Humic substances biological activity at the plant-soil interface: from environmental aspects to molecular factors.

Humic substances (HS) represent the organic material mainly widespread in nature. HS have positive effects on plant physiology by improving soil structure and fertility and by influencing nutrient uptake and root architecture. The biochemical and molecular mechanisms underlying these events are only partially known. HS have been shown to contain auxin and an “auxin-like” activity of humic substances has been proposed, but support to this hypothesis is fragmentary. In this mini-review we want to give an overview over present data concerning the molecular structure and the biological activity of humic substances, with special emphasis on the hormone-like activity.