Dispersion of Pure Clay Minerals as Influenced by Calcium/Magnesium Ratios, Sodium Adsorption Ratio, and Electrical Conductivity

Sodium concentration and the soil solution Ca/Mg ratio may influence soil dispersion and water movement. This study investigated the impact of electrical conductivity (EC), Na adsorption ratio (SAR), and Ca/Mg ratios on dispersion thresholds of pure clay minerals (montmorillonite, illite, and kaolinite). Replicated laboratory studies were conducted where the impact of SAR (1, 5, 12, or 24), EC (0–16 dS m−1), and specific Ca/Mg ratios on clay dispersion was determined. The results showed that Ca/Mg ratios did not influence clay dispersion of montmorillonite and kaolinite. For illite at a SAR of 12, however, dispersion was reduced by reducing the Ca/Mg ratio from 1/0, 2/1, and 1/2 to 0/1. Dispersion thresholds were <6 dS m−1 across all clays. At a SAR of 12, dispersion occurred for EC < 2 dS m−1. Kaolinite exhibited little, if any, dispersion. These results indicate that in soils having little or no organic C (i.e., subsoils), Mg does not influence dispersion thresholds and that dispersion risks can be reduced by adopting practices that maintain the EC > 2 dS m−1.