Binary and Ternary Exchange of Potassium on Calcareous Montmorillonitic Soils

Abstract

Simultaneous K exchange with Ca, Mg, and Na occurs in soils, but most of the published information is obtained from binary systems. The theoretical question of whether selectivity coefficients obtained in binary systems are valid for ternary systems has not yet been clarified; however, no significant difference has been found between selectivity coefficients in binary and ternary systems. Potassium exchange with Mg plus Ca was studied in two calcareous soils with variable concentrations of Na, using batch and miscible-displacement methods. Preference for K over the divalent cations was found in both soils, regardless of Na concentration and the experimental methods. Modified Gapon (k c) and Vanselow (ft v) selectivity coefficients were calculated for each cation pair in binary and ternary systems. In both soils, the k v and the k c decreased with increasing K fraction in the solution, but the k G value was less affected than the k v. A simple mathematical solution of the Gapon binary equations, K-(Mg + Ca) and Na-(Mg + Ca), in combination with an assumption of constant cation-exchange capacity (CEC), was used to predict the amount of exchangeable K as a function of solution composition in a ternary system. The high correlation of predicted exchangeable K with the directly measured values shows that binary selectivity coefficients can be used in multicationic systems on calcareous montmorillonitic soils. "DOTASSIUM EXCHANGE REACTIONS with each of the A cations Ca, Mg, or Na on clay minerals and soils have been investigated intensively (Bolt, 1979; Sparks and Huang, 1985). Irrigation water, especially sewage effluents and brackish waters, may contain considerable quantities of Na, K, Ca, and Mg. These cations undergo simultaneous exchange reactions with each other on soil surfaces. The possible reactions involving K-Ca-Mg, K-Na-Ca, and K-Na-Mg in soils and the study of their effect on K distribution between the solution and the solid phases is extremely important in plant nutrition. Theoretical understanding of the relationship between ternary and binary exchange systems in clays and soils has been enhanced in the last decade. Chemical and geochemical models were adapted to soils for predicting cation exchange in a ternary system using binary-exchange data (Elprince and Babcock, 1975; Elprince et al, 1980). Chu and Sposito (1981), using data obtained from experiments with pure clays and with careful analytical methods, found that the effect of ternary systems on binary-exchange coefficients was not greater than experimental error. In subsequent studies on ternary cation exchange on clay minerals and soils, Sposito and others (

Ca-K-Na exchange equilibria on a smectitic soil: Modeling the variation of selectivity coefficient

“…These values show that the soil preferred Kover Ca in this range of the exchanger phase composition. Similar trends in the variation in C KKlca were reported for other smectitic soils (Wada and Weerasooriya 1990;Feigenbaum et al 1991;Wada and Odahara 1993). On the other hand, the cKNalca decreased rapidly from 1.6 at ENa =0.01 and took negative values at ENa > 0.03, indicating that the selectivity reversed at around ENa = 0.03.…”

“…In addition, there was a significant difference in the range of variation; 1.6 to -0.5 for the binary system and -1.2 to -0.95 for the ternary system. The In C KNaf(ca+Mg) val ues calculated from the experimental data for N a exchange with (Ca + Mg) in the (Ca + Mg)-K-N a system (Feigenbaum et al 1991) also differed markedly from those for the (Ca+ Mg)-Na system, although Feigenbaum et al (1991) did not refer to this fact. All these findings suggest that the coexisting K affected the N a exchange with Ca.…”

“…4. Feigenbaum et al (1991) also reported that the presence of Na did not affect significantly the In C KK/Ca values for two smectitic soils. The constancy or near constancy of In C KNa/ca has been repeatedly reported for smectites and smectitic soils as reviewed by Fletcher and Sposito (1989).…”

“…Feigenbaum et al (1991) studied the cation exchange equilibria on two smectitic soils and showed that the Ca-Mg-K-Na system can be treated as a quasi-ternary system comprising (Ca + Mg), K, and N a, i.e., Ca and Mg can be regarded as identical cations. As shown in the previous section, the selectivity of the present soil for Ca and Mg were quite similar and this allows to adopt the approximation used by Feigenbaum et al (1991). Therefore, we dropped Mg and examined only the ternary Ca-K-Na exchange equilibrium.…”

“…To date, however, there are few experimental studies on ternary systems involving K + in spite of the plant nutritional importance of K+ in soils (Feigenbaum et al 1991). Therefore, in the present study, exchange experiments on the Ca-K-Na, Ca-K, and Ca-Mg systems were carried out to test whether the binary data can be used to predict the exchange equilibria in the ternary system.…”