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SCIENCE PROGRESS 



supposition falls to the ground, because when small quantities 

 of ammonium salts are added to untreated soils the whole 

 of the added nitrogen is recovered as ammonia and nitrate. 

 Hence we must conclude that the treatment has induced an 

 increased production of ammonia. 



Several considerations show that the production of ammonia 

 subsequent to the small initial gain on heating or treating with 

 toluene is mainly the work of micro-organisms. The curves 

 belong to the type associated with bacterial, rather than purely 

 chemical, change. Soil which has been heated to 125 C. (at 

 which temperature all organisms are killed) shows no increase 

 in ammonia-content after the first small gain. There is no 

 rapid period of gain if enough toluene is left in to inhibit 

 bacterial action nor if the water supply is insufficient. There 

 may be some enzyme action — indeed the result of leaving toluene 

 in suggests that there is — but the chief factor is clearly bacterial 

 activity. 



Further evidence was obtained by counting the number of 

 bacteria in the various soils that develop on Koch's gelatine 

 plates. The numbers given below show that the bacteria 

 increase pari passu with the amount of ammonia ; we may 

 therefore associate the increased production of ammonia with 

 the increased numbers of bacteria. Direct experiments showed 

 that addition of ammonium salts to unheated soil led to no 

 such increase as is observed here. 



The new flora arising after partial sterilisation was found to 

 be more active than the original flora in effecting the decompo- 

 sition of nitrogenous organic matter such as peptone and in 

 hydrolysing urea. But there is no evidence that the species 



1 After 4 days ; 9 days count lost by plates liquefying. 



