IMPORTANCE OF NON-SYMBIOTIC NITROGEN FIXATION 111 



of energy would be left for the use of nitrogen-fixing bacteria and 

 that these would have to use the nitrogen from the atmosphere is 

 not very conclusive. Under field conditions, however, some 

 nitrogen may be added to soils through the agency of bacteria. 

 Fixation has been noted even when animal manure was the organic 

 substance with which the soil was treated. The soil conditions 

 and the nature of the growing crop probably exert the determining 

 influences in many cases. It has been stated that in some soils 

 as much as 40 pounds of nitrogen become added to soils annually 

 as a result of the activities of non-symbiotic nitrogen-fixing bac- 

 teria. Certain soils in regions of deficient rainfall show particu- 

 larly striking increases in nitrogen. 



Due to the fact that it is almost impossible to detect even 

 appreciable increases in the nitrogen content of soils (such as 40 

 pounds per acre per year), the actual agricultural importance of 

 the non-symbiotic organisms is practically unknown. With the 

 numerous facts in mind, it may be assumed that, in most soils, 

 the increase in the nitrogen content as a result of the activities of 

 these nitrogen-fixing bacteria is rather limited. 



There are two conditions not yet considered under which the 

 activities of these organisms may tend to increase the store of com- 

 bined nitrogen in the soil: (1) In the neighborhood of growing 

 roots of plants there is an excretion of soluble carbohydrates and 

 addition of other residues to the soil which may serve as food for 

 the bacteria. Plants rapidly consume most of the available com- 

 bined nitrogen from this portion of the soil. These two factors, 

 namely, the presence of available sources of energy and a nitrogen 

 minimum, would favor the rapid development of Azotobacter and 

 Clostridium and lead to nitrogen fixation. (2) A similar relation- 

 ship may exist between certain green algae and the nitrogen-fixing 

 bacteria. The former ^x the carbon from the carbon dioxide of the 

 air (by photosynthetic agencies at the surface of the soil) and liber- 

 ate some of the synthesized carbohydrates, which can be used as 

 sources of energy by the nitrogen-fixing bacteria; the latter fix 

 the nitrogen and liberate some of it in a combined form which 

 can be used by the algae. 



The reaction of the soil is one of the factors of major impor- 

 tance in determining the presence or absence of Azotobacter, as 

 shown in Table 26. Under the great diversity of soil conditions, 

 the reaction may limit the distribution of the organisms more fre- 



