Potassium–calcium exchange in electropositive oxisols: description of exchange sites

Abstract

Potassium–calcium exchange was studied in batch experiments carried out with 2 oxisols exhibiting positive charge balance. The experimental data were quantitatively described with the Rothmund–Kornfeld formulation of the Gaines–Thomas approach, and the permanent and variable surface negative charges were measured using the caesium-adsorption method. For both soils, no appreciable involvement of permanent negative charges was observed in the potassium–calcium exchange, which, in turn, seemed to occur solely on the variable negative charges. The preference for potassium over calcium exhibited by both soils was well described by the Rothmund–Kornfeld formulation of the Gaines–Thomas approach. It was hypothesised that the exchange sites could be divided into 2 groups with different potassium selectivities. The proportions and selectivities of these exchange site groups were estimated combining the Rothmund–Kornfeld formulation with the Dufey–Delvaux multisite model. For both soils, there was excellent agreement between experimental and modelled data and it was possible to estimate the amounts of exchange sites (cmol_c/kg) presenting greater and lower potassium selectivity. The existence of variable negative charge pools more accessible to K than to Ca ions but not evenly accessible to the former was considered as a possible cause of the non-ideal behaviour of the studied soils in relation to the potassium–calcium exchange.