Losses of nitrogen fertiliser under oil palm in Papua New Guinea: 2. Nitrogen transformations and leaching, and a residence time model

Abstract

Nitrogen fertiliser is an expensive input to oil palm in Papua New Guinea, and prone to leaching due to the about 3000 mm/year of rainfall. Transfer function theory is used to describe this leaching, and to devise ways of reducing it. Four variants of a leaching experiment were conducted at 2 sites to parameterise and check the theory. The experiment involved the application of ammonium chloride to an area of 25 m2, and then from 6 days to 5 months later taking soil samples at 200-mm intervals down to 2 m depth and analysing them for chloride, ammonium, and nitrate. Background concentrations were obtained by contemporaneous sampling nearby. In one variant of the experiment 353 mm of rain in 6 days moved nearly half the applied nitrogen to below 400 mm depth. Nitrification was rapid, with ammonium half-lives ranging from 2 to 16 days once the soil was wet. The theory is used to demonstrate how the fertiliser residence time in the root-zone can be increased by applying it in certain months and about 2 m from the trunk where there is less throughfall.