248 SOILS AND SOIL WATER RELATIONS 



2. The Organic Matter of the Soil. — ]\Iost soils contain organic matter 

 which has been derived principally from the partial decomposition of plant 

 residues. Small quantities may also originate from animal residues and excre- 

 tions. The proportion of organic matter present may vary from almost none 

 as in some sand deposits to 95 per cent or better in some peat soils. In ordi- 

 nary agricultural soils the amount present seldom exceeds 15 per cent. In 

 forests organic matter comes from falling leaves, dead branches and trunks 

 of woody plants, roots which die and decay underground, and from dead 

 herbaceous vegetation. In grass^dands the underground roots and rhizomes 

 as well as the aerial parts of the plants all contribute their quota to the or- 

 ganic matter of the soil. In well managed agricultural soils attempts are 

 made to maintain the organic matter content by supplying them with organic 

 fertilizers. 



The organic matter is the seat of most of the micro-biological processes 

 occurring in the soil. One of the most important of these is the oxidation of 

 the organic matter, a process due largely to the metabolic activities of bacteria 

 and fungi, although a limited amount of purely chemical decomposition prob- 

 ably also occurs. Under conditions which are exceptionally favorable for the 

 activities of micro-organisms the organic matter of the soil is oxidized com- 

 pletely and disappears. For this reason the organic matter of soils in tropical 

 regions, particularly when under cultivation, is very low. Even in more tem- 

 perate regions cultivation of the soils generally results in a rapid reduction in 

 organic matter content, due principally to the better aeration induced by 

 tillage. 



As a result of the decay process there is present in most soils organic 

 matter in various stages of decomposition. A large proportion of the organic 

 material which is added to some soils survives in the form of a dark-colored 

 amorphous substance called humus. Humus is composed principally of the 

 degradation products of the cellulose and lignin derived from plant remains. 

 The accumulation of humus in soils is furthered by conditions unfavorable 

 to the oxidative decomposition of organic matter. The decomposition of 

 organic matter in bogs, ponds, swamps, and water-logged soils under condi- 

 tions which are largely if not entirely anaerobic results in the production of 

 relatively large quantities of humus, frequently of the type which is called 

 peat. Where the organic matter supplied is distinctly acid, as under conifer- 

 ous forests, or heath plants, humus usually accumulates as a definite layer at 

 the surface. Decomposition of the organic remains under these conditions 

 is largely effected by fungi. In prairie and steppe regions, where a grassland 

 vegetation is predominant, humus usually accumulates in considerable quan- 

 tities as a result of the decay of both underground and aerial organs of plants. 



