The Anaerobic Form 199 



actual work performed by these organisms in the field. 

 We do not know how they are affected by various 

 modes of soil treatment, tillage, and crop-rotation. We 

 know that they are influenced in their growth by the 

 mechanical composition of the soil, by the amount of 

 humus present, and by the proportion of moisture. 

 While found in both open and compact soils, their growth 

 is favored by the less thorough aeration of the latter. 

 As to their distribution in the different soil- layers, it 

 will probably be found that they occur at greater depths 

 in sandy loams than in heavy clays or clay loams. 



The aerobic nitrogen-fixing bacteria. — These organisms, 

 described by Beyerinck in 1901, consisted of two species, 

 Azotobacter chroococcum and Azotobacter agilis (Fig. 30). 

 Three additional species, making five in all, were de- 

 scribed by the writer in 1903 and 1904. They were all 

 large bacilli, quite characteristic in their appearance> 

 and widely distributed in arable soils. Being distinctly 

 aerobic in character, they develop only when air is 

 freely admitted. Their power to fix atmospheric nitrogen 

 is more pronounced than that of Clostridium Pastori- 

 anum, and the highest yield of combined nitrogen thus 

 far recorded has been given by Azotobacter Vinelandii 

 (Fig. 31), isolated from a New Jersey soil. 



From the standpoint of crop-production, this cir- 

 cumstance is of great significance, since the fixation 

 of nitrogen is accomplished at the expense of the humus. 

 The latter furnishes the food and energy to the bacteria 

 and they use it up in their growth. For the fixation of 

 nitrogen a smaller quantity of humus suffices for the 

 azotobacter species. Azotobacter Vinelandii can fix, 



