Herviyanti, .-., Prasetyo, T. B., Juniarti, .-., & Rezki, D. (2017). Activation Unproductive Coal Powder with Urea to Improve Chemical Properties of Ultisols. International Journal on Advanced Science, Engineering and Information Technology, 7(3), 957–963. https://doi.org/10.18517/ijaseit.7.3.2152