PLANT NUTRITION 253 



has disappeared. Hiinuis is charactei i/ed as an amorphous, dark- 

 colored, nearly odorless iiKilerial, lia\ing no definite chenn'cal composi- 

 tion. 



The carbon content ol hunuis, ahoui 55 to 58 per cent, is greater 

 than the carbon content ol the original plant and animal matter from 

 which it is formed. 1 he nitrogen content of humus is variable, usu- 

 ally falling in the range of 3 to G per cent. The ratio of carbon to 

 nitrogen is therefore between 20 to 1 and 10 to I, depending on the 

 kind of organic matter from which it was derived, the stage of decom- 

 position, the nature and depth of the soil, and the climatic conditions 

 imder which it was formed. 



Since humus originates from the decomposition of ])lant, animal, 

 and microbial residues, compounds are found in it which are similar 

 lo, or tlerived from, the compounds of the parent material. Thus sig- 

 nificant amounts of the following four groups of compounds are 

 found: (1) Lipides and related materials. Fats, waxes, higher alcohols, 

 resins, sterols, organic acids, and pigments are found in rather small 

 amounts. (2) Carbohydrates and their derivatives. Polysaccharides 

 such as cellulose, pentosans, hexosans, polyuronides, glycosides, starch, 

 and sugars are present in varying amounts. (3) Proteins and other 

 nitrogen compounds. This group of compounds, which includes pro- 

 teins, proteoses, peptones, amino acids, amines, purines, and py- 

 rimidine bases, is present in fairly large amounts. (4) Lignin (see 

 page 186) and its derivatives. Lignin is one of the most stable com- 

 pounds found in plants. It constitutes 25 to 30 per cent of the wood 

 of trees and 15 to 20 per cent of straw. Lignin is present in large 

 amounts in humus. Thus the two major groups of compounds found 

 in humus are proteins and lignins. Probably most of the humus 

 found in soils results from a combination of lignin with bacterial 

 proteins and is commonly known as a lignoproteinate. 



Humus is nearly insoluble in water, but a part of it is soluble in 

 dilute alkali solutions. Many of the important properties of humus 

 depend on the fact that, alone or with inorganic substances of the 

 soil, it forms complex colloidal systems. The properties and impor- 

 tance of soil colloids will be discussed later. 



Two important groups of humus substances have been recognized: 

 (1) nutrient or temporary humus, which is easily broken down and 

 which serves as a source of carbon for microorganisms; (2) persistent 

 or maintenance humus, which is slowly converted to other substances 

 and forms the important organic colloidal material of soil. 



Organic matter has several important functions in soils. It serves as 

 a substrate for the growth of microorganisms and improves the physi- 



