Abstract
Humic substances (HS) are an essential component of soil organic matter and are considered a promising basis for the development of environmentally safe bioproducts used in sustainable agriculture. In the context of global soil degradation and the increasing demand for enhanced productivity of agroecosystems, interest in natural compounds capable of improving soil fertility and stimulating plant growth is steadily growing. This review summarizes current scientific data on the origin, structure, physical and chemical properties, as well as biological activity of humic substances, as well as their role in the functioning of soil ecosystems. It is shown that humic substances are formed during the humification of organic residues and are characterized by a complex molecular structure containing various functional groups that ensure their high chemical and biological activity. Due to their ability to participate in complexation, ion exchange, and electron transfer processes, humic substances regulate key soil processes, including nutrient cycling, carbon sequestration, soil structure formation, and detoxification of pollutants. Particular attention is given to the physiological effects of humic substances on plants. It is demonstrated that they exhibit hormonelike properties, stimulating root system development, activating metabolic processes, and increasing plant resistance to abiotic stress. In addition, humic substances enhance the bioavailability of macroand micronutrients, improve soil water retention capacity, and promote the activity of soil microbiota. The review also examines modern methods for obtaining humic acids from natural carbon-containing materials, including chemical, mechanochemical, hydrothermal, biochemical methods, as well as process intensification techniques using ultrasound, microwaves, and electromagnetic fields. The advantages and limitations of various technologies, as well as their environmental aspects, are discussed. Thus, humic substances are considered multifunctional natural compounds with significant potential for improving soil fertility, enhancing the sustainability of agroecosystems, and developing biologically active products for agriculture. Their use opens new opportunities for the advancement of environmentally oriented agricultural technologies and for addressing current challenges in food security and environmental protection.