DEPRESSION OF PLANT AND BACTERIAL GROWTH 125 



limiting plant growth happens to be the supply of nitrogenous plant 

 food we may expect to find a close connection between bacterial 

 activity and soil fertility; if on the contrary the limiting factor is 

 something else such as water supply, lack of phosphates, etc. no 

 such connection is necessary. Even here, however, a connection may 

 exist, for bacteria are living things, affected by the same circumstances 

 that influence plants. 



Three distinct cases therefore arise : 



(1) Bacterial activity may show no sort of relationship with soil 

 fertility, because fertility is limited by some factor other than the 

 nitrogen supply. 



(2) Bacterial activity may be directly related to soil fertility but 

 the relationship is only accidental, both bacteria and plants being 

 affected by the same limiting factor. 



(3) Bacterial activity may be directly related to soil fertility and 

 the relationship is causal, fertility being limited by the amount of 

 ammonia and nitrate produced by the bacteria. 



Instances of the first are common in arid and semiarid districts. 



The second case is not unfrequent. An admirable illustration is 

 afforded by the experiments of Crowther and Ruston on the effect of 

 acid rain-water on plant growth (72). The pots were watered with 

 solutions of sulphuric acid, some being of the same order of concentra- 

 tion as the Leeds rain-water. The acid depressed the growth not 

 only of plants but of bacteria also, and the effect is very similar in 

 both cases (Table L.). 



TABLE L. EFFECT OF ACIDULATED WATER ON THE GROWTH OP PLANTS AND 

 BACTERIA. CROWTHER AND RUSTON (72). 



At first sight this* looks like a close relationship between bacterial 

 activity and plant growth. But the figures in the last column show 

 that the failure of the crop is not due to the failure of the bacteria to 



