12 AGRONOMY 



destroyed almost as fast as made, and no such difference in 

 color may be seen except in the occasional low, wet lands. 

 The soil in arid lands, however, is often more fertile than that 

 of humid regions and can be cultivated to a greater depth. 

 This is mainly because the scanty rainfall has not washed out 

 and carried away the food materials in it. Such soils need 

 only to be irrigated to give abundant crops. 



The subsoil. The soil is commonly regarded as extending 

 downward as far as traces of organic matter or humus are 

 found. In humid regions this is from a few inches to several 

 feet. Below this is the subsoil, lighter in color, more compact, 

 and consisting almost entirely of rock fragments. It is there- 

 fore little adapted to growing crops, though rich in the mate- 

 rials needed by plants. While not of itself able to produce 

 good crops, it forms a storehouse of food materials upon which 

 the plant can draw, and, slowly breaking down under the at- 

 tacks of wind and weather, gradually becomes part of the 

 soil itself. 



Origin of the soil and subsoil. If one digs down far enough 

 anywhere on the earth, he comes at last to the solid rock. 

 This is called bed rock. Here and there it comes to the sur- 

 face, forming outcrops, such as may be seen in the cliffs of 

 broken country or where a rapid stream has carved out a 

 deep valley. Usually, however, it is buried under a thick 

 deposit of rock fragments, called mantle rock, which has been 

 derived from the bed rock in various ways. Sections of man- 

 tle rock may be seen in railway cuts, gravel pits, brickyards, 

 and the openings for quarries and mines. Since the bed rocks 

 range from soft and porous sandstones and limestones to 

 compact and flinty granites, the mantle rock may differ in 

 composition according to the locality, and the soil derived 

 from the mantle rock necessarily partakes of the same char- 

 acteristics. Soils derived directly from the underlying bed 

 rock are known as residual or sedentary soils ; those brought 



