ashed out of the
soil. The work of the nitrifying germs is called nitrification.

To do their work well the nitrogen-fixing germs and the nitrifying
germs require certain conditions.

The soil must be moist.

The soil must be well ventilated to supply nitrogen for the
nitrogen-fixing germs and oxygen for the nitrifying germs.

The soil must be warm. Summer temperature is the most favorable. Their
work begins and continues slowly at a temperature of about forty-five
degrees and increases in rapidity as the temperature rises until it
reaches ninety or ninety-five.

The nitrifying germs require phosphoric acid, potash and lime in the
soil.

Direct sunlight destroys these bacteria, therefore they cannot work at
the surface of the soil unless it is shaded by a crop.

From this we see that these bacteria or germs work best in the soil
that has conditions necessary for the growth and development of plant
roots.

DENITRIFYING GERMS

These germs live on the coarse organic matter of the soil. Like the
nitrifying germs they need oxygen, and when they cannot get it more
readily elsewhere they take it from the nitric acid and nitrates. This
allows the nitrogen of the nitrates to escape as a free gas into the
air again, and the work of the nitrogen-fixing and nitrifying germs is
undone and the nitrogen is lost. This loss of nitrogen is most apt to
occur when the soil is poorly ventilated, because of its being very
compact, or when the soil spaces are filled with water. This loss of
nitrogen by denitrification can be checked by keeping the soil well
ventilated.

CHEMICAL PROPERTIES OF A FERTILE SOIL

By the term chemical properties we have reference to the chemical
composition of the soil, the chemical changes which take place in the
soil, and the conditions which influence these changes.

The sand, clay and humus of the soil are made up of a great variety of
substances. The larger part of these act simply as a mechanical
support for the plants and also serve to bring about certain physical
conditions. Only a very small portion of these substances serve as the
direct food of plants and the chemical conditions of these substances
are of great importance.

In Chapter VIII we learned that plants are composed of several
elements and that seven necessary elements are taken from the soil.
These seven are nitrogen, phosphorus, potassium, magnesium, calcium,
iron and sulphur.

Now a fertile soil must contain these seve