Nine trials testing the application of N, P, K and Mg fertilizers to oil palm were carried out at different locations on mineral soils in both North and South Sumatra, and their effect on the contents of both major and minor elements were monitored. This article reports the effect of N, P and K fertilizers on the trace element status of the palms. In four of the trials on old palms in North Sumatra, urea fertilizer significantly reduced the leaf levels of both copper and zinc, in some cases below the established critical levels. In addition, superphosphate fertilizer significantly reduced leaf copper in four of these trials whilst muriate of potash fertilizer significantly depressed leaf zinc in two trials. In contrast, N and P fertilizers had no effect on leaf trace element levels in five trials on younger palms in South Sumatra, but in four of the trials, K fertilizer significantly reduced leaf zinc. There were no consistent effects of N, P and K fertilizers on leaf levels of B, Mn or Fe.
Changes in soil properties due to the fertilizer applications did not appear to explain these results. Furthermore, the effects of fertilizers on trace element levels were not generally seen in the rachis, suggesting that the uptake of trace elements into the palms was not significantly affected. The reduction in leaf copper and zinc levels may therefore have been due to a physiological effect of the fertilizer nutrients inside the palms which influenced the transfer of these trace elements to the leaves. Leaf concentrations of other trace elements were not generally affected by fertilizers, indicating that the results for copper and zinc were not due to a dilution effect resulting from increased growth.
In the trials on older palms in North Sumatra, yields had generally fallen with time, and Ganoderma incidence had increased in the treatments where trace elements had been depressed by the major fertilizers. It is concluded that continuous application of N, P and K fertilizers is likely to have an adverse effect on the trace element status of oil palms, which if not corrected may result in reduced yields and increased disease incidence.