Potassium-Fixing Clay Minerals as Parameters that Define K Availability of K-Deficient Soils Assessed with a Modified Mitscherlich Equation Model

The objective of this study was to elucidate how changes on illite K and vermiculite content affect cation exchange capacity (CEC) and K availability and to evaluate the ability of different extraction methods to predict respective soil K fixation and release capacities. A greenhouse pot experiment with winter wheat was conducted on K-deficient soils with different K application rates and soil K was extracted with ammonium acetate (NH4OAc) and sodium tetraphenylboron (NaBPh4). Illite K and vermiculite contents were determined and a modified Mitscherlich equation was applied to predict critical levels of K availability with respect to a minimum soil K status which could be considered as a constrain fixation factor (z factor). The study showed that z factor was strongly correlated with NaBPh4-K of K-deficient soils (r = 0.97). In addition, illite K and vermiculite content satisfactorily predicted both the saturation ratio of NH4OAc-K (KSR) (r2 = 0.83) and K buffering properties of K-deficient soils (r2 = 0.75), while illite K alone well predicted available NaBPh4-extracted K (r2 = 0.78). Finally, strong correlations were found between critical levels of K addition and K availability of NaBPh4-K (r = 0.92). The study suggests that potassium-fixing clay minerals can predict K availability of K-deficient soils, while CEC found to improve the above relationships. Furthermore, the amount of K that represents a minimum K status (z factor) consists of both exchangeable and non-exchangeable K, documenting the superiority of NaBPh4 over the conventional method of NH4OAc in predicting fixation capacities.