438 NATURE AND PROPERTIES OF SOILS 



of the nitrogen of a normal inoculated legume comes from 

 the soil and two-thirds from the air. He also assumes that 

 one-third of the nitrogen of the plant exists in the roots. Al- 

 though both of these assumptions are questionable, they sug- 

 gest the reason why the removal of the tops of legumes as 

 forage allows no accumulation of nitrogen in the soil. 



According to Hopkins, the nitrogen in the tops of legumes 

 is a rough measure in general of the nitrogen fixed. On such 

 an assumption, the growth of red clover should facilitate the 

 fixation of about 40 pounds of nitrogen for every ton of air- 

 dry material. On the same basis, the figure should be about 

 50 pounds for alfalfa, 43 pounds for cowpeas, and 53 pounds 

 for soybeans. These figures, even though they are obviously 

 incorrect, give some idea of the importance of B. radicicola in 

 nitrogen fixation. The growth of an average leguminous 

 crop under proper conditions probably is accompanied by a 

 fixation of 80 to 100 pounds of nitrogen.- Of the three nat- 

 ural methods by which atmospheric nitrogen may be fixed 

 by the soil that facilitated by the nodule organisms seems 

 at first thought to be considerably the most important. It 

 must be remembered, however, that with an average rotation 

 a legume occupies the land but one or two years in three to 

 six. Moreover, the gain of nitrogen in a fertile soil is but 

 slight unless the crop is turned under as a green-manure. 

 Unless so used the chief advantages of growing a legumi- 

 nous crop lie in the increase of soil organic matter, the 

 ready and favorable decay of the roots and stubble, and the 

 opportunity of growing a high protein crop without ma- 

 terially depleting the soil nitrogen. 



241. Soil inoculation for legumes. — Although the inocu- 

 lation of the soil with free-fixing organisms has not proven 

 of value, since such organisms are always present and suffi- 

 ciently active if soil conditions are favorable, the inoculation 

 with nodule bacteria is of considerable practical importance. 

 Such organism may never have been present in a soil or may 



