A comparison of the compositional differences between humic fractions isolated by the IHSS and exhaustive extraction procedures

Humic substances, defined in terms of humic acids (HAs), fulvic acids (FAs), and humin, are the major components of soil organic matter (SOM), the major contributor to sequestered carbon (C). Most studies of SOM/natural organic matter (NOM) have been carried out on extracts in sodium hydroxide, and little research has focused on extractions with solvents, such as urea solutions and DMSO (dimethyl sulphoxide). In the present study, the products of the conventional International Humic Substance Society (IHSS) (NaOH solution) extraction amounted to 33.2% of the SOM in an organic soil, whereas those from sequential exhaustive extractions with NaOH, NaOH plus urea, and dimethyl sulphoxide (DMSO) amounted to 69.7% of the SOM. Unlike the aqueous extractants, the DMSO + H ₂SO ₄ combination extracted humin components that were insoluble in the aqueous media.