Apriliani, S., S. Sudradjat, and S. Yahya. “Optimization of N, P, and K Single Fertilizer Package for Oil Palm Aged Four Years”. International Journal of Sciences: Basic and Applied Research (IJSBAR), vol. 36, no. 1, Sept. 2017, pp. 202-1, https://www.gssrr.org/index.php/JournalOfBasicAndApplied/article/view/7973.