146 



SOIL PHYSICS AND MANAGEMENT 



becomes very slow, and still later ceases almost entirely. The supply 

 of oxygen is somewhat low in the soil and the conditions are not 

 favorable for complete oxidation of the vegetable matter. The 

 partial oxidation produces organic matter of varied composition. 

 In this change the hydrogen and oxygen content of the vegetable 

 matter becomes less, while the proportion of carbon and nitrogen 

 increases. The organic matter of the soil under different conditions 

 may contain from three to twenty times as much nitrogen as the 

 original material. This change may be carried so far that ultimately 

 carbonized material may be produced that is similar to coal or char- 



FIG. 76. FIG. 77. 



~T 



FIG. 76. Specimens of charcoal and charcoal-like material found in soils. 

 FIG. 77. Specimens of coal found in soils. (Bulletin 90, Bureau of Soils.) 



coal 1 (Figs. 76 and 77). This does not undergo further change. 

 The humus accumulates more rapidly in very moist soils than in 

 comparatively dry ones. 



The following table from Hilgard shows the changes in the for- 

 mation of coal, probably somewhat similar to those changes of 

 organic matter in soils : 



Progress of H unification, and Formation of Coal * (Moisture and Ash Omitted 



From Calculations) 



