A. Kasno, A. Rachim, I. Iskandar, and J. S. Adiningsih, “Relationship between K/Ca Ratios in the Soil Solution with the Dynamic of K in Ultisol and Vertisol of Upland Area”, J. Il. Tan. Lingk., vol. 6, no. 1, pp. 7–13, Apr. 2004, doi: 10.29244/jitl.6.1.7-13.