D. F. Campos, N. B. de Miranda, D. C. Serrano, A. G. Gehrke, and V. Botton, “Extraction of humic and fulvic acids from leonardite for agricultural biostimulant applications”, Eng. Sust. J., vol. 1, no. 1, p. e006, Jul. 2026, doi: 10.68418/apj47y03.