Problem statement:The oil palms are mainly grown in the humid tropics with high rainfall. Soluble Nitrogen (N) and Potassium (K) fertilizers are commonly required by the oil palm plantations to maximize palm productivity due to the highly weathered soils with low fertility. Thus, leaching losses of N and K nutrients may be unavoidable and these nutrients may move further downward and eventually cause groundwater pollution. This study reports the leaching of N and K nutrients in a mature oil palm field as affected by fertilizer rates and soil depths and its effect on groundwater quality during the monsoon period in Tawau soil depths of 60 and 120 cm, respectively. The transformation of NH 4 -N to NO 3 -N was not a major process during the monsoon period. The leaching losses of inorganic N were 1.0 and 1.6% of the applied fertilizer for N1P2K0 and N1P2K1 respectively. For K, the leaching losses were 5.3 and 2.4% for N0P2K1 and N1P2K1 respectively. The concentrations of NH 4 -N, NO 3 -N and K in groundwater ranged from 0.23-2.7, 0.07-0.25 and 0.63-9.54 mg L −1 , respectively. Conclusion/Recommendations: N and K concentrations in the soil solution decreased with soil depth and their leaching losses were related to rainfall pattern, fertilizer treatment and nutrient uptake by roots. Groundwater quality was not affected by the applications of N and K fertilizers at the optimum rates for mature oil palms.

In Malaysia, four soil conservation practices are oft en recommended for non-terraced oil palm (Elaeis guineensis Jacq.) plantations. Th ese practices are oil palm empty fruit bunches (EFB), Ecomat (a compressed EFB mat; ECO), and pruned oil palm fronds. Th ese three oil palm residues are used as organic mulching materials. Th e fourth method is silt pits (SIL) which are soil trenches to collect nutrients from runoff water and later redistribute them back into the soil. Nonetheless, the relative eff ectiveness of these four methods in improving soil and oil palm properties have never been studied. A 3-yr fi eld experiment was consequently conducted to determine their relative eff ects on increasing soil chemical properties (pH, cation exchange capacity, organic C, total N, available P, and exchangeable K, Ca, and Mg) and oil palm nutrition levels (N, P, K, Ca, and Mg). Biomass decomposition rate and nutrients release rate in the fi eld by the three mulching materials were also determined. Results showed that EFB mulching was signifi cantly better than the other three soil conservation practices in improving nearly all of the measured soil and plant parameters. Empty fruit bunches was most eff ective partly because of the combined eff ects of higher amounts of dry matter added and the higher nutrient concentrations in the EFB than in other mulching materials. Silt pitting was found not to be as eff ective as EFB because SIL could only trap and return nutrients back into the soil, whereas EFB could do both: trap nutrients and release additional nutrients into the soil as it decomposes.