P dynamics in vertosols-factors influencing fertiliser P availability over time and the implications for rate, application method and residual value

Take home messages • Fertiliser P is an increasingly important contributor to profitable cropping and efficient use of available water and nitrogen • The method of P application, soil characteristics and seasonal moisture availability will significantly impact the recovery of fertiliser P by plants. Starter P is important but only supplies a small proportion of the crop total P requirement in the year of application • Dispersing or spreading P fertiliser through large soil volumes to increase root exposure to fertilised soil reduces the risk of P precipitation but enhances the risk of low crop recovery due to strong P sorption to soil surfaces. This risk is enhanced when soil P reserves are strongly depleted and occurs somewhat independent of PBI • Conversely, applying P in concentrated bands limits the risks of irreversible P sorption but increases the chances of precipitating insoluble P minerals – particularly when fertiliser bands are acidic and/or co-application of other cations like K increases calcium availability in the soil solution • Fertiliser strategies need to balance the competing risks and rewards from differing volumes of soil P enrichment, while ensuring adequate available P in both topsoils and immediate subsoil layers to meet water limited crop demands • Closer spaced, less concentrated P bands applied more frequently hold the keys to more effective crop P use in these soils and will increase the return of P in crop residues to enhance the background P supply in topsoil layers