THE ORGANIC MATTER OF THE SOIL 121 



It is evident that linmid soils not only contain tlie greater 

 amounts of organic matter, but also excel in humus. The 

 humus of the arid regions, however, is richer in nitrogen, due 

 to the character of the decomposition going on. As a conse- 

 quence the nitrogen in the soil of humid regions is not greatly 

 in excess of that in the soils of drier climates. The percentage 

 of humus not only decreases in the lower depths of the soil, 

 but also changes in composition, becoming poorer in nitrogen 

 the deeper the soil. 



64. The influence of organic matter on the soil. — The 

 effects of organic matter on soil and plant conditions are as 

 numerous as they are complex. Some of the influences are 

 direct, others are indirect. As the specific gravity of organic 

 matter is low, the first effect of its addition would be to lower 

 the specific gravity of the soil. The organic matter tends also 

 to spread the individual particles of soil farther apart, especi- 

 ally in a clay. Such action will markedly influence the volume 

 weight. 



The loosening effects of organic matter are especially ap- 

 parent in such soil as clay* On the other hand, because or- 

 ganic matter has a higher cohesive and adhesive power than 

 sand, it performs the function of a binding material with tlje 

 latter soil, a condition much to be desired in a material pos- 

 sessing such loose structure. 



As the water capacity of organic matter is very high, a soil 

 rich in organic constituents usually possesses a high water- 

 holding power. This makes possible greater volume changes 

 both on drying and in the presence of excessive moisture. The 

 granulating effects of wetting and drying and freezing and 

 thawing are, therefore, accelerated. The increased water ca- 

 pacity of the soil, resulting from the presence of organic ma- 

 terials, is of great importance in drought resistance, while 

 the black color imparted by the humus tends to raise the 

 heat absorptive power of the soil. 



The better tilth induced by the presence of organic matter 



