128 THE FORAGE AND FIBER CROPS IN AMERICA 



of symbiosis is not well understood. That tubercles contain a 

 high percentage of nitrogen is established. Whether the host 

 plant secures its supply of nitrogen as a result of the life 

 processes of the bacteria, or as a result of the death and de- 

 composition of the bacteria, is not definitely known. 



136. Acquirement of Nitrogen without Legumes. Arable 

 soils may gain nitrogen without the aid of legumes. If sterilized 

 soil is allowed to stand without growing any plants, the nitrogen 

 content will not increase; but if unsterilized soil in good tilth 

 is allowed to stand, the nitrogen content may increase. 



A considerable number of bacterial species in the soil possess 

 in a more or less marked degree the power of utilizing ele- 

 mentary nitrogen for their growth. There is some reason to 

 suppose, even, that Bacillus radicicola, the organism which 

 leads to the formation of tubercles on the roots of legumes, 

 may grow in the soil outside of the legume tubercles, and cause 

 there the fixation of slight amounts of nitrogen. Of the soil 

 organisms known to possess the ability to fix atmospheric 

 nitrogen, there are two distinct groups that are particularly 

 prominent. The first of these groups is represented by Clostri- 

 dium pasteurianum, an anaerobic butyric ferment isolated and 

 described by Winogradsky. 1 The second group is represented 

 by the Azotobacter species, particularly Asotobacter chroococ- 

 cum Beyerinck 2 and A. vinelandii Lipman and A. bcyerincki 

 Lipman. 3 The exact significance of these organisms in the 

 economy of soil nitrogen is still to be determined. Some of 

 the recorded data show that cultivated soils may at times gain 

 very considerable quantities of combined nitrogen without the 

 intervention of legumes. Thus Kuhn * was able to grow non- 

 leguminous crops on the same land for 20 years without a 



iCompt. rend. 1894, 118, p. 353. 

 2 Centr. f. Bakt. Vol. VII (1901), Part II, p. 561. 

 'New Jersey Sta. Rpts., 1903, 1904, 1905. 

 Centr. f. Bakt., Vol. VI (1901), Part II, p. 601. 



