4 PRINCIPLES OF SOIL MICROBIOLOGY 



the bacteria; actually they belong to the fungi and are so far known 

 to be represented in the soil by one extensive genus Actinomyces. 



Bacteria predominate, in numbers and in the variety of activities, 

 over all the other groups of microorganisms. This was the reason why 

 the earlier microbiologists named the whole science of soil microbiology 

 "soil bacteriology." It has long been recognized, however, that the 

 soil population consists of various microorganisms other than bacteria, 

 so that the more comprehensive term is fast coming into general use. 

 Since the bacterial activities in the soil do not coincide with their 

 taxonomic groupings, these organisms may be classified on the basis 

 of their physiology for the sake of convenience in treatment. As a 

 major division, the bacteria can be separated into two large groups: 



Chart I. The microflora and microfauna of the soil 



(1) autotrophic, and (2) heterotrophic forms. Living organisms that 

 require for their nutrition substances which have been built up by other 

 organisms are called heterotrophic. The heterotrophic saprophytic 

 bacteria consume, for their energy and for the building up of their 

 protoplasm, the organic compounds of plant and animal bodies. Organ- 

 isms like the green plants and certain bacteria that can thrive on purely 

 inorganic substances and obtain their carbon from the carbon dioxide 

 of the atmosphere are called autotrophic. But while the green plants 

 derive their energy photosynthetically, the autotrophic bacteria derive 

 their energy from the oxidation of purely inorganic substances, or 

 chemosynthetically. The autotrophic group of bacteria is represented 

 in the soil by smaller numbers and by much fewer species than the 

 heterotrophic group, but it includes forms which are of greatest impor- 



