Humic substances (HSs) are chromogenic organic assemblies that are widespread in the environment, including soils, oceans, rivers, and coal-related resources. HSs are known to directly and indirectly stimulate plants based on their versatile organic structures. Their beneficial activities have led to the rapid market growth of agronomical HSs. However, there are still several technical issues and concerns to be addressed to advance sustainable agronomical practices for HSs and allow growers to use HSs reliably. First, it is necessary to elucidate the evident structure (component)–function relationship of HSs. Specifically, the core structural features of HSs corresponding to crop species, treatment method (i.e., soil, foliar, or immersion applications), and soil type-dependent plant stimulatory actions as well as specific plant responses (e.g., root genesis and stress resistance) should be detailed to identify practical crop treatment methodologies. These trials must then be accompanied by means to upgrade crop marketability to help the growers. Second, structural differences of HSs depending on extraction sources should be compared to develop quality control and assurance measures for agronomical uses of HSs. In particular, coal-related HSs obtainable in bulk amounts for large farmland applications should be structurally and functionally distinguishable from other natural HSs. The diversity of organic structures and components in coal-based HSs must thus be examined thoroughly to provide practical information to growers. Overall, there is a consensus amongst researchers that HSs have the potential to enhance soil quality and crop productivity, but appropriate research directions should be explored for growers’ needs and farmland applications.

avoid food shortages. However, the implementation of intensive agricultural practices such as excessive irrigation, tillage, and chemical fertilizers to increase crop productivity has devastated soil and farmland quality. [1,2] Particularly, acidification, salinization, and organic matter decomposition in soils typically coincide with agriculture practices, which adversely affect crop productivity and soil fertility. [3][4][5] Additionally, the inefficient absorption of conventional chemical NPK (i.e., nitrogen, phosphorus, and potassium) fertilizers by plants are among the main drivers of environmental eutrophication. [6] Therefore, many efforts are being made to develop novel strategies to overcome these drawbacks, including new types of fertilizers that can both maximize crop nutrient use efficiency and restore soil quality. [7,8] Novel smart fertilizer structures should not only be sensitive to crop rhizosphere, thus selectively releasing NPK or other crop macronutrients, but should also be made with non-toxic and low-cost materials. Crop nutrient use is maximized when the sensing ability of smart fertilizers is coupled with slow-releasing activity,

Structurally stable humic substances (HSs) in soils are tightly associated with soil fertility, and it is thus important to understand how soil HSs are naturally formed. It is believed that microbial metabolism on plant matter contributes to natural humification, but detailed microbial species and their metabolisms inducing humic functionality (e.g., direct plant stimulation) need to be further investigated.

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