Humic materials from an organic soil: A comparison of extractants and of properties of extracts

Five extraction procedures and thirteen extracting reagents, which included dipolar aprotic solvents, organic chelating agents, pyridine, ethylenediamine, sodium hydroxide, ion-exchange resins and two salts (sodium pyrophosphate and ethylenediamine hydrochloride), were used to extract humic materials from an organic soil. Extractabilities increased in the general order: salts < organic chelating agents < dipolar aprotic solvents < pyridine < ethylenediamine = sodium hydroxide, and the amounts of the soil organic matter extracted by the reagents in the series ranged from 13 to 63%. Gel chromatography techniques indicated that extracts in dipolar aprotic solvents were predominantly of intermediate and low molecular-weight values, and it is suggested that the more highly oxidised soil humic materials were extracted in these. The more efficient solvents extracted materials with a range (high—low) of molecular-weight values. Data from elemental analysis and from E.S.R. measurements indicated that ethylenediamine altered the chemical nature and the composition of extracts. Dipolar aprotics, by the same criteria, were found not to alter the humic extracts, and can be regarded as mild reagents for the extraction of a less representative (of the total) fraction of soil organic matter. Sodium hydroxide in solution, despite its oxidation effects, was the best of the reagents tested for isolating extracts which were representative of a wide range of soil humic substances.