CHEMICAL STUDIES ON SOIL HUMIC ACIDS .5. DEGRADATION OF HUMIC ACIDS WITH POTASSIUM HYDROXIDE

Seventeen samples of soil humic acids, two fractions of soil fulvic acid sample, and several related compounds such as lignin, tannin, flavonoid and artificial humic substances were decomposed in conc. KOH solution at 180°C. Succinic acid, glutaric acid, phloroglucin, p-hydroxybenzoic acid, vanillic acid, protocatechuic acid, 3,4-dihydroxy-5-methoxybenzoic acid, and gallic acid were detected in the degradation products of humic acids. The amounts of these degradation products were discussed in relation to the degree of humification or the sources of the humic acid samples. Succinic acid also resulted from glucose, polymaleic acid, and the humic acid and humin prepared from glucose, but glutaric acid resulted only from glucose humic acid and glucose humin but not from glucose and polymaleic acid. Succinic acid and glutaric acid were supposed to result from the same structural portions in humic acids because of the very significant positive linear correlation between their amounts. p-Hydroxybenzoic, vanillic, protocatechuic, and 3,4-dihydroxy-S-methoxybenzoic acids were presumed to result mainly from lignin structure in humic acids. Soil humic acids yielded small amounts of gallic acid although the yields by hydrolysable tannins were in large amounts. The yields of above-mentioned degradation products from humic acids decreased with increasing degree of humification. Phloroglucin resulting from ftavonoids including condensed tannins were also found in the degradation products of humic substances. Its yield showed no linear correlation with RF value of humic acid, and is presumed to be rather related to the vegetation at the sites of soil sampling. © 1979 Taylor & Francis Group, LLC.