136 



Humus: Nature and Formation 



Fig. 59. 



Slight Medium Good 



Degree of decomposition of peat 



Decomposition of plant constituents in the jjrocess of peat formation 

 (from Maliutin). 



vegetable remains. The surface layer, or litter, is made up of partly 

 decomposed leaves, needles, roots, twigs, cones, and other tree 

 residues. This layer is superimposed on another layer of partly 

 decomposed plant residues and is underlain by a third layer of 

 thoroughly decomposed material which is said to be completely 

 humified. The total organic layer of the surface of forest soil is 0.5 

 to more than 6 inches deep. In evergreen forests, the largely organic 

 surface layers are usually not mixed with the inorganic soil layers; 

 the former are referred to as the "raw hiuuus" or "duff." In decidu- 

 ous forests, the organic residues and their decomposition products 

 are well mixed with the inorganic part of the soil, giving rise to a 

 type of soil known as "mull." This soil is less acid and is more active 

 biologically. 



