Showing papers in "Transactions of The Faraday Society in 1922"

Section II.—Papers and discussion on “soil moisture”. The system soil-soil moisture

Abstract: The system soil-soil moisture is exceedingly complex. On the one hand there is the soil-a mass of particles of all shapes and sizes, of varying chemical composition, and intimately associated with both organic and inorganic colloidal materials; on the other hand there is the soil moisture, holding in solution certain substances, some of which are essential, others detrimental, to plants and organisms. The system as a whole is subject to ever-changing meteorological conditions, as well as the effects of growing vegetation and organisms, and, at frequent intervals, abrupt changes in many factors occur owing to manurial and cultivation operations. Practically all of the factors are interdependent-if one of them changes, the remainder will also change in the necessary amount to suit the new conditions. Another way of stating this is that soil phenomena are dynamic in character, not static. When the relations between the soil and its moisture content were first examined the obvious hypothesis to take as a starting point was that the moisture was distributed in a thin continuous film over and around the soil particles. This view was eventually set out by Briggs (I) over twenty years ago, who treated the water films from the standpoint of the surface tension over their curved surfaces. This treatment is closely modelled on the wellknown experiments of Reinold and Rucker (2) on soap bubbles which hardly need further mention here. It is instructive, however, to consider one case. Imagine a long row of wet spheres each of the same radius and touching one another, and let the row be raised from a horizontal to a vertical position. The liquid will drain downwards and drip from the bottom sphere. When equilibrium is established, the curvature of the annular ring of water between adjacent spheres will diminish, and the actual volume of this ring will increase from top to bottom of the row, the difference of pressure between adjacent rings being that necessary to support the weight of the film connecting them. Hence in a cylinder of moist soil, made up of coarse particles, there is a progressive increase in moisture content from top to bottom; in a finer soil this gravitational effect is much less marked owing to the smaller interstices between the grains. For each case, however, and for a given depth of soil, a distinction can be drawn between gravitational water, which drains away under the action of gravity, and capillary water, which is capable of movement over the soil particles under the forces of surface tension, whenever the equilibrium is disturbed. Briggs further distinguishes hygroscopic water, which, as the name implies, is the