Phosphorus Availability and Speciation in the Fertosphere of Three Soils over 12 Months

Identifying the effects of soil properties on the transformations of phosphorus (P) in highly concentrated fertiliser bands is a critical component in improving P fertiliser use efficiency, but little is known regarding the kinetics of P transformation, including over longer periods of time that are relevant to multiple cropping seasons. In the present study, we utilised a 12-month soil-fertiliser incubation experiment to simulate deep-placed banded P fertilisers, investigating three contrasting soils from Australia, three P sources, and the co-application of potassium (K). Overall, concentrations of soil solution P in the fertosphere decreased substantially over time regardless of soil type and P form. In the Ultisol and Vertisol, this progressive decrease was due to the precipitation of aluminium-phosphates (and, to a lesser extent, magnesium-phosphates) as well as ongoing sorption reactions, whilst in the alkaline Alfisol this progressive decrease was due to precipitation of calcium-phosphates. Despite P precipitation and sorption reactions, concentrations of potentially-plant available P measured by isotopically exchangeable P within 24 h (Ee24h) remained relatively constant between 1 and 12 months for the Ultisol and Vertisol, with this indicating that the ongoing sorption reactions of P in these fertospheres of non-calcareous soils may not necessarily decrease plant availability over time. We also found that co-application of K resulted in further reduction in P availability in all three soils and was mediated by K displacement of other exchangeable cations, thereby increasing precipitation reactions with dominant minerals. To improve P use efficiency in semi-arid cropping environments, optimising P form as a function of soil type is critical, especially for systems that are reliant on subsoil moisture and where highly concentrated fertiliser bands are applied infrequently to supply P over multiple growing seasons.