Fig. 3.8. Conceptual illustration of soil water retension

Consider a soil sample in contact with porous stone that allows only the water movement into and out of the soil. The soil is connected to a flexible burette filled with water. The water level in the burette, in the beginning of the experiment, is at the same level as the saturated soil surface as shown in Fig. 3.8. When the soil surface is raised from the previous level, the soil is still saturated due to capillary rise of water in the soil pores against the gravity. The soil starts to desaturate when the level of the soil is increased due to drainage of the pore water under gravitational force. However, the soil still holds some water in the pores due to osmotic and adsorption mechanisms. The relative contribution of these two mechanisms responsible for soil water retention in soils are discussed here, in detail. The dominant pore water retention mechanism at relatively high values of water content or very low values of suction is becomes capillarity. This is governed primarily by pore size distribution in the soils. At low values of water content and correspondingly high values of suction, osmotic mechanism plays an important role in retaining the water.

Osmotic mehchanism

It is well known that the surface of the clay particles is negative charged due to isomorphous substitution. Several cations present in the diffuse double layer around the clay surface, in exchangeable state, due to the negative charge. The concentration of these exchangeable cations at the surface of the interacting particles is higher as compared to that in the solution. This concentration difference drives the water molecules to diffuse toward the interlayer in an attempt to equilize concentration. This mechanism of water retention in clayey soils is referred to as an osmotic mechanism.