HYGROSCOPIC WATER.
Soils artificially dried so as to deprive them of all their moisture, when exposed to moist air absorb water vapor with great energy at first; both the rapidity of absorption and the amounts absorbed, when full time is given, varying greatly with their nature. Sandy soils, broadly speaking, absorb the smallest amounts; while clayey soils, and those containing much humus, or finely divided ferric hydrate, take up the largest proportion.
The figure expressing the amount of aqueous vapor absorbed at the standard temperature of 15° Cent., is called the coefficient of moisture absorption. For one and the same substance, this coefficient rises as the grain becomes finer, the surface being correspondingly increased ([see chapt. 6]).
The table below indicates the effect of the three substances mentioned in increasing moisture absorption as compared with a very sandy soil from the pine woods of Mississippi, and a gray silt or “dust” soil from Washington, very fine-grained but poor both in humus and ferric hydrate. (For details of the physical composition of the Mississippi soils see table in [chapt. 6, p. 93]). A highly ferruginous soil from Oahu shows plainly the effect of that substance.
TABLE SHOWING INFLUENCE OF SILT, SAND, CLAY,
FERRIC HYDRATE, AND HUMUS ON
MOISTURE ABSORPTION.
| 248 | 79 | 238 | 230 | 246 | 220 | 215 | ||
|---|---|---|---|---|---|---|---|---|
| Miss. Pine Hills Sandy Loam. | Wash’n Dust Soil. | Miss. White Pipe Clay. | Miss. Flat- woods Clay Soil. | Misc. Ferru- ginous Clay Soil. | Oahu Ferru- ginous Laterite. | Miss. Marsh Muck. | Miss. Marsh Soil. | |
| % | % | % | % | % | % | % | % | |
| Hygr. Moisture | 2.48 | 4.92 | 9.09 | 9.33 | 18.60 | 19.66 | 21.00 | 15.40 |
| Clay | 2.94 | 1.27 | 74.65 | 25.48 | 28.15 | ? | Tr. | Tr. |
| Ferric Hydrate | 1.64 | .15 | 12.10 | 41.00 | ||||
| Humus | .55 | .44 | 0.00 | .50 | little | 3.33 | 66.10 | 19.83 |
| Finest Silts (.01-.0250 mm.) | 60.10 | 45.04 | 23.15 | 68.60 | 40.33 | 45.66 | 33.94 | 8.70 |
| Sands, f. and c. (.0250-.50 mm.) | 31.20 | 42.40 | .20 | 4.70 | 15.671 | 70.18 |
It will be noted that the greater fineness of grain in the Washington dust soil induces a higher absorption of moisture than occurs in the sandy soil from Mississippi, although the latter contains more clay. Comparison of the figure for the Mississippi pipe-clay and clay soil with the ferruginous soils, from the same state and from Oahu, indicate plainly the influence of the ferric hydrate in increasing absorption; although in the latter case the clay determination was not made, because of the excess of ferric hydrate. The influence of humus is plainly shown in the case of the marsh muck and soil, neither of which contain any appreciable amount of either clay, or ferric hydrate in the finely diffused condition. The relatively slight difference in the absorptions of muck and soil is due to the only partial humification of the organic matter in the former, while in the soil the humification is sensibly complete, and the sand forming the body of the material serves to render it more loose.
These data, referring to natural materials, while not as complete as could be desired, are sufficient to prove the facts, and seem preferable to any artificially devised imitation of their kind.
Influence of Temperature, and Degree of Air-Saturation.—The amount of moisture absorbed varies materially both with the temperature, and with the degree of saturation of the air to which the soil is exposed. Schübler, Knop and other earlier observers, operating with earth exposed to air only partly saturated, and with soil layers of considerable thickness (in watch glasses), found that the absorption decreased as the temperature increased, according to a law formulated by Knop. The writer found that under the conditions established in the experiments of Knop and others, the air was not nearly saturated,[70] so that these determinations are marred by ineliminable faults, the more as the soils used are only designated in general terms, as “garden soil,” “loam,” “peaty land,” etc., without any definite indication of their actual physical or chemical constitution. The writer therefore undertook to correlate these coefficients, determined with respect to completely saturated air, with the physical composition of certain soils, as determined by means of the methods heretofore described.
Some of the data so obtained are given in the table of physical soil composition on [page 93, chapt. 6]. They have since been extensively supplemented by additional determinations, but without materially changing the coefficients approximately corresponding to the several designations accepted in farm practice. Experiments conducted by the writer have conclusively shown that Knop’s law of decrease of absorption with rise of temperature not only is not true for fully saturated air, but must be reversed; the fact being that the amount of water absorbed by the soil increases in a fully saturated atmosphere (i.e., in presence of excess of water) as the temperature rises, at least between 15 and 35 degrees Cent. Thus, fine sandy soil which at 15° absorbed 2% of moisture, took up 4% at 34°; while loam soil absorbing 7% at 15°, showed nearly 9% at 35°; an increase of 2% in each case. But in partially saturated air[71] it was found that, as stated by Knop, the amounts absorbed steadily decrease, though not according to the law announced by him. Taking as a unit the moisture absorbed at 15°, it was found that in air three-fourths saturated, ¾ of the unit was taken up by the soil; at half saturation, nearly the proportional amount; but at one-fourth saturation the earths absorb materially more than a similar proportion, being then capable of withdrawing moisture from greatly undersaturated air. Since air thus undersaturated occurs not uncommonly in the arid regions of the world, the fact that the soil cannot be farther dried by such air of the same temperature, is of some practical significance.