about the roots of plants to allow the air
to penetrate and give an opportunity for the chemical changes constantly
undergoing in the soil. Then, too, the work of the earthworms in
loosening the soil, and thus adding to its porosity, should not be
overlooked. In this soil workshop, too, live and labor certain minute
organisms that make it their business to enrich the soil by helping the
rootlets to assimilate the nitrogen of the air.

Since the soil is composed mostly of ingredients that come from the
decomposition of rocks, it follows that is must be of very complex
composition. Fortunately, however, there are only a few of the
ingredients of the soil that are of interest to the agriculturist, as
only a few of the elements, as they are called, go to make up the plant
structure, or at least only a few are essential ingredients of the
plant. Nitrogen, though very abundant in the air, is not abundant in the
soil. In fact, the soil has to depend largely on the nitrogen compounds
that are washed out of the atmosphere in small quantities by the rain.
Another source of nitrogen is the action of certain bacteria, that make
little sacs on the rootlets and, living on the juices of the plants, fix
the nitrogen of the air, and thus fertilize the soil; especially on
plants of the leguminous family, as peas, beans, and clover.

Silicon, which with oxygen makes ordinary sand, is essential to the
growth of plants and is everywhere found in abundance. Sulphur, united
with oxygen and the metals to form sulphates, is generally abundant
enough. The same may be said of chlorine, which, united with sodium or
potassium, is always present in our prairie soils. Phosphorus, as it
occurs in the phosphates, is one of the most essential ingredients of a
fertile soil. Calcium and magnesium are found in combination as
carbonates and sulphates, and, though essential, are usually abundant,
especially where limestone rocks underlie the soil and outcrop in so
many places. Potassium is found united with chlorine or sulphuric acid.
It is one of the elements that is most liable to be exhausted from the
soil by a succession of crops. Sodium exists almost everywhere. It is
one of the elements of common salt, and, though much like potassium,
cannot take the place of the latter in plant nurture. Iron is abundant
and at the same time necessary in small quantities. The elements above
mentioned, together with oxygen, are to be found in the ashes of plants.
Besid