THE SOIL 7 



movements of the earth's crust ; and glacial deposits 

 left by the great ice sheet; or finally they may have 

 been formed in situ by the slow weathering and de- 

 composition of the underlying rocks. Bog soils are 

 also worthy of special mention since they are mostly 

 formed in situ from the remains of aquatic or semi- 

 aquatic plants. Sand dunes and other shifting de- 

 posits made by the wind usually have but little 

 agricultural value. 



Kinds of Soils. — For practical purposes soils are 

 usually classified according to the size of the particles 

 of which they are composed. Among farmers they 

 are spoken of as gravelly, sandy, loamy, or clayey, 

 and intermediate grades are expressed by combina- 

 tions of these terms, as sandy-loams, loamy-clay, etc. 

 Soil physicists make a more accurate classification, 

 which is expressed in the following table: — 



clay, soil particles, .005 mm. or less in diameter, 

 fine silt, soil particles, .01-.005 mm. in diameter, 

 silt, soil particles, .05-. 01 mm. in diameter, 

 very fine sand, soil particles, .1-05 mm. in diameter, 

 fine sand, soil particles, .25-. 1 mm. in diameter, 

 medium sand, soil particles, .5-.25 mm. in diameter, 

 coarse sand, soil particles, 1-.5 mm. in diameter, 

 gravel, soil particles, over 1 mm. in diameter. 



In describing soils the shape of the soil particles is 

 also taken into consideration. Of course most soils 

 contain particles of different sizes. In making a 

 mechanical analysis of a soil these particles are sepa- 

 rated according to the sizes in the above table, and 

 the percentage of each is determined. The adapta- 



