88 AGRICULTURAL BACTERIOLOGY. 



eral conclusions only appear, as yet, to be sufficiently estab- 

 lished to warrant their insertion here. 



1 . The process of denitrification is one of extracting oxygen 

 from nitrogen compounds and it does not occur so vigorously 

 if the bacteria are supplied with an abundance of atmospheric 

 oxygen. Consequently denitrification is not so prominent in 

 soils or manure heaps which are well supplied with air as they 

 are where air is excluded. On the other hand, these bacteria 

 do not appear to thrive in an absolute exclusion of oxygen, 

 some of the denitrifying bacteria acting best in the presence 

 of oxygen, while others benefit by its absence. 



2. The denitrifying bacteria need for their action a supply 

 of some easily assimilable carbonaceous material. The pres- 

 ence of lactic acid greatly stimulates their denitrifying power 

 since it furnishes them with carbon and energy. It is also a 

 fact that the presence of hay and straw in manure increases 

 the denitrification, a fact supposed at first to be due to the ad- 

 dition to the manure of many denitrifying bacteria which were 

 adhering to the straw. It is now regarded as due to the fact 

 that the straw supplies the carbonaceous material necessary to 

 furnish the bacteria with carbon and energy. From this it 

 follows that the more completely the manure heap can be kept 

 free from straw the less the nitrogen loss by denitrification. 



3. The denitrification which occurs in soil as it exists in our 

 fields is much less than early experiments indicated. If the 

 amount of organic matter is large, denitrification is excessive, 

 and early experiments were conducted under conditions of 

 large amounts of organic matter. When the organic matter 

 is small in amount, as is the case in the ordinary soil, the denitri- 

 fication is very slight. Consequently denitrification is a phe- 

 nomenon of little importance in ordinary soil, while it is the 

 cause of a large amount of nitrogen loss in the manure heap. 



Nitrogenous Decomposition in the Soil. Let us now picture 

 to ourselves what is going on in the soil at all times. Within 



