R. Khalid, K. S. Khan, G. Shabbir, M. Yousaf, and S. Y. Naz, “Status of Plant Available Sulphur and Its Relationship to Other Soil Characteristics in Pothwar Soils: Plant Available Sulphur in Soil”, Pakistan Journal of Scientific & Industrial Research, vol. 52, no. 2, pp. 84–90, Apr. 2009.