DECOMPOSITION OF ORGANIC MATTER IN THE SOIL 397 



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 all 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 (under certain conditions) and in market gardening 

 where manure is used at the rate of 45,359 kg. to 54,431 kg. (50 to 60 

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



ANALYTICAL AND SYNTHETICAL REACTIONS 



AMOUNT OF BACTERIAL SUBSTANCE IN THE SOIL. Various decom- 

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

 as analytical in that protein, carbohydrates and fats are split into more 

 simple compounds. At the same time, the microorganisms concerned 

 in the decomposition processes multiply very rapidly and fashion the 

 complex compounds of their cell-substance out of the simple cleavage 

 products in their medium. In other words, analytical and synthetical 

 reactions proceed hand in hand in the soil. 



While it is not definitely known how large a proportion of the soil 

 humus consists of the dead and living cells of microorganisms there 

 is a mass of indirect evidence to show that these cells form a very con- 



