Origin of the effect of pH on the saturated hydraulic conductivity of non-sodic soils

The effect of pH on the saturated hydraulic conductivity (K) of repacked columns of two non-sodic soils (Healaugh and Fagaga soils) was examined. The K value was greater for the Fagaga soil than for the Healaugh soil which is attributed to the difference in the amount of free iron oxides between the soils. The K values reached a maximum when the pH was close to the point of zero net charge (PZNC) (3.7 and 4.8 for the Healaugh and the Fagaga soil, respectively) and decreased on either side of these pH values. The effect of pH on K was related to its effect on surface charge. As the pH of variable-charge soils approaches the PZNC, the net surface charge decreases resulting in flocculation and maintenance of a high K. Conversely, as the pH deviates from the PZNC, the net surface charge increases, resulting in dispersion and a decrease in K. There was a negative relationship between the amount of dispersed clay and the K values, suggesting that clay dispersion and the resulting clogging of pores decreases K in these soils.