V. THE SOIL 



Soil Defined. Soil consists of finely divided rock fragments 

 with which air, water, certain living organisms, and parts of decay- 

 ing plants or animals are mixed. On it cultivated plants may be 

 grown. Ordinarily, the term is applied to the first six to twelve 

 inches of the earth's surface in which plants may be grown. 



The Subsoil. The harder and colder earth under the top soil 

 or surface layer is called the subsoil. It differs from the top soil 

 in that it contains less vegetable matter, is less finely divided, 

 and is more compact. Sometimes there is a sharp line of demarca- 

 tion in the color of the two portions; when the deeper soil is 

 brought to the surface it is found to be unproductive in some cases. 

 However, in arid regions these distinctions do not always appear. 

 In Arizona and some of the other western States the soil from a 

 depth of thirty feet or more is frequently found to be quite pro- 

 ductive. The subsoil renews the minerals depleted in the top 

 soil and also acts as a retaining medium for the roots of plants and 

 trees and as a storehouse of moisture. 



It is estimated that the first eight inches of soil on each acre 

 contain over three thousand pounds of nitrogen, nearly four 



thousand pounds of phos- 

 phoric acid, and over seven- 

 teen thousand pounds of 

 potash. Still, the soil itself 

 furnishes no more than 10 

 per cent of the weight of 



Relative positions of soil, subsoil, and underlying plants and often much leSS. 

 rock strata: s, soil and subsoil; 55, sandstone; TT _ ,. 



sh, shale; LS, Limestone. Hard Pan. Sometimes 



the subsoil after becoming 



very closely packed and dry forms a kind of hard layer or 

 stratum of earth which we call hard pan. Beneath this we find 

 rich porous earth, but it is of no service to the plant unless in 

 our cultivation we break through the hard pan. In some parts 



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