128 THE PRINCIPLES OF SOIL MANAGEMENT 



(b) The remainder of the organic material, after 

 extraction, is composed of the fresh and partially 

 decomposed fragments of plant and animal remains 

 more or less stained. It is a light chaffy material, which 

 by decay may be changed to humus, but in this condition 

 is not subject to direct loss. 



57. Weight. The organic material is the lightest 

 constituent in the soil. Warington gives the specific 

 gravity of humus as 1.2 to 1.5, as compared with 2.68 

 for the mineral constituents, and Hilgard reports its 

 volume weight when dry as .33, as compared with 

 about 1.1 for clay and 1.5 for sand. Therefore, in pro- 

 portion as a soil contains humus, it is lighter in weight. 

 On the basis of the above figures, a cubic foot of humus 

 would weigh about twenty-one pounds. Muck and peat, 

 however, contain mineral matter washed in with the 

 organic material and their volume weight is higher. 

 It ranges from twenty to forty-five pounds per cubic 

 foot, according to the stage of decay when dry. 



58. Absorption properties. In the form of humus, 

 organic matter has a very large absorptive power for 

 gases and salts in solution, similar to that shown by 

 powdered charcoal. It is much greater than that of 

 even clay soils, and for this reason its addition to soil 

 increases this important property. 



59. Volume changes. Like clay soil, when humus 

 is dried, it shrinks very greatly, and conversely, when 

 it is moistened it expands. In humus this property 

 is much more pronounced than in even the heaviest 

 clay. Warington reports the shrinkage of a very pure 

 clay, in drying from a saturated state, to be 18 per cent 



