CHAPTER III 

 THE SOIL POPULATION AND ITS DISTRIBUTION 



The Occurrence of Microbes in Soil. — Because of the 

 fact that microbes are so small in size and that many of them 

 are capable of growing on a variety of foods in a wide range of 

 environments, they are found universally distributed in nature. 

 They are present in great abundance in water, in various food- 

 stuffs, upon all growing and dead plants, in the digestive tracts 

 of animals, upon dust particles, and in soil. Because of their 

 great abundance in nature and the ease with which they are dis- 

 tributed by means of air, water, moving animals, living and dead 

 plants, as well as through man's activities, it is reasonable to 

 assume that in soils of the same composition in identical environ- 

 ments the numbers and abundance of types of microorganisms 

 should be nearly identical. 



However, microbes do not develop actively upon all sub- 

 strates where they may be located. The mere fact that many 

 bacterial cells are found upon clothing and dust does not indicate 

 that they grow and develop upon these substrates; it merely 

 indicates that clothing and dust are carriers of these microbes 

 and that the organisms can be distributed in nature through 

 these vehicles. Likewise certain microbes may be found in the 

 soil merely because they have been introduced there by dust, 

 waters, or through man's activities, as in the application of 

 manures of domesticated animals. The fact that a certain microbe 

 is found in the soil is no proof that it is a normal inhabitant of the 

 soil. The presence of large numbers of Bacterium coli in a soil 

 may merely indicate that this organism has recently reached the 

 soil in animal excreta and not that it finds the soil a favorable 

 medium for its development. 



One should thus differentiate between microbes found in the 

 soil as a result of their fortuitous introduction and those which 

 lead a normal existence in the soil. If conditions are not favor- 



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