Introduction: Potassium (K) is an essential plant nutrient, and its availability in soils is governed by dynamic interactions among soluble, exchangeable, and non-exchangeable forms. In calcareous soils, variations in clay mineralogy can substantially influence K retention, release, and supply capacity. K quantity–intensity (Q/I) relationships provide valuable information on the equilibrium status of K in the soil–solution system; however, their ability to explain the release behavior of non-exchangeable K remains insufficiently understood. Furthermore, clay mineralogy, particularly the type and abundance of 2:1 clay minerals, plays a critical role in controlling K dynamics and availability. Therefore, the objective of this study was to investigate the role of clay mineralogy and K quantity–intensity relationships in explaining non-exchangeable K release and evaluating K supply capacity in selected calcareous soils of the Shahrekord Plain.
Materials and Methods: 30 soil samples (0–30 cm) were collected from agricultural fields in the Shahrekord Plain. Based on clay content, exchangeable K, non-exchangeable K, and cation exchange capacity (CEC), 15 soil samples were selected for further investigation. The physical and chemical properties of the soils, as well as different forms of K, were determined. K quantity–intensity (Q/I) curves were then established for each soil by equilibrating the samples with solutions having different K activity ratios. The K concentration in the equilibrium solution was subsequently measured as a function of the K activity ratio, and Q/I parameters were calculated. Non-exchangeable K release was evaluated after 2017 h using the sequential extraction method, and the clay mineralogical composition of the soils was determined by X-ray diffraction (XRD) analysis.
Results and Discussion: The results showed that soluble K, available K (extracted with 1 N NH₄OAc), and non-exchangeable K ranged from 11.5 to 27.5, 168 to 403, and 527 to 931 mg kg⁻¹, respectively. The K quantity–intensity parameters also exhibited considerable variability among the soils. The equilibrium activity ratio of K (ARₖ) ranged from 0.011 to 0.027 (mmol L⁻¹)⁰·⁵, while the potential buffering capacity of K (PBCₖ) varied from 215.8 to 417.7 (mmol kg⁻¹)/(mmol L⁻¹)⁰·⁵. The amount of non-exchangeable K released after 2017 h ranged from 242.5 to 345.0 mg kg⁻¹. Mineralogical analyses revealed that micas, chlorites, smectites, kaolinite, quartz, and mixed-layer mica–chlorite were the dominant clay minerals in the studied soils. The results indicated that differences in K behavior among the soils were mainly related to the type and abundance of 2:1 clay minerals. Soils rich in mica exhibited a greater capacity for K retention and buffering due to the presence of specific K adsorption sites, whereas higher smectite contents enhanced the release of non-exchangeable K. Furthermore, significant correlations and regression relationships between Q/I parameters and non-exchangeable K release demonstrated that these parameters, in addition to describing the equilibrium status of K in the soil–solution system, could explain part of the behavior of non-exchangeable K release.
Conclusion: The findings demonstrated that K quantity–intensity parameters are useful not only for describing the equilibrium status of K in the soil–solution system but also for explaining part of the long-term release behavior of non-exchangeable K. In addition, clay mineralogy, particularly the type and abundance of 2:1 clay minerals, played a decisive role in controlling K dynamics. Therefore, the combined use of quantity–intensity parameters and mineralogical information can provide a more comprehensive assessment of potassium supply capacity and improve K management in calcareous soils.