SYMBIOTIC NITROGEN FIXATION 165 



inoculated and non-fnoculated legumes from like areas of normal 

 soils and as a result of pot experiments. Computed on these data 

 a four- ton alfalfa crop adds 132 pounds, a four-ton crop of 

 clover adds 107 pounds, a four-ton crop of cowpea hay adds 115 

 pounds. This nitrogen is added to the soil, provided the total 

 crop is plowed under. 



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 had been grown and plowed under, where- 

 as the Rhode Island Experiment 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 q 

 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 

 sub-soil, when planted to clover continuously and reseeded every 

 two years, doubled in nitrogen in ten years. This was a yearly 

 gain of nitrogen of fifty pounds per acre. 



Shall we Inoculate? — ^The early experiments demonstrated 

 that legumes assimilate atmospheric nitrogen only when properly 

 inoculated. Since that time much has been written on soil inocu- 

 lation. However, it is being found that in the majority of cases 

 where the physical and chemical conditions of the soil are opti- 

 mum, 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 car- 

 ried into the soil with manure or dust. 



When legumes are being grown on a piece of land for the first 



