Spatial Variability of Soil Inorganic N in a Mature Oil Palm Plantation in Sabah, Malaysia

Abstract

The identification and understanding of soil factors influencing yield variability of oil palm enable their efficient management. Soil samples were therefore collected from a fertilizer response trial on oil palm to study the spatial inorganic N distribution and some selected soil chemical properties as affected by long-term N fertilizer applications. The experiment was conducted on mature oil palms grown on Kumansi family (Typic Paleudults) soil in Tawau, Sabah, Malaysia. The soil samples were taken from 2 areas; with and without N treatments for 8 years. They were analyzed for total N, NH₄⁺-N, NO₃^--N, exchangeable K, and pH. Semivariance analysis was used to characterize the spatial variance of soil NH_4⁺-N and NO₃^--N while point kriging method was used to illustrate their spatial distributions. Results showed that application of N in the palm circle increased soil NH₄⁺-N above 150 mg kg^-1 at 0 to 15 cm depth. In unmanured plot, the NH₄⁺-N contents were similar in the different sites within a palm area although the frond heap area tended to have higher NH₄⁺-N probably due to the N return from the decaying cut fronds. The coefficient of variations for both soil NH₄⁺-N and NO₃^--N exceeded 30% even within each microsite of palm circle, interrow, frond heaps, and harvesting path. Semivariance analysis showed that the maximum range of soil NH₄⁺-N could be reached at 10 m and 90 m in areas with and without N respectively, indicating that the application of N fertilizer reduced its spatial variability in mature oil palm agroecosystem. The kriged soil map showed localized spots of high NH₄⁺-N content, which corresponded to the palm circles where N fertilizer was applied. Gradual changes in soil fertility were observed in area without N, moving from northern to southern portion of the field. Long-term applications of N caused significant downward movement of NH₄⁺-N and NO₃^--N to the lower soil depth. They also decreased the soil pH from 4.2 to 3.7, and caused leaching of K to the lower depth. Fertilizer should be broadcast in the interrow in mature oil palm agroecosystem to reduce spatial N variation and other detrimental effects.