Yield Responses of Oil Palm to NPK Fertiliser Application on an Alluvial Soil in Indonesia

A 33 factorial fertiliser trial established on DxP oil palms planted on riverine alluvium in North Sumatera province, recorded significant yield responses to fertiliser application. Of the three major nutrients evaluated, the quantum of yield increase was greatest with phosphate (P). fertiliser. Significant yield responses were recorded to P application in six out of the seven years of harvesting. On average, P application increased fresh fruit bunch (ffb) yields by 45 per cent or 7.4 tonnes/ ha/year. Yield response to nitrogen (N) was intermediate between that of phosphate and potassium (K). Significant yield responses were recorded to N fertiliser in five out of seven years of harvesting and N application increased yields by 10 per cent or 2.0 tonnes/ha/year. A positive NxP interaction was also observed. Maximum yield response to N only occurred in the presence of P fertiliser and vice-versa. Yield response to K was low, with significant responses recorded in only two out of seven years of harvesting. On average, K application increased ffb yield by only 4 per cent or 0.8 tonnes/ha/year. However, there were strong indications that K was required for the maximum expression of the yield response to both N and P, as the highest yields were only attained with the inclusion of K fertiliser. Economic analysis clearly indicated the profitability of applying fertilisers even at low CPO and high fertiliser prices. Maximum profitability was achieved with the treatment receiving the first level of N and Mg and second level of P and K (NPK,Mg), which was also the highest yielding. Keywords: Alluvial soil, Indonesia, fertilisers, oil palm.