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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 + .

Effects of soil solution cation concentrations and ratios on hydraulic properties must be understood in order to model soil water flow in reactive soils or develop guidelines for sustainable land application of wastewater. We examined effects of different ratios and concentrations of the cations Ca2+, Mg2+, Na+, and K+, using hydraulic conductivity measurements in repacked soil cores, as an indicator of soil structural stability. We examined widely used indices—sodium, potassium, and monovalent cation absorption ratios (SAR, PAR, MCAR)—which assume that the flocculating effects of Ca2+ and Mg2+ are the same, and the dispersive effects of Na+ and K+ are the same. Our laboratory measurements indicate that at any given values of MCAR, the reductions in soil hydraulic conductivity with decrease in electrolyte concentration are not identical for different cation combinations in solution. The hydraulic conductivity curves showed a marked lateral shifting for both the surface and subsurface soils from a winery wastewater application site. This indicates that MCAR is inadequate as a soil stability parameter in soil solutions containing a mixture of Na+, K+, Ca2+, and Mg2+. We employed an unpublished equation that was proposed by P. Rengasamy as a modified index of soil stability for mixed cation combinations, using calculated relative flocculating powers of different cations (‘CROSS’, cation ratio of structural stability). Our observation of lateral shift in hydraulic conductivity measurements at any value of MCAR appears to relate to changes in CROSS values for all cation combinations tested, except for K–Mg solutions, for which a more generalised CROSS equation with modified parameters seems more suitable for calculating the CROSS value. Appropriate modified parameters for use in this generalised CROSS equation were determined empirically, using the experimental data. We derived a combination of threshold electrolyte concentration and CROSS values required to maintain high hydraulic conductivity for the soils at a winery wastewater application site. The potential use of this relationship in developing management practices for sustainable wastewater management at the site is discussed. Further research on the applicability of CROSS and generalised CROSS equations for other soils in the presence of different mixed cation combinations is needed.