14 SOILS: PROPERTIES AND MANAGEMENT 



very superficial and affects the earth to but relatively 

 shallow depths However, from the fact that it provides 

 a medium for crop growth and at the same time is largely 

 instrumental in maintaining the fertility of this medium, 

 its agencies and processes become of great significance. 



The forces of weathering, while very diverse not only 

 as to action but also as to product, permit of an outline 

 so clear that the true relationships at once become ap- 

 parent. This classification may be made under two 

 heads, mechanical and chemical, as follows : — 



Forces of weathering 



I. Mechanical changes, or disintegration 



A. Erosion and denudation 



Water, wind, ice 



B. Temperature 



Heat and cold, and frost 



C. Plants and animals 



II. Chemical changes, or decomposition 



A. Oxidation and carbonation 



B. Deoxidation 



C. Hydration 



D. Solution 



10. Water. — The three great agencies of erosion and 

 denudation are water, wind, and ice. They are instru- 

 mental not only in the breaking up of rocks, but also in 

 transporting the resultant materials. Water is especially 

 of importance, as its denuding effects are very rapid when 

 viewed over geological periods. It is estimated that 

 the United States is being planed down at the rate of 

 one inch in seven hundred and sixty years. This is rapid 

 enough to dig the Panama Canal in seventy-three days. 



