Denitrificatiou in Soil 183 



methylamino, diiiu'th\ laminr, and egg albumin, lie concluded that 

 all forms of organic nitrogen have to be transformed first into am- 

 monia before the\ can be nitrified. This was found to hold true also 

 for calcium cyanamide. When the processes of nitrate formation 

 from ammonium salts and from amino acids are compared, the latter 

 is found to take place more slowly. This is probably due to the fact 

 that the amino acids ha\ e to be changed first to ammonia and also 

 to the fact that some of the nitrogen is stored away in the microbial 

 cells which use the carbon of the amino compounds as a source of 

 energy. 



When ammonium sulfate is used as a source of nitrogen for nitrate 

 formation and the reaction of the soil is acid to begin with, there will 

 be an increase in acidity in absence of sufficient buffer or base, as a 

 result of formation of nitric acid from the oxidation of the ammonia 

 and the accumulation of the residual sulfuric acid. Nitrate accumu- 

 lation will proceed until the reaction of the soil has reached a pH of 

 about 4.0. The amount of nitrate formed under these conditions 

 depends upon the initial reaction of the soil and its buffer and base 

 content; the higher the buffer and base content of the soil, the larger 

 will be the amount of nitrate formed for a certain change of reaction. 

 The continuous use of ammonium sulfate as a fertilizer without the 

 addition of lime will, therefore, lead to a gradual increase in soil acid- 

 ity. However, nitrates may be found even in very acid soils. This 

 was explained by Hall and associates as due to the fact that, under 

 acid conditions, nitrate formation takes place in films surrounding 

 the small isolated particles of CaCOs. The addition of CaCOs has, 

 therefore, a decidedly stimulating effect on nitrate formation, par- 

 ticularly in acid soils. In alkaline soils which are deficient in organic 

 matter, CaCOs may have the opposite effect, since it tends to liberate 

 from ammonium salts free ammonia, which retards nitrification. 



Conditions that tend to promote nitrate formation in the soil are 

 temperature of 27.5 ^C, an abundant supply of air (oxygen), proper 

 moisture supply, a favorable reaction (pH greater than 4.6), pres- 

 ence of carbonates or other buffering agents, and absence of large 

 quantities of soluble organic matter. The nature of the crop grown 

 also influences the nitrate content of the soil. 



Denitrification in Soil 



Just as aerobic conditions in soil favor oxidation processes, so do 

 anaerobic conditions (exclusion of free oxygen) fa\'or processes of 



