72 



SOILS AND THEIR USES 



members of the clover family to accomplish the same purpose. 

 Each of these makes its host plant more thrifty and productive. 

 See the nodules on the roots of soy bean in figure 59. These are 

 full of bacteria. The farmer is thus enabled to introduce into 

 any field the bacteria which will be most helpful to the member 

 of the legume family which he wishes to grow. If alfalfa, for 

 example, has never been grown upon a certain field, the most suit- 



able bacteria for this crop may 

 be introduced by spreading 

 upon the field a few hundred 

 pounds per acre of soil from'an 

 old alfalfa field. This should be 

 done under favorable condi- 

 tions, as when the sun is not 

 shining. The top dressing of soil 

 is harrowed in to mix it with the 

 other soil. The rapid multiplica- 

 tion will soon thoroughly inocu- 

 late the whole mass, resulting in 

 great benefit to the alfalfa crop 

 as soon as it begins to grow. 



Pure cultures are sometimes 

 used for inoculation of soils. 

 Bacteria for this purpose may be 

 grown under favorable condi- 

 tions in laboratories and applied 

 to the field in different ways. 

 The seed to be sown may be 

 treated with water containing 



FIG. 59. A soy bean nitrogen factory, bacteria Or SOme Soil may be SO 

 The free nitrogen of the air is changed by the +__+ J <* C rr^irl nrrm thofinlrl 



bacteria in the root nodules into forms that treated and spread upont ne neici. 

 f r ****** ^^ The cost of such pure cultures is" 



so great that the natural soil 

 method first mentioned is usually preferred. 



Air in Soils. The presence of living organisms in the soil re- 

 quires ventilation not only for their own existence, but in order 

 that they may accomplish the work which they do. Bacteria 

 which change nitrogen into nitrates could not do so without the 

 presence of oxygen. It has long been observed that air is neces- 

 sary for the germination of seeds and for the growth of roots of 

 farm and garden crops. 



