H. Herviyanti, T. Prasetyo, J. Juniarti, S. Prima, and S. Wahyuni, “The Role of Powder Sub-bituminous Coal with Sodium Hydroxide (NaOH) to Improve Chemical Properties of Ultisols”, Int. J. Adv. Sci. Eng. Inf. Technol., vol. 8, no. 5, pp. 2052–2058, Oct. 2018.