Borkert, Clóvis Manuel, et al. “Fertilization Response and Potassium Availability for Soybean in Oxisol”. Pesquisa Agropecuaria Brasileira, vol. 32, no. 10, Oct. 1997, pp. 1009-22, doi:10.1590/S1678-3921.pab1997.v32.4749.