DECOMPOSITION OF ORGANIC MATTER IN THE SOIL 335 



The Actinomyces reduce nitrates to nitrites, but they do not cause 

 any loss of free nitrogen, for the nitrites are utilized by the organisms, 

 and complete denitrification does not take place. Thus these organ- 

 isms may prevent the leaching out of nitrates and nitrites in the, soil, 

 or the active denitrification by other organisms. \ 



Relation to Environment. — Nitrate reduction is favored by insuffi- 

 cient aeration, as well as by an abundance of readily decomposable 

 organic matter. In fine-grained, compact soils nitrate formation and 

 nitrate reduction may alternate, depending upon the more or less 

 complete replacement of soil air by water. Similarly, in soils receiving 

 excessive amounts of animal manure denitrifying bacteria may cause 

 the reduction of nitrates. In greenhouse soils excessive moisture, as 

 well as excessive amounts of organic matter, may be present and may 

 prevent the accumulation of nitrates. It has also been shown by 

 Niklevski that, contrary to opinions previously held, denitrification 

 may occur in manure heaps. In the better aerated surface portion of 

 manure heaps conditions are favorable for the oxidation of ammonia 

 to nitrites and nitrates. The nitrous acid may combine with ammonia 

 to form ammonium nitrite, the latter decomposing, spontaneously, into 

 water and nitrogen gas. It is very likely, also, that the nitrites and 

 nitrates are reduced by the denitrifying bacteria in manure. On the 

 other hand, in manure kept moist under the feet of cattle nitrite and 

 nitrate formation is prevented and losses by denitrification are not 

 likely to occur. 



The economic significance of denitrification was overestimated at 

 one time, on account, largely, of the assertion of Wagner in 1895 that 

 in aU soils receiving applications of horse manure, the nitrates in the 

 soil itself as well as those added in commerical fertilizers are almost 

 certain to be destroyed by denitrification. Subsequent experiments 

 by many investigators demonstrated that under field conditions, deni- 

 trification is a factor of slight moment; however, in the greenhouse, 

 in the manure heap (imder certain conditions) and in market gardening 

 where manure is used at the rate of 45)359 ^^g- to 54,431 kg. ($0 to 60 

 tons) per acre, the danger of denitrification is real. 



Analytical and Synthetical Reactions 



Amount or Bacterial Sxtbstance in the Soil. — Various decom- 

 position processes in the organic matter of the soil may be designated 



