8 BTJLLETIISr 180^ U. S. DEPAETMENT OF AGEICULTURE. 



This classification holds for only very limited conditions. It 

 shows the relative absorptive power of the soils named, other condi- 

 tions being the same. 



The depth of the soil is the ultimate measure of the amount of 

 water it is capable of absorbing. When the soil is saturated the addi- 

 tional water falling on it runs off over its surface, carrying away soil 

 particles. A thin layer of soil, underlain at shallow depths by an 

 impervious layer, becomes saturated quickly and erosion at the sur- 

 face is most active. The depth of plowing in cultivated areas has 

 much to do with the depth of the soil and the amount of water neces- 

 sary to saturate it. 



Vegetation affects the amount of water absorbed by the soil by 

 retaining the water for a longer time on the surface, giving it a better 

 opportunity to be absorbed. An additional effect is that the soil 

 is kept more or less open by the roots penetrating it, and these roots 

 form channels along which the water may be conducted to the sub- 

 soil. The vegetation further affords protection to the soil in that 

 it retards the movement of the water flowing over the surface and 

 prevents the removal of soil particles. 



All these factors influencing the absorption of water by the soil 

 are under the control of man, with the single exception of the pre- 

 cipitation. However, this factor is fairly constant as to quantity, 

 although slightly less so as to character, for any given locality. If 

 2 or more inches of rain fall during 24 hours, much of it wiU be ab- 

 sorbed by the soil, but if the same amount of rain faUs during 1 or 2 

 hours, only a small part wiU be absorbed. Since the movement of 

 water within the soil meets with considerable frictional resistance, 

 this movement is rather slow. If the water moves into the lower 

 layers at a rate slower than that at which water is furnished to the 

 surface, the upper layer of soil soon becomes saturated and the addi- 

 tional water runs off over the surface. Again, if precipitation 

 occurs in the form of rain, it is much less likely to be totally absorbed 

 than if in the form of snow. The melting snow supplies water to the 

 soil so gradually that it has ample time to be totally absorbed. 



In the Southern States probably the most important factor 

 influencing absorption is the character of the precipitation. This 

 is mainly in the form of rain and is quite heavy at times. This means 

 that generally this factor is most unfavorable for the retention of 

 water by the soil and to prevent its flowing off the other factors 

 must be made as favorable as possible. In the mountainous regions 

 of Virginia, North CaroHna, South Carolina, and Tennessee vegeta- 

 tion exerts great influence. Where the forests have been cut off 

 the steep hillsides rapid erosion has followed, and in some places 

 the soil has been removed down to the underlying bare rock. Other 



