120 AGRICULTURAL BACTERIOLOGY 



cellulose is also made available to the nitrogen-fixing bac- 

 teria by the cellulose-fermenting bacteria through the for- 

 mation of sugars. 



It has been maintained that the nitrogen-fixing power of 

 a soil could be increased by inoculation with cultures of 

 these organisms. Such a hope has not yet been realized. 

 Their action in the soil must be favored by the establish- 

 ment of a suitable environment. A sufficient supply of 

 both phosphorus and potassium is essential, as is organic 

 matter from the fermentation of which they may obtain the 

 energy necessary for the combination of the nitrogen. As 

 noted in the discussion on manures, it is desirable to add 

 to the soil as much organic matter as possible, irrespective 

 of whether it contains any of the four elements that are 

 known to be most important from the standpoint of the soil. 

 One important role of the organic matter is to favor the 

 growth of these nitrogen-fixing organisms. There is good 

 reason to believe that these two groups of bacteria are im- 

 portant factors in the maintenance of the nitrogen content 

 of the soil, rather than simply scientific curiosities, as some 

 have considered them. The nitrogen fixed by these groups 

 of bacteria is built into organic matter, which must be 

 ammonified and nitrified before the nitrogen drawn from 

 the air can be used by the green plant. 



Leguminous plants. It has long been recognized that 

 the leguminous plants have different properties from the 

 grains and grasses in that they are able to produce luxuri- 

 ant crops on lands on which the non-legumes will make but 

 a meager growth. They also seem to enrich the soil, since 

 the yield of the crop following them is often greater than 

 that obtained from the same land on which a non-legume 

 had been grown. This property has led to the inclusion of 

 some type of leguminous plant in most systems of crop 

 rotation. It was found by Liebig, the father of agricultural 



