THE ORGANIC MATTER OF THE SOIL 



127 



to become mixed with the surface soil; the remainder 

 is a result of root extension and subsequent decay. The 

 organic matter of the surface soil is derived from the 

 tops and the roots of plants growing on it, while that of 

 the subsoil is very largely a result of root extension and 

 subsequent decomposition. The relationship between the 

 humus content of three soils and the roots developed is 

 shown by the following data presented by Kostytscheff 1 

 and quoted by Hilgard 2 and Wollny 3 : — 



Root Content and Percentage of Humus in Three 

 Russian Soils 



89. Composition of plants. — It is usual, in classifying 

 the materials composing plant tissue, to group them under 

 three heads — carbohydrates, fats and oils, and proteins. 



1 Kostytscheff, M. P. Les Terres Noires de Russie. Annales 

 de la Sci. Agron., Tome II, pp. 165-191. 1887. 



2 Hilgard, E. W. Soils, p. 130. New York. 1906. 



3 Wollny, E. Die Zersetzung der Organischen Stoffe, p. 194. 

 Heidelberg. 1897. 



