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 



