Wien effect measurements were used to study the effect of organic matter on the interactions between divalent cations and soil clay particles of two black soil samples containing organic matter (OM) at 54.4 and 12.3 g kg-1 in the top (0-20-cm) and bottom (100-120-cm) horizons, respectively, and a sample of OM-free black soil, all saturated with Cd2+, Cu2+, Pb2+, and with Ca2+ as a reference cation. The weak-field electrical conductivities of suspensions of the top and bottom horizons and OM-free black soil samples were 0.021 to 0.033, 0.011 to 0.021, and 0.0065 to 0.0082 mS cm-1, respectively. The mean free binding energies of the cations in the same soil sample suspensions were 5.5 to 7.3, 7.3 to 9.3, and 9.6 to 10 kJ mol-1, respectively. The mean free adsorption energies of all cations increased with field strength and were in the order OM-free > bottom horizon > top horizon. At field strengths >100 kV cm-1, in the top-horizon soil, the adsorption energies of Ca were 0.21 to 0.72 kJ mol-1, those of Cd and Cu were similar to one another at 0.01 to 0.25 kJ mol-1, and those of Pb were close to zero, while in the bottom horizon soil, the adsorption energies of the various cations were in descending order: Ca > Cd > Pb > Cu, and in the OM-free soil the order of the adsorption energies of the various cations were Cd ? Cu ? Ca > Pb. The humus basically increased the negative electrokinetic potentials of the clay-size-fraction particles of the three black soil samples saturated with Ca, Cd, Cu, or Pb. © Soil Science Society of America, 5585 Guilford Rd., Madison Wl 53711 USA.
