Another and not so readily intelligible effect observed by King is that when the surface-soil is wetted, moisture may be withdrawn toward the surface from the lower layers. In one experiment he found that when water was applied on the surface so as to add two pounds of water to each surface foot in several soils, at the end of 26 hours there had been an increase of three pounds in the same, and a loss of one and three quarter pounds from the second and third feet. The cause of this translocation is probably a “distillation” of the subsoil moisture toward the cooled soil; the fact that it occurs is of practical interest, since it seems to show that wetting the upper portion of the soil by cold rain or irrigation may tend to raise additional supplies from below. At the change of seasons we not uncommonly find, in digging tree holes or wells, a wet streak at from 9 to 18 inches below the surface, caused evidently by the condensation of subsoil moisture, at the limit of a cold zone resulting from the penetration of unseasonable temperature (“cold snap”) from above. Such movements of soil-moisture by means of evaporation and recondensation within the soil can of course take place even when the minimum of liquid absorption has been reached and direct capillary movement has ceased. It is, as it were, dew within the soil.
Proportion of Moisture Available to Growing Plants.—Not all the capillary moisture contained in soils is available to plants, as can readily be seen from the fact that many plants, especially when growing in pots, begin to wilt while the soil still appears visibly moist. The limit of wilting differs greatly in different plants, and in the open ground it is difficult to ascertain that limit, because the deeper roots continue to supply moisture from moister substrata. Hence potted plants wilt while the soil appears much moister than when the same grow in the field. King[79] has determined the amounts of moisture down to 43 inches in a Wisconsin soil in which clover and corn were at the wilting point, as in the following condensed table:
| Clover. | Maize. | Fallow ground. | |
|---|---|---|---|
| First 12 inches, clay loam | 8.44 | 7.03 | 17.01 |
| Second 12 inches, reddish clay | 12.84 | 11.79 | 19.86 |
| 4 to 30 inches, sandy clay | 13.52 | 10.84 | 18.56 |
| 40 to 43 inches, sand | 9.53 | 4.17 | 15.90 |
It is plainly shown here that the roots of clover and corn were unable to utilize the higher moisture-content of the subsoil-clay to the same extent as the smaller amounts present in the surface foot, and in the sandy substrata. Evidently the moisture in the clay soil was more tenaciously retained.
This is doubtless due, as King shows, to the equal thinness of the moisture film remaining on the soil grains in either case; the number of grains, and therefore the aggregate surface holding these films, being much greater in the clay than in sands; hence the higher water content.
It is interesting to compare these figures given by King for clover and maize at the wilting-point, and fallow ground adjacent, with those given by Eckart (Rep. Expt. Sta. Haw. Sugar Planters’ Ass’n., 1903) for those affording good growing conditions for sugar-cane on the (highly ferruginous) soils of that station. The plots were irrigated at the rate of one, two and three inches of water per week, allowance being made for the rainfall. Two inches proved, on the whole, to give the best average results for production. The moisture determination of the soil under the two-inch regime gave an average moisture content of 29.13% in the first foot of soil. It is not stated what was the hygroscopic coefficient of that soil, but it was probably very high; in the neighborhood of 21.5%, judging by the determinations made with six Hawaiian soils at the California Station. This would indicate about 7.63% of free moisture as the optimum for sugar-cane.
Moisture-requirements of Crops in the Arid Region.—Plants (particularly broad-leaved ones) which have made a brash growth during a period of abundant moisture, will wilt quickly when sunshine returns, and take some time to adapt themselves to the drier conditions. On the other hand, plants accustomed to dry air and scanty soil-moisture, will not wilt or suffer under what would elsewhere be considered very rigorous conditions. Loughridge[80] has made numerous determinations of moisture in soils in which crops were beginning to suffer, and others on similar soils that were growing normally, and found that in general, not only were the differences in moisture content considerably less than in the case above quoted from King’s observations, but that the amounts of free moisture required by various crops in the arid climate of California were surprisingly small.
The tables below show the results of observations made by Loughridge during several drought years in California; so arranged as to show the differences of moisture content for the same crop in different soils. It will be observed that in all cases where a crop growing on a clay soil could be compared with the same on a lighter soil, the moisture required to keep the crop in good condition was very much greater in the clay than in the loam or sandy soils. In the case of apples, e. g., 8.3% of water was abundant to keep the trees in excellent condition on a loam soil, while on a clay soil holding 12.3% the condition was very poor. That this difference is due in the main to the difference in the hygroscopic-moisture coefficient of the respective soils, is plainly apparent in several cases. It is therefore not the total moisture content, but the free moisture present in excess of what is held by hygroscopic absorption, that determines the welfare of the plant.
By determining, first, the total moisture in the soils, as taken in the field, then, after allowing them to become air-dry, determining the maximum of hygroscopic moisture they would absorb ([see p. 198]), Loughridge found by difference the amount of free moisture, or liquid water which must be present in the soil to prevent the crops from suffering. An exceptionally good opportunity for these observations was offered by the dry season of 1898, during which crops suffering and not suffering, on identical lands, could easily be found. The determinations were always made for each foot of the upper four feet of the land in the immediate neighborhood of the trees or among the field crops. The first table exemplifies the method of procedure; the second gives the summary of results for the several crops and trees, as calculated from observations made during the season.
TABLE SHOWING CONDITION OF
CROPS ON VARIOUS SOILS UNDER
DIFFERENT MOISTURE-CONDITIONS.