PHYSICAL, COMPOSITION AND CLASSES OF SOILS S/ 



The figures given represent per cent. ; the minus sign ( ) repre- 

 sents, less, and the plus sign (-(-) represents more; and the sign 

 (-) when used between two figures, thus 20-50, gives limiting 

 values, and should be read from 20 per cent, to 50 per cent. Thus, 

 25 + means 25 per cent, or more; 25 means less than 25 per 

 cent. 



For example, a soil containing less than 20 per cent, silt and 

 clay and over 50 per cent fine sand, would be called a fine sand. 

 A soil containing over 50 per cent, silt and clay particles and over 

 30 per cent, clay would be called a clay soil. 



Relation of Chemical Composition to Soil Texture. The effect 

 of the physical composition of a soil is modified by the chemical 

 character of its constituents. The three important modifying 

 constituents are organic matter, colloidal clay, and carbonate of 

 lime. 



Organic matter binds a loose soil, and lightens a heavy soil, 

 and thus reduces the difference between them. When there is a 

 large quantity of organic matter, the mechanical analysis loses 

 much of its significance. 



Carbonate of lime (i to 2 per cent.) also lightens a clay soil 

 and otherwise modifies its properties. For example, 1 the follow- 

 ing pairs of soils had similar physical composition but differed 

 decidedly in properties : 



It is very well known that calcareous clay soils are more easily 

 cultivated and break up better than similar soils deficient in lime. 



The clay particles, so called, may be composed of quartz dust, 



hydrated oxide of iron, gelatinous silica, carbonate of lime. 



hydrated silicates, and of true clay, or hydrated silicate of 



alumina. These substances have different properties, and the 



1 Hall, Jour. Agr. Sci., 1911, p. 187. 



