ch and similar plants are legumes. These legumes get the
nitrogen from the air in a very curious and interesting manner. It is
done through the aid of bacteria or germs.

Carefully dig up the roots of several legumes and wash the soil from
them. On the roots will be found many small enlargements like root
galls; these are called nodules or tubercles. On clover roots these
nodules are about the size of the head of a pin while on the soy bean
and cowpea they are nearly as large as a pea (see Fig. 34). These
nodules are filled with bacteria or germs and these germs have the
power of taking nitrogen from the air which finds its way into the
soil. After using the nitrogen the germ gives it to the plant which
then uses it to build stem, leaves and roots. In this way the legumes
are able to make use of the nitrogen of the soil air, and these germs
which help them to do it by catching the nitrogen are called
nitrogen-fixing germs.

The work of these germs makes it possible for the farmer to grow
nitrogen, so to speak, on the farm.

By growing crops of legumes and turning them under to decay in the
soil, or leaving the roots and stubble to decay after the crop is
harvested, he can furnish the following crop with a supply of nitrogen
in a very cheap manner and lessen the necessity of buying fertilizer.

NITRIFYING GERMS

Almost all the nitrogen of the soil is locked up in the humus and
cannot in that condition be used by the roots of plants. The nitrogen
caught by the nitrogen-fixing germs and built into the structure of
leguminous plants which are grown and turned under to feed other
plants cannot be used until the humus, which is produced by their
partial decay, is broken down and the nitrogen built into other
substances upon which the root can feed. The breaking down of the
humus and building of the nitrogen into other substances is the work
of another set of bacteria or germs called nitrifying germs.

These nitrifying germs attack the humus, break it down, separate the
nitrogen, cause it to unite with the oxygen of the air and thus build
it into nitric acid which can be used by plant roots. This nitric acid
if not immediately used will unite with lime or potash or soda or
other similar substances and form nitrates, as nitrate of lime,
nitrate of potash or common saltpetre. These nitrates are soluble in
water and can be easily used by plant roots. If there are no plant
roots to use them they are easily lost by being w