This study evaluated the molecular weight (mw) as well as the proportions of humic and fulvic acid fractions in humic substances (H5) extracted with 0.1 M NaOH and a chelating (Chelex 100 chelating ion exchange resins) (Na form), via High-Performance Size Exclusion Chromatography (HPSEC). Samples of a Dusky Red Latosol were collected from three areas: under orange-cultivated forest with and without liming. Standards of sodium polystyrene sulfonate and acetone were used to calibrate the molecular weight and retention time ratio. All chromatograms of both NaOH- and resin-extracted humic substances contained two peaks: one peak eluted at the same time as the reference HAs (mw = 27700 daltons) and the other at the same time as the reference aquatic FAs (mw = 1330 daltons). The chromatograms showed no difference in the distribution of soil humic substances from the different agricultural systems studied. However, the relative proportions of humic and fulvic acid fractions differed between NaOH- extracted and resin-extracted humic substances, with more humic acid fractions in the resin-humic material and greater proportions of fulvic acid fractions in the NaOH-humic extracts. The proportions of humic and fulvic acid fractions obtained by using the method proposed by SISH were close to those determined by HPSEC in the Chelex 100 ion exchange resin-humic substances, showing a potential use of this separation technique to the analyses of molecular weight and proportions of humic and fulvic acid fractions from soil humic substances.
humic acid; fulvic acid; humic substances extraction method; Latosol