Colberg, Octavio, and Alberto Beale. “Four Levels of Nitrogen Fertilization in Two Onion (Allium Cepa) Varieties”. The Journal of Agriculture of the University of Puerto Rico 75, no. 1 (January 1, 1991): 1–10. Accessed October 2, 2022. https://revistas.upr.edu/index.php/jaupr/article/view/3542.