Distribution of soil P fractions in oil palm plantation of different planting ages in Pahang, Malaysia ○N.P. Tan 1), M.K. Wong 2), Y. Yusuyin3), A.B. Abdu4), K Iwasaki5), A.R. Zaharah1), Sota Tanaka5) 1)Fac. Agriculture, Universiti Putra Malaysia, 2)Felda Global Ventures Research & Development Sdn Bhd, 3)Ehime Univ.,4)Fac. Forestry Universiti Putra Malaysia, 5)Kochi University Dr. TAN Ngai Paing, Department of Land Management, Fac. Agriculture, Universiti Putra Malaysia. ngaipaing@upm.edu.my Introduction In Malaysia, application of phosphorus (P) fertilizer is crucial for oil palm cultivation because of the inherent low P in soils. Within an oil palm field, P input, mostly phosphate rocks, is applied around the canopy circle of palm trees (called as weeded circle) while some portion of the plant P could be expected to return to soil through decomposition of fronds that are harvested at the same time as the fruit bunches and piled up in-between palm trees (frond heap). No P is applied at the operation path for harvesting and transporting (harvest path). A study was conducted to evaluate the fate of soil P at these three micro-sites in oil palm fields with different planting ages . Materials and Method Study area Tun Razak Agric. Serv. Central Pahang, Malaysia Study site Oil palm field with 5 years (OP5), 10 years (OP10) and 18 years (OP18) planting age. Sampling points (Micro sites) Frond heap, Weeded cirlce, Harvest path Sampling depth 0-3cm, 3-10 cm, 30-40 cm Analysis Bray II P, Total P, Modified sequential fractionation, P-adsorption isotherm analysis P management in field Prior transplanting the young oil palm trees from nurseries, P (phosphate rocks) was broadcasted to the soils for the establishment of N-fixing legumes in the field. Later, P was applied to the weeded circle around transplanted palm trees. 0.5 g soil 0.5M NaHCO3 (16hours) Available, labile P Digest, Ammomium persulfate Total P Harvest path Bicarbonate extracted P Frond heap Soil pellet Inorganic P (Pi) Precipitate, Sulfuric acid 0.1M NaOH (16hours) Fe, Al, associated-P, non-labile P Digest, Ammomium persulfate Total P Hidroxide extracted P Soil pellet Weeded circle Inorganic P (Pi) Precipitate, Sulfuric acid 1M HCl (16hours) Ca associated primary-P, available P *Organic P (Po)was determined as total P minus inorganic P Ca- Pi Soil pellet Micro sites of study area Modified Tiessen and Moir (1993) sequential fractionation P fractions at Weeded circle Total P and Bray II P Discussion In 5 years old site, the higher amount of P at harvest path and frond heap was attributed by residual effect from P application prior planting. In 10 and 18 years old site, continuous P-application around weeded circle resulted higher P. Results of analysis suggested that some P at weeded circle was undissolved portions of phosphate rocks. Meanwhile, contrary to our expectation, presence of roots at weeded circle contributed more organic P than frond heap. Frond heap contributed both inorganic and organic P in soil to a lesser extent than weeded circle. Langmuir adsorption isotherm at weeded circle in OP 10 Conclusion Most portion of P fertilizer was remained undissolved and accumulated with time and dissolved portion was adsorbed on Al and Fe oxide and organic matter at the surface soils. After saturation of such adsorption sites, soluble P move downward to subsoils. For fertilizer application, application of finer grain/reactive phosphate rocks should be more concentrated at the early planting stage to promote saturation of P adsorption by Al and Fe oxide in soils in order to increase the fraction of avail. P; and at the later stages the P fertilizer application rate can be reduced. Upon replanting stage, the large variation of P-distribution at the micro sites during the previous planting should be taken into consideration in order to efficiently use the available nutrients in the soils.