Structure-Property-Function Relationship of Humic Substances in Modulating the Root Growth of Plants: A Review

The structural identity of humic substances (HS) in the soil is a highly debated issue in the soil sciences. Although the complexity and structural characteristics that determine the functions of HS justify their further study, there is enough scientific evidence explaining the presence of these compounds as a group of structures that are formed by humification and have unique chemical characteristics. This review presents scientific information that follows the structure-property-function relationship with the objective of better explaining the nature of HS. On the basis of the spectroscopic characterization of a number of humic acids, together with the use of chemometric techniques, it is shown that, although the sources of origin are different, HS have a unique structural pattern that is different from that of any other group of soil compounds. From this structural pattern, it is possible to understand how fragments with greater lability in HS can reach root surfaces and interact with the cell membrane, regulate oxidative metabolism, and stimulate root growth in plants. Lability and recalcitrance are properties arising from the characteristics and suprastructural organization of HS and can define the type and intensity of the bioactivity of HS in plants. In this review, we demonstrate that there are methods for studying HS by which a deeper understanding of the functions of these substances on the basis of their chemical properties is possible. Therefore, the understanding of this complex system allows the connection of the scientific elements that justify the existence of these compounds in the soil.