Soil–root cross-talking: The role of humic substances

he biological activity of humic substances (HS) has been elucidated in the last 40 years. Growthenhancement from HS has been demonstrated in several plants in the laboratory and the field.Morphogenesis effects have also been investigated and include induction of lateral root forma-tion and root hair initiation in intact plants and stimulation of root and shoot development intreated cell calluses. HS enhance nutrient use efficiency, aiding assimilation of both macro andmicroelements and promoting plant growth by the induction of carbon, nitrogen, and secondarymetabolism. The review aims are to: (1) shed light on the mechanism by which plants ‘‘talk’’ withsoil through humic substances, (2) elucidate the plant responses to the stimulatory effects of HS,the regulatory circuits that allow plants to cope with humus, and the feedback between plantcommunity structures, and (3) show (in light of recent debate about the alkaline extraction of soilhumic substances) the plant capability to acquire biologically active substances from soil. It willbe shown that plants modify soils, creating and maintaining favorable habitats for growth andsurvival. Therefore, organic substances exuded by roots are not a wasteful loss of carbon andenergy. They represent an evolved strategy by which plants ‘‘talk’’ to the soil. The mobilization ofbioactive organic/humic substances from bulk soil or bulk humus is critical to plant and soilhealth.