Effects of organic matter on the rate of potassium adsorption by soils

Abstract

Soil organic constituents may strongly affect the kinetics of soil chemical processes, including K exchange reactions. We investigated the influence of organic matter on the rate of K adsorption by selected soils (Ferric Ultisol, Orthic Ultisol and Vertisol) using a H2O2 treatment and a K ion-selective electrode technique. In the reaction period of 0–30 s, in which the adsorption was too fast for one to determine rate coefficients of K adsorption, the amount of K adsorbed by the untreated soils was 158–363 mg kg–1, compared with 0.5–47 mg kg–1 for the treated soils. In the reaction period of 30–120 s, K adsorption data based on the first-order kinetics show that rate coefficients of K adsorption by the untreated soils were 47 × 10–5 s–1 (Ferric Ultisol), 59 × 10–5 s–1 (Orthic Ultisol) and 61 × 10–5 s–1 (Vertisol); by contrast, after H2O2 treatment, the rate coefficients were 23 × 10–5 s–1 (Ferric Ultisol), 17 × 10–5 s–1 (Orthic Ultisol) and 42 × 10–5 s–1 (Vertisol). Similar treatment effects were observed for the reaction period of 120–600 s, though the difference in the rate coefficients between the treatments was not as great as that for the reaction period of 30–120 s. These results indicate that organic matter considerably promotes the initial fast rate of K adsorption and has more easily accessible adsorption sites for K compared with mineral constituents of the soils. Key words: Organic matter, kinetics, potassium adsorption, adsorption site, accessibility