Short‐term phosphorus sorption and desorption in contrasting cropped Vertisols

Abstract

AbstractVertisols are important cropping soils in tropical and subtropical areas, but in many regions, decades of cropping has substantially reduced concentrations of plant‐available phosphorus (P), especially in the subsoil layers. Phosphorus behaviour in P‐depleted Vertisols has received comparatively little attention, and the availability of P following the addition of inorganic P fertilisers at different concentrations is poorly understood. In this study, we evaluated short‐term P sorption and desorption behaviour in cropped Vertisols in relation to specific soil physical and chemical properties. We collected the surface and subsurface of 15 Australian soils with a broad range of physical and chemical properties, comprising nine Vertisols, three Ferralsols, two Lixisols and one Calcisol. For each soil, we generated sorption and desorption curves (fitted with a Freundlich equation), determined soil physical and chemical properties likely to influence P sorption and evaluated the relationships between the measured soil properties and the Freundlich equation sorption coefficients. The P sorption curves differed drastically between soils, with the sorption equation coefficients (aS × b) significantly correlated with the P buffering index (PBI) and clay content. Clay content itself was correlated with citrate‐extractable Fe and Al oxides and BET surface area. Vertisols formed on basaltic parent materials had greater Fe and Al oxide concentrations, resulting in an overall greater P sorption capacity. Sorption and desorption hysteresis were mostly small. The reacting materials in these soils probably had limited ability to continue to react with P. The Vertisols differed in their capacity to replenish P in the soil solution by desorbing different proportions of previously sorbed P, although the proportion of desorbable P generally increased with greater concentrations of sorbed P. These results suggest that for fertiliser management in these soils, smaller volumes of P enrichment combined with higher P concentrations may result in a greater P recovery by the crop.