del Valle-González, Reinaldo, and Miguel Santiago-Córdova. “Nitrogen Rates and Sources and Guinea Negro Yam in an Ultisol in Puerto Rico”. The Journal of Agriculture of the University of Puerto Rico 74, no. 1 (January 1, 1990): 29–35. Accessed December 22, 2024. https://revistas.upr.edu/index.php/jaupr/article/view/6525.