Abstract
Humic acids (HAs) extracted from the surface horizons of some Venezuelan soils were characterized by elemental and functional group analyses and visible, Fourier transform infrared (FT-IR), fluorescence and electron spin resonance (ESR) spectroscopies. The elementary composition of Venezuelan HAs was generally similar to that of HAs of temperate soils but carbon content was lower and oxygen content was higher. The HA from Entisol under natural palm savannah showed the highest acidity, whereas the lowest content of acidic functional groups was exhibited by the HA from Vertisol under flooded savannah. Both HAs from savannah soils were characterized by a pronounced aliphatic character and strong fluorescence intensity. The HAs originated from cultivated Inceptisol and Alfisol were characterized by fluorescence spectra indicating the presence of highly condensed, conjugated and substituted aromatic rings, and contained high concentration of free radicals. The ESR spectra also showed differences related to the vegetation cover of the soils. A high proportion of iron was present in the HA from the Amazonas Spodosol, which also showed a pronounced aliphaticity and low concentration of organic free radicals.
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