The development of variable rate application of fertiliser from a fixed wing topdressing aircraft

New Zealand hill country has a regular requirement for application of phosphorus (P) and sulphur (S) fertilisers usually as superphosphate. There are different requirements for fertiliser within farms with flatter slopes carrying more stock and needing a greater rate than steeper slopes which carry less stock. For the last 70 years aerial application of fertiliser has been mainly uniform with one rate applied over the whole area of a block or farm. As part of Clearview Innovations (PGP), Ballance Agri-Nutrients and Ministry of Primary Industry have developed a system for fixed wing aircraft to apply variable rates of fertiliser during flights across contrasting topographic areas. This system comprises a hydraulically-operated hopper gate linked to a hopper gate controller which in turn is linked to a computer and GPS guidance system, into which a fertiliser application prescription map is inputted. The prescription map uses GPS points to delineate the boundaries between areas that are flat (0-12 degrees), easy (13-25 degrees) or steep (>25 degrees), plus non-productive areas and exclusion zones. Calculated net benefits for the same amount of product applied using either variable rate (VR) or uniform rate application show an advantage of $36 to $166/ha/yr for VR rate application for low and medium fertility farms. Keywords: hill country, phosphorus fertilisers, variable rate, uniform rate