Towards a Single-Point Approach for Estimating Potassium Quantity-Intensity Parameters in Soils at Different K Saturation Ratios

The objective of this study was to obtain sufficient estimates of quantity-intensity (Q/I) parameters using a single-point method, and to evaluate the role of cation exchange capacity (CEC) and Gapon selectivity coefficient (K G ) as parameters that improve prediction model’s estimation accuracy or explain its limitations under conditions of K deficiency. Regression models were applied on raw data derived from previews Q/I experiments conducted on 20 soils of northern Greece at initial and K-depleted treatment states. Relationships between soil K adsorbed or released (ΔΚ), or K activity ratios at each K addition (AR K ) and respective values at equilibria (-ΔΚ 0 , AR e K ) were tested to find the best fitting models. CEC was included in regression models to improve their performance, while K G parameter was assessed for initial and K-depleted soils. The study showed that Q/I parameters were well predicted with the single-point method for a wide range of K saturation ratios, by applying 0.01 M CaCl 2 and 10 mg L −1 K addition. Furthermore, CEC significantly improved model’s estimation accuracy. Under the ΔG breakpoint of − 14.67 kJ mol −1 , specific adsorption sites of the soil inner-sphere region weaken the single-point prediction efficacy, as was clearly indicated by the K G formation under conditions of K depletion. The study suggests that the proposed method is applicable for estimating Q/I parameters for soils covering a wide range of K activity ratios. Moreover, CEC and K G parameters constitute useful tools in improving prediction model’s estimation accuracy, or explaining their limitations under conditions of K deficiency.