234 THE LIVING ORGANISMS OF THE SOIL [chap. 



In an untreated soil the numbers of bacteria present, 

 though showing variations with temperature, moisture, 

 aeration, and similar factors,, never undergo any 

 permanent and pronounced change. Exceptionally 

 favourable conditions may cause a temporary rise, 

 which is often followed by a considerable decrease. 



Clearly, then, the effect of the treatment had been 

 to increase markedly the capacity of the soil for bacterial 

 activity, and the increased fertility was due to the 

 larger numbers of the bacteria present, which break 

 down the nitrogenous reserves into ammonia and other 

 forms assimilable by the crop. 



Various hypotheses may be advanced to account for 

 the increase in bacterial numbers ; the treatment may 

 have so altered the organic reserves that they become 

 more readily available as bacterial food, a stimulus 

 may have been given to the bacteria themselves, so 

 that they became more effective, certain injurious or 

 competing races may have been selectively removed, or 

 again, toxins inhibiting the development of the bacteria 

 may have been destroyed. As regards the first 

 hypothesis, the experiments did show that in soil 

 treated to ioo C. some of the organic matter had been 

 altered, and bodies like ammonia had been set free. 

 Germination and the growth of seedlings in such soil 

 is often depressed for a time, the plant recovers later 

 and the expected outburst of growth follows ; but there 

 is an initial period of depression due to these active 

 products of decomposition. 



But none of the hypotheses indicated are consistent 

 with all the facts, which only become explicable on the 

 assumption that the treatment has brought about the 

 removal of some factor inhibiting the free development 

 of the bacteria in untreated soils. In normal soils 

 untreated, there is something present which keeps the 



