K. Radhika, S. Sivagnanam, J. Sekar, Y. Muralidharudu, Pradip Dey and A. Subba Rao, R. S. P. S. K. A. Soil Fertility Appraisal for Villupuram District of Tamil Nadu Using GPS and GIS Techniques. JISSS 2018, 66 (2).