Relationship between oil palm yield and soil fertility as affected by topography in an Indonesian plantation

Typically, Indonesian oil palm plantations rest on rolling topography. There is limited information on how topography affects soil fertility and oil palm yields. A study was conducted to evaluate these relationships in a commercial oil palm plantation located in South Sumatra, Indonesia. Two sites with differing past management history and fertility regimes were each partitioned into three topographic positions. At each topographic position, yields were recorded at 10-day intervals over a period of 2 years. Leaf and soil samples were collected from corresponding points spaced at 36.4 m (x direction) and 8.7 m (y direction) using a systematic scheme. Leaf analysis was performed to quantify nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), and calcium (Ca), and soil analysis was carried out to determine pH, organic carbon (C), extractable P, exchangeable K, Mg, and Ca, effective cation exchange capacity (ECEC), and texture. The collected data were subjected to exploratory, univariate, and bivariate analyses, as well as analysis of variance. Empirical production functions based on measured variables were defined for each topographic position. Results showed that average yields at both study sites varied with topographic position. At site 1 (Sungai Pelepah Estate), the sideslope and the summit consistently gave higher yields than the toeslope. At site 2 (Sri Gunung Estate), a yield gradient was observed with the highest yield occurring at the toeslope and the lowest yield from the summit. Soil fertility varied across topographic positions at both sites. The measured leaf/soil variables showed varying levels of optimality/sufficiency across topographic positions. In most cases, leaf and soil variables showed comparable performance as yield predictors. Validation of the calibrated models showed reasonable accuracy for the toeslope of site I and all three positions at site 2.