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, vetch 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 washed out of the soil.  The work of the nitrifying germs is called nitrification.