226 Nitrogen Fixation— Symbiotic 



4. Excretion may occur fairly early in the development of the 

 plant, before sloughing of nodules or roots would be expected. 



Wilson further developed this concept by considering the effect of 

 length of day upon the excretion of nitrogen by the leguminous plant, 

 as measured by absorption by the nonlegume (Table 51). 



Table 51. Influence of Length of Day upon Fixation of Nitrogen 



(from Wilson) 



Associated barley control 36.7 19.8 



Importance of Symbiotic Nitrogen Fixation in the Soil 



Though the amounts of nitrogen fixed nonsymbiotically under field 

 conditions are still subject to doubt, the symbiotic fixation of nitro- 

 gen is of great economic importance. The amount of nitrogen 

 added to the soil by leguminous plants depends entirely upon the 

 abundance of available nitrogen in the soil. The poorer the soil is 

 in nitrogen and the richer it is in lime, phosphorus, and potash, the 

 greater will be the gain in nitrogen from the growth of legumes. 

 The nature of the legume, soil conditions, and season will affect the 

 amount of nitrogen fixed (Table 52). 



Tahle 52. Influence of CaCOs upon Nitrogen Fixation by Alfalfa 

 Grown in Sand (from Jensen) 



Warington demonstrated in 1891 that an increase of about 350 

 pounds of nitrogen per acre may be obtained as a result of the growth 



