Spectroscopy characterization of humic acids isolated from amazonian dark earth soils (terra preta de índio) - Cunha, T. J. F.; Novotny, E. H.; Madari, Beata Emoke; Martin-Neto, Ladislau; Rezende, Maria Olimpia de Oliveira; Canelas, L. P.; Benites, V. de M. (Amazonian dark earths: wim sombroek's vision, 2009)