Vertisols 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 differ-
ent 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) signifi-
cantly 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 propor-
tions 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.