54 THE SOIL POPULATION AND ITS DISTRIBUTION 



organisms as are capable of developing upon the media which are 

 used. Since the various microbes form typical colonies, the 

 method allows the determination of the abundance of specific 



Fig. 40. — Colonies of bacteria and actinomyces (below), and fungi (above), 

 developing on plates as used in counting these organisms in soils. 



types of organisms in the soil. Each medium is selective for cer- 

 tain groups of microbes, and each set of conditions under which the 

 organisms are allowed to develop upon the medium is selective for 

 certain organisms. A medium containing proteins will favor the 



