Particle coatings affecting the wettability of soils

The fundamental principles underlying the process of wetting show that a reduction in the surface tension of a solid substance to be wetted reduces the wettability. Conversely, a reduction in the surface tension of the applied liquid increases the wettability. The wetting of a soil, which exhibits water-repellency in an air-dry state, can sometimes be achieved by leaving water in contact with this soil. There is some evidence that this wetting is achieved by an interaction of the solid and the liquid phase, leading to a reduction in the surface tension of the liquid. The phenomenon of advancing and receding angle of contact had led to an hypothesis that the angle of contact changes as the soil wets up or dries out. Some evidence is presented which does not support such an assumption, as the angle of contact seems fixed at a moisture content just above air-dry. There is some evidence that under certain conditions an improvement in the base status of the soil may lead to improved wettability. Particle coatings by hydrophobic films are in some cases responsible for the observed water-repellency. There is some evidence that this film is strongly adsorbed. No quantitative relationship could be detected between the characteristics of substances extracted by chloroform from a problem soil and the degree of unwettability of such a soil.