Humus in Different Soils 143 



ht'CMi brought iimlcr tultixatioii. Light-textured soils eontuin less 

 organie matter than soils of heavier texture. Typieal analyses oi 

 sandy loams ga\e, on an a\erage, 0.06 per cent nitrogen, which is 

 equi\alent to 1.03 per cent organic matter; clay loams in the same 

 series of soils were found to contain 0.1 per cent nitrogen or 1.72 

 per cent organic matter. The stage of development of a soil influ- 

 ences considerably its organic matter content. 



Humus is not distributed evenly through the soil depth. It is 

 largely concentrated in the upper 10 or 12 inches, or the surface soil 

 layer, and decreases rapidly in the subsoil. In brown forest soil and 

 in the red and yellow soils, there is a rapid drop in organic matter 

 content below the upper 6 inches; it becomes \'ery low at compara- 

 tively shallow depths. In prairie soils, chernozems, and chestnut- 

 brown soils, there is a gradual decrease of organic matter with depth. 



It has already been emphasized that the accumulation of humus or 

 its further decomposition depends entirely upon the \'egetation, the 

 soil, its topography, and the environmental conditions, especially the 

 climatic, under which humus has been produced. These are the 

 major factors which influence the abundance and nature of the 

 humus. In heavy soils, especially those rich in lime, humus will be 

 fixed and will tend to decompose only \ery slowly. In light sandy 

 soils, humus will not accumulate to so great an extent. Under favor- 

 able conditions, humus will decompose more rapidly; under certain 

 conditions, it may also be readily leached out, as illustrated by the 

 process of podzolization. 



The nature of the vegetation has a marked effect upon the accumu- 

 lation of humus. In pasture and prairie soils, where the land is cov- 

 ered by a growing crop and is not cultivated, the gradual disintegra- 

 tion of the roots in a soil environment lacking suflBcient oxygen will 

 give rise to a considerable amount of humus. If this soil is high in 

 lime, the humus produced will be fixed and will gradually accumu- 

 late. This results in the course of time in a type of soil which is 

 designated as "chernozem." In forest vegetation, the annual drop of 

 lea\es, needles, and other tree residues will bring about an extensive 

 accumulation of organic matter on the surface of the soil. If this 

 organic matter is gradually worked in with the underlying mineral 

 part of the soil, a phenomenon brought about largely through the 

 activities of the earthworms, insects, and other animals inhabiting 

 some of these soils, a type of soil will result which is designated as 

 "mull"; this soil is characterized by one type of humus. If the tree 

 residues are not mixed with the underlying mineral layers, however. 



