Summary.—Two salient points are revealed by even a cursory inspection of the preceding diagram, viz.:
1. The downward percolation is most rapid in the same soils in which the capillary ascent is quickest, that is, in the coarse, sandy soil.
2. The rapidity of percolation decreases materially as the wetted soil column increases in length.
The first point is readily foreseen and needs no comment. As regards the second, it results from the fact that as the wetted column lengthens the frictional resistance increasingly counteracts the effects of the hydrostatic pressure from above, until the water’s descent becomes but little more rapid than would be its lateral diffusion, or its ascent at the end of a similar column supplied by capillary rise from below. In both cases the frictional resistance has so far counteracted the effect of gravity that the capillary coefficients of the soil-material become the controlling factors of the water movement.
Influence of Variety of Grain-sizes.—King (Physics of Agriculture, pp. 159, 160), compared the rapidity of the percolation of water through definitely graded pure sands on the one hand, and a sandy loam and a clay soil on the other. The materials were arranged in 8-foot columns fully saturated with water at the outset, and then allowed to drain freely. The following abridged table shows the tenor of his results:
TABLE SHOWING RELATIVE RAPIDITY OF
PERCOLATION IN PURE SANDS AND SOILS,
IN INCHES OF WATER DRAINED OFF.
| Diameter of Uniform Sand Grains. | First 30 minutes. | Second 30 minutes. | Total in one hour. |
|---|---|---|---|
| .475 mm. | 10.25 | 4.68 | 14.93 |
| .155 “ | 5.67 | 4.52 | 10.19 |
| .083 “ | 1.21 | .85 | 2.06 |
| Soils. | First 21-23 hours. | First 10 days following. | Second 10 days following. | Total in about 505 hours. |
|---|---|---|---|---|
| Sandy loam | 2.64 | 5.07 | .91 | 8.62 |
| Clay loam | 1.96 | 2.11 | .49 | 4.56 |
This table is very instructive in showing the great difference in the rapidity of percolation in materials of uniform, even-sized grains, as compared with such as contain particles of many different sizes, in which the interspaces of the larger ones are filled more or less closely by the smaller sizes of particles ([see chapter 7, p. 109]). While it is true that we have no definite physical analysis of the soils here used, the differences are so great as to be sufficiently striking. Compare the percolation through the sand of .155 mm. uniform grain-size (a fine sand), during the first half hour, with that through the sandy loam during the first 21 hours. Twice as much water has passed from the sand as from the soil in one forty-second part of the time. Comparing similarly the finest sand, .083 mm. in diameter, with the clay loam, we find the difference to be as one to seventy-three. It is thus evident that but for the variously assorted sizes of the soil-particles, water would not be held long enough to supply plant growth.
Percolation in Natural Soils.—In artificial percolation experiments, as well as during a fall of rain, the gradual settling of the fully wetted soil-column produces a compacting of that portion of the mass, that increasingly impedes the downward penetration. The effect of this under natural conditions is readily seen in the fact that after the first, rapid absorption of falling rain by the soil when in good tilth, there is a gradual slackening of the process even when the rain is fine and slow, causing a perceptible increase of the runoff until, should the rain continue for some time, the absorption becomes so slow as to cause all, or nearly all the water to drain off the surface. The soil is then called “saturated,” having really arrived at that point right at the surface, and to a depth varying according to the duration and amount of rain, and the natural perviousness of the land.