Ifansyah, H. 2014. Soil pH and Solubility of Aluminum, Iron, and Phosphorus in Ultisols: the Roles of Humic Acid. JOURNAL OF TROPICAL SOILS. 18, 3 (Sep. 2014), 203–208. DOI:https://doi.org/10.5400/jts.2013.v18i3.203-208.