Rosolem, C. A., et al. “Effect of the Soil Ca Mg, Ca K and Mg K Ratios on Sweet Sorghum Production”. Pesquisa Agropecuaria Brasileira, vol. 19, no. 12, Apr. 2014, pp. 1443-8, doi:10.1590/S1678-3921.pab1984.v19.16216.