Interpretation of electrical conductivity in a clay soil containing salts

The use of directly measured electrical conductivity of soil to provide an index of soil salinity is discussed. It is suggested that if an anion exclusion mechanism within the microfabric of a wet clay soil can lead to a non-uniform, but stable, range of salt concentrations, then the validity of such an index would be jeopardized. Experiments involved the monitoring of soil electrical conductivity during leaching and diffusion of salts from stable clay-soil aggregates packed in columns and cells. The equilibrium values of conductivity are inversely related to exchangeable cation valence, but are not affected by the type of anion involved. The soil conductivity was often much smaller than the value inferred from the amount of salts actually extracted from the columns or contained in the small isolated electrical conductivity cells. The results are consistent with the operation of a salt exclusion mechanism in the smaller pores of the soil fabric. With monovalent cations and relatively low electrolyte concentrations, the trapping of salts within the microfabric is maximized, while in contrast, with aluminium and aged hydrogen clays the exclusion effect is almost completely suppressed, in accord with double-layer theory. If salts are retained against concentration gradients within the fine structure units of the soil, current-transmitting regions between the units are of correspondingly high resistance, and this is reflected in a soil electrical conductivity value that is low relative to the amount of electrolyte between the electrodes. The observed soil conductivity may, however, bear a simple relationship to the salt fraction of the soil actually 'available' to plant roots.