H. Herviyanti, G. Gusnidar, and M. Harianti, “Using of Humic Matter from Low Rank Coal to Increase Phosphorous Fertilizer Efficiency and Production of Corn at Oxisol”, Int. J. Adv. Sci. Eng. Inf. Technol., vol. 3, no. 5, pp. 350–354, Oct. 2013.