SOMJS PHYSICAL FROPERTmS OF THE SOIL 115 



gravity for that soil. A laboratory determination may be 

 made by putting the soil into a receptacle of known volume 

 and weighing it. From the weight of the absolutely 

 dry soil and the weight of an equal volume of water, the ap- 

 parent specific gravity may be calculated. This method 

 will give only approximate results, however, as the struc- 

 tural relationships are more or less artificial. The only 

 reliable method is the one first described. 



83. Actual weight of a soil. — With the apparent 

 specific gravity of a soil known, its weight in pounds to 

 the cubic foot may be found by multiplying by 62.42. 

 Soils may vary in weight from 68 to 80 pounds for clays 

 and silts to 100 to 110 pounds for sand. The greater the 

 humus content, the less is this weight to the cubic foot. 

 A muck soil often weighs as little as 25 or 30 pounds. 

 This weight, of course, is for absolutely dry soil and does 

 not include the water present, which may be much or 

 little, according to circumstances. The actual weight 

 of a soil is often expressed in acre-feet. An acre-foot of 

 soil refers to a volume of soil one acre in extent and one 

 foot deep. In the same way we may have an acre-eight- 

 inches or an acre-six-inches. The weight of an acre-foot 

 of soil usually varies from 3,500,000 to 4,000,000 pounds ; 

 granulation and organic matter may modify this con- 

 siderably. The value of knowmg the actual weight of 

 a soil lies in the possibility of calculating thereby the 

 amount of water, the amoimt of humus, or the actual 

 number of pounds of the mineral constituents present in 

 the soil Such information affords a ready means of 

 comparing two soils as to their crop-producing capabilities. 



84. Pore space in soil. — The pore space in soil is 

 due largely to structural conditions. As already em- 

 phasized, the coarser the soil, the smaller is the aggregate 



