K and Mg retention and losses, as affected by soil and other site factors

Results of a series of seven long-term NPKMg fertilizer trials in different Sumatran sites showed variations in oil palm (Elaeis guiineensis Jacq.) responses to fertilisers. Identification of important site factors and quantification of their contribution on the response variations is studied. The information is essential to improve the current fertilizer recommendation system used for larger area of commercial fields with similar characteristics. Soil type, mineralogy and topography, and rainfall were possible factors that generated the variation. Soil retention and losses of the applied fertilisers by water mechanisms were studied in additional trials. The trials included comparisons of soil nutrients from added and zero K and Mg plots, runoff and leaching at sites with different rainfall and slopes, and a pot observation. Of the fertilisers applied the soils retained 9-41% K and 73-102% Mg. The effect of rainfall was greater than soil on nutrient loss. P. Rambong with double rainfall than Bah Lias received six and eight times greater loads of leaching and runoff, respectively. Consequently lost K and Mg by runoff on the same flat topography were respectively 1% and 2% at Bah Lias, and 18% and 10% at P. Rambong. Deep drainage removed the applied K and Mg by 2% and 5% at Bah Lias, and 15% and 23% at P. Rambong. The effect of land steepness exacerbated runoff losses of K and Mg to 28% and 13% at the higher rainfall site. Some possible measures to implement in the field included improved fertilizer application policy, and soil and water conservation