Lugo-López, M. A., and R. Abrams. “High Yields of Non-Fertilized Protein-Rich Pigeon Peas on Tropical Soils of Low Inherent Fertility of Puerto Rico: An Explanation of a Paradox”. The Journal of Agriculture of the University of Puerto Rico 65, no. 1 (January 1, 1981): 21–28. Accessed December 7, 2022. https://revistas.upr.edu/index.php/jaupr/article/view/7558.