Green Humic Acid and its Possible Origin as a Fungal Metabolite

Abstract

SPECTROSCOPIC investigations of soil humic acids in the visible region have hitherto been of little value in indicating the nature of these compounds. Humic acids usually show an almost featureless absorption curve in which optical density increases with decreasing wavelength over the visible scale. Work on Japanese humic acids^1,2, however, has disclosed the widespread distribution of a form described as “P type”, which is commonly present in podzolic soils and alpine grassland soils, and may also be detected in forest brown earths and yellow earth soils. P type humic acid has absorption bands near 620, 570 and 450 mµ, which are of variable intensity in humic acids from different horizons and from different soil types. This absorption occurs because there is a green pigment which can be separated from the remaining dark brown humic complexes by chromatography on columns of cellulose powder². The purified green pigment from Japanese humic acids has λ_max near 615, 570, 450 and 430 mµ in aqueous alkali. Green humic acid is easily soluble in alkali, concentrated sulphuric acid and pyridine, moderately soluble in methanol, ethanol and nitrobenzene, and weakly soluble in chloroform, ether and tetrachloroethane. Work in Japan (personal communication) suggests a possible relationship to a perylene quinone. It may be related to a pigment previously isolated from irregular olive green patches in Australian lateritic and podzolic soils. This pigment was considered to be a hexachloro-polynuclear quinone of the dihydroxyperylenequinone or dihydroxydinaphthylquinone type³.