Soil pH and Solubility of Aluminum, Iron, and Phosphorus in Ultisols: the Roles of Humic Acid - Ifansyah, Hairil; Department of Soil Science, Agriculture Faculty, Lambung Mangkurat University Jl. A.Yani Km.36 Banjarbaru Kalimantan Selatan (UNIVERSITY OF LAMPUNG, 2014)