THE SOIL AS A CULTURE MEDIUM 625 



ganic matter may be too slow and the farmer may use drainage, tillage, 

 liming and manuring to intensify aeration. Insufficient aeration favors 

 reduction processes. 



The mineral composition of the soil. While nitrogen and, to a large 

 extent, sulfur are present in the soil almost entirely in organic form and 

 the carbon utilized by the heterotrophic organisms is also of complex 

 organic nature, the other elements are largely or entirely of inorganic 

 origin. Phosphorus exists in the soil as apatite (Ca 5 (P0 4 )3-Cl or 

 Ca 5 (P0 4 ) 3 -F), calcium phosphate (Ca 3 (P0 4 ) 2 ), iron and aluminum phos- 

 phates, as well as in various organic combinations. Potassium occurs 

 in the soil in orthoclase and microcline feldspar (KAlSi 3 8 ), in mus- 

 covite mica (KH 2 Al 3 Si 3 0i 2 ) and in hydrated and non-hydrated alum- 

 inum silicates and to a less extent in organic combination. Calcium 

 exists in the soil in various minerals such as calcite, plagioclase, 

 feldspar, hornblende and augite, and in absorbed compounds with 

 kaolinite, etc. 



Table 65 shows the composition of some typical American soils com- 

 pared with the composition of the lithosphere. 12 



It is only seldom that the mineral constituents become limiting fac- 

 tors to the development of microorganisms in normal soils. These 

 minerals influence the activities of the organisms by modifying the soil 

 reaction, the concentration of the soil solution and by serving as direct 

 nutrients and in some cases even as sources of energy. 



The physico-chemical role of organic matter in the soil. Organic matter 

 plays a manifold role in the soil. Most soil microorganisms find in it 

 their source of energy and other nutrients. Its influence on soil texture 

 is of especial importance. Organic matter helps to loosen a clay soil 

 and add body to a sandy soil. It is best to apply only undecomposed 

 organic matter to heavy soils, since the large quantities of C0 2 pro- 

 duced in the process of decomposition tend to make the heavy soil porous. 

 However, it is best to subject the organic matter first to partial de- 

 composition before applying it to sandy soils, so as not to make the 

 soil too open. The colloidal "humus" seems to have a cementing ef- 

 fect upon the coarser soil particles. It also exerts a protective effect 

 upon clay, so that greater concentrations of electrolytes are required 

 for its flocculation. 



12 Robinson, W. O. The inorganic constituents of some important American 

 soils. U. S. Dept. Agr. Bui. 122. 1914. Methods of chemical and mechanical 

 soil analysis are discussed by Russell and others (see list of texts). 



