126 Humus: Nature and Formation 



field, orchard, and garden soil humus, of pasture soil humus; of forest 

 humus, comprising not only the litter, but also the underlying humus 

 layers; of highmoor, lowmoor, and forest peat humus; of composts 

 produced from stable manures and from other farm residues; of 

 sewage sludge and garbage humus; and of water humus or marine 

 humus, that form of organic matter which is found in flowing and 

 in standing fresh- or salt-water basins. 



Some plant residues decompose very rapidly in the soil but leave 

 comparatively little humus, whereas others decompose much more 

 slowly and leave large amounts of humus. Not only the nature of 

 the plant material, but also its age or degree of maturity influences 

 the rapidity of its decomposition. Among the soil factors that influ- 

 ence formation of humus from the plant residues, the mechanical 

 composition of the soil; its physical conditions, notably its texture; 

 and its chemical properties, especially reaction and presence of avafl- 

 able nutritive elements, are most important. The formation and 

 nature of humus are also influenced by the system of crop rotation, 

 by fertilizer treatment, by utilization of green manures, by abun- 

 dance of animals on the farm, by climatic conditions, and by other 

 factors. 



Humus may be considered the more or less final and stable product 

 into which some of the plant and animal residues are transformed in 

 the process of decomposition. Humus is not an absolutely resistant 

 product, since it is also decomposed, but only very slowly, in the soU. 

 The rate of its destruction, however, is far less than that of the plant 

 and animal materials from which it originated. 



Humus thus represents a natural organic system, in a state of a 

 more or less dynamic equilibrium. Since humus originates from 

 plant, animal, and microbial residues, its composition depends upon 

 the chemical nature of these residues. Since humus is formed as a 

 result of various decomposition processes, its composition will also 

 depend upon the microorganisms concerned in the decomposition of 

 the residues, and upon the conditions under which this process takes 

 place. Because of these factors, a number of humus types are pres- 

 ent in nature. One is thus able to difterentiate between the humus 

 of lowmoor and of highmoor peats; between the humus in conifer- 

 ous and deciduous forests; between the humus found in mineral soils, 

 notably of the podzol, chernozem, and gray-desert types; and be- 

 tween the humus present in lake and in marine bottoms. Although 

 all these forms of humus vary markedly in chemical composition. 



