SOME PHYSICAL PROPERTIES OF THE SOIL 113 



gravity of a purely mineral soil varies only between 

 narrow limits, these being from 2.6 to 2.8. Nor has 

 fineness any appreciable effect, as shown below by Whit- 

 ney's l determinations on the various separates : — 



Specific Gravity 



Fine gravel (2-1 mm.) 2.647 



Coarse sand (1-.5 mm.) 2.655 



Medium sand (.5-25 mm.) . . . . . 2.648 



Fine sand (.25-. 10 mm.) 2.659 



Very fine sand (.10-.05 mm.) .... 2.680 



Silt (.05-.005 mm.) 2.698 



Clay (below .005 mm.) 2.837 



The only marked variation here observed is in the clay 

 separate, and this may be due to the concentration of the 

 iron-bearing silicates in this grade. However, for all 

 practical purposes the average absolute specific gravity 

 of a mineral soil may be placed at about 2.70. One 

 condition that may vary this is the quantity of organic 

 matfer present. As the specific gravity of the soil humus 

 usually ranges from 1.2 to 1.7, the more humus there is 

 present, the lower will be the absolute figure for a given 

 soil. A purely organic soil, such as muck or peat, pre- 

 sents a variable absolute specific gravity ranging from 

 1.5 to 2.0, according to the amount of wash it has received 

 from external sources. Some humus-loam soils may 

 drop as low as 2.1. Nevertheless for general calculations 

 the average arable soil may be considered to have an 

 absolute specific gravity of about 2.70. 



82. Apparent specific gravity. — Since all soils contain 

 mqre or less pore space, depending on textural and struc- 



1 Whitney, M. Some Physical Properties of Soils. U. S. 

 D. A., Weather Bur., Bui. 4, p. 34. 1892. 



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