Deep P bands – the solution to subsoil decline or just a useful supplement?

Abstract

Increasing removal of phosphorous (P) via grain harvests, shallow fertiliser placement and direct drill tillage systems have collectively produced soil P profile distributions that are strongly stratified and increasingly deficient in subsurface layers. Nutrient supply in these layers is a critical success factor for cropping systems that rely on stored soil moisture. Banding of P fertilisers into the 10-30cm profile layer has produced strong crop yield responses in Vertosols in CQld, particularly in seasons where topsoils are dry for extended periods. The relationship between P uptake and grain yield is typically linear, with little evidence of luxury P accumulation in either biomass or grains. We hypothesize that root proliferation around deep P bands rapidly dries these P-rich zones and limits P acquisition unless rain events are large enough to rewet those profile layers. While residual fertiliser benefits are strong, crops are still reliant on declining indigenous P reserves to achieve water-limited yield potential.