114 SOILS: FROPERTIES AND MANAGEMENT 



tural conditions, the actual weight of the absolutely dry 

 soil in any volume is of great importance. This is ex- 

 pressed as is the absolute specific gravity of any material, 

 the weight of an equal volume of water being used as a 

 unit. Because of their tendency to granulate, fine soils 

 have a very large amount of pore space, as has been shown 

 in the discussion of structure ; it is to be expected, there- 

 fore, that they will weigh less in any particular volume 

 than will soils made up of large particles. Coarse soils 

 are heavy soils, as far as 'weight is concerned. Mineral 

 soils may range in apparent specific gravity^ from 1.10 

 to 1.20 for clay to 1.65 to 1.75 for sand. Humous loams 

 may drop as low as 1.00, and muck often reaches the 



i^ioiinillL 



niiiiB>i 



Fig. 19. — Cylinder for deter- 

 mining the apparent specific 

 gravity of soil in the field. 

 The cutting edge at (6) is 

 drawn in somewhat to pre- 

 vent excessive friction be- 

 tween the sides of the cylin- 

 der and the entering soil core. 



low figure of .40. The appar- 

 ent specific gravity is always 

 expressed on the basis of abso- 

 lutely dry soil. 



In the field the absolute 

 specific gravity of a soil may 

 be determined by driving a 

 cylinder of known volume into 

 the ground and obtaining 

 thereby a core of natural soil 

 (see Fig. 19). By weighing 

 the soil and then determining 

 the amount of water that it 

 holds, the amount of absolutely 

 dry soil may be ascertained. 

 Dividing this by the weight 

 of an equal volume of water 

 gives the apparent specific 



^See Whitney, M. Some Physical Properties of Soils. 

 U. S. B. A., Weather Bur., Bui. 4. 1892, 



