The effects of fertilizer application rates on leaf nutrient contents and yield of oil palm were investigated at the Agricultural and Technological College Plantation in Trang province during May 1998 - June 2001. A five-year-old oil palm plantation, planted on the Na Tham soil series (Fine loamy, mixed, isohyperthermic Oxic Plinthudults) with spacing 9x9x9 m, was selected for study. A randomized complete block design with three replications with 20 palms/replication was used. The treatments included six different rates of fertilizer application. The rates of fertilizer were as follows: T1 (farmer practice), T2 (40% of application rate in T4), T3 (70% of application rate in T4), T4 (urea 2,750 g/plant; triple super phosphate 1,500 g/plant; potassium chloride 4,000 g/plant; kieserite 1,000 g/plant; borate 80 g/plant), T5 (130% of application rate in T4) and T6 (170% of application rate in T4). The high leaf nutrient contents of N, P and K at the range of 2.6-2.8%, 0.16-0.18% and 1.13-1.18%, respectively, were found in the high nutrient application rate treatments (T5, T6). However, the amounts of leaf Ca and Mg in T5 and T6 decreased from 0.75-0.80% and 0.33- 0.37% at the beginning of experiment to 0.65-0.70% and 0.22-0.24%, respectively, at the end of the experiment. Small increases of leaf sulphur and boron up to about 0.20-0.22% and 16-19 mg/kg were also found in the high rate of fertilizer treatments. Accumulated fresh fruit bunch yield (FFB) increased according to increasing rate of fertilizer application. Accumulated FFB yield of 268.4 kg/plant in the low fertilizer rate (T1) (farmer practice) and 278.8 kg/plant in T2 were found compared with the highest yield of 370.2 kg/ plant in the highest fertilizer application treatment (T6) for the 3 years experiment. Regarding the economic return, the medium rate of fertilizer application (T3) which achieved an accumulated FFB yield of 338.0 kg/ plant gave the highest profit with the VCR (Value: Cost ratio) of 2.53.
