Marbun, Posma, Zulkifli Nasution, Hamidah Hanum, and Abubakar Karim. 2016. “Classification of Inceptisol Soil on Robusta Coffee Plantation in Silima Pungga ”. International Journal of Sciences: Basic and Applied Research (IJSBAR) 30 (5): 20-27. https://gssrr.org/JournalOfBasicAndApplied/article/view/6499.