Structural characteristics of humic substances as related to nitrate uptake and growth regulation in plant systems

Humic extracts with distinct chemical and physical-chemical characteristics, obtained by various soil extractants and from different sources, were used to study their biological activity. The humic structural features were related to the rate of nitrate uptake by barley seedlings and growth regulation in watercress and lettuce. Chemical analysis of functional groups, molecular weight distribution, and NMR quantitative spectra were used to characterize the humic extracts. Results showed that the most effective humic fraction on both plant nitrate uptake and hormone-like activity had the highest acidic functionality and the smallest molecular size, whereas both the aliphatic and aromatic content of extracts did not appear to play a role. Low molecular size fractions obtained from the original material by disaggregating the humic macromolecule through the formation of a sort of inverse micelle by acetic acid addition, showed results that confirmed the effectiveness of the combination of high acidity and low molecular size in influencing the biological activity of the plant system tested.