Marbun, P., Z. Nasution, H. Hanum, and A. Karim. “Classification of Inceptisol Soil on Robusta Coffee Plantation in Silima Pungga”. International Journal of Sciences: Basic and Applied Research (IJSBAR), vol. 30, no. 5, Dec. 2016, pp. 20-27, https://www.gssrr.org/index.php/JournalOfBasicAndApplied/article/view/6499.