THE SOIL PARTICLE 93 



equal volume of water, the volume weight may be calculated. 

 This method will give only approximate results, however, as 

 the structural relationships are more or less artificial.^ 



50. Actual weight of soil. — When the volume w^eight of 

 a soil is 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 sands. The greater the organic 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 soil may also be 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. The standard usually adopted is 2,000,000 pounds, 

 being the weight of average soil to a depth of 6V3 inches. 

 The value of knowing the actual weight of a soil lies in the 

 possibility of calculating thereby the amount of water, the 

 amount of organic matter, or the actual number of pounds of 

 the mineral constituents present in the soil. Such informa- 

 tion affords another means of comparing two soils. 



51. Pore space of soil. — The pore space of soil is occu- 

 pied by air and water in constantly varying proportions. The 

 amount of this pore space is determined by the texture and 

 the structure of the soil. As already emphasized, the coarser 



^ A comparison of the four methods is given by Israelsen, 0. W., A 

 New Method for Determining Volume Weight; Jour. Agr. Res., Vol 

 XIII, No. 1, p. 32, 1918. 



Average Volume Weight of Tehama Clay to a Depth of 60 Inches. 



Laboratory method on disturbed soil 1.35 ± .008 



Rubber tube method 1.74 ± .010 



Iron cylinder method 1.73 



Paraffin-immersion method 1.73 ± .035 



