456 SOILS: PROPERTIES AND MANAGEMENT 



B. denitrificans alpha and B. denitrificans beta reduce 

 nitrates with the evolution of gaseous nitrogen. 



375. Nitrate-assimilating organisms. — In addition to 

 the nitrate-reducing bacteria already mentioned, there are 

 other bacteria which also utilize nitrates ; but, like higher 

 plants, these convert the nitrogen into organic nitrog- 

 enous substances. However, as they operate in the 

 dark and cannot obtain energy from sunlight, they must 

 have organic acids or carbohydrates as a source of energy. 

 While these bacteria cannot be considered as nitrate 

 reducers, they help to deplete the supply of nitrates when 

 conditions are favorable for their development. What 

 these conditions are is not well understood, nor can any 

 estimate be made as to the extent of their operations. 



376. Denitrification. — The term denitrification may 

 be used to include both the process of nitrate reduction 

 and that of nitrate assimilation (see Fig. 61). 



Most of the denitrifying bacteria perform their func- 

 tions only under a limited amount of oxygen, while others 

 can operate in the presence of a more liberal supply ; 

 but, in general, thorough aeration of the soil practically 

 prevents denitrification. Straw apparently carries an 

 abundant supply of denitrifying organisms, and also 

 furnishes a supply of carbohydrates which favor their 

 action; so that stable manure is very likely to undergo 

 denitrification, and straw or coarse stable manure are 

 conducive to the growth of denitrifying bacteria in the soil. 



Under ordinary farm conditions, denitrification is 

 of no significance in the soil where proper drainage and 

 good tillage are practiced. Warington 1 showed that if 



1 Warington, R. Investigations at Rothamsted Experi- 

 mental Station. U. S. D. A., Office of Exp. Sta., Bui. 8, p. 64. 

 1892. r - 



