A BULLETIN 447, U. S. DEPARTMENT OF AGRICULTURE. 
depth to which they extend depend somewhat upon the manner in which the 
soil has been dried. Where a close-drilled crop has been grown, they are small 
and numerous and break the soil into small lumps to a depth of about 15 inches. 
Beiow this depth the soil is generally compact and practically free from cracks, 
no matter how dry it may be. 
When the soil becomes wet it expands, and thus the cracks are closed. When 
any excess of the expanding force in the cracked layer occurs, the whole force 
of expansion in the uncracked area has the effect of crowding the soil particles 
closer together. For this reason the wet soil is always compact throughout and 
tree from open spaces. 
Considering these structural differences between the wet and the 
dry soil, it can readily be seen that the moisture content of the soil 
may have a great influence upon water movement; but a study of 
water movement in both the wet and the dry soil is necessary in 
order to obtain information as to how water penetration takes place 
under actual field conditions. 
On the Belle Fourche project it has been found in field practice 
that the water content of the surface soil at the time the water is 
appled determines to a great extent the quantity that will be ab- 
sorbed, especially when the water is applied rapidly. When both 
the surface soil and the subsoil are dry, over an inch of rain, even if 
it comes in a torrential manner, will be absorbed with very little 
loss, because much of the water makes its way into the soil through 
the cracks. On the other hand, when the surface soil is wet these 
cracks are closed and a rain of as little as one-fourth of an inch may 
be largely lost by run-off. Any rain falling on a wet surface must 
fall very slowly in order to be absorbed. 
That the condition of the surface soil determines the amount of 
water absorbed is especially true when irrigation water is applied. 
A comparatively small rain, by wetting the surface and causing the 
cracks to close, often stops irrigation. There are times when it is 
possible to irrigate successfully in the afternoon, when an attempt 
to irrigate during the forenoon of the same day has resulted in the 
run-off of practically all the water applied. _ 
These facts indicate that water movement through this soil when 
wet is very slow. 
RATE OF MOVEMENT OF WATER IN LOOSE, SATURATED SOIL. 
In order to make actual determinations of the rate at which water 
moves in this soil when it is saturated, the following experiment was 
performed: 
Sections of blotting paper were fitted, as bottoms. into a number of cans 
that were open at both ends. Each can was filled with a composite sample 
of a foot section of soil and then immersed in water. After the soil had 
become thoroughly saturated the cans were removed from the water and placed 
upon a screen. All the soil was then removed except a 3-inch layer in the bot- 
tom of each can. 
