CHAPTER IX 



THE FORMATION, COMPOSITION, AND FERTILITY OF 



SOILS 



62. Formation of Soil. At one period in the history of 

 the earth, its entire crust was igneous rock. Many agencies 

 have been working through untold ages to develop the 

 soil into its present condition. The weathering of rock 

 has proceeded at different rates, being governed by the 

 activities of the agencies involved. Weathering is brought 

 about by chemical, physical, and biological means. The 

 ' agencies of weathering are the atmosphere, water, heat, 

 cold, gravitation, electrical discharges, and organisms. 



The chief action of the atmosphere is chemical, brought 

 about mainly through oxidation and the solvent effects 

 of carbon dioxide in solution. The atmosphere also exerts 

 physical influences that hasten rock decay. Winds blow 

 particles against other particles, producing abrasion, and 

 by blowing against trees and plants, cause them to act 

 as levers, which press the soil particles against each other, 

 thus causing them to grind and wear one another away. 



Water, in addition to its solvent action, has a physical 

 influence on the soil, in that rainfall causes the rubbing 

 together of soil particles, thus producing some erosive effect. 

 Surface water causes the wearing of soil particles against 

 each other and thus increases their fineness. The erosion 

 of the land tends to reduce the soil nearer to a base level. 



Heat and cold have the same physical effects on rocks 

 that they have on other substances, rocks expanding when 

 heated and contracting on cooling. The units which go 

 to make up rocks are minerals. Different minerals have 

 different rates of expansion and contraction, consequently, 



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