THE SOIL. 35 



ness, then, of the soil grains is one means of 

 determining different kinds of soils. However, 

 there are other things to be taken into considera- 

 tion. 



Some rocks, such as granites, are so hard that, 

 when ground to pieces, they are not easily reduced 

 to a fine powder, but remain hard and sharp 

 grains of considerable size. Such are sandy soils. 

 Sand grains are not only comparatively large, but 

 of such a nature that they are not readily dissolved; 

 hence, they give up their food very slowly. Other 

 rocks, such as limestone, and the clays, are easily 

 reduced to a fine powder, forming soils of very 

 small grains, with a great water-holding capacity. 



Soils are called sandy when a large per cent, of 

 sand enters into their composition; and clay soils, if 

 they are composed largely of clay. Where sand 

 and clay are mixed, it gives a sandy clay, or a clayey 

 sand according as the sand or the clay predominates. 



"Light" and "heavy" soils are terms applied on ac- 

 count of the ease or difficulty with which they are 

 worked, and not on account of weight. A sandy 

 soil weighs more per cubic foot than a clay soil, but 

 it is called a light soil on account of its loose texture; 

 and a clay soil heavy on account of its stiffness. 



Humus soils are those composed mostly of decay- 

 ing organic matter, the remains of plants and ani- 

 mals. These soils predominate in regions where 

 organic decomposition goes on but slowly. The 

 best examples of humus soils are peat and black 



