Biological activity of humic substances extracted from soils under different vegetation cover

Comparisons were made between the chemical compositions of humic substances extracted from three soils covered by different vegetation and their biolo gical activities assayed using 15- and 30-day-old seedlings of Pinus sylves tris and Picea abies. The growth, "alpha-amylase and invertase activity wer e affected by humic fractions and by gibberellic acid (GA), indicating that humic matter had a gibberellin-like activity. The isoenzymatic polymorphis m in the electrophoretic patterns of esterase was influenced in a similar w ay by all humic fractions and by indoleacetic acid (IAA), which also sugges ted that the humic fractions exhibited an auxin-like activity. The humic fr actions extracted from the grassland, exhibiting higher amounts of phenolic and a considerable amount of carboxyl carbon, showed the best metabolic ef fect. The forest humic fractions, characterized by lower phenolic carbon co ntent, appeared less effective in influencing plant metabolism, whereas the grassland-forest humid substances proved to be even more less effective. T he auxin- and the gibberellin-like activities were related to a high conten t of phenolic and carboxylic groups. These results presented evidence that the biological activity of the humic substances was attributed to their che mical structure and to their functional groups, which could interact with h ormone-binding proteins in the membrane systems, evoking a hormonelike resp onse.