The Effect of Organic Matter on Potassium-Calcium Cation Exchange Selectivity in a Calcareous Soil

Organic matter is one of the most important factors affecting soil properties, and understanding its effect on potassium and calcium selectivity can help the management of these two essential nutrients. This study examined the effect of organic matter on potassium and calcium selectivity in a binary exchange system in soil with dominant illite mineralogy. Organic matter was added to soil at a rate of 2%, and then the soil was incubated for ten months. The experiment was performed at an ionic strength of 0.1 M using solutions with a different equivalent fractions of potassium (ẼK) and calcium (ẼCa) between 0 and 1. Then Vanselow (KV), Gapon (KG) and Davis (KD, n = 2, 4, 6) selectivity coefficients and thermodynamic parameters including exchange equilibrium constant (Kex) and free energy changes of the exchange reaction (ΔG°ex) were calculated using exchange data with and without considering the effect of calcium carbonate dissolution on exchangeable calcium values. Comparison of K-Ca exchange isotherms with non-preference isotherm (NP) showed that potassium was preferred over calcium up to ẼK≃0.8, before and after adding organic matter, however at higher ẼK, calcium was preferentially selected. All selectivity coefficients increased with increasing equivalent fraction of potassium in the exchange phase (EK), which indicates that these selectivity coefficients are not true equilibrium constants. Moreover, Kex was less than 1, and ΔG°ex was positive, indicating a preference for potassium over calcium by soil. Also, after the addition of organic matter Kex, increased, and ΔG°ex decreased, indicating a decrease in soil preference for potassium. In addition, results showed that considering the effect of calcium carbonate dissolution on exchangeable calcium value has a significant impact on selectivity coefficients and thermodynamic parameters of exchange in calcareous soil. This highlights the need to pay attention to the dissolution of calcium-containing minerals in the soil during exchange studies.