Potassium availability: An approach using thermodynamic parameters derived from quantity-intensity relationships

Extraction methods of exchangeable and non-exchangeable K were combined with data obtained from quantity-intensity isotherms in order to propose a prediction method of K availability, based on free energy of exchange and potential available illite K. This method aimed to overcome limitations arising from the complexity of K release and fixation dynamics which makes conventional extraction methods often inadequate. Winter wheat was grown in a greenhouse pot experiment until K-depletion and soil K was assessed with ammonium acetate (NH4OAc) and sodium tetraphenylboron (NaBPh4, one- and five-min incubation period). Q/I isotherms were constructed for the soils before and at the end of the K-exhaustion experiment. Potassium availability was assessed as a reference base with a net K pool contribution approach, by subtracting from the initial quantity of extracted K of each extractant at the beginning of pot experiment, the final quantity of extracted Κ at the end of the experiment (ΔΚex, ΔΚ-NaBPh4), for estimating the exact amounts of exchangeable and non-exchangeable K that contribute to plant nutrition. Q/I isotherms revealed a constant relationship between labile K (−ΔΚ0) and free energy of exchange (ΔG) (r2 = 0.89), which allowed ΔG prediction for a wide range of soils (r = 0.9). Furthermore, the present study suggests that soil solution K is dynamically connected with both exchangeable and non-exchangeable K of soil which is supported by significant relationships that were found between ΔG and ΔΚ-NaBPh4 (r2 = 0.86). Finally, ΔG could be used as a reliable predictor of K availability, if CEC and illite K are also taken into consideration (r2 = 0.89).