266 Microbes and You 



cally, that it would be impossible to go into adequate detail in order 

 to give the student a complete and clear idea of what transpires in 

 this medium hour after hour, year on end. A few of the funda- 

 mental concepts relative to soil microbiology can be presented, 

 however. 



The very food we eat comes directly or indirectly from the 

 soil. We may grow crops to be consumed directly, or we may 

 feed these crops to lower animals which in turn furnish us with 

 meat, hides, wool, and many other useful items. 



Too often the layman looks upon the soil as an inert aggrega- 

 tion of minerals and organic matter, but in reality soil is dynamic 

 —changing every second as microscopic and macroscopic organ- 

 isms act upon available food materials and upon each other. The 

 better medium that the soil provides, the higher will be the bac- 

 terial count, and there is a direct relationship between bacterial 

 count and soil fertility. It is just as important for the soil to ful- 

 fill the prerequisites of a good medium as it is for a bacteriologist 

 to compound foods which meet the needs of the organisms he is 

 attempting to cultivate in the laboratory. Unless temperature and 

 oxygen supply are properly controlled, however, an otherwise 

 satisfactory medium may not support the growth of the desired 

 organisms. When a farmer drains, plows, limes, and fertilizes a 

 soil, he may not realize all of the technical reasons involved, but 

 he is, in effect, trying to create a favorable environment for the 

 cidtivation of organisms. 



Because of the diversity of life found in rich soils, no single 

 diet can be expected to supply all of the requirements of so many 

 different appetites. Most soil bacteria prefer complex food mate- 

 rials which they can attack (analyze) with the aid of enzymes; 

 such bacteria are known as heterotrophs. Another group of or- 

 ganisms, AUTOTROPHS, demand simple food elements which they 

 can build up (synthesize) to suit their peculiar requirements. 

 Waste products excreted by one organism may be sought as food 

 by still other bacteria. Little or nothing is wasted in the soil. 



When bacteria attack complex organic matter, the presence of 

 oxygen is an important prerequisite for insuring oxidation of or- 



