138 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



saturated environment is insufficient to satisfy the requirements of 

 these strictly aerobic bacteria. The addition of hme to an acid 

 soil, or the drainage of a water-logged soil, will, therefore, mate- 

 rially favor the development of nitrifying organisms and the 

 process of nitrification. The organisms may be found at some 

 depth in soils, but naturally are much less abundant and little 

 active in the lower layers, on account of the less aerobic conditions 

 and the lack of materials upon which to feed. In arid soils these 

 bacteria are more abundant in deeper layers than in soils of humid 

 regions, since plant roots penetrate to greater depths and moisture 

 conditions are more favorable at considerable distances below the 

 surface. 



TABLE 32 



Relationship between Soil Reaction and Abundance of Ammonia- 

 Oxidizing Organisms (from Wilson) 



pH Abundance of organisms 

 6.2 Less than 1,000 



6.4 3,500 



6.6 6,280 



6.8 25,000 



7.0 35,000 



The numbers of nitrifying organisms in soils vary greatly, 

 depending upon a number of factors ; in cultivated soils they have 

 been found to range from a few hundred to more than a milhon 

 cells per gram. The data in Table 32 show that with increasing 

 acidity there is a pronounced decrease in the abundance of these 

 organisms. In many cases the reaction of the soil affects nitrify- 

 ing organisms only indirectly by modifying plant development, 

 which, in turn, affects the soil organisms. 



Because of the limited number of organisms associated with the 

 process of nitrification and the similarity of their characteristics 

 of growth, the soil conditions under which nitrate accumulates 

 are much more restricted than for ammonia formation. Nitrifica- 

 tion is affected much more by modification of soil reaction, 

 aeration, moisture, and salt concentration than the process of 

 ammonia formation is altered by such changes. The treatment of 

 soils with steam or volatile antiseptics, known as processes of 

 partial sterilization of soils, eliminates most of the nitrite- and 

 nitrate-forming organisms, while leaving many of the ammonia- 



