SOIL EROSION IN THE SOUTH. 7 
mated that 53 per cent reaches the sea. When the mean annual 
rainfall on mountain topography is 40 inches, the run-off approaches 
30 inches; if the rainfall is 25 inches, the run-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 falling 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 water 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 be 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. 1 
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. 
50 per cent silt and clay, less than 30 per cent clay. 
Nearly all. 
Large part. 
Very little absorbed. 
Loams, silts, and clay loams. . . 
From values given by Whitney, Bui. 78, Bureau of Soils, U. S. Dept. Agr., p. 12. 
