HYDRAULIC CONDUCTIVITY, DISPERSION AND OSMOTIC EXPLOSION IN ARID-ZONE SOILS LEACHED WITH ELECTROLYTE SOLUTIONS

Abstract

The maintenance of an appropriate soil structure and favorable soil hydraulic conductivity (HC) is critical for the sustainability of irrigated soils. Clay dispersion and/or swelling satisfactorily explain the observed reductions in HC at relatively low electrolyte concentrations (C) and/or relatively high sodium adsorption ratios (SAR). However, uncertainties still remain with regard to the mechanisms responsible for observed reductions in HC at relatively high C and/or low SAR values. Our objective was to determine the effect of a range of C and SAR values on clay dispersion and HC of four calcareous, micaceous, arid-zone soils and to ascertain the contribution of the osmotic explosion effect to the observed HC reductions at high electrolyte concentrations. We determined the saturated HC and clay dispersion in the leachates of soil and soil:sand (1:2 w/w) columns equilibrated at successively diluted electrolyte concentrations of 500, 100, 50, 10, 5, 3, and 1 meq L−1 of SAR 0 and 10 (meq L−1)0.5, followed by deionized water (C < 0.01 meq L−1). The observed decreases in HC at C levels below the flocculation value (FV, i.e., minimum C without clay dispersion) of these soils were attributed to clay dispersion and partial plugging of conducting pores. A total dispersed clay of less than 2 g kg−1 soil (i.e., less than 0.8% of the total soil clay in the column) was responsible for a cumulative 72% reduction in HC, indicating that very small quantities of dispersed clay had a dramatic effect on the water-conducting properties of these soils. Significant decreases in HC also occurred at C levels well above those at which dispersed clay appeared in the leachate. The estimated threshold electrolyte concentrations (Ct at which HC starts to decrease) were higher than the corresponding FV of these soils, indicating that clay dispersion was not responsible for the observed reductions in HC. The steepest reductions in HC originated in systems where the steepest concentration gradients developed between the micropores and the macropores (i.e., in the soil:sand columns, close to the advancing front of the displacing solution and for changes in electrolyte concentrations from 500 to 100 meq L−1). We concluded that an osmotic explosion effect (i.e., the osmotic water movement into, macroscopic swelling within, and the destruction or slaking of the aggregate) was responsible for the observed HC reductions at these high electrolyte concentrations.