COMPOSITION OF SOILS. 33 



true character in reference to the amount of plant-food 

 contained in it, unless it is accompanied by a descrip- 

 tion of its general character, whether sandy, clayey, or 

 peaty. 



It will be observed from the foregoing, too, that the 

 ordinary idea of the weight of soil has reference to the 

 physical character, rather than actual weight. A sandy 

 soil is called " light," and a clay soil " heavy ; " while in 

 reality a sandy soil is heavy, and a clay soil is light. 



The sandy soil is called light because it is easier to 

 work, — the particles of sand are readily separated from 

 each other, and no particular force is required ; while in 

 the case of clay soils the particles adhere tenaciously, and 

 it requires considerable force to separate them. 



Classes of Soil Constituents. — The constituents of 

 a soil may again be divided into three classes, — first, 

 those which serve mainly as a mechanical support for the 

 plants, like sand, clay, limestone, and gravel; second, 

 dormant or reserve substances, which not only act the 

 same as the first class, but are capable of changing to 

 such a form as to furnish nutrition to the plant; these 

 are of both vegetable and mineral origin; and third, 

 active constituents, or those directly available to plants. 



The mechanical constituents constitute the bulk of all 

 soils — frequently over ninety-five per cent. The second 

 class, or dormant, are contained in most soils in rela- 

 tively small amounts as compared with the first class, 

 though the quantity that a soil may contain varies con- 

 siderably, depending upon its formation. 



The active constituents, those that are immediately 

 available to the plant, are never present in even the 



