ORGANIC MATTER. COMPOSITION 121 



of decay. Leaf-mold is the partially decomposed layer 

 of leaves, twigs, etc., found on the surface of the ground, 

 usually in well-drained forest areas. Decomposition 

 is very incomplete. Humus is the black or brown pul- 

 verent material resulting from a considerably more 

 advanced stage of decay than is represented by leaf- 

 mold. When wet, it forms a very fine, gelatinous mass 

 of a colloidal nature. Peat represents large and usually 

 deep accumulations of plant remains in the early 

 stages of decay. Disintegration has usually been stopped 

 by the saturation of the mass. The products of bacterial 

 and fungicidal action have accumulated, until the 

 organisms are killed, and any further growth is pre- 

 vented until the water is removed and more thorough 

 aeration is introduced. Plant tissues are plainly evident. 

 When the peat results from a particular kind of plant, 

 the name of the latter may be affixed as moss peat. 

 Peat is generally unproductive as a soil. Muck repre- 

 sents a much more advanced stage in the decay of peat. 

 It has a black or brown color, more closely resembling 

 humus, due to the large proportion of the latter which 

 it contains. Plant tissues are much less apparent. It 

 is generally productive, or will very quickly become so 

 under drainage and cultivation. 



53. Chemical composition. There is no definite 

 chemical composition of the organic matter in the soil. 

 It is as variable as the materials from which it is derived 

 and the conditions under which it is formed. It is 

 composed of a great variety of carbon compounds, 

 into which enter nitrogen and all of the mineral ele- 

 ments which are necessary to plant and animal growth. 



