SOILS AND FERTILIZERS 



by the real specific gravity. 6 Ordinarily cultivated soils 

 have a pore space range from 30 to 60 per cent of the 

 volume of the soil, depending upon the conditions to 

 which the soil has been subjected. 



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

 varies from those hardly distinguishable with the micro- 

 scope to coarse rock fragments and determines the type 

 of a soil as sand, 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 di- 

 ameter. 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. 

 Arable soils, in general, contain from 5 to 20 per cent 

 of skeleton. 



The fine earth is composed of six grades of soil parti- 

 cles. The names and sizes are as follows : 



Soils are mechanical mixtures of various-sized par- 

 ticles. In most soils there is a predominance of one 



