash in the soil._--The plant finds potassium in potash which
exists in the soil. Potash like phosphoric acid often exists in forms
which the plant cannot use but may be made available to a certain
extent by tillage, the addition of humus, and the addition of lime to
the soil.

_Lime in the soil._--Most soils contain the element calcium or lime,
the compound in which it is found, in sufficient quantities for plant
food. But lime is also of importance to the farmer and plant grower
because it is helpful in causing chemical changes in the soil which
tend to prepare the nitrogen, phosphoric acid and potash for plant
use. It is also helpful in changing soil texture.

The chemical changes which make the plant foods available are
dependent on moisture, heat, and air with its oxygen, and are
therefore dependent largely on texture, and therefore on tillage.

When good tillage and the addition of organic matter and lime do not
render available sufficient plant food, then the supply of available
food may be increased by the application of manure and fertiliz