IO2 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



of Soils. The surface soil is the upper layer of the earth, and 

 continues until there is a decided change in physical character. 

 In other words, the distinction between soil and subsoil is based 

 upon differences in mechanical composition. 



In humid sections, the subsoil is not well suited for the growth 

 of plants. If it is exposed by removal of the surface soil, it is 

 in most cases unproductive until it has been subjected to 

 atmospheric influences for some time. Organic matter added to 

 the soil, such as manure, is said to aid in converting the raw sub- 

 soil into productive soil. If too much subsoil is mixed with the 

 surface soil, by deep plowing, the productiveness of the soil may 

 be decreased. In arid regions, according to Hilgard, 1 the soil is 

 suitable for plants to the depth of three to ten feet, or more. 

 Material from the depth of eight feet has been put on gardens 

 and served well the first year. In preparing land for irrigation, 

 the land is leveled without regard to the subsoil, and no bad 

 effects are noticed. These practices would be injurious to humid 

 soils. The difference is due to the greater depth of penetration 

 of air and the roots of plants, under arid conditions. 



Penetration of Roots. The depth to which roots penetrate 

 into the soil depends upon the condition of the soil and subsoil, 

 the climate, and the kind of plant. The roots of plants grown in 

 humid regions penetrate only a comparatively short distance. In 

 arid climates it is necessary that the roots penetrate deeply, in 

 order that they may endure drought. 



According to Hilgard, the roots of the hop have been found, in 

 arid climates, to penetrate to the depth of 18 feet; roots of wheat 

 and barley may reach to 4 to 7 feet in sandy soil ; roots of grape 

 vines have been found at the depth of 22 feet below the surface. 

 Thus in arid climates, where a drought of five or six months pre- 

 vails during the growing season, the roots of plants, by penetrating 

 to considerable distances, will secure water in the depths of the 

 soil. The depth to which plants send their roots affects the 

 quantity both of water and of plant food at their disposal. Differ- 

 ences in the needs of plants for food may, in part, be due to 

 1 The Soil, p. 163. 



