SOIL EROSION 



359 



the total pore space in the soil. This fact explains the rapid ab- 

 sorption by the coarse-grained soils and the slow action of the line- 

 grained ones. However, if the latter are loose and open from 

 recent tillage their absorption compares favorably with that of 

 coarser soils. Unless the finest-grained soils (clay loams and clays) 

 are exceptionally well supplied with organic matter and limestone 

 the beating of raindrops breaks down the granules, diminishing the 

 size of the pores, thus rendering the soil less absorbent. As a result 

 a large amount of water is lost from even moderate slopes. 



3. Vegetative Covering. The surface soil of a natural forest 

 is usually covered with leaves and twigs, which protect it from 



Fio. 174. Two hundred s<|u:ire miles <>l mi-i- forested mountains in China, which a cen- 

 tury ago paid rich revenue mi ihcir lumber product, (Bailey Willis.) 



erosion. It suffers little so long as this natural protection remains 

 undisturbed (Fig. 171). Natural prairies are usually protected in 

 this way by a good sod of native grass. The rain drops do not 

 usually strike the soil direct and thus destroy the granules, as they 

 tend to do in cultivated fields. When this covering which nature 

 provided is removed or destroyed erosion takes place. 



4. Character of the Rainfall. A gentle rain will be absorbed 

 entirely by almost all soils, since it does not come more rapidly than 

 the water can percolate through the soil, thus preventing complete 

 saturation of the surface. A heavy rain falling on medium- or fine- 

 grained soils soon saturates the surface and then absorption bv the 

 soil cannot take place anv faster than percolation from the surface 



I I 



into the lower strata. 



Results of Erosion. It is quite impossible to determine with 



