50 THE SOIL POPULATION AND ITS DISTRIBUTION 



ents will be more accurately appraised and their activities will be 

 more profitably utilized. 



Summary of Methods. — The present methods for studying 

 soil microorganisms can be listed as follows: 



(1) Those methods which are used for the determination of 

 the number of microorganisms in the soil as well as the specific 

 nature of some of these organisms. These methods can be 

 conveniently subdivided : 



(a) Direct microscopic methods. 



(6) Cultural methods, based upon the development of 

 microorganisms upon solid media in plates or in tubes. By 

 adjusting the concentration and nature of specific nutrients, 

 such as the energy source, nitrogen supply, and other inor- 

 ganic salts, by modifying the hydrogen-ion concentration and 

 air supply, these media can be prepared so as to allow the 

 development of either large numbers of a variety of microbes, 

 or only representatives of specific groups. 



(c) Cultural methods, based upon microbial growth or 

 upon a specific chemical transformation brought about by 

 microbial development in a medium containing a specific 

 chemical substance, after inoculation with high dilutions of 

 soil. Either liquid or solid media are employed for such use. 



(2) Methods which are devised primarily for obtaining 

 information about the activities of the total soil flora and fauna, 

 or of only one specific transformation, irrespective of the nature 

 of the organisms concerned. 



(3) Methods for determining the availability in soil of 

 elements essential for plant growth. These will be discussed 

 in Chapter IX. 



Direct Microscopic Methods. — The direct microscopic 

 methods have been devised with the idea of determining the total 

 abundance of organisms in the soil in order to supplement cultural 

 methods which are selective. A small particle of soil or a dilute 

 soil suspension is first mixed with a dilute solution of agar or gela- 

 tin, dried and fixed upon a slide, and then stained with acid dyes 

 such as rose bengal or erythrosine. The bacteria can be distin- 

 guished and counted with the aid of a microscope, since they are 



