Samuels, George, J. Vélez-Santiago, and M. Manzano. “A Comparison of Ammonium Sulfate and Urea As Tobacco Fertilizers”. The Journal of Agriculture of the University of Puerto Rico 44, no. 1 (January 1, 1960): 35–39. Accessed April 26, 2024. https://revistas.upr.edu/index.php/jaupr/article/view/12104.