Linking Phosphate Sorption Kinetics With Hysteresis Ratio in Soils of Varying Weathering Intensity

ABSTRACT Phosphorus (P) sorption, desorption hysteresis and sorption rates have been extensively studied, but often in isolation. The interrelationship amongst these processes, particularly how the rate of phosphate sorption relates to the hysteresis ratio, remains unclear. This study aimed to investigate this relationship and identify which parameter serves as a better indicator for managing P availability in soils. We analysed 30 soil samples from five soil orders in India – Entisol, Inceptisol, Vertisol, Alfisol and Ultisol – spanning a gradient from young to highly weathered soils. Phosphorus hysteresis ratios were calculated using a modified Freundlich equation, and sorption rates were determined in selected soils by reacting them with P solutions over time and fitting data to the equation, , where S is the amount sorbed, c is the solution concentration, t the time of contact, and a , b 1 and b 2 are parameters. The P sorption–desorption curves obtained from the isothermal sorption experiment showed substantial variation, leading to a wide range of hysteresis ratios, which remained stable over time. Our results revealed an exponential relationship between hysteresis ratio and the sorption parameter ‘ b ’, and an inverse relationship with Colwell extractable P. Soils with low native P showed both higher sorption rates ( b 2 ) and hysteresis ratios and their relationships are exponential. The sorption equation parameter ‘ a ’ was inversely related with the parameter b . We infer that younger, more heterogeneous soils exhibit lower ‘ b ’ values, whilst highly weathered soils with less site diversity have higher ‘ b ’ values. Variations in hysteresis ratio emphasized differences in P retention ability and its subsequent diffusion into particles. The study highlights that hysteresis ratio and b 2 , both measuring the rate of reaction, are crucial indicators of P behaviour in soils but the hysteresis ratio is a more reliable predictor for guiding fertilizer P management in diverse soil types. The study highlights how soil weathering – and thus soil order – influences sorption characteristics by altering surface heterogeneity.