A method for isolating soil organic matter after the extraction of humic and fulvic acids

The aim was to propose a method for the extraction of additional soil organic matter after alkaline removal of humic acid and fulvic acid. The additional organic matter was isolated from a volcanic soil and an alluvial soil. The soils, deprived of humic and fulvic acids, were treated twice with a 50% glycerol aqueous solution, once with 0.5 M citric acid and once with 0.5 M sodium hydroxide + 0.1 M sodium pyrophosphate solution. From the glycerol and sodium hydroxide + sodium pyrophosphate solutions, three fractions were extracted. They were characterised using Fourier transform infrared and 1H-nuclear magnetic resonance spectroscopy, UV–VIS spectrophotometry and chemical techniques. The analyses were also performed on the humic acid extracted from the same soils. The fractions extracted from the two soils have different yields (21.2% for the volcanic and 8.3% for the alluvial soil) and chemical characteristics. The fractions from the volcanic soil were more aromatic in nature than their corresponding humic acid (about 30% aromatic H for the fractions and 7.7% for the humic acid). The fractions from the alluvial soil did not show differences in aromatic group content from their corresponding humic acid (about 13% aromatic H for the fractions and 17.9% for the humic acid). In general, the fractions had a lower N content and greater C and ash content, as well as greater acidity, than the corresponding humic acids. Finally, for each soil, the citric acid fraction showed some chemical similarity to the corresponding humic acid.