8 CIRCULAR NO. 113, BUREAU OF PLANT INDUSTRY. 



form nitrate into nitrite, perhaps also into ammonia, and certainly 

 many of them are capable of breaking up the molecule entirely and 

 giving off free nitrogen gas. This group of organisms if functioning 

 in this manner constantly would soon destroy the most fertile land, 

 but although these organisms are found generally, they are not usu- 

 ally very plentiful and they do not become active denitrifiers unless 

 afforded certain definite conditions for growth. The requirements of 

 different species of the denitrifying group vary considerably; most of 

 them will transform nitrate nitrogen into nitrite if organic material, 

 such as beef broth or plant extracts, or even manure, be supplied them 

 together with nitrate nitrogen. This fact illustrates the danger of 

 applying nitrate fertilizers and manure simultaneously to a crop. 

 Especially if the soil is moist or water-logged at the time there ensues 

 a rapid development of nitrites with consequent injury to the crop. 

 If nitrates and manure are to be applied to a given field, the manure 

 should be applied weeks or, preferably, months in advance, in order 

 to allow the preliminary decomposition of the proteid compounds in 

 the manure to proceed to a point where they will no longer be food 

 for denitrifying bacteria. 



NITROGEN-FIXING BACTERIA. 



The nitrogen-fixing bacteria are those which have the power of 

 combining the atmospheric nitrogen with their other food materials 

 and forming proteid nitrogen. Their activity can be enhanced by the 

 improvement of the soil tilth, and sometimes also by the addition of 

 carbohydrates, such as sugar and molasses. It is probable, however, 

 that cellulose which has been decomposed to some soluble form is as 

 good a food as any of the sugars produced commercially. Since the 

 development of these free-living, nitrogen-fixing bacteria is presumed 

 to be largely responsible for the continued fertility of agricultural land 

 and since it is probable that these bacteria are the chief cause of the 

 undiminished fertility of the lands of the intermountain district in the 

 Northwest, where continuous grain cropping has produced no apparent 

 diminution hi soil nitrogen, both the importance of this group of 

 bacteria and the probable correlation of the development of nitrogen- 

 fixing bacteria with cellulose-dissolving bacteria are obvious. 



NODULE-FORMING BACTERIA. 



An important subdivision of nitrogen-fixing bacteria and one 

 which is perhaps better known than any other group of soil bacteria 

 is the nodule-forming bacteria of the legumes. These bacteria when 

 associated with the roots of the leguminous plants and a few non- 

 leguminous plants form nodules upon the roots which are capable of 

 using as food large quantities of atmospheric nitrogen. The recipro- 



[Cir. 113] 



