67 



been influenced by no factors other than the natural inorganic status of 

 the soil, its humus content, and its physical condition. To be sure, the two 

 first named factors may be of controlling importance we are inclined to 

 believe the supply of humus is an all important one in comparison with 

 which the others are relatively insignificant. The only difference in the 

 above figures which would warrant conclusions is that between the clay soils 

 and those of lighter texture. And that fixation in these heavy soils should 

 exceed that of the lighter ones is not without its element of surprise, yet 

 the difference is quite well defined even the clay loam exceeds the loams 

 and the sandy loams in fixing nitrogen. 



Eather than asserting the superiority of clay soils or of any special soil 

 type in fixing nitrogen, the results summarized in Table III more poignantly 

 demonstrate the indifference with which that process is affected by such 

 factors. This fact has been impressively presented in many instances in 

 the present study. 



By singling out from Table I the results from a few cultivated soils re- 

 gardless of their origin and noting their power of fixation, it is difficult to 

 ascribe any influence whatever to soil types. Such examples are presented 

 in Table IV. 



TABLE IV. Nitrogen Fixation ~by a Number of Cultivated Soils of 



Various Types. 



No. [ Soil Types 



Milligrams Nitrogen fixed 



per 100 e. c. Media 



with 10 gms. soil 



Under proper cultivation and a sufficient supply of humus disregard- 

 ing for the moment certain other factors it appears that the type of a soil 

 would have negligible influence on its power for free nitrogen fixation. 



SUMMARIZING DISCUSSION. 



The examination of ninety-three soils, eighty-eight of which were col- 

 lected in pairs of virgin and cultivated samples, showed clearly the super- 

 iority of cultivated soils for nitrogen fixation. Though some of the virgin 

 samples excelled in the addition of nitrogen, the average increase, where 

 induced by cultivated soils, was appreciably higher than where it resulted 

 from virgin soils. Tables I and II summarize these results. 



