♦/2-. 



Associative and Aiita<>:()iiistic Effects 

 of Soil Microors>:aiiisins 



Microorganisms live in the soil, not in the form of pure cultures, 

 hut as complex populations. Each particle of soil, no matter how 

 small, contains more than one type of organism. Many of these 

 organisms depend upon one another for direct and indirect nutrients; 

 some compete with one another for energy sources and for the ele- 

 ments and compounds used as nutrients. This results in the forma- 

 tion of numerous associations among the soil microorganisms in 

 which \'arious relationships exist, some favorable to one another 

 and others injurious. The abundance, in the complex soil population, 

 of each type of organism, the rate of its multiplication, and its physio- 

 logical activities are greatly influenced by the presence and abun- 

 dance of other organisms. 



What Is a Soil Microbiological Population? 



The quantitative and qualitative composition of a complex popu- 

 lation is controlled by the nature and availability of the nutrients; 

 the physical, chemical, and biological nature of the habitat; and the 

 environmental conditions, especially aeration, temperature, and mois- 

 ture supply. This is also true of the soil microbiological populations 

 (Table 58). The examination by suitable methods of a sandy or 

 clay soil, which is as free from organic materials as possible, will 

 reveal a microbiological population that is very limited in numbers 

 and types. Snow, for example, made a study of the abundance of 

 microorganisms in wind-blown soils. She found as few as 17,000 

 organisms per gram of soil containing about 0.3 per cent organic 

 matter; these organisms were largely bacteria, together with some 

 (10-15 per cent) actinomycetes and a few (0.56-2.0 per cent) fungi. 

 Another soil with 0.45 per cent organic matter gave, on the average, 



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