228 Nitrogen Fixation— Symbiotic 



not been grown previously on the same soil. Hiltner obtained by 

 inoculation an increase of 1.7-.31 times the yield for lupines and 15- 

 80 times for serradella. On the average, there may be a gain of 

 50-100 pounds of nitrogen per acre of soil due to the growth of 

 legumes. Lipman and Blair obtained a gain of 54 pounds annually 

 over a period of 7 years from the growth of legumes in rotation with 

 corn, potatoes, oats, and rye in cylinders. 



Hopkins reported that a 3-ton crop of cowpea hay adds 86 pounds 

 of nitrogen per acre, a 25-bushel crop of soybeans with 2.25 tons 

 of straw adds 106 pounds, a 4-ton clover crop adds 106 pounds, and 

 a 4-ton alfalfa crop adds 132 pounds. On the average, about two- 

 thirds of the nitrogen in the legumes grown in the soil is obtained 

 from the air. Under optimum conditions and on a relatively poor 

 soil, as much as 400 pounds of nitrogen may be added per acre 

 yearly. In a light sandy soil, clover was reported to produce an 

 annual gain of 50 pounds of nitrogen. If the crop is removed, the 

 nitrogen content of the soil may not be greatly increased, since the 

 amount fixed may be just sufficient to fulfill the need of the tops. 

 Perennial legumes, like alfalfa, may not show an increase in soil 

 nitrogen, although the nitrogen is higher than in the same soils 

 upon which grains are grown. 



Because of the associated bacteria, the economic importance of 

 legumes in agriculture is so great that it has been said many times 

 that, had the whole subject of soil microbiology contributed nothing 

 more of practical value than a knowledge of the legume bacteria, it 

 would have more than fully justified itself. 



Selected Bibliography 



1. Allen, E. K., and Allen, O. N., Biochemical and symbiotic properties of the 

 rhizobia, Bad. Revs., 14:273-330, 1950. 



2. Allison, F. E., and Ludvvig, C. A., The cause of decreased nodule formation 

 on legumes supplied with abundant combined nitrogen, Soj7 Sci., 37:431- 

 443, 1934. 



3. Bond, G., Quantitati\c obser\ations on the fixation and transfer of nitrogen 

 in the soy bean, Ann. Botany, 50:559-578, 1936. 



4. Chen, H. K., and Thornton, II. G., The structure of "ineffecti\e" nodules 

 and its influence on nitrogen fixation, Proc. Roy. Soc. London, B, 129:208- 

 229, 1940. 



