FIXATION OF NITROGEN 



125 



Average Fertilizing Constituents and Digestible Nutrients 

 to the Ton 



Inoculation of the soil. It is, of course, evident that if 

 the soil docs not contain the bacteria that form the nodules 

 and bring the free nitrogen of the air to the service of the 

 pl;mt, the legume must draw all of its supply from the soil. 

 If the crop is removed, the soil will be more rapidly de- 

 pleted of its nitrogen content than with a grain crop, and 

 if the crop is turned under, no increase in the nitrogen 

 content of the soil will have taken place. If the legumes 

 are to be used to enrich the soil, the fields must contain the 

 bacteria. The recognition of this fact led to the inocula- 

 tion of the soil. At first soil from a field on which the 

 legume in question had been grown was employed. Later 

 the use of pure cultures of the bacteria was attempted. 

 For many years it met with little success, due, apparently, 

 to the fact that the bacteria on artificial cultivation lost 

 their ability to form the nodules. More recently improved 

 methods of growing the bacteria in the laboratory have been 

 devised, and at present the use of artificial cultures for the 

 inoculation of legumes is successful. 



The success attained in the use of the cultures for the 

 inoculation of seed depends on many factors, chief among 



