Lugo-López, M. A., and Raúl Pérez-Escolar. “Attempts to Explain the Variability in the Cation-Exchange Capacity of Soils in Terms of Particle Size and Organic-Matter Levels”. The Journal of Agriculture of the University of Puerto Rico, vol. 53, no. 4, Oct. 1969, pp. 369-73, doi:10.46429/jaupr.v53i4.11150.