Soil Permeability in Relation to Sodicity and Salinity [and Discussion]

Difficulties encountered in the irrigation of sodium-affected soils or in the reclamation of sodic--saline soils can be circumvented by controlling the electrolyte level in the irrigation water, which prevents deterioration of the soil structure. The threshold concentration concept simply expresses the minimum level of electrolyte required to maintain the soil in a permeable condition for a given degree of sodium saturation of the soil colloids. It is shown that decreases in permeability at concentrations below the threshold concentration are occasioned by increased clay-particle interaction owing to the extensive development of diffuse double layers which give rise to enhanced swelling. Mechanical failure is associated with this swelling and deflocculation of clay particles. This behaviour of sodium-affected soils is considered in relation to the operation of van der Waals forces, hydration forces and osmotic repulsive forces arising from the development of diffuse double layers on particle surfaces. Considerations of ion-exchange equilibria between particle surfaces and the soil solution (irrigation water) are discussed in relation to the threshold concentration concept, and the application of the concept in irrigation practice is illustrated by reference to several case histories.