42 BULLETIN" 355, U. S. DEPARTMENT OF AGRICULTURE. 
the same as those which live on sweet clover, and the nodule bacteria 
of the true clovers have also been found to be interchangeable for 
purposes of inoculation. The bacteria of cowpeas and soy beans are 
not interchangeable nor can they be used for inoculating any other 
of the leguminous plants. It often happens in a particular field 
that bacteria of the right kind are not present to form nodules on a 
species of legume which is being grown on the field for the first time. 
It is then necessary to supply these bacteria. This is done in two 
different ways: 
(1) A culture of bacteria is used. This culture is made by trans- 
ferring some bacteria from a plant nodule to a substance suitable for 
their growth. Under right conditions of temperature and air these 
bacteria multiply very rapidly, and in a comparatively short time 
the growing medium will contain millions of the microorganisms and 
is then called a culture. This culture growth of nodule bacteria 
needs to be handled by trained people in order that it may be kept 
pure. The United States Department of Agriculture, several of the 
State agricultural experiment stations, and many commercial firms 
have been growing cultures for agricultural use. These cultures, with 
directions for their use, are shipped direct to farmers by express or 
parcel post. The cultures are most commonly applied to seeds just 
before sowing. The methods are very simple and easy to carry out. 
(2) Soil is used for inoculation. Soil to be used for this purpose 
should be taken from a field in which are growing, or have recently 
grown, healthy plants containing a good supply of the nodule bacteria 
desired. Nodules occur largely in the surface soil, ordinarily in the 
first 5 or 6 inches. In securing soil for inoculating, the first inch or so 
should be scraped away and the soil to the next few inches of depth 
should be taken. From 200 to 400 pounds per acre of inoculated soil 
can be scattered over a field before sowing and harrowed in, or the 
soil containing bacteria can be stirred up in water and after settling 
the liquid can be poured off and used to inoculate seeds much as the 
cultures are used. With either method of inoculation care must be 
taken not to permit too intense heat from the sun to kill the bacteria. 
This can be avoided by harrowing under the seeds or soil-carrying 
inoculation soon after sowing them, or by doing the work early in 
the morning or late in the afternoon. 
Amount of nitrogen fixed in the soil by legumes. — Tne fixation of 
nitrogen through the action of tubercle-forming organisms growing 
on the roots of legumes is the only practical method available to the 
farmer for storing this essential element in the soil. It must not be 
supposed, however, that all the nitrogen used by leguminous plants 
in their growth is secured in this way from the nitrogen of the soil 
air. Soluble nitrates of the soil are absorbed by growing clover and 
alfalfa, for example, just as they are by corn and cotton. But while 
