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, […]