Exchange equilibria of potassium in soils, V. Effect of natural organic matter on K-Ca exchange

Abstract

Exchange equilibria of K-Ca were studied in five soil samples (Anglberg surface, ABS; Anglberg subsurface, ABSS; Ottenhofen surface, OHS; Ottenhofen surface limid, OHSL; Buchhofen surface, BHS) from the Federal Republic of Germany. These samples had a wide difference in their clay content, clay mineralogy, CEC, K-fixation capacity, organic matter (OM), specific surface area, and surface charge density (SCD). Untreated and H 2O 2-treated samples were Ca-saturated and equilibrated with KCl+CaCl 2 solutions at a constant chloride concentration of 0.025 M 1 −1 in the equilibrium solutions. The experimental results were described, using the Gaines and Thomas, and Babcock and Duckart thermodynamic approaches, and various selectivity quotients. Out of five, two soil samples showed a decrease in the Gapon ( K G ) and Vanselow ( K V ) selectivity quotients in the lower K-saturation (i.e., 0–15 exchangeable potassium percentage, EPP, in ABS; 0–30 EPP in ABBS) zone on oxidation of OM. In the remaining three soil samples, K G and K V increased with H 2O 2-treatment for the whole exchange isotherms. The Gaines and Thomas standard free energy for K-Ca exchange ( ΔGo) decreased in all but one (i.e., ABSS) and Babcock and Duckart ( ΔGo′) in all the soil samples on OM oxidation. The values of ΔGo were less negative than those of ΔGo′ for comparable soil samples. The positive or negative effect of OM on K-preference has been ascribed to the “resultant” of the two opposing phenomena, viz., an increase in SCD and “low K-specific adsorption sites” (causing a decrease in K-preference), and an increase in the internal: external surfaces due to organo-mineral colloids (causing an increase in K-preference).