Perkasa, Achmad Yozar, et al. “Effect of Various Levels of NPK Fertilizer on the Yield Attributes of Soybean (Glycine Max L.) Varieties”. Journal of Tropical Crop Science, vol. 3, no. 1, Feb. 2016, pp. 7-12, doi:10.29244/jtcs.3.1.7-12.