Soil mineralogy and potassium forms distribution and release in some calcareous soils of the Darab Region, Iran

Understanding the distribution and release dynamics of potassium (K) in calcareous soils is essential for efficient nutrient management in arid and semi-arid regions. This study investigates K forms and their release behavior in ten agricultural soil pedons from the Darab region in southern Iran, characterized by diverse physiographic settings and soil orders (Entisols, Inceptisols, Aridisols, and Vertisols). Dominant clay minerals identified through X-ray diffraction included illite, smectite, and chlorite. The surface horizons showed greater average concentrations of soluble K (0.63%), exchangeable K (5.06%), and non-exchangeable K (6.5%), whereas structural K was most abundant in subsurface horizons (89.32%). Cumulative K release, assessed through successive CaCl₂ extractions, ranged from 267.7 to 572.4 mg/kg in surface soils and 145.2 to 433 mg/kg in subsurface layers. Release kinetics of non-exchangeable K were effectively described by the power function and pseudo-second-order models, indicating both rapid and sustained release phases. Among the studied soils, pedon 6 (Inceptisol) exhibited the highest release rate, as reflected in its b parameter suggesting enhanced K availability and faster equilibrium attainment. These findings underscore the critical influence of soil mineralogy on potassium dynamics and support the development of site-specific fertilization strategies.