144 Humus: Nature and Foiination 



a soil will result which is designated as "raw humus"; this may be 

 due to the nature of the vegetation and type of soil. The surface 

 layer of the raw humus soil may undergo considerable leaching, 

 whereby certain of its organic and inorganic constituents are re- 

 moved and are reprecipitated in the subsurface; a type of soil is thus 

 formed which is designated as a "podzol." 



A comparison of the chemical nature of the humus in different 

 soil types shows that, in warmer regions, the carbon-nitrogen ratio 

 of the humus tends to become narrower. This is due to a greater 

 decomposition of the carbohydrates and other nonnitrogenous con- 

 stituents of the humus and the greater accumulation of the proteins, 

 the latter originating largely through microbial synthesis. 



The two major factors which control the abundance and nature 

 of the humus in different soil types are, first, the formation and 

 accumulation of the humus, as influenced by nature of vegetation, 

 type of soil, aeration of soil, reaction, and environmental factors, 

 especially temperature and rainfall; and, second, the decomposition 

 of the humus, as controlled by the treatment of the soil, especially 

 drainage and cultivation, farming practices, and climate. 



Humus and Soil Fertility 



Among the various factors which contribute to the fertility of 

 the soil, none occupies a more prominent place than soil organic 

 matter. It has a fourfold effect upon the soil: 



1. It serves as a storehouse of plant nutrients: the slow but gradual 

 decomposition of the organic matter by microorganisms results in 

 the liberation of a continuous stream of carbon dioxide; of available 

 nitrogen as ammonia, which is soon changed to nitrate; of phos- 

 phorus; and of other elements essential for plant growth. 



2. It has important physical effects upon the soil: it improves the 

 soil structure; it provides better aeration; it has a binding effect 

 upon the soil particles; it increases the water-holding capacity of the 

 soil; it helps the soil to absorb more heat; it increases the buffering 

 properties of the soil, preventing rapid increases in acidity or 

 alkalinity. 



3. It has certain chemical effects upon the soil constituents, such 

 as rendering phosphorus and other elements more soluble, and 

 neutralizing substances which tend to be toxic to plants; it has also 

 a very high base-holding power. 



