Rocha Façanha, Arnoldo, et al. “Humic Acids Bioactivity: Effects on Root Development and on the Plasma Membrane Proton Pump”. Pesquisa Agropecuaria Brasileira, vol. 37, no. 9, Sept. 2002, pp. 1301-10, doi:10.1590/S1678-3921.pab2002.v37.6470.