Chemistry of Fulvic Acid, a Soil Humic Fraction, and its Relation to Lignin

Abstract

Fulvic acid, a water-soluble soil humic fraction, was extracted from a Podzol soil and exhaustively methylated so as to make it soluble in benzene. The resulting material was separated repeatedly by column, thin layer, and preparative gas chromatography until relatively pure components were obtained. The latter were then identified by comparing their mass and micro-i.r. spectra and their gas chromatographic retention times with those of standards of known structures.Twenty-one phenolic and benzene-carboxylic acids were identified as methyl ethers and esters. In addition, a number of polymethoxy-benzene polycarboxylic acids of as yet unknown identity were isolated.A number of the compounds isolated have been reported to arise from the alkaline KMnO4 oxidation of lignin. Since we did not oxidize the fulvic acid in the laboratory, it is likely that it originated from products resulting from the chemical and/or biological oxidation of lignin in the soil. Methylation of the fulvic acid was necessary to make the compounds extractable by organic solvents. Since methylation reduces hydrogen-bonding in the fulvic acid, it is suggested that in fulvic acid phenolic and benzenecarboxylic acids are joined by hydrogen-bonds to form a polymeric structure of yet unknown conformation. Any weakening of hydrogen-bonding will lead to a break-up of this structure.