Nursyamsi, Dedi. 2009. “Effect of Oxalic Acid, Na+, NH4+, and Fe3+ on Release of Fixed Potassium and Basal Distance of Smectite in Smectitic Soils”. JOURNAL OF TROPICAL SOILS 14 (3):177-84. https://doi.org/10.5400/jts.2009.v14i3.177-184.