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The relative effects of Na + and K + on soil structural stability are not clearly defined by the existing literature. This is an important issue for the application of winery wastewater on soils as it contains high levels of K + and varying levels of Na + . To evaluate the relative effects of Na + and K + on soil structural stability both surface and subsoil from a land application site for winery wastewater were used to assess changes in soil hydraulic conductivity in repacked soil columns. The soil was rich of smectite, 51–56%, with minor presence of illite, 5–8%, and kaolinite 10% clays. Solutions with sodium adsorption ratio (SAR) and potassium adsorption ratio (PAR) of 5–40, where the monovalent cation was Na + or K + and the divalent cation was Ca 2 + or Mg 2 + were used to leach the soil columns, at electrolyte concentrations ranging from 2.5 to 640 meq L − 1 . In both surface and subsoil, percolating solutions with PAR or SAR, comprising all cation combinations, of 20 and 40 caused a decrease in hydraulic conductivity as electrolyte concentrations reduced. However, in PAR solutions the decreases in hydraulic conductivity were significantly smaller than the corresponding SAR solutions. These results indicated greater soil stability in the presence of K + relative to Na + .