34 Microbiological Population of the Soil 



the soil is far from constant, that it undergoes a number of variations 

 and changes which may be seasonal and even more frequent, and 

 that these changes are subject to a variety of influences. Thus the 

 many discrepancies in the interpretation of results obtained by dif- 

 ferent investigators, using different methods, were due more fre- 

 quently to the procedure employed than to underlying fundamental 

 differences in the soil population. The depth of the soil was found 

 (Tables 2 and 3) to influence not only the total number of organ- 

 isms, but especially the distribution of the various constituent groups. 



Table 2. Distribution of Microorganisms in the Different Horizons of a 

 PoDZOL Soil Profile (from Gray and Taylor) 



Microorganisms in thousands per gram dry soil. 



Table 3. Bacteria and Actinomycetes at Various Depths of Soil 

 (from Waksman) 



Numbers in thousands per gram, determined by plate method. 



Composition of the Soil Microbiological Population 



As has been pointed out, most of the investigators who devoted 

 themselves, as late as the beginning of this century, to the study 

 of occurrence and activities of soil microorganisms looked upon the 

 bacteria as responsible for the most important, if not all, soil processes. 

 A mere reading of the papers dealing with soil microorganisms, 

 published during the final decade of the last century and the first 

 decade and a half of this century, will tend to prove this generaliza- 



