Specific ion effects on soil water movement

Soil water movement is closely related to soil tillage. However, the development of a solid understanding of the physical mechanism by which soil ionic composition influences soil water movement remains a challenge for the scientific community. In this study, the specific ion effects of Li+, Na+ and K+ on soil water movement in a permanently charged soil were examined, and the vertical infiltration experiment was employed. The results provided evidence of strong specific ion effects, for which cationic polarization appears to be responsible. The velocity of the wetting front in the presence of K+ was up to 18.0 and 1.52 times faster than that in the presence of Li+ and Na+, respectively; the water diffusion coefficient values in the presence of K+ were up to 2300 and 9.451 times higher than those in the presence of Li+ and Na+, respectively. At electrolyte concentrations ≤0.01 mol L−1, water could not infiltrate into the soil in the presence of Li+. Knowledge of the interaction forces of soil particles combined with the experimental results on soil water movement indicates that the specific ion effects can be explained by cationic polarization. We concluded that the different polarizabilities among the three cations resulted in different soil electric field strengths for a given soil under a given electrolyte concentration, which led to differences in soil pore and aggregate stability, and thus differences in the soil water movement. Our results imply that it may be possible to regulate soil water movement by adjusting ionic polarization in a soil.