42 BULLETIN 355^ U. S. DEPARTMENT OF AGEICULTURE. 



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 cultm'e is made by trans- 

 ferring some bacteria from a plant nodule to a substance suitable for 

 then- growth. Under right conditions of temperature and air these 

 bacteria multiply very rapidly, and in a comparatively short time 

 the growing medium will contain milUons 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 apphed 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 setthng 

 the hquid can be poured off and used to inociQate seeds much as the 

 cultures are used. With either method of moculation 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 hy legumes. — Tne fixation of 

 nitrogen through the action of tubercle-forming organisms growing 

 on the roots of legimies 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 



