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

The maintenance of an appropriate soil structure and favorable soil hy draulic conductivity (HC) is critical for the sustainability of irriga ted 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, u ncertainties still remain with regard to the mechanisms responsible fo r 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 va lues on clay dispersion and HC of four calcareous, micaceous, arid-zon e soils and to ascertain the contribution of the osmotic explosion eff ect 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 dil uted electrolyte concentrations of 500, 100, 50, 10, 5, 3, and 1 meg L (-1) of SAR 0 and 10 (meg L(-1))(0.5), followed by deionized water CC < 0.01 meg L(-1)). The observed decreases in HC at C levels below the flocculation value (FV, i.e., minimum C without clay dispersion) of th ese soils were attributed to clay dispersion and partial plugging of c onducting 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 respons ible for a cumulative 72% reduction in HC, indicating that very small quantities of dispersed clay had a dramatic effect on the water-conduc ting properties of these soils. Significant decreases in HC also occur red at C levels well above those at which dispersed clay appeared in t he 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 originate d in systems where the steepest concentration gradients developed betw een the micropores and the macropores (i.e., in the soiI:sand columns, close to the advancing front of the displacing solution and for chang es in electrolyte concentrations from 500 to 100 meg L(-1)). We conclu ded 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.