792 BACTERIA : THEIR WORK 



lo. Denitrification. — A large number of different species 

 of bacteria have been isolated from the soil, air, well-water, 

 dung, and other sources, which are capable of destroying 

 nitrates by a process of reduction. In some instances the 

 reduction, or denitrification as it is termed, results in the 

 formation of nitrites which remain in the medium in which the 

 process is going on : in others, the gases, nitric and nitrous 

 oxides, or even free nitrogen, are produced, in all of which cases 

 there is a loss of nitrogen into the surrounding air. 



The particular amount and character of the denitrification 

 depends upon the species of bacteria involved in the process, 

 and also upon the presence of easily oxidisable organic matter : 

 without the latter the reduction cannot proceed. 



Many of these bacteria only carry on their work under anaer- 

 obic conditions, hence denitrification is often energetic in 

 water-logged soilS from which air is excluded : some species 

 are, however, able to reduce nitrates even in the presence of 

 oxygen. 



After nitrification has taken place in the soil of ordinary 

 arable land, there is no fear of loss of nitTOgen through deni- 

 trification, for the nitrates are not produced in the former 

 process until the organic material has been oxidised and the 

 conditions for denitrification have passed away. 



Several observers have noticed that the addition of very 

 large amounts of fresh dung to soils already containing nitrate 

 of soda has diminished the yield of produce below that obtained 

 from similar soil to which dung has not been applied. The 

 decrease in the crop is doubtless due to the denitrification 

 of the nitrate, and the consequent loss of nutrient nitrogen. 

 Some authorities assume that the peculiar action of the dung 

 in such cases is mainly due to its containing very large numbers 

 of the denitrifying organisms, others maintain that the dele- 

 terious effect of the dung is owing to the oxidisable organic 

 matter derived from the straw and undigested vegetable tissue 



