Quantifying the aggregation-dispersion boundary condition in terms of saturated hydraulic conductivity reduction and the threshold electrolyte concentration

Marginal quality saline sodic water will be important for agricultural production in water limited environments, and has been demonstrated as suitable for irrigation on a soil-specific basis. Suitability is usually demonstrated as the threshold electrolyte concentration (CTH), defined as a 10-20% reduction in saturated hydraulic conductivity. Others have suggested that the aggregate-dispersion boundary may be used as this threshold, which is also known as the threshold turbidity concentration (CTU). Using a saturated hydraulic conductivity approach, this work sought to quantify the extent of reduction at the CTU and compare this to traditional CTH approaches to define the practicality of the thresholds. The CTU was determined as the point where dispersed clay was detected, and subsequently compared to the CTH with the difference between these compared within the measured domain. The reduction in saturated hydraulic conductivity from a Ca dominant stable condition was determined at each threshold value. It was found that saturated hydraulic conductivity at the CTU reduced by between 44 and 78% for the five Vertisol soils investigated, demonstrating that the CTU varied between soils and was substantially more than the 10-20% reduction in hydraulic conductivity at the CTH. Discussion on application of these thresholds to practical irrigation is provided, and suggests that irrigation water quality application can be optimised on a soil-specific basis. Results reinforce that management guidelines should not be based on the CTU, or at the aggregation-dispersion boundary.