Mobility and lability of phosphorus from highly concentrated fertiliser bands

In order to improve crop nutrition, phosphorus (P) fertiliser is increasingly being applied in concentrated bands. Although information is available regarding the sequence of reactions that occur within the band itself, comparatively little is known regarding the reactions controlling P diffusion and availability further away from fertiliser bands. In particular, no previous experiments have utilised in situ methods to examine P speciation with increasing distance from the band. The present study used two contrasting soils in an incubation experiment to investigate the impact of soil characteristics, form of P fertiliser, and the co-application of potassium (K) on changes in P speciation and availability up to 40 mm away from the fertosphere (i.e., volume of soil which includes the fertiliser band and the fertiliser-enriched soil immediately adjacent band). Precipitation of aluminium-P (Al-P) minerals were important for regulating P availability within the fertosphere itself, with this precipitation of Al-P minerals largely driven by changes in fertosphere pH (soluble Al) and solution P. However, adsorption processes were important for controlling P diffusion from the band, primarily by decreasing con centration gradients between the fertosphere and bulk soil. The choice of P fertiliser form was more important in the soil with a low P sorption capacity, where the pH of the fertiliser saturation solution strongly influenced precipitation reactions and P availability. Co-application of K with P fertilisers had the greatest impact on P availability in the soil with a higher concentration of exchangeable cations – these cations could be displaced by the K and precipitate with P. This study therefore demonstrates that P distribution is influenced by soil char acteristics, but that P availability is primarily impacted by fertiliser choice and the composition of the nutrient blend in the fertosphere – these findings have important implications for improving P use efficiency in cropping systems.