12 SOILS AND FERTILIZERS 



to which it has been subjected. Usually about 50 

 per cent, of the volume of a soil is air space. A cubic 

 foot of soil from a field which has been well cultivated 

 weighs less than from a field where the soil has been 

 compacted. Hence it is that soils have both a real 

 and an apparent specific gravity. The apparent spe- 

 cific 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 the Soil Particles. — The size of the 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 the 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 

 amount of fine earth and skeleton is variable. Arable 

 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 : 



