Nature and Functions of Humus 



127 



the>' repicstMit a t\pc of organic matter which has a number of com- 

 mon physical, chemical, and biological pi'operties. An appreciation 

 of the chemistry of humus is based largely upon a knowledge of the 

 processes of decomposition of plant and animal residues under 

 natural conditions. 



Fig. 55. Humus podzol (from Thom). 



Nature and Functions of Humus 



Humus possesses certain properties that distinguish it from the 

 plant and animal materials from which it has been formed. These 

 properties of humus can be briefly characterized as follows: it is 

 dark brown to black in color; it is virtually insoluble in water, al- 

 though a part of it may go into colloidal solution in pure water; it 

 dissolves to a large extent in dilute alkali solutions, especially on 

 boiling, giving a dark-colored extract; a large part of this extract 

 precipitates when the alkali solution is neutralized by mineral acids. 



