316 SYMBIOTIC NITROGEN FIXATION 



four-ton crop of clover adds 107 pounds, a four-ton crop of cowpea 

 hay adds 115 pounds. 



These are the quantities of nitrogen which reach the soil under 

 ideal conditions, but some may be lost under natural conditions 

 with the drainage, and possibly by other means. The New Jersey 

 Experiment Station has reported a gain of 200 pounds per acre where 

 crimson clover has been grown, whereas the Rhode Island Experi- 

 ment Station, as a result of pot culture experiments, reports a gain of 

 400 pounds per acre yearly. This experiment extended over five 

 years, and legumes were grown both in the summer and in the 

 winter. The tops of the summer legumes (cowpeas and soybeans) 

 were removed from the soil, while the winter legumes (vetch) were 

 turned back into the soil. 



Shutt, in pot and plat experiments extending over two years in 

 which mammoth red clover was grown on soil and turned under, 

 showed a gain of 179 pounds of nitrogen per acre to a depth of 9 

 inches in the pot experiments and 175 pounds to a depth of 4 inches 

 in the plat experiments. A light sandy loam with a sandy subsoil, 

 when planted to clover continuously and reseeded every two years, 

 doubled in nitrogen in ten years. This was a yearly gain of nitrogen 

 of 50 pounds per acre. 



Soil Inoculation. The early experiments demonstrated that 

 legumes assimilate atmospheric nitrogen only when properly inocu- 

 lated. Since that time much has been written on soil inoculation. 

 However, it is'being found that in the majority of cases where the 

 physical and chemical conditions of the soil are optimum, the 

 ordinary legume bacteria are already present. This is especially 

 true where that particular legume has been grown in that district 

 for some time. The legume organism may have been in the virgin 

 soil, having come from the native legume, or carried into the soil 

 with manure or dust. When a new legume is being introduced into 

 a district, one should see that the soil is properly inoculated for the 

 members of that group. Successful seed inoculation can be per- 

 formed with fresh properly prepared artificial cultures, but in many 

 cases this has not proved successful and in the majority of cases 

 inoculation with soil known to be infected is to be preferred. The 

 method suggested by the Illinois Experiment Station for large seeds, 

 such as soybeans, is very satisfactory. The seeds are thoroughly 

 moistened by a 10 per cent, solution of glue and sufficient dry 

 pulverized infected soil sifted on to absorb all of the moisture. The 

 seed is shovelled over a few times. Such infected seed should be 

 planted very soon or else spread out to dry to prevent mould action. 

 Neither infected seed nor soil should be long exposed to bright sun- 

 shine, as this is very destructive to the bacteria. 



Where the old plants are to be inoculated a few hundred pounds 

 of soil may be obtained from an old infected field spread on the new 

 field and harrowed. 



