Ifansyah, Hairil. “Soil PH and Solubility of Aluminum, Iron, and Phosphorus in Ultisols: The Roles of Humic Acid”. JOURNAL OF TROPICAL SOILS 18, no. 3 (September 15, 2014): 203–208. Accessed August 2, 2025. https://journal.unila.ac.id/index.php/tropicalsoil/article/view/188.