Soils 57 
Few of these minerals occur separately; they are 
usually combined to form the different igneous and sedi- 
mentary rocks, which, on decomposing, form soils. Each 
one has its effect on the resulting soil. Granite, con- 
taining a potash feldspar, gives a soil rich in potash and 
also high in phosphoric acid, which comes from small 
apatite crystals. Eruptive rocks as a class decompose 
slowly, but usually form highly productive soils. Hard 
limestone dissolves slowly, but the softer varieties go into 
solution readily. Limestone soils, from which much of 
the lime has been leached, form some of the richest soils. 
Many of the better sugar-beet sections of America have 
soil high in lime. Sandstone soils are often poor, but this 
depends on the material cementing the grains together. 
Claystone soils are usually rich in plant-food, but are too 
heavy for the best growth of sugar-beets. Hardpans 
are formed where an excess of alkali accompanies the 
clay. 
Soils are formed from minerals and rocks by the various 
chemical and physical agencies of rock decay known as 
weathering. The most important of these agencies are: 
(1) heat and cold, (2) water, (3) ice, (4) the atmosphere, 
and (5) plants and animals. Their action is both me- 
chanical and chemical, the mechanical causing the break- 
ing up of the rock into finer fragments, and the chemical 
causing a change in the actual composition of the material. 
CLASSIFICATION OF SOILS 
Soils may be classified according to their origin as either 
sedentary or transported. Sedentary soils are of two 
