It has generally been considered sufficient to cover with water the bottom of the space in which absorption was to occur. The writer found that in order to insure uniform results, it was necessary to cover the entire inner surface of the vessel with wet blotting paper, and even then to exclude carefully all circulation of air by padding the joints with such paper. When only the bottom of the box was covered, samples placed at different levels above the water surface gave discordant results. It was also observed that whenever the thickness of the soil layer exceeded about one millimeter, a long time was required for full saturation; during which inevitable changes of temperature would bring about a deposition of dew on the soil, greatly exaggerating the absorptive coefficient.
In the chamber used at the California station for soil saturation, dimensions 12 × 18 × 19 inches high, the same soil was exposed on a shelf close to the surface of the water, another midway up, a third near the lower surface of the cover; liquid water being in the bottom of the chamber, and the rest covered with wet blotters. It was found that despite these precautions, the lowest soil layer absorbed in the same time as much as ¾% more than the uppermost one.
[71] The partial saturation to a definite extent was effected by means of solutions of calcium chlorid of different degrees of concentration, according to the determinations of Wüllner (Pogg. Ann.). These solutions were placed in a wide, flat dish, over which a layer of soil 1 mm. in thickness was exposed, all being covered with a bell glass lined inside with the same solution, so as to insure equal saturation.
[72] E. A. Mitscherlich (Bodenkunde für Land-und Forstwirthe, p. 156 et al.) claims that all determinations of soil hygroscopicity thus far made are grossly incorrect on account of the dew liable to be condensed on the soil layer from fully saturated air, as the result of slight changes of temperature. He therefore would have all such determination made either in an air-vacuum, or over a 10% solution of sulfuric acid.
Such dew-formation, however, cannot happen to any appreciable extent under the conditions maintained in the writer’s work, viz, absorption within a thick-walled (two-inch) wooden box of the dimensions given above, and sunk in the ground in a cellar in which the temperature varies only a few tenths of a degree during 24 hours. The soil layer of one millimeter thickness being put down in the morning, the 7 hour absorption period falls at the time of slightly rising temperature, as an additional precaution against dew-deposition. Mitscherlich fails, moreover, to show that this source of error produces any wide or serious discrepancies except under such long absorption periods as he finds it necessary to use because of the great thickness of his soil layers. It is doubtful whether the limits of errors in soil sampling do not greatly exceed any of those involved in the writer’s method, and whether such accuracy as is attempted by Mitscherlich is of any practical significance.
[73] Simple as this operation appears to be, it is found to be by no means easy to expel with certainty every small air bubble without resorting to means which would destroy the natural condition of the soil; such as boiling, or the use of the air-pump. These determinations cannot therefore lay claim to great accuracy.
[74] The ascent is of course most rapid, in the large tubes almost instantaneous, when the capillary space is entirely clear; but in the complex system of connected air spaces in soils, the curved paths and the friction obstruct the movement.
[75] I. e., uniform between the narrow limits given.
[76] Ad. Mayer (Agriculturchemie 2, p. 141) designates this minimum content of liquid water as the “absolute” water capacity of the same; but it is not obvious wherein this factor is better entitled to this name than would be the maximum (see Wollny’s Forsch., 1892, p. 1.). M. Whitney (Rep. Proceedings Ass’n Agr. Coll. & Exp’t St’ns, Nov. 1904) gives as a new observation the fact that in soils approaching the drought condition water “does not obey the ordinary physical laws as we recognize them in capillarity.” This evidently refers simply to the well-known phenomenon mentioned above.
[77] This figure represents only a temporary condition; the full height of 46 inches was not reached until the 195th day.