Reduction in Hydraulic Conductivity in Relation to Clay Dispersion and Disaggregation

The saturated hydraulic conductivities (HCs) of three California soils (Arlington, Fallbrook, and Pachappa Haplic; Durixeralfs, Typic and Mollic Haploxeralfs, respectively) were determined at successively diluted electrolyte solutions with a constant sodium adsorption ratio (SAR) of 0 or 10, followed by deionized water. When these soils were Ca‐saturated and then leached with deionized water, very little clay dispersion (0–0.05 g soil column −1 ) occurred and yet the HC was reduced to about 2, 24, and 72% of the maximum HCs for Fallbrook, Arlington, and Pachappa soil columns, respectively. At SAR 10, replacement of the most dilute solution (2.5 mol m −3 ) with deionized water resulted in a rapid reduction in the HC to zero for both the Arlington and Fallbrook soils. The appearance of clay in the effluent solutions indicated that the dispersed clay was mobile in the soil columns and thus not plugging a major part of the soil pores. The major cause of reductions in HC is concluded to be the reduction of large conducting pores as a result of aggregate failure (slaking). This conclusion is supported by the fact that the HC of Fallbrook soil, which contained the least stable aggregates, was reduced to zero despite its having the greatest proportion of macropores; and that clay dispersion did not cause plugging of conducting pores for the Pachappa soil, which had the most stable aggregates despite its having the highest proportion of micropores.