PHYSICAL PROPERTIES OF SOILS 13 



It is estimated that an acre of soil to the depth of 

 one foot weighs in round numbers from 2,500,000 to 

 4,200,000 pounds, depending upon the chemical com- 

 position, size of soil particles, and state of compac- 

 tion. 



The weight per cubic foot of soils in situ generally 

 exceeds the weight derived from the apparent density 

 of the dry soil ; this is because of the tendency of soils 

 in the field to become compacted. While a dry clay 

 soil reduced to a powder may show an apparent weight 

 of 70 pounds per cubic foot, the field weight (air-dry 

 basis) may range from 80 to 98 pounds, depending upon 

 the degree of compactness. 



Between the soil particles are * non-capillary or pore 

 spaces occupied by air or water. If the soil be con- 

 sidered a homogeneous mass without air spaces, it will 

 have an absolute specific gravity of about 2.6 ; with the 

 air spaces its apparent specific gravity is about 1.2. 

 That is, in its natural condition a soil weighs about 1.2 

 times heavier than the same volume of water. The 

 porosity of a soil is determined by dividing the apparent 



