SOIL EEOSION IN THE SOUTH. 7 



mated that 53 per cent reaches the sea. When the mean annual 

 rainfall on mountam topography is 40 inches, the rmi-ofF approaches 

 30 inches; if the rainfall is 25 inches, the rmi-off is about 12 inches; 

 and if there is 15 inches rainfall, the run-off is less than 5 inches. 



All effort should be directed toward lessening the surface run-off 

 and increasing the quantity of water soaking into the soil. If all the 

 water faUing on the surface of a given area were absorbed by the 

 soil, there could be no erosion. It is the water flowing over the 

 surface that must be controlled to prevent damage from excessive 

 soil washing. 



The amount of water which the ground absorbs depends upon the 

 slope, the character or condition of the soil, the nature and amount 

 of vegetal covering, and the amount and character of precipitation. 

 Perhaps the slope has the greatest influence of any of these factors, 

 but even this may be more than balanced by the character of soil, 

 the precipitation, and the vegetation. As has been previously 

 pointed out doubling the slope results per se in greatly increasing 

 the erosion, but the increased velocity of v/ater flowing down the 

 slope makes the erosive power about 32 times greater. 



The character of a soil greatly influences the amount of water ab- 

 sorbed by it. Soils vary in composition from light or sandy soils 

 to heavy or clayey soils. The difference is in the size of particles 

 composing them. The loams lie between the two extremes and rep- 

 resent varying mixtures of the coarser and finer particles of soil. 

 While it is true that the actual pore space in a clay soil is much greater 

 than that in a sandy one, the size of the individual spaces is much 

 smaller in the case of the clay, so that the movement of water within 

 the clay is slower than in the sandy soil. The sandy soils, therefore, 

 absorb rainfall more readily than the heavier soils. The power of 

 a soil to absorb water rapidly depends not so much upon the total 

 amount of pore space as upon the size of the individual spaces. Of 

 course, the absorptive capacity should be such that all of the inter- 

 spaces are not filled by the rainfall at any particular time. The 

 size of the interspaces may bo increased in the heavier soils by the 

 introduction or incorporation of organic matter. Upon moderately 

 rolling land the following classification shows the relative capacity 

 of the soils for absorbing an ordinary rainfall : 



Class. 



Composition.' 



Amount of water ab- 

 sorbed. 



Sands 



Less than 20 per cent silt and clay; 25 to 50 per 



cent sand. 

 20 to 50 per cent silt and clay, 25 to 50 per cent 



sand. 

 60 per cent silt and clay, less than 30 per cent clay. 



Nearly all. 



Sandy loams 



Large part. 



Loanu, silts, and clay loams. . . 



Very little absorbed. 



' From values given by Whitney, llul. 78, IJurcau of Soils, U. S. Dcpt. A^r., p. V2. 



