Background and aims Besides general effect of organic residues on soil quality and plant crop, hormonal direct effect on plant growth by extracted humic acids of organic materials is interesting and profitable theme. In the present work, we studied on direct interaction between humic acid and root growth, depending on different origin of organic materials. Methods All extracted humic acids of four organic materials (sewage sludge, compost sewage sludge, municipal solid waste, compost municipal solid waste) were characterized chemically by elemental analyses, ion pair chromatography (ICP), size exclusion chromatography (HPSEC), solid-state nuclear magnetic resonance (C-13-CPMAS-NMR) and quantification of IAA. Later, different morphological effects on maize (principal root growth, lateral root growth, root area, root mitosic site, root dry weight and H+-ATPase activity of plasma membrane) were analyzed. Results All humic acids samples promoted root growth and proton pump activity in maize vesicles, especially those composted samples, which contained more carboxylic groups and had a more hydrophobic character, produced preferentially morphological and biochemical effects. Conclusion The conformational dynamics of humic hydrophobic associations in the rhizosphere may release auxin-like plant growth promoters and enhance plant biochemical activities. These organic wastes represent a renewable source of humic acid for use as plant root promoter.
