Aerial topdressing of granular fertilizer is common practice on New Zealand hill country farms because of the challenging topography. Ravensdown Limited is a New Zealand fertilizer manufacturer, supplier and applicator, who are funding research and development of differential rate application from aircraft. The motivation for utilising this technology is to improve the accuracy of fertilizer application and fulfil the variable nutrient requirements of hill country farms.  The capability of this system to apply fertilizer at the target rate within an intended boundary was measured in two trials. Collectors were placed in a grid and nested grid formation over two sheep and beef farms in the North Island, New Zealand. Granular fertilizer was applied at two rates on the farm. The collected fertilizer was weighed and geo-statistical kriging was completed over the trial area. This produces a proof of placement map.  Proof of release maps are produced using recorded aircraft data.

Proof of release maps are highly detailed compared to placement maps. Release maps are based on thousands of flight recorded data points, while placement maps were created using 130 – 180 collectors. The release maps showed a clear transitional boundary with a noticeable change in rate between the two zones. In contrast, the average application rate found from the ground truth trial was lower than expected. Therefore the proof of placement maps predicted under-application in the trial area. High speed photometry of a collector showed that the collector was not able to fully capture all particles. A significant number ricocheted off the inside surface of the collector and bounced out. The average collected application rates of each zone improved when collection error was considered. The CV ranged from 34% to 56% for each zone. Variable wind conditions contributed to variation in the spread pattern. Therefore, although the differential rate application system is capable of applying fertilizer at different rates, wind speed and direction have a significant impact on the ground distribution and will remain a limitation of the system.