Objectives We summarized the humic acid structures and the current research effort on using humic acid to promote root growth and provided a theoretical basis for further humic acid utilization.

Main advances  1) Humic acid has a complex structure and diverse functions. It comprises C, H, O, N, and S elements. The acid is a polycondensate of polyvalent phenolic aromatic compounds and N compounds. Its pyrolysis products mainly include alkanes, saturated alcohols, unsaturated linear alcohols and pyridines, unsaturated linear alcohols and pyridines. The mechanism of humid acid formation is mainly based on amino sugar condensation theory, polyphenol theory, and lignin theory. Humic acid is characterized by nuclear magnetic resonance, diffuse reflectance Fourier transform infrared spectroscopy, surface-enhanced Raman spectroscopy, X-ray absorption near-edge structure, and X-ray absorption spectroscopy. Our review shows that humic acids from various sources contain aromatic carbon, carboxyl carbon, carbonyl carbon, hydroxyl carbon, and other structures. Chemical modification can change the functional composition of humic acids, and oxidation is an important process for increasing the number of oxygen-containing functional groups in humic acids. Oxidation can increase the H/C ratio and O/C ratio in humic acids or coal, reduce the molecular weight of humic acid, and increase the number of oxygen-containing groups such as carboxylic acid and alcohol amine, ester, ether, and so on. 2) Humic acid contains many active functional groups and has high physical, chemical, and biological activities. Its active functional group has two-way regulation on the availability of nutrients, affecting plant growth’s primary and secondary metabolic processes. Its impact on plant growth is closely related to its structure. Humic acid can promote crop nutrient absorption and improve soil nutrient contents, comprehensively regulate the growth environment of crops, and improve the nutrient absorption, assimilation, and utilization of crops. 3) The response of roots to humic acid is the initial driving force for promoting plant growth. It can affect root growth and development by affecting root morphology, root nutrient absorption, and root gene expression. The effect of humic acid on crop roots is affected by its source, concentration, molecular weight, functional group, structure, and composition.

Recommendations and Prospects  It is very important to strengthen research on the structural characteristics of humic acid and improve the understanding underlying the mechanism of its effect. The research methods on how humic acid improves crop growth regulation, especially the mechanism of regulating crop roots, are far from reality. Therefore, establishing a non-destructive method to restore the structural characteristics of humic acid and studying its mechanism on the growth of crop roots are worthy of attention.