184 



Transformation of Nitrogen 



reduction. Either organic or inorganic compounds may be formed 

 as a result of these processes, depending upon the composition of 

 the medium. It is not necessary for the soil to be saturated with 

 water for the conditions to be anaerobic. Winogradsky demon- 

 strated, by the development of anaerobic nitrogen-fixing bacteria. 



10 



20 30 40 50 

 Years of cultivation 



60 



70 



Fig. 78. Nitrogen le\'el of soils culti\atcd for a number of years (from Jenny). 



that, when a soil contains water equivalent to only about 40 per cent 

 of its moisture-holding capacity, anaerobic bacteria find conditions 

 favorable for their development even at the very surface of the soil. 

 The disappearance of nitrates in soil as a result of activities of 

 microorganisms may be due to three groups of phenomena: first, 

 direct utilization of nitrates by microorganisms as sources of nitro- 

 gen, in the presence of sufficient energy material; second, reduction 

 of nitrates to nitrites and ammonia in the process of nitrate assimila- 

 tion; third, utilization of nitrates as sources of oxygen (nitrates as 



