No abstract

Summary Specific ion effects are now thought to be important in nature. We studied the specific ion effects on soil particle transport during rainfall simulation (150 mm hour−1, 110 minutes) in sodium nitrate (NaNO3), potassium nitrate (KNO3) and caesium nitrate (CsNO3) solutions. The results showed marked differences in the intensity of soil particle transport in Na+, K+ and Cs+ systems. The differences increased sharply with the decrease in electrolyte concentration, which indicated strong specific ion effects on soil transport and suggested that the differences could not be explained by ionic size, hydration effect or dispersion force. The cationic non‐classical polarization in a strong electric field increases the Coulomb attractive force between the cation and clay surface, and further adversely decreases the strength of the electric field. With the absolute effective charge coefficients, γ, of Na+ (1.110), K+ (1.699) and Cs+ (2.506), we recalculated the true surface potentials of soil particles in NaNO3, KNO3 and CsNO3 solutions. The true surface potentials decrease sharply with the increase in ionic non‐classical polarization, and then the electrostatic repulsive pressure between particles in the soil should decrease sharply. Comparison of fitting the equation for transport intensity in NaNO3 solution with that in KNO3 and CsNO3 solutions showed clearly that the soil electric field controlled the aggregate breakdown and particle transport. The results suggested that the stronger the non‐classical polarization for cations in the soil, the weaker is the electrostatic field that forms and the soil erosion at the same solution concentrations.

It is well known that humus markedly increases soil aggregate stability, but at the same time strongly decreases the flocculation of clay particles in suspension. These seemingly inconsistent observations suggest the need for a deeper understanding of the physical mechanisms that govern clay–humus interactions. In this research, soil samples from an Entisol were used to explore the role of cationic polarization in humus‐increased soil aggregate stability and sodium (Na+) and potassium (K+) ions were used to characterize weak and strong polarization, respectively. The results showed that strong cationic polarization has a critical role in increased soil aggregate stability in the presence of humus. We concluded that, without cationic polarization, the effects of humus alone on soil aggregate stability were weak in the presence of monovalent metal cations. When we compared the individual contributions of humus and strong cationic polarization, the latter proved much more important than humus in increasing soil aggregate stability. The strongest increase in soil aggregate stability occurred when strong cationic polarization was coupled with humus. The combined analysis of activation energy of soil flocculation in suspension and soil aggregate stability in the presence of humus indicated that humus increased the long‐range electrostatic repulsive force of soil particles, and increased the short‐range attractive force of soil particles. Consequently, humus decreased the flocculation of soil particles in suspension but increased soil aggregate stability; all effects were adjusted by the strength of cationic polarization. Highlights Elucidation of physical mechanisms of cation–surface interactions that determine clay–humus interactions. Cations at the particle surface of the clay–humus complex are strongly polarized. Cationic polarization has a critical role in humus‐increased soil aggregate stability. Without cationic polarization, the effect of humus alone on soil aggregate stability was weak.

No abstract