Potassium and magnesium losses via runoff and leaching under oil palm at contrasting sites

High rainfall and weathered soils in the humid tropics create challenges for nutrient management in agriculture. Fertiliser trials in Sumatra have shown variable oil palm responses, possibly due to nutrient losses via runoff and deep drainage. This study quantified losses of K and Mg in both pathways to support nutrient balance calculations and facilitate site-specific fertiliser recommendations. Losses of K and Mg were measured in plots with 23-year-old oil palm on flat and steep slopes at two locations with contrasting average annual rainfall (1668 mm at Bah Lias and 4094 mm at P. Rambong) in North Sumatra. The plots received 118 kg N, 51 kg P, 127 kg K and 31 kg Mg ha −1 a −1 applied in two doses per year. Runoff and deep drainage were measured and analysed for sediment, litter and dissolved K and Mg following rainfall events. As a proportion of rainfall, runoff was 12 and 2% and deep drainage was 61 and 23%, at P. Rambong and Bah Lias, respectively. At both sites on flat terrain, losses of K and Mg through deep drainage exceeded those via runoff when expressed as fractions of fertiliser applied. Total K and Mg losses were similar within sites, equivalent to 33 and 30% of inputs at P. Rambong but just 4 and 6% at Bah Lias. Rainfall data could be used to predict losses. Knowledge of the loss mechanisms enabled formulation of strategies to reduce losses. Additional field conservation measures, such as contour frond stacking or stop bunds, combined with modification of fertiliser inputs were proposed.