CHAPTER XXIV 



The Soil as a Medium for the Growth and Activities of 

 Microorganisms 



To be able to understand the processes resulting in the soil from the 

 activities of microorganisms, we must understand the nature of the 

 medium, the soil, in which these organisms live, act and reproduce. 



The soil as a culture medium. The soil is a complex system consisting 

 of (a) mineral particles formed as a result of mechanical and chemical 

 decomposition of the various mineral constituents of native rocks; 

 (6) organic matter of plant, animal and microbial origin, in the process 

 of decomposition or resisting further decomposition; (c) compounds 

 formed as a result of interaction of substances produced in the decom- 

 position of organic matter with materials resulting from the disinte- 

 gration of the inorganic soil complexes; (d) soil moisture, containing in 

 solution C0 2 , oxygen and other gases, and various organic compounds 

 and inorganic salts; and finally (e) soil atmosphere. 



The soil is regarded 1 as a mineral framework, the particles of which 

 are coated with a jelly-like layer of organic and inorganic materials 

 present in a colloidal condition, the soil solution being present partly 

 in the colloidal layer and partly in a free condition. The microbial 

 population of the soil lives largely in the colloidal layer and partly in 

 the soil solution. The activities of the microorganisms living in the 

 soil as a medium are affected by the nature of the different ingredients 

 of the medium and by the various conditions influencing them. 



The complex medium supplies to the various organisms inhabiting 

 it the necessary nutrients, of an organic and inorganic nature, con- 

 sisting of the elements oxygen, hydrogen, carbon, nitrogen, phosphorus, 

 sulfur, potassium, magnesium, calcium and iron; and, to a less extent, 

 chlorine, silicon, sodium, aluminum and manganese. The availability 



1 Page, H. J. The part played by organic matter in the soil system. Trans. 

 Faraday Soc, 17: 272-287. 1922. Bouyoucos recently called attention to the 

 fact that the colloids in the soil do not exist entirely as a coating around the 

 soil grains, but are also independent components; sand particles may not be 

 covered with colloidal gel (Soil Sci., 21: 481^88. 1926.) 



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