J. A. . Amu, H. . Yatim, I. . Tatu, and H. A. . Katili, “Analysis of Soil Fertility in Oil Palm Plantation (Elaeis guineensis Jacq) Smallholder Farmers in East Luwuk District, Banggai Regency: Soil fertility of smallholder oil palm plantation”, CELEBES Agricultural, vol. 3, no. 1, pp. 72–81, Oct. 2022.