The extraction of humic substances (HS) from A-horizons of two soils from Rio Grande do Sul (Oxisol, LR, and Mollisol, BZ) and from an organic sludge (LO) was investigated, employing two methods: (1) the HS were extracted with 0.5 mol L-1 NaOH, after treating the sample with 0.5 mol L-1 HCl; (2) the extraction with 0.15 mol L-1 sodium pyrophosphate solution (pH 7) was introduced before the alkaline and after the acid treatments. In each step, the contents of HS, fulvic acids (FA) and humic acids (HA) were determined by spectroscopic and gravimetric methods, and the contents of co-extracted Fe, Al, Si and Ca in the extracts of method 2 were measured by atomic absorption. The elemental composition of HA was determined and their chemical structure and characteristics were evaluated from the C/N, H/C and O/C ratios and from UV/Vis spectroscopy data. In the Oxisol, method 1 extracted a greater amount of HS, and in method 2 the pyrophosphate step extracted only AF. On the other hand, method 2 was more efficient in the Mollisol, and the greatest yield was obtained in the pyrophosphate step. These results suggest that in the Oxisol, the HS interact mainly through ligand exchange reactions and hydrogen bonding, while in the Mollisol, cation bridges are also an important mechanism. The NaOH-extracted HA showed higher H/C and lower O/C ratios than the pyrophosphate-extracted ones, indicating that the neutral solution solubilized HS with higher aromatic character and oxygenation grade.
neutral extraction; alkaline extraction; UV/Vis spectroscopy