CHAPTER XXVI 



Transformation of Organic Matter in the Soil 



Nature of soil organic matter. The organic matter of the soil comprises 

 a great mass of substances of plant, animal, and microbiological origin. 

 Some of these substances are undecomposed and include the living or 

 recently dead roots of plants and microorganisms, others are ingredients 

 of the original organic matter resistant to decomposition, others are 

 intermediary substances formed from the original constituents of the 

 organic matter, and the remainder are products synthesized by the 

 activities of the soil microorganisms. Soil organic matter is, therefore, 

 complex in composition and varies according to the original materials, 

 extent of their decomposition, environmental conditions under which 

 the processes of decomposition are taking place, and the organisms con- 

 cerned in these processes. 



The natural materials which contribute chiefly to the formation of 

 the soil organic matter are plant roots, branches, stems, leaves, plant 

 seeds, dead bodies of all sorts of plants and animals, manures, and the 

 bodies of microorganisms. These natural organic substances consist of 

 proteins and other nitrogenous compounds, of carbohydrates (celluloses, 

 hemicelluloses, compound celluloses, starches, pectins, di- and mono- 

 saccharides, glucosides) and their derivatives, of fats, waxes, lignins, 

 tannins, resins, alkaloids and mineral matter. However, these various 

 constituents of the original organic matter are found in the soil only to a 

 limited extent. Most of the organic materials actually found in the 

 soil consist of substances more or less resistant to decomposition and, 

 to a less extent, of materials undergoing decomposition. Quantita- 

 tively and qualitatively, this organic matter depends upon the activi- 

 ties of the soil microorganisms, which are in their turn influenced by 

 environmental soil conditions including moisture, aeration, soil reaction, 

 and presence of available nitrogen and mineral elements. 



The average content of organic matter in the soils of the United States 

 is 2.06 per cent and of the subsoils 0.83 per cent. Very little is known 

 concerning the chemical composition of this organic matter, although 

 a large number of chemical substances have been isolated from the soil. 



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