SIZE OF SOIL PARTICLES II 



gravity of a soil is sometimes less than half of the real 

 specific gravity. The specific gravity of different soils 

 as given by Shoen is as follows : 6 



Specific gravity. 



Clay soil 2.65 



Sandy soil 2.67 



Fine soil 2.71 



Humus soil 2.53 



4, Size of Soil Particles. The size of soil particles 

 varies from those hardly distinguishable with the 

 microscope to coarse rock fragments. The size of the 

 particles determines the character of the soil as sandy, 

 clay, or loam. The term ' fine earth ' is used to 

 designate that part of a soil which passes through a 

 sieve with holes 0.5 mm. (0.02 inch) in diameter. 

 Coarse sand particles and rock fragments which fail 

 to pass through the sieve are called skeleton. The 

 amounts of fine earth and skeleton are variable. Ara- 

 ble soils, in general, contain from 5 to 20 per cent, of 

 skeleton. 



The fine earth is composed of six grades of soil 

 particles. The names and sizes are as follows : 



Millimeters. Inches. 



Medium sand 0.5 to 0.25 0.02 to o.oi 



Fine sand 0.25 to o.i o.oi to 0.004 



Very fine sand- ... o.i to 0.05 0.004 to 0.002 



Silt 0.05 to o.oi 0.002 to 0.0004 



Fine silt o.oi to 0.005 0.0004 to 0.0002 



Clay 0.005 and less 0.0002 and less 



Soils are mechanical mixtures of various-sized par- 

 ticles. In most soils there is a predominance of one 

 grade, as clay in heavy clay soils, and medium sand 

 in sandy soils. No soil, however, is composed entirely 



