The spatial distribution of soil nitrogen determines responses of Sorghum bicolor to banded phosphorus fertiliser

Context The grain-growing areas of north-eastern Australia are a major producer of grain for human and livestock consumption, but declining soil nitrogen (N) and phosphorus (P) fertility is increasing fertiliser requirements to sustain productivity. Adding a concentrated zone of fertiliser P to the subsoil (i.e. a ‘deep P’ band) is an effective strategy to increase plant P uptake in farming systems reliant on stored soil water. However, crop responses to deep P with contrasting soil N status remain unclear. Aim This study aimed to assess responses of sorghum (Sorghum bicolor) to fertiliser P with contrasting distributions of soil N. Method A lysimeter experiment was conducted in semi-controlled environment, where sorghum was grown to physiological maturity in P depleted Vertosol with contrasting fertiliser N and P additions. Key findings Responses of sorghum to deep P were optimised when bands were placed in N enriched soil in the 0–20 cm layer, producing comparable biomass to when P was dispersed throughout the soil volume. Localised root proliferation around the deep P band was maximised when bands were placed into N-enriched soil, however plant P uptake was only 77% of that with dispersed P. Conclusions Sorghum responses to deep P were affected by the distribution of soil N within the top 60 cm of the soil profile, with maximum dry matter production, N and P uptake occurring when high concentrations of N and P were co-located in the 0–20 cm layer. Implications Adequate N status of the upper soil profile is required to optimise sorghum responses to deep P.