SOIL ORGANISMS 425 



soil through the combined influence of cropping and drainage. 

 This is equivalent to a removal of about 520 pounds of sodium 

 nitrate as far as the nitrogen contained is concerned. 



While the removal of nitrogen from the soil is due very 

 largely to the phenomena just referred to, the loss of nitro- 

 gen through reduction demands a certain amount of atten- 

 tion. Reduction includes the change of nitrates to nitrites, 

 to ammonia and even to free nitrogen.^ In the same way 

 nitrites may be reduced to ammonia and the latter to ele- 

 mental nitrogen. When the process is carried to completion 

 there is opportunity for an escape of some nitrogen to the at- 

 mospheric air. The loss of nitrogen is not the important con- 

 sideration, however. The interference with plant nutrition, 

 which naturally occurs, is much more serious and justifies 

 the attention which the phenomena have received from bac- 

 teriologists. 



The number of organisms that are capable of accomplishing 

 one or more of the reduction processes is very large. This is 

 due to the facultative character of the soil flora, which is 

 able to alter its functions to suit the conditions. Thus B. 

 mycoides, which is a normal decay and ammonifying organ- 

 ism, may, under anaerobic conditions become a vigorous re- 

 ducing agent. Other specific reducing organisms are: — B 

 ramosus and B, pesUfer, B. suhtilis, B, mesenferious vul- 

 gatus, B. de^itrificanSf and many others. It is probable that 

 fungi also are able to effect the transformation. 



Most of the reducing bacteria perform their functions only 

 in presence of a limited amount of oxygen^ while others can 

 operate in the presence of a more liberal supply. In general, 

 thorough aeration of the soil impedes the process to a consid- 

 erable degree. Straw apparently carries an abundant supply 

 of such organisms, and it is consequently possible to reach a 



* The reaction may be iUustrated empirieally as foUows: 



4HN0, = 2H2O H- 2N2 + 30;,. 

 HNO3 + H,0 = NH3 + 20.. 



