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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.

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Rice plants accumulate cadmium (Cd2+) within the grain, increasing the danger of human exposure. Natural materials have been used in soil remediation, but few studies have examined the risks (based on the bioavailability of these metals to plants) of using these materials, so the practice remains controversial. In the present study, we evaluated the effectiveness of biochar produced from sugarcane bagasse, vermicompost (VC), vermicompost solid residue (VCR) and humin for remediation of Cd2+-contaminated soils. We characterized the interactions between these materials and Cd2+ and evaluated their capacity to alter Cd2+ availability to rice plants. Our results show that under the conditions in this study, biochar and humin were not effective for soil remediation. Although biochar had high Cd2+ retention, it was associated with high Cd2+ bioavailability and increased Cd2+ accumulation in rice plants. VC and VCR had high Cd2+ retention capacity as well as low Cd2+ availability to plants. These characteristics were especially notable for VCR, which was most effective for soil remediation. The results of our study demonstrate that in the tested materials, the bioavailability of Cd2+ to plants is related to their structural characteristics, which in turn determine their retention of Cd2+.