540 NATURE AND PROPERTIES OF SOILS 



teria are in turn much influenced by plant and soil conditions, 

 such as amount of organic matter, presence of nitrates, acidity 

 and the like. Hopkins 1 estimates that about one-third of the 

 nitrogen in a normal innoculated legume comes from the soil 

 and two-thirds from the air. He also considers that one-third 

 of the nitrogen exists in the roots. 



Both of these assumptions are questionable and at best 

 tentative. The amount of nitrogen fixed by legume organisms 

 is extremely variable, probably more so than that assimilated 

 by the azotobacter and allied groups. Again the percentage 

 of the nitrogen held in the roots of legumes is by no means 

 the same for all species. The amount varies within the species 

 with age, degree of maturity and, season. The Delaware in- 

 vestigations 2 show that the proportion of the total nitrogen 

 of the plant occurring in the roots may be as low as 6 per cent, 

 in case of cowpeas and as high in the roots of alfalfa as 42 

 per cent. A range from 6 to 28 per cent, of the total nitrogen 

 of crimson clover was noted in the roots under different condi- 

 tions. 



According to Hopkins, the nitrogen found in the tops of 

 legumes will be a rough measure of the nitrogen fixed by the 

 nodule organisms. When the crop is turned under, this will 

 represent the gain to the soil. If the preceding assumption 

 is correct, red clover turned under would actually add about 

 50 pounds of nitrogen for every ton of air-dry substance util- 

 ized, alfalfa about 50, cowpeas 43, and soybeans 53 pounds. 

 These figures, even though they may be far from correct, at 

 least give some idea of the possible addition of nitrogen by 

 green-manuring practices, and show how the soil may be en- 

 riched by such management. As in the case of farm manures, 

 the indirect effects of such a procedure on the physical and 

 bacteriological properties of the soil may over-ride the direct 



1 Hopkins, C. G., Soil Fertility and Permanent Agriculture, p. 223; 

 Boston, 1910. 



a Penny, C. L., The Growth of Crimson Clover; Del. Agr. Exp. Sta., 

 Bui. 67, 1905. 8 



