Flux meters for quantifying the leaching of agrichemicals on the island of Tongatapu, The Kingdom of Tonga

van der Velde, Marijn [UCL] Green, Steve Gee, Glendon Clothier, B. [HortResearch Palmerston] Snow, V. Manu, V. Menoniti, Vunivesi Vanclooster, Marnik [UCL] et al.[show all]

Intensification of agriculture in Tonga, notably through squash-pumpkin production, has lead to increased use of agrichemicals. These agrichemicals, both fertilisers and pesticides, pose a risk to the fragile environment and water resources of the raised atolls of Tonga. We have used water flux meters (WFM) to monitor both the quantity and quality of drainage leaving the rootzone of squash pumpkin growing on Tongatapu. Here we discuss the results from non-suction WFMs in relation to the loss of nitrogen during the establishment phase of the squash. During this establishment of the squash, over days-of-year (DOY) 200-245, some 350 mm of rain fell, and about 70% of this left the rootzone as drainage in this humid environment. The concentration of nitrate-N in the drainage water was around 50 ppm, indicating that not only had the initial fertiliser dressing of 62 kg-N/ha been lost, but also some of N mineralised from the ploughed-in grass had also drained away. Eliminating the initial N-fertilisation at planting would reduce the nitrogen load on the groundwater, and save on fertiliser costs.