268 SOIL CONDITIONS AND PLANT GROWTH 



proved most valuable ; numerous instances are given in the 

 Rothamsted papers on soil sterilisation (241). 



This discrepancy between field and laboratory experience 

 is cleared up by a closer examination of the nature of the 

 relationship between bacterial activity and plant growth. 

 The connection, as already pointed out, lies in the fact that 

 bacteria decompose the organic matter of the soil and make 

 new plant food out of old plant residues. If the factor 

 limiting plant growth happens to be the supply of nitrogenous 

 plant food or the rate of decomposition of the plant residues 

 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 phos- 

 phates, 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 : — 



(i) Bacterial activity may show no sort of relationship with 

 soil fertility, because fertility is limited by some factor other 

 than the nitrogen supply or rate of decomposition of plant 

 residues. 



(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 or by other decompositions which they effect. 



Instances of the first are common in arid and semi-arid 

 districts. 



The second case is not unfrequent. An admirable illustra- 

 tion is afforded by the experiments of Crowther and Ruston 

 on the effect of acid rain-water on plant growth (71). The 

 pots were watered with solutions of sulphuric acid, some being 

 of the same order of concentration 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 LXX.). 



