CHAPTER VIII 

 THE ORGANIC MATTER OF THE SOIL 



ONE of the essential differences between a soil and a 

 mass of rock fragments lies in the organic content of the 

 former. Organic matter is a necessary constituent in 

 order that finely ground mineral material may be desig- 

 nated as a soil and that it may grow crops successfully. 

 Physical condition depends largely on the presence, 

 and chemical reaction is greatly accelerated by the de- 

 cay, of organic matter. In the process of soil formation 

 its addition is more or less a secondary step. In residual 

 debris the amount of organic matter held by the growing 

 soil increases as the process of weathering goes on ; in 

 glacial soils, however, the matrix, or skeleton of the soil, 

 is already formed before there is an opportunity for humus 

 to become incorporated therein. The final result from 

 the mixing of the minerals carrying numerous weathered 

 and altered products with the decayed or partially de- 

 cayed organic matter that is sure to accumulate, must 

 be a mass much more complicated than either of the 

 original constituents. It is hardly necessary to further 

 emphasize the complexity of the average soil, the reasons 

 therefor, and the difficulties in studying the question. 



88. The source and distribution of organic matter. - 

 The source of practically all soil organic matter is plant 

 tissue. Some of this matter accumulates from the above- 

 ground parts of plants that have died and fallen down 



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