PurnamayaniR., SabihamS., .S., and DarusmanL. K., “Value of the Zero Point of Charge (ZPC) and Its Relation with Potassium Sorption in Coastal Peatland of Jambi and Central Kalimantan”, J. Il. Tan. Lingk., vol. 6, no. 2, pp. 75-82, Oct. 2004.