On the chemical structure and plant growth-promotion effects of environmental humic acids.

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Forth humic acid samples were extracted from various types of soils, including aquatic sediment. Each source of humic acid was analysed for elemental conpositions and contents of carboxyl, low pka carboxyl, carbonyl, phenolic and alcoholic hydroxyl groups. From the visible and ^1H-NMR spectra of various types of humic acids, optical properties and proton compositions (aromatic, methoxyl, lactone, aliphatic-<alpha>, -<beta>, -<gamma>) were compared.Factor analysis clarified that the humic acid was first characterized by the contents of aliphatic chains and nitrogenous components, followed by functional groups contents and optical properties. Soil groups of the source of humic acids could be judged by discriminant analysis on the above characteristics.Structural analysis developed in coal chemistry was modified to apply to humic acids. Average molecular structure parameters (f_a H_<au>/C_a, <sigma>, n) were calculated from elemental composition, functional groups composition and proton composition. The higher the humification digrees were, the larger the condensation degrees were and the shorter the side chain lengths were.Number-average molecular weight of well humified humic acids were lower than low humified ones. Surface tension of humic acids solution were measured. High molecular weight fraction and the humic acids holding high number-average molecular weight exhibit high surface activity.To test the effect of humic acids on plant growth, rice was cultivated in water aculture containing nutrients. Ion uptake was estimated by the analysis of exudate and plant tops. The uptake of magnesium was enhanced by humic acids. Most effective humic acid was extracted from Entisol and characterized by the followings; low condensation of aromatic rings, long side chain, high number-average molecular weight and high surface activity.