Impact of Contrasting Rates of Phosphate Fertiliser Addition in Two Vertisols: Effects of Fresh Application and Residual Fertiliser on Sorption and Plant Uptake

Cropping systems in many sub-tropical and tropical regions rely on phosphorus (P) fertilisers to maintain crop yields. However, crop responses are often spatially and temporally inconsistent. This study evaluated P availability from dispersed applications of fresh phosphate fertiliser and their residual effect in two contrasting Vertisols - a major soil type for crop production. We collected soils that had received mono-ammonium phosphate (MAP) fertiliser four years prior to sampling (residual), as well as unfertilised soils to which we applied MAP at equivalent rates (‘fresh’). Soil P tests quantified the potential P availability arising from the residual and fresh P applications, and P sorption curves were generated to describe the P sorption/desorption dynamics. A 45-day pot trial evaluated wheat growth and P uptake from fresh and residual P applications in both soils. Soil tests and sorption curves reflected differences in P addition in the two soils, although the sorption characteristics did not differ appreciably in response to residual P. Crop growth and P uptake strongly increased with increasing P availability, with greater responses for the freshly applied P. Low P recovery from aged fertiliser applications demonstrated that the availability of residual P for crop uptake after dispersed applications is limited in Vertisols. P-tests and sorption measurements could collectively capture potential soil P reactions and plant P availability in two contrasting Vertisols. Residual P benefits from dispersed applications remained low. Enrichment in a defined soil volume (i.e., banding) merits further investigation in this soil type.