CHAPTER VII. 



THE DENSITY AND VOLUME-WEIGHT OF SOILS. 



ASIDE from the humus-substances the specific gravity of the 

 common soil constituents, taken individually, do not vary 

 widely; kaolinite being the lightest (2.60), feldspar next 

 (2.62); then quartz (2.65), calcite (2.72). Mica and horn- 

 blende range (according to their iron contents) from 2.72 

 to over 3.0. The average specific gravity of soils of ordinary 

 humus content only will thus range between 2.55 and 2.75 ; 

 sandy soils approaching very closely to that of quartz alone. 



Volume-Weight. The specific gravity of the soil is, how- 

 ever, of little practical consequence compared with the " volume- 

 weight," i. e., the weight of the natural soil as compared with 

 an equal bulk of water. A cubic foot of water weighs 62^/2 

 pounds ; a similar volume of soil usually weighs more, but in 

 the case of peaty lands may actually (when dry) weigh less. 

 The extreme range is from no pounds for calcareous, and 

 somewhat less for siliceous sand, to as little as 30 to 50 pounds 

 in the case of peaty and swamp soils. It may be conveniently 

 remembered that while average arable loams range from 80 to 

 about 95 pounds per cubic foot, " heavy " clay soils range from 

 75 pounds down to 69. observed by the writer in the case of 

 certain alluvial soils, poor in humus, 1 of the Sacramento river, 

 California. Manured garden soils, and the mold surface soil of 

 deciduous forests, generally contain so much humus as to 

 depress their weight considerably, varying according to their 

 state of tilth from 66 to 70 pounds per cubic foot. 



IV eight per acre-foot. As for practical purposes and calcu- 

 lations it is often desirable to know approximately the weight in 

 pounds of an acre (43.560 square feet) one foot deep, it is 

 convenient to remember that in the case of sandy land, this 

 weight (per "acre-foot") may be assumed at four millions 

 of pounds; for loams, at 3'j millions; for clay lands, 3^.4 



1 This remarkable soil seems to have been derived from the finest "slickens " of 

 the hydraulic gold mines. 



107 



