E. J. Russell and H. B. Hutchinson 



Table 13. Number of organisms per gram of soil. 



139 



Change in ammonia and nitrate. 



§ 37. There is clearly some factor in the untreated soils which 

 limits bacterial activity and which is put out of action by heating or 

 by treatment with toluene. Other experiments lead to the same 

 conclusion. 



Arguments have been adduced in § 8, and need not be here repeated, 

 against the view that the limiting factor is a toxin. 



Ammonia produced from urea {as mgms. of nitrogen). 



§ 38. The limiting factor is probably biological since it takes time 

 to operate when it is re-introduced into a toluened soil (§ 36). It occurs 

 to a less extent in the extract : thus when an extract of untreated soil 



