144 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



moisture contents are unfavorable to aerobic processes in general 

 (Fig. 62). 



A number of other factors, such as temperature, concentration 

 of salts in the soil solution, and previous drying of soil, exert pro- 

 nounced effects on the rapidity of formation of nitrate from 

 ammonia, either produced from the decomposition of organic 



5 10 15 



Moisture Content- Per Cent 



Fig. 62. — Influence of moisture upon nitrate formation 



(after Traaen). 



matter present, or recently introduced into the soil, or added to the 

 soil in the form of inorganic salts. It was found, for example, that 

 drying of soil followed by remoistening causes nitrates to form more 

 rapidly. 



In general, the bacteria which bring about the processes of 

 transformation of ammonia to nitrate are among the most sensitive 

 of soil organisms. Conditions which can be readily withstood by 

 other bacteria may prove injurious to these organisms. This is 

 due to no small extent to their very specific nature. 



