Effects of sodium adsorption ratio and electrolyte concentration on soil saturated hydraulic conductivity

Abstract

Sodium adsorption ratio (SAR) and electrolyte concentration (EC) can significantly affect water movement in soil. However, the theoretical mechanisms behind it are rarely studied. In the present study, an analytical function between electrostatic repulsive pressure (PEDL) and SAR/EC was derived in Na-Ca mixed electrolyte solutions or soil water. Soil water SAR and EC controls PEDL through surface potential and midpoint potential between two adjacent soil particles, which consequently influences the saturated soil hydraulic conductivity (Ks). Theoretical calculations show that the PEDL decreases with increasing EC and increases with increasing SAR, and the reduction in Ks is correlated with the repulsive PEDL. Five soils of different dominant clay minerals, specifically kaolinite (K), illite (I) and montmorillonite (M), were used to investigate the relationship between PEDL and change of Ks. The soils were packed in columns and leached with a serial of solutions (constant EC values of 0.5, 1, and 2 ds m−1, respectively) with increasing SAR from 0 to infinity. Our study showed that the Ks decreases with increasing PEDL. At the inflection point of the percent of Ks reduction (Kred) vs. PEDL curve, the critical PEDL values are 8.358 ∼ 8.569 atm and Kred > 92% for the Mungana soil (M), Yarrandoo soil (M, K) and Dunholm soil (M), while the average critical PEDL value is about 12.34 atm and Kred is only ∼51.75% for the Urrbrae soil (I) and Timberlea soil (K). For a given soil, the PEDL is the determinant factor, while other factors such as soil texture and clay type can also affect Ks.