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

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