I. Iskandar and I. Irwanti, “Use of Volcanic Sand as a Natural Fertilizer: Increasing Release of NutrientsUse of Volcanic Sand as a Natural Fertilizer: Increasing Release of Nutrients”, J. Il. Tan. Lingk., vol. 5, no. 1, pp. 23–28, Apr. 2003, doi: 10.29244/jitl.5.1.23-28.