598 PEINCIPLES OF SOIL MICROBIOLOGY 



growth of leguminous plants in the presence of the proper bacteria. 

 Poor soils are usually found to give larger gains than rich soils. Soils to 

 which lime and phosphorus compounds have been added show greater 

 increases in combined nitrogen than soils where those minerals were lack- 

 ing. Inoculated soils give better results than uninoculated, particularly 

 if the legume in question or the related forms have not been grown pre- 

 viously on the same soil. Hiltner, 160 for example, obtained an increase 

 of 1.7 to 31 times the yield for lupines and 15-80 times for serradella as 

 a result of inoculation with the proper organism. On the average, there 

 may be a gain of 50 to 100 pounds of nitrogen per acre of soil due to the 

 growth of legumes. Lipman and Blair 161 found a gain of 54 pounds 

 annually over a period of seven years from the growth of legumes in 

 rotation with corn, potatoes, oats and rye in cylinders. 



According to Hopkins, 162 a 3-ton crop of cowpea hay adds 86 pounds of 

 nitrogen per acre, a 25-bushel crop of soybeans with 2\ tons of straw 

 adds 106 pounds, a 4-ton clover crop adds 106 pounds and a 4-ton alfalfa 

 crop adds 132 pounds. At least two-thirds of the nitrogen in legumes 

 grown on normally productive soils 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 per year. 163 The net yearly gain per 

 acre from the growth of clover on a light sandy soil was found to be 50 

 pounds of nitrogen. 164 From 120 to 250 mgm. of nitrogen are fixed 

 per plant of red clover and alfalfa. 165 If the tops are removed, the nitro- 

 gen content of the soil may not be increased, since the amount fixed 

 may be just sufficient to fulfill the need of the tops. In the case of per- 

 ennial legumes, like alfalfa, there may not be an actual increase in soil 

 nitrogen, as compared with uncultivated soils, although the nitrogen is 

 higher than in the same soils upon which grains are grown. 166 



160 Hiltner, 1904 (p. 128). 



161 Lipman, J. G., and Blair, A. W. Cylinder experiments relative to the 

 utilization and accumulation of nitrogen. N. J. Agr. Exp. Sta. Bui. 289. 1916. 



162 Hopkins, C. Nitrogen bacteria and legumes. 111. Agr. Exp. Sta. Bui. 94. 

 1904. 



i6j Wheeler, H. J. Cooperative experiments in alfalfa culture. R. 1. Agr. 

 Exp. Sta. Bui. 152. 1912. 



164 Shutt, F. T. Nitrogen enrichment of soils. Experiment Farms Rpt., 

 Ottawa. 1912, 144-146. 



166 Brown, P. E., and Stallings, J. H. Inoculated legumes as nitrogenous 

 fertilizers. Soil Sci., 12: 365-407. 1921. 



166 Swanson, C. O., and Latshaw, W. L. Effect of alfalfa on the fertility 

 elements of the soil in comparison with grain crops. Soil Sci., 8: 1-39. 1919. 



