Evaluating the fraction of electrically associated cations on surfaces of soil particles by extrapolation of strong-field Wien effect measurements in dilute suspensions

Purpose For agricultural production and environment protection, it is cations loosely bound to the soil particles that have a great significance in short-term processes of adsorption–desorption, exchange, and transport. It is beneficial to be able to evaluate the fractions of these cations in order to correctly predict potential pollution of soils by heavy metals and availability of plant nutrients. Materials and methods The homionic suspensions of yellow-brown soil (YB) and black soil I (BI) saturated with Na + and Ca 2+ and three subsamples of black soil II (BII) saturated with Ca 2+ and Cd 2+ were prepared to determine the electrical conductivity (EC) of the suspensions. On the basis of electrical conductivity vs. field strength (EC-E) curve, the fraction of electrically associated cations on surfaces of soil particles was evaluated by extrapolation of strong-field Wien effect measurements in dilute suspensions. Results and discussion The maximum dissociation degree ( α max ) of Na + adsorbed on surfaces of yellow-brown soil and black soil I was about 0.21, which is approximately twice as much as those of Ca 2+ (0.07–0.10) adsorbed on surfaces of two soils. The soil type was not the main factor in evaluating α max , and the valence of the cations was. For divalent cations, α max of Ca 2+ and Cd 2+ adsorbed on soil particles with different contents of organic matter descended in the order: top black soil II > bottom black soil II > OM-free bottom black soil II. Conclusions The relatively small fractions of electrically adsorbed cations—about 0.2 for Na + and 0.1 for Ca 2+ on yellow-brown and black soils particles indicated that even for the more loosely adsorbed Na + ions, most of the cations in the double layers of soil particles were adsorbed strongly by other, more specific mechanisms and cannot be stripped off into the solution, which would increase its electrical conductivity in a strong applied field.