8 BULLETIN" 180, U. S. DEPARTMENT OF AGRICULTURE. 
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 f ailing on it runs off over its surf ace, 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 will be ab- 
sorbed by the soil, but if the same amount of rain falls during 1 or 2 
hours, only a small part will 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 
solLso 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 Carolina, 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 
