nts of sand, silt, clay and humus
contained in the soil.

The power of admitting a free circulation of air through its pores is
also an important property of a fertile soil, for air is necessary to
the life and growth of the roots. This property is dependent also on
texture.

Two other important properties of a fertile soil are power to absorb
and power to hold heat. These depend upon the power of the soil to
take in warm rain and warm air, and also upon density and color. The
denser or more compact soil and the darker soil having greater power
to absorb heat.

The compactness of the soil which gives it greater powers to absorb
heat weakens its powers to hold it, because the compactness allows
more rapid conduction of heat to the surface, where it is lost by
radiation.

The more moisture a soil holds, the weaker is its heat-holding power,
because the heat is used in warming and evaporating water from the
surface of the soil.

These important properties or conditions of moisture, heat and air,
are, as we have seen, dependent on soil texture and color, which in
turn are dependent upon the relative amounts of sand, clay and humus
in the soil. We are able to control soil texture and therefore these
physical properties to a certain degree by means of tillage and the
addition of organic matter or humus (see Chapter IV).

BIOLOGICAL PROPERTIES OF A FERTILE SOIL

Biology is the story or science of life; and the biological properties
of the soil have to do with living organisms in the soil.

The soil of every fertile field is full of very small or microscopic
plants called bacteria or germs. They are said to be microscopic
because they are so small that they cannot be seen without the aid of
a powerful magnifying glass or microscope. They are so small that it
would take about 10,000 average-sized soil bacteria or soil germs
placed side by side to measure one inch.

A knowledge of three classes of these soil germs is of great
importance to the farmer. These three classes of germs are:

Nitrogen-fixing germs.

Nitrifying germs.

Denitrifying germs.

NITROGEN-FIXING GERMS

We learned in Chapter VIII that nitrogen is one of the necessary
elements of plant food, and that although the air is four-fifths
nitrogen, most plants must take their nitrogen from the soil. There
is, however, a class of plants called legumes which can use the
nitrogen of the air. Clover, alfalfa, lucern, cowpea, soy bean, snap
bean, vet