2- . 



The Microbiological Population 

 of the Soil as a Whole 



Composition of the Soil 



The soil represents a medium or substrate in which numerous 

 microorganisms Hve and bring about a great variety of processes 

 which are responsible for continuation of the cycle of life in nature. 

 A normal soil is made up of solid, liquid, and gaseous constituents. 

 These can be broadly divided into five groups: 



1. Mineral Particles. These vary greatly in size and in the de- 

 gree of their mechanical and chemical disintegration. They include 

 particles ranging from large pebbles to fine sand, clay, and silt. 



2. Plant and Animal Residues. These comprise the freshly fallen 

 leaves and other plant stubble and dead remnants of a variety of 

 insects and other animal forms. Some of the materials are largely 

 undecomposed; still others are partly or thoroughly decomposed, so 

 that the original structure can no longer be recognized. In the last 

 state they are spoken of as humus or humified materials. 



3. Living Systems. These include the living roots of higher plants; 

 the great number of living animal forms, which range from protozoa, 

 insects, and earthworms to rodents, as well as the numerous algae, 

 fungi, actinomycetes, and bacteria. 



4. Water. The liquid phase of the soil, comprising both free and 

 hygroscopic water, contains in solution varying concentrations of in- 

 organic salts and certain organic compounds. 



5. Gases. The soil atmosphere consists of carbon dioxide, oxygen, 

 nitrogen, and a number of other gases in more limited concentrations. 



The composition of a typical podzol soil is shown in Table 1. 

 The microbiological population which inhabits the soil, together 

 with the roots of higher plants and with animal forms, makes the 



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