It is curious that in this case the “clay” showed a rise markedly below that of the finest granular sediment, despite the extreme fineness of its particles. This proves plainly that the physical nature of colloid clay is unlike that of the granular sediments; as has been repeatedly mentioned above.
Maximum and Minimum of Water-holding Power.—It is clear that at the base of the columns of soils just considered, the maximum of water-absorption of which the soil is capable will have been brought about; while at the top of the same column, the minimum of possible liquid absorption (continuous films of water) will exist. The same minimum moisture-condition will be produced when a limited quantity of water is placed with a large mass of soil; the moisture will spread to certain limits, until the surface films of water have all acquired uniform tension; and will then cease to extend, except by evaporation and hygroscopic absorption.[76] It is clear that the same condition will be brought about in the course of time at the top of a soil column in which water has percolated from above; and hence the minimum mentioned, aside from evaporation, represents approximately the usual condition of the soil near the surface within a variable time after a rain, or irrigation, when the descending water column has attained a length corresponding to the height to which the water would have risen from below in a tube arranged as shown on [p. 205]. It is therefore a condition of very frequent occurrence in the arid region.
Capillary Water held at Different Heights in a Soil Column.—To determine the amounts of water held in the different portions in columns of soils in which water ascends by capillary rise, the following plan was adopted by the writer in collaboration with Loughridge (Rep. Calif. Sta. 1892-4, p. 99).
Instead of glass tubes the soils to be tested were placed in copper tubes one inch in diameter, divided into segments six inches long, and flattened on one side. In the flattened side a slot half an inch wide was left, and glass plates, held in position by rubber elastics, were cemented on the slotted side by means of paraffin, to prevent a sifting-out of the soil. The short sections can be connected at the ends like joints of stove-pipe, and the earths can be easily introduced in proper, even condition. It was thus possible to gain access to any portion of the column at any time, for the taking of samples.
| WATER CONTENTS OF SOIL COLUMNS AT VARIOUS HEIGHTS ABOVE WATER LEVEL. | |||
|---|---|---|---|
| No. | 233 | 1197 | 1679 |
| Height above Water Level. | Sandy Soil, Morano. | Sandy Alluvium, Gila. | Adobe, Berkeley. |
| 47 inches | 4.33 | ||
| 42 inches | 10.26 | ||
| 36 inches | 11.99 | ||
| 30 inches | 15.26 | ||
| 24 inches | 21.39 | 10.26[77] | |
| 18 inches | 27.63 | 29.48 | |
| 12 inches | 3.93 | 32.48 | 33.04 |
| 6 inches | 14.15 | 35.04 | 38.47 |
| 3 inches | 38.49 | ||
| 1 inch | 24.34 | 36.64 | 44.41 |
Since gravity limits the capillary ascent in a progressive ratio, as shown in diagram 39, it is obvious that the true maximum saturation can exist only in a very short (strictly speaking, an infinitesimally short) vertical column. The least practicable height for experimental work being about 1 cm. (⅖ in.), the writer has adopted for the purpose of rapid determination of this factor, the use of a brass cylinder 1 cm. high and of such width as to contain, for the sake of convenience, 25 or 50 cm. of soil. This cylinder has a finely perforated bottom, which may be covered with filter paper; after being filled with soil which has been struck level, and weighing, it is immersed to 1 mm. depth in distilled water and allowed to rest for an hour; then quickly dried outside and beneath with filter paper, and again weighed. The amount of water found by difference should for all practical purposes be referred to the volume, not to the weight, of the soil, so as to eliminate the error arising from the varying specific gravity of the latter.
In most cases the surface of the soil in the sieve cylinder remains level after wetting; but sometimes it swells so as to rise above its dry level, even to the extent of nearly 30% ([see chapter 7, p. 114]). This happens especially in strongly ferruginous soils. In the case of “black alkali” soils, in wetting an enormous collapse sometimes takes place ([see chapter 22]).
If it be desired to determine also the minimum liquid absorption (see below), the surface of the wet soil is first covered with air-dry soil, to absorb the surplus moisture, and finally with soil previously saturated with hygroscopic moisture; the added soil being each time thrown off and finally the surface “struck” level with a tense silk thread before weighing. Corrections must be applied for the usual increase in weight, from the addition of soil, and for the hygroscopic moisture.
While the minimum of liquid absorption can thus be determined quickly, without awaiting the capillary ascent of a water column, and if sufficient time is given can also be determined in higher columns, as proposed by Mayer (Wollny’s Forsch. Vol. 3), the maximum cannot thus be determined without gross inaccuracy. In determinations made by the writer it was found that the figures for the minima of very different soils (clayey and sandy) of the arid region, differ proportionally much less than do the respective maxima. In few of these soils it was found to exceed about 10 per cent, and it scarcely fell below 4 per cent even in very sandy soils. A very deep, sandy soil, which had been irrigated in May, and upon which no rain had since fallen, showed in July in the second foot, upon which rested ten inches of fully air-dried soil free from vegetation, a water-percentage of eight per cent.[78]
Capillary Action in Moist Soils.—In the preceding discussion the case of columns of air-dry soils, so common in the arid regions, has been considered. It is obvious that a soil column holding the minimum of capillary water may be of any height; so that when, as happens in the open field, the rain water soaks down beyond the range of capillary rise in a given soil, the upper portions of the latter, above that range, will remain at the minimum of moisture-content so long as it is not depleted by evaporation. King has made extended observations on soil columns ten feet high and moistened throughout the mass. Capillary movement takes place in moist soils much more rapidly than in dry ones, although when sufficient time is given the final adjustment will of course be the same. King’s experiments showed that evaporation at the surface of the tenfoot columns caused a sensible depletion of the water content originally existing at the depth of ten feet, in the course of 314 days. While so slow a movement might not be of any benefit during the growth-period of shallow-rooted annual crops, the fact shown is of importance to permanent plantings, as of trees and vines.