112 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



quently than any other condition. If the reaction is more acid 

 than pH 6.0, Azotobacter is seldom present; when the soil is limed 

 and the reaction is changed to pH 6.0 or above, Azotobacter will 

 develop quite readily, when present or when introduced. The 

 Clostridium types can resist a somewhat greater acidity than Azo- 

 tobacter (such as pH 5.2) and occur under a greater variety of 

 conditions and in considerably greater numbers. 



TABLE 26 

 Soil Reaction and Occurrence of Azotobacter (prom Gainey) 



Soil conditions which appear to be aerobic, that is, well-aerated 

 soils with a good crumb structure and with optimum moisture 

 content, may be a very favoral^le habitat for the anaerobes. The 

 conditions within the aggregates of soil particles or close to decom- 

 posing portions of soil organic matter may be far from aerobic. 

 Aerobic organisms may consume the oxygen about their cells 

 during growth and create sufficiently anaerobic conditions to per- 

 mit neighboring cells of Clostridium to develop. Such associative 

 development of various organisms is probably more common in 

 soils than development of single species of organisms entirely 

 separated from one another. At least, one need not postulate the 

 fixation of nitrogen by the anaerobic types only under bog condi- 

 tions or in water-logged soils. In fact, the fixation of small 

 amounts of nitrogen in soils by the group of non-symbiotic nitrogen- 

 fixing bacteria may be as much the result of the action of the Clos- 

 tridium as of the Azotobacter. 



