CHAPTER XV 

 General Principles of Microbial Metabolism 



Metabolism as a whole. To be able to understand the chemical 

 processes taking place in the soil as a result of the activities of micro- 

 organisms and to learn how to control these processes so as to produce 

 conditions which make a soil productive and thus benefit the growth 

 of higher plants, we must understand the metabolism of the various 

 groups of soil microorganisms. The biological changes produced in the 

 soil fall under the class of chemical reactions. However, the biolo- 

 gist is dealing with dynamic phenomena, while the chemist considers 

 chiefly static phenomena. This is the reason why a chemical analysis 

 of a soil is far from sufficient to give us information as to productivity 

 of the soil, or the rapidity with which the nutrients necessary for the 

 growth of higher plants become available. We must consider not 

 only chemical changes as such, but also the course or rate of change. 

 This can be done and the information, subject to a host of variable 

 factors, can be properly interpreted only when the metabolism of the 

 organisms concerned is taken into consideration. 



The metabolism of the leading groups of soil microorganisms can be 

 considered under the transformation of carbon, of nitrogen and of 

 mineral compounds. From the point of view of soil productivity, 

 various groups of soil microorganisms may be considered to play im- 

 portant roles in certain specific transformations, depending on the 

 nature of the organism and nature of the medium. The various 

 transformations in the soil dovetail and, for a proper understanding 

 of the resulting phenomena, metabolism should always be considered 

 as a whole. 



The carbon source is used by the heterotrophic microorganisms as 

 a source of energy and as a source of carbon for structural purposes, 

 namely, for the building up of the microbial cell. In both cases the 

 carbon is required in the form of complex organic compounds, such 

 as carbohydrates, hydrocarbons, fats, fatty acids, proteins and their 

 split products including amino acids and acid amides. Some organisms 

 prefer one group of compounds and some another, while some can 

 utilize a variety of substances as sources of carbon. A great many 



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