pf 



"**' 



v*^ 



CHAPTER V 

 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 necessary in order that mineral material 

 may become a soil and that it may grow crops successfully. 

 The physical condition of soils depends largely on the pres- 

 ence of organic matter and chemical reaction is greatly ac- 

 celerated by its decay. 



In the process of soil formation the addition of organic 

 materials is more or less a secondary step. In residual debris 

 the amount of organic matter held by the growing soil in- 

 creases 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 organic matter to become 

 incorporated in it. The final result from the mixing of min- 

 erals and their weathered and altered products with the 

 decayed or partially decayed organic matter that is sure to 

 accumulate, is a mass much more complicated than either 

 of the original constituents. The complexity of the average 

 soil has already been sufficiently stressed. 



54. The source of soil organic matter 1 and the char- 

 acter of plant tissue. — The source of practically all soil 



1 The soil organic matter includes not only all compounds contained 

 in the original vegetable and animal tissues but also those existing in 

 the partially decayed portions of such material. Carbon dioxide, methane 

 and like compounds are usually not considered as a part of the soil 

 organic matter. In this respect, the above definition is narrower than 

 that for organic chemistry, which is the chemistry of carbon compounds. 

 For a very good review of literature on soil organic matter, see Morrow, 

 C. A., The Organic Matter of the Soil: A Study of the Nitrogen Distri- 

 bution in Different Soil Types; Dissertation, Univ. Minn., 1918. 



99 



