128 Humus: Nature and Formation 



Certain constituents of humus may also dissolve in acid solutions 

 and may be precipitated at the isoelectric point, which is at pH 4.8. 



Chemically, humus contains a somewhat larger amount of carbon 

 than do plant, animal, and microbial bodies; the carbon content of 

 humus is about 55-60 per cent, usually averaging 58 per cent. 

 Humus contains considerable nitrogen, from 3 to 6 per cent; these 

 figures may frequently be lower, as in certain highmoor peats, which 

 contain only 0.8-1.0 per cent nitrogen; they may also be higher, 

 especially in certain subsoils, where they may reach 10-12 per cent. 



Humus contains the elements carbon and nitrogen in a ratio which 

 is close to 10:1; this is true of many soils and of the humus in sea 

 bottoms. This ratio varies somewhat with the nature of the humus, 

 the stage of its decomposition, the nature and depth of soil from 

 which it has been obtained, and the climatic and other environ- 

 mental conditions under which it has been formed. Humus is not 

 chemically static or nonvariable, but is rather in a dynamic condition, 

 since it is constantly formed from plant and animal residues and is 

 continuously decomposed further by microorganisms. Humus serves 

 as a source of energy for the development of various groups of 

 microorganisms, and as a result of its decomposition a continuous 

 stream of carbon dioxide and ammonia is given off. 



Humus possesses a high capacity for base exchange, the ability 

 to combine with various other inorganic soil constituents, to absorb 

 water, and to swell. It is also characterized by other physical and 

 physicochemical properties that make it a highly valuable constitu- 

 ent of natural substrates, such as soils, which support plant and 

 animal life. 



The importance of humus in the soil is manifold: it serves as a 

 source of nutrients for plant growth; it modifies, in various ways, the 

 physical and chemical nature of the soil; it regulates and determines 

 the nature of the microbial population and its activities, by supply- 

 ing various organic and inorganic nutrients essential for growth of 

 these organisms and by making the soil a more favorable substrate 

 for their development. An abundance of humus in the soil is 

 practically equivalent to a high rate of fertility of the soil. Humus 

 characterizes the soil type, since differences in its origin, abundance, 

 and chemical nature result in the development of a particular type 

 of soil. 



Humus may be looked upon as a storehouse of important chemical 

 elements essential for plant life, especially of carbon and nitrogen, 

 and to a less extent of phosphorus, calcium, magnesium, iron, man- 



