Direct SEM observations indicate that reduction in hydraulic conductivity of soils leached with mixed NaCl-CaCl2solutions of low electrolyte concentrations are related to fine particle dispersion and the formation of a more continuous network of these particles at higher SAR values. This causes blockage of the larger pores in the soil and reduces the rate of water flow by increasing the proportion of small size pores. The changes in a sandy and in a clayey soil were quite similar. Dispersion of the fine particles seems to start at low SAR values (< 8), but the formation of the network becomes apparent only at SAR > 16.