In view of the highly variable composition of soils and of the doubtless varying hygroscopic properties of their several physical constituents, it is not to be expected that any one numerical law will hold good exactly for all kinds of lands. Mineral powders, colloidal clay, ferric hydrate, aluminic hydrate, the zeolites, humus, and other hydrates known to occur, doubtless each follow a different law in the absorption of moisture and gases; so as to modify the hygroscopic properties[72] of the soil in accordance with their relative predominance in each case. (See table of absorption of gases, [chapter 14]).
Utility of Hygroscopic Moisture to Plant-growth.—The early experimenters considered the hygroscopic moisture of the soil to be of very great importance to the welfare of crops. Within the last twenty-five years much doubt has been cast upon this claim, even to the extent of stating that “the hygroscopic efficacy of soils must be definitely eliminated from among the useful properties” (Mayer’s Agriculturchemie, vol. 2, p. 131). Yet Mayer himself concedes the cogency of the experiments made by Sachs, which proved that dry soil immersed in a (probably not even fully) saturated atmosphere is capable of supplying the requirements of normal vegetation; thus explaining the obvious beneficial effects on vegetation of the summer fogs prevailing in portions of the arid region, e. g.; on the coasts of California and Chile.
Mayer’s experiments relied upon to prove the uselessness of hygroscopic moisture to plant growth, were carried out in flower-pots, in which it was plainly shown that the plants wilted before even the visible liquid (capillary) moisture of the earth was entirely exhausted. But this simply proves that under such artificial conditions, plants cannot withdraw moisture from the soil rapidly enough for their needs. In nature, and notably in the arid regions, the chief supply of water is received through the deep-going main roots, while the bulk of the active feeding roots of the plant may be surrounded by almost air-dry soil; under which conditions, as Henrici (Henneberg’s Journ., 1863, p. 280) has shown, slow growth and nutrition occurs even in such plants as the raspberry, a native of humid climates. But in the arid region this is the normal condition of the native vegetation through most of the rainless summer. That a higher moisture-coefficient does not necessarily imply that a larger amount of moisture can be withdrawn from the soil by the plants, is undoubtedly true in some, but not in all cases; for in soils rich in humus, the moisture is more freely shared with the roots than in non-humous, clay lands.
The higher moisture-absorption is however of the most unquestionable service in the case of the occurrence of the hot, dry winds that so frequently threaten the entire crops of some regions. In this case the soil containing the greater amount of moisture requires a much longer time to be dried, and heated up to the point of injury to the roots, than in the case of sandy soils of low absorptive power, whose store is exhausted in a few hours and then permits the surface to be heated up to the scalding point, searing the stems and root crowns. That such injury occurs much sooner in sandy lands than in well-cultivated clay soils, is a matter of common note in the arid region.
Summary.—The significance of hygroscopic moisture in connection with plant growth may then be thus summarized:
1. Soils of high hygroscopic power can withdraw from moist air enough moisture to be of material help in sustaining the life of vegetation in rainless summers, or in time of drought. It cannot, however, maintain normal growth, save in the case of some desert plants.
2. High moisture-absorption prevents the rapid and undue heating of the surface soil to the danger point, and thus often saves crops that are lost in soils of low hygroscopic power.
Capillary Water.
The liquid water held in the pores of the soil, in the form of surface films representing the curved surface seen in capillary tubes, and therefore tending to cause the water to move upwards, as well as in all other directions, until uniformity of tension is established, is of vastly higher importance to plant growth than hygroscopic moisture. It not only serves normally as the vehicle of all plant food absorbed during the growth of the usual crops, but also, as a rule, to sustain the enormous evaporation by which the plant maintains during the heat of the day, a temperature sufficiently low to permit of the proper operation of the processes of assimilation and building of cell tissue.
Comparatively few plants have roots adapted to healthy action while submerged in water, excluding them from free access of the oxygen of the air; and when such roots are formed by plants not naturally growing in water or swampy ground, they differ so far from earth-roots in their structure that when transferred to soil they usually die, normal earth-roots being gradually formed instead. Conversely, there is for all land plants a definite time-limit beyond which their roots cannot live, or at least remain healthy, in submersion. Thus grain fields will with difficulty recover from a week’s total submersion; while young rice fields will resist considerably longer. When in the resting (winter) condition vineyards will bear submergence for thirty-five and even forty days, deciduous orchards about three weeks; but when in the growing condition, injury is suffered much more quickly.