Humic acid (HA), a key component of soil organic matter, has emerged as a multifunctional biostimulant that enhances soil quality, nutrient availability, plant growth, and stress resilience. The complex supramolecular structure of HA influences soil physicochemical properties by improving aggregation, increasing cation exchange capacity, and enhancing water and nutrient retention. These changes facilitate greater uptake of essential macro- and micronutrients, supporting improved photosynthesis, root development, and biomass accumulation. At the physiological and molecular levels, HA modulates hormone signaling, activates antioxidant defenses, and primes stress-responsive pathways that enhance tolerance to drought, salinity, and other abiotic stresses. HA’s ability to regulate reactive oxygen species (ROS) homeostasis, maintain ion balance, promote osmolyte accumulation, and activate pathways such as Salt-Overly-Sensitive (SOS) or other stress-regulatory networks has been demonstrated across diverse crop species. Despite these benefits, variability arising from differences in HA source materials and extraction methods remains a major challenge for consistent application. Future research integrating multi-omics approaches, improved formulation strategies, and large-scale field validation will be essential for elucidating unknown HA’s regulatory mechanisms and maximizing its agricultural potential. Collectively, current evidence positions HA as a promising biostimulant capable of enhancing crop productivity and resilience within sustainable agricultural systems.

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ABSTRACT
Ⅰ. INTRODUCTION
Ⅱ. METHOD
Ⅲ. SOURCES AND CHARACTERISTICS OFHUMIC ACID
Ⅳ. PHYSICOCHEMICAL AND BIOLOGICALMECHANISMS BY HUMIC ACID
    1. Soil physicochemical improvements
    2. Nutrient availability and chelation
    3. Enhancement of microbial activity
    4. Direct effects on plant physiology
    5. Molecular mechanisms
Ⅴ. THE ROLE OF HUMIC ACID INENHANCING PLANT STRESS TOLERANCE
Ⅵ. HUMIC ACID IN FORAGE CROP ANDGRASSLAND MANAGEMENT
Ⅶ. FUTURE PRESPECTIVES
Ⅷ. ACKNOWLEDGEMENTS
Ⅸ. REFERENCES

• Septi Anita Sari(Division of Applied Life Science (BK21four), Plant Biological Rhythm Research Center, Plant Molecular Biology and Biotechnology Research Center, Research Institute of Life Science, Institute of Agriculture and Life Science, Gyeongsang National University, Jinju 52828, Republic of Korea)
• Ade Citra Aulia(Division of Applied Life Science (BK21four), Plant Biological Rhythm Research Center, Plant Molecular Biology and Biotechnology Research Center, Research Institute of Life Science, Institute of Agriculture and Life Science, Gyeongsang National University, Jinju 52828, Republic of Korea)
• Youngju Kim(Division of Applied Life Science (BK21four), Plant Biological Rhythm Research Center, Plant Molecular Biology and Biotechnology Research Center, Research Institute of Life Science, Institute of Agriculture and Life Science, Gyeongsang National University, Jinju 52828, Republic of Korea)
• Woe-Yeon Kim(Division of Applied Life Science (BK21four), Plant Biological Rhythm Research Center, Plant Molecular Biology and Biotechnology Research Center, Research Institute of Life Science, Institute of Agriculture and Life Science, Gyeongsang National University, Jinju 52828, Republic of Korea)
• Joon-Yung Cha(Division of Applied Life Science (BK21four), Plant Biological Rhythm Research Center, Plant Molecular Biology and Biotechnology Research Center, Research Institute of Life Science, Institute of Agriculture and Life Science, Gyeongsang National University, Jinju 52828, Republic of Korea) Corresponding author
• Daeyoung Son(Department of Plant Medicine, Institute of Agriculture and Life Science, Gyeongsang National University, Jinju 52828, Republic of Korea)
• Min Gab Kim(Research Institute of Pharmaceutical Science, College of Pharmacy, Gyeongsang National University, Jinju 52828, Republic of Korea)