32 
Research Bulletin No. 5 
than from a surface which has been loosened by cultivation. The 
smooth, compact soil has less chance to hold the water and the 
dry soil offers more resistance to the penetration of the water 
than does a soil that is more open. Where the surface has been 
cultivated, the water runs into the openings made by the imple- 
ment used, and is held until it can soak into the soil. Where the 
surface has not been stirred, there are no open spaces to catch 
the water. As soon as the soil at the surface has been filled, it 
can hold no more water except as the water is removed from the 
surface by penetration. The water which falls above this amount 
will be lost as run-off unless the surface is level. Obviously the 
amount of run-off from any rain is governed largely by the type 
of soil and character of the rain. A loose, sandy soil offers so 
little resistance to the penetration of water that there is little 
run-off. The soil particles composing the sandy soil are larger 
than those of heavier soil, and consequently the open spaces are 
larger and water readily penetrates into the soil. It is only dur- 
ing excessively heavy rains that run-off is seen in real sandy soil. 
The resistance to the penetration of water increases as the soil 
becomes heavier or more like clay. All gradations are found, 
from the sand, which offers so little resistance, to the gumbo, 
where penetration is extremely slow. We have observed that 
water will stand for several days over a heavy gumbo spot, and 
be lost mainly by evaporation rather than by penetration ; often 
penetrating only a few inches into the soil. 
The character of the rain has also an important influence on 
the amount of water which will be retained from any given rain. 
The more rapidly the water falls on the surface the greater the 
run-off. During heavy, torrential rains the water falls so rapidly 
that it does not have time to penetrate into the soil and must be 
lost as run-off. The beating drops of water often puddle or clog 
the surface, making it more difficult for the water to enter. It 
is a matter of common observation that a heavy downpour of 
rain frequently does less good than a much lighter rain that falls 
more slowly. The type of rain that is really most beneficial is the 
rain of sufficient amount and falling slowly enough to be carried 
into the soil to a depth where it will not be exposed to the rapid 
surface evaporation. 
Of equal importance with the two factors above mentioned 
in governing the amount of water held in the soil from any given 
rain, is the amount of water present in the soil when the rain 
comes. A very dry soil offers much greater resistance to the 
downward movement of water than does a moist soil. In a very 
