40 Microbiological Population of the Soil 



bers may represent only a very small fraction of the total soil popula- 

 tion, such as 1 per cent or even less. This is due to the fact that 

 many organisms actually present in the soil in a living state are 

 unable to develop in the artificial culture media employed for their 

 evaluation. In some cases, very special procedures have to be de- 

 vised before one is able to determine even the presence of a certain 

 organism in the soil. 



On the other hand, the microscopic methods may give a highly 

 exaggerated picture of the abundance of the microbiological popula- 

 tion, since the counts may include not only living but also dead cells 

 of various organisms. Hence counts of 1-10 billions of bacteria per 

 gram of soil (Table 5), as frequently reported by use of these 



Table 5 Microorganisms in 1 gm of Soil as Determined by the Direct 

 Microscopic Method (from Richter) 



Numbers in thousands per gram. 



Bacteria 



methods, may not be fully correct. These and similar methods may, 

 therefore, also be open to criticism, since they do not give an ac- 

 curate picture of the soil microbiological population. 



Whatever the methods used, however, for e\'aluating the num- 

 bers of soil microorganisms, it may well be recognized that the actual 

 living mass of microorganisms in the soil is considerable. Most of 

 the organisms found in the soil are indigenous members of the 

 population. Some are found there because they seek shelter and 

 protection beneath the soil surface. Others are carried into the soil 

 by dead and dying plants and animals, by wind, or by rain, to live 

 or to die there. Numerous groups of organisms pass their whole 



