THE NITRIFYING BACTERIA 575 



Under special circumstances there occurs in the soil a 

 process the reverse of nitrification, that is, a reduction of 

 nitrates and nitrites to lower compounds and ultimately 

 to free gaseous nitrogen. This so-called " denitrification," 

 while the result of bacterial activity is not dependent upon 

 such specific varieties of bacteria as is nitrification. For 

 instance, true denitrification is known to be an attribute 

 of bacillus coli communis, of bacillus fluorescens lique- 

 faciens, of bacillus pyocyaneus, and of bacillus typhosus. 

 While this group of species ordinarily develop under 

 free access of oxygen they can develop without it and 

 secure their necessary oxygen from such oxides of nitro- 

 gen as nitrates and nitrites, thus reducing them. It seems 

 probable that certain products of bacterial growth have 

 also a reducing action on soil nitrates. Denitrification 

 occurs most often and most actively in soils containing an 

 excess of undecomposed organic matter. 



In addition to nitrification and denitrification there is 

 seen in the soil a phenomenon resulting in "nitrogen fixa- 

 tion." In some instances this results from the symbiotic 

 activities of bacteria and higher plants, in others it appears 

 to be peculiar to certain definite species of bacteria acting 

 alone. While a discussion of the extreme agricultural 

 importance of these phenomena would be of great interest, 

 yet this is scarcely the place to undertake it. 1 



The unusual nature of the nitrifying bacteria, demanding 

 as they do special methods for their cultivation, renders 

 them of sufficient technical interest to justify for purposes 

 of illustration a more or less detailed description of one of 

 them. 



These very important and interesting nitrifying organisms, 



1 See Bacteria in Relation to Country Life, by Lipman-MacMillan, 1911. 



