Recent findings on the characterization of humic substances extracted from soils from widely differing climatic zones

To obtain information on the effect of climate on the analytical characteristics and chemical structure of humic substances, humic and fulvic acids were extracted from soils formed under Arctic, cool temperate, subtropical and tropical climates. All humic materials were extracted, fractionated, purified and analysed by the same methods. These included elementary and functional group analyses, spectrophotometry in the ultra-violet, visible and i.r. regions, Electron Spin Resonance (ESR) spectrometry and chemical degradation with alkaline KMnO4 and alkaline CuO. The oxidation products were separated by solvent extraction and chromatographic methods, and identified by mass spectrometry and micro-i.r. spectrophotometry. The analytical, spectrophotometric and spectrometric data, and the information provided by chemical degradation, showed that humic substances formed under widely differing climatic conditions had essentially similar chemical structures and characteristics. Major humic 'building blocks' were in all instances complex phenolic and benzene-carboxylic structures. Most of the aliphatics were n-fatty acids which appeared to be esterified to phenolic OH groups. Recent physico-chemical studies done in our laboratory show that humic and fulvic acids behave like linear, flexible polyelectrolytes that are readily aggregated, most likely with the aid of H-bonding, at low pH but dispersed, because of increased dissociation of functional groups, at higher pH. Thus, it becomes apparent at this time that humic and fulvic acids are not single molecules but molecular associations of phenolic and benzene-carboxylic compounds ('buildings blocks') of microbiological, polyphenolic, lignin and condensed lignin origins. The 'building blocks' appear to be held together by weak linkages, mainly H-bonds. (author)