270 SOIL CONDITIONS AND PLANT GROWTH 



OT million of bacteria per grm. in the raw moorland soil, 

 but 7 millions in similar soil that had been cultivated and 

 manured. 



So close is the similarity between ordinary crops and 

 azotobacter in relation to soil acidity that an azotobacter test 

 is used in Denmark for determining the need of lime (p. 242). 



It may often be difficult in practice to determine whettfer 

 the relationship between the bacterial numbers and plant 

 growth is causal or accidental, but the principle is perfectly 

 clear ; the relationship is causal only when the plant growth is 

 limited by the supply of compounds produced by bacterial 

 activity, or the rate at which plant residues or harmful 

 substances such as phenol, thiocyanates, etc., are decomposed 

 by bacteria. The recognition of this central principle greatly 

 facilitates investigation, for it shows the futility of haphazard 

 attempts to correlate bacterial activity and plant growth over 

 a set of soils that are not strictly comparable. The better 

 course is to narrow down the problem and confine it to the 

 elucidation of the connection between bacterial activity and 

 nitrate production. 



Effect of Soil Conditions on Bacterial Numbers and on 

 Nitrate Production. 



On general grounds it might be expected that the soil 

 bacteria would be affected by external conditions in much 

 the same way as plants, and to a considerable extent this 

 happens. Most of the effects observed with growing plants 

 have been paralleled in the case of the soil bacteria. There are, 

 however, nearly always differences between the effects produced 

 on the bacterial numbers and on the amount of work done : 

 one may be increased but not the other. Discussion of the 

 effects of external' conditions on bacterial numbers is much 

 hampered by the paucity of data. Observers have often been 

 content to make counts once in twenty or thirty days, and 

 they have usually assumed that changes in numbers are slow. 

 Cutler and Crump have shown (73^) that this supposition 

 is unfounded : changes proceed rapidly from day to day. 



