668 
Journal of Agricultural Research voi. xxvii. No. 9 
was absorbed in the treated basin within about five hours. The photo¬ 
graph shown in the figure was taken 54 hours after the water was put 
on. It shows that the water was still standing in the untreated basin 
while the soil in the treated basin was beginning to dry and crack. 
There is another condition of impermeability sometimes encountered 
in irrigated land that differs from those described above in that it occurs 
in a zone of limited thickness somewhat below the surface of the soil. 
This condition, which has been referred to as “hardpan” or “plowsole,” 
is sometimes found in soils that have not been irrigated, and it also 
develops under certain conditions in irrigated lands ( 10 ). 
The formation of hardpan appears to be due to the precipitation of 
substances from the soil solution at the point below the soil surface 
where evaporation takes place. Such precipitation may be due in part 
to evaporation and in part to the loss of carbon dioxid from the soil 
solution. The formation of indurated layers in the soil, sometimes 
referred to as “caliche,” is probably due largely to the escape of carbon 
dioxid and the consequent precipitation of calcium carbonate. These 
subsurface impermeable layers often interfere seriously with the penetra¬ 
tion of water. When they are not too thick and hard, they are often 
broken up by deep cultivations; otherwise blasting may be resorted to 
if conditions justify that expense. 
The existence of an underground water table in effect constitutes a 
barrier to the downward percolation of water through the soil unless 
the drainage conditions are such that the whole body of underground 
water is free to move. Furthermore, when the underground water 
stands close to the surface of the ground, it suffers losses by evapora¬ 
tion, and consequently deposits its dissolved material at the point where 
it evaporates. This deposition of dissolved material from underground 
water is one of the most prolific sources of trouble in irrigated land. 
The movement of water through saturated soil is usually very slow. 
Consequently, readjustments of underground water levels that have been 
disturbed by additions from percolating water are very sluggish. The 
rate of these readjustments is influenced by the same factors of soil 
texture and physical condition that have been shown to affect the move¬ 
ment of water through subsaturated soil. In the absence of informa¬ 
tion to the contrary, we find it natural to assume that underground 
water seeks its level just as open water does. This is no doubt true, but 
it does so very slowly. If we make a survey of underground water con¬ 
ditions in an irrigated field, we find that the upper limit of the saturated 
zone is not level, but that in some places the free water stands much 
higher than in others. 
THE ACCUMULATION OF UNDERGROUND WATER 
The occurrence of soluble material in irrigated soils in harmful quan¬ 
tities may be taken as a definite indication that the irrigation water does 
not move downward through the soil. It is obvious that if there were a 
cumulative downward movement of water the soluble material would 
be carried away. It does not appear to be essential that this downward 
movement should be continuous or that any large proportion of the 
water applied to the soli should pass below the root zone. But it doe§ 
seem certain that at least occasionally some water should pass on. 
Unless this is so it is inevitable that in time, either near or remote, the 
