Mar. i, 1924 
Movement of Water in Irrigated Soils 
631 
It will be seen from the table that the rate of water penetration was 
six to eight times as fast in the samples that had not been subjected to the 
action of the alkaline salts as it was in the other sample. The penetration 
rate on these unaffected clay soils is at first nearly twice as great as 
that of the sandy loam soil (No. 337) shown in Table VI. Furthermore, 
the penetration rate was better sustained in the clay soil than in the 
slowly permeable sandy loam, as is shown by the depth of water penetra¬ 
tion in 30 days. 
The results of the laboratory experiments with dry pulverized soils in 
glass tubes afford a means of stating in definite terms what is continually 
to be observed in the field. They serve to establish two facts which have 
long been known to irrigation farmers: (1) That some soils absorb 
water readily while others do not; and (2) that the rate of water absorp¬ 
tion is affected by factors other than the texture of the soil. 
ablK VIII .—Penetration of water in each of four samples of heavy clay soil from the 
Belle Fourche project in South Dakota 
Time elapsed. 
Depth of penetration (indies). 
Soil No. 338. 
Soil No. 339. 
Soil No. 340. 
Soil No. 341. 
1 hour . . 
0. 8 
4.2 
9-7 
17.8 
22. 2 
3-4 
7. 6 
3. 7 
6 hours. 
i- 5 
8. 2 
1 day. 
2. 3 
14-3 
18.5 
22. 5 
25. 8 
3 °. 1 
37-4 
15-3 
* 9 - 3 
23. 2 
26. 0 
2 days. 
2.8 
a davs. 
3 * 5 
26.8 
6 days. 
v 
3-8 
3 °. 2 
34 * 6 
42. 1 
10 davs . 
29. 6 
38. 2 
■30 davs .. . . 
O w ... 
THE PERCOLATION OF IRRIGATION WATER 
In the preceding pages reference has been made to the penetration of 
water into pulverized dry soil. Another situation which has to be dealt 
with in irrigation practice is the movement of water through a soil 
already saturated or nearly so. This movement of water through a 
saturated soil is here designated as percolation. The justification for 
making a distinction in the present use of these two terms is merely one 
of convenience. The movement of water into a dry soil may be held to 
be percolation as truly as the movement of water through a saturated soil. 
In the first case, the moving water partially or completely displaces air, 
and in the second case it displaces the water already present. 
It is a matter of convenience, however, to distinguish the two condi¬ 
tions by specific terms. In the application of irrigation water to a field 
the purpose ordinarily is to restock the soil reservoir with water for the 
use of crop plants. In this process the chief concern is that the water 
applied to the surface of the field shall enter the soil promptly and be 
held within the root zone for subsequent use by crop plants. 
There are situations which occur not infrequently in irrigated lands 
where the movement of water through a saturated soil has to be con¬ 
sidered as a special problem, and the factors which influence this move¬ 
ment need to be understood. When the water that is held in the soil 
contains so much dissolved salt as to be injurious to plant growth, it 
