164 



BACTERIA AND SOIL FERTILITY 



available and still gain from the turning under of properly in- 

 oculated legumes. (4) The legume must be properly inoculated; 

 otherwise, it obtains its nitrogen as do other plants. (5) The 

 soil must be a suitable home for the legume and bacteria, that is, 

 it must have a correct reaction, moisture, temperature, and aeration 



Fig. 34. — Influence of legumes on crop yield. The two shocks of wheat on 

 the left were grown on 1/20 acre after a legume as green manure, the one 

 on the right was grown on 1/20 acre without a legume as green manure. 

 (After Lipman and Blair.) 



for maximum nitrogen fixation. Hence, we can expect to find a 

 wide divergence in the results reported by investigators. 



Frank, in 1891, found that soil which had been green-manured 

 with legumes showed an appreciable gain of nitrogen. And it is 

 a well-known fact that in sand culture experiments wherein the 

 nitrogen of the soil is very low that much more nitrogen may be 

 removed in the legume crop than was found at first in the soil, 

 and after the removal of the crop the soil may have gained in 

 nitrogen. But what would happen in normally productive soil? 

 The most reliable data now existing are contributed by the Illinois 

 Experiment Station and indicates that two-thirds of the nitrogen 

 in legumes grown on soils of normal productive power is obtained 

 from the air. These figures were obtained from the analysis of 



