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ISSN : 2287-5824(Print)
ISSN : 2287-5832(Online)
Journal of The Korean Society of Grassland and Forage Science Vol.45 No.4 pp.275-284
DOI : https://doi.org/10.5333/KGFS.2025.45.4.275

Mechanistic Insights into Humic Acid-Mediated Enhancement of Crop Productivity and Stress Tolerance

Septi Anita Sari1, Ade Citra Aulia1, Youngju Kim1, Daeyoung Son2, Min Gab Kim3, Woe-Yeon Kim1, Joon-Yung Cha1*
1Division 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
2Department of Plant Medicine, Institute of Agriculture and Life Science, Gyeongsang National University, Jinju 52828, Republic of Korea
3Research Institute of Pharmaceutical Science, College of Pharmacy, Gyeongsang National University, Jinju 52828, Republic of Korea
* Corresponding author: Joon-Yung Cha, Division of Applied Life Science (BK21four), Gyeongsang National University, Jinju 52828, Republic of Korea. Tel: +82-55-772-0092, Fax: +82-55-772-2633, E-mail: jycha@gnu.ac.kr
December 5, 2025 December 18, 2025 December 22, 2025

Abstract


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.


Biostimulant , Crop productivity , Humic acid , Stress response

초록

    Ⅰ. INTRODUCTION

    Humic substances, including humic acid (HA), are environmentally friendly biostimulants widely recognized for enhancing plant growth, nutrient uptake, and stress tolerance (Canellas et al., 2023). These compounds originate from the natural decomposition of plant and animal residues through long-term humification processes (Canellas and Olivares, 2014). Humic substances are broadly classified into humic acids, fulvic acids, and humin (MacCarthy, 2001), among which humic acid represents the most bioactive fraction. HA is structurally complex and heterogeneous, containing abundant functional groups such as carboxyl, phenolic, carbonyl, and quinone groups (de Melo et al., 2016). Owing to this chemical diversity, HA plays significant roles in improving soil structure, regulating nutrient dynamics, and supporting plant physiological functions (Xu et al., 2021;Li et al., 2025).

    Global agricultural challenges—such as climate change, soil degradation, and heavy reliance on chemical fertilizers—have shifted considerable attention toward HA as a sustainable soil amendment. HA has been reported to enhance soil aggregation, increase water-holding capacity, and improve nutrient retention. In addition, HA directly influences plants by stimulating root development, modulating hormone-like signaling, promoting membrane permeability, and enhancing tolerance to environmental stresses including salinity, drought, heat, and heavy metal toxicity (Antu et al., 2025).

    Recent studies also highlight HA’s ability to increase microbial diversity in the rhizosphere, thereby supporting nutrient cycling and plant–microbe interactions essential for soil health (Ren et al., 2022). As interest in HA rapidly increases— driven in part by the global shift toward sustainability—there is a need for an integrated understanding of its physicochemical, biochemical, and molecular modes of action. This review summarizes current scientific insights into the sources, structural features, mechanisms, and stress-mitigating roles of HA, with emphasis on its potential application for sustainable crop production and grassland ecosystems.

    Ⅱ. METHOD

    This review followed a structured systematic approach adapted for narrative and mechanistic plant science research. Literature from 2000-2025 was collected in major databases (PubMed, Web of Science, and Google Scholar) using keywords related to humic substances, crop and forage productivity, and abiotic stress. Eligible studies examined humic substances in crop or forage species reported physiological, biochemical, molecular, or soil outcomes. Key data on species, stress conditions, application methods, measured traits, and proposed mechanisms were extracted. Findings were synthesized qualitatively, focusing on HA effect on biochemical effects, soil improvements, redox regulation, molecular mechanisms, and signaling involved. Only studies with clear experimental design, appropriate controls, and statistically supported results were considered. The aim was to highlight representative evidence of humic acid-mediated mitigation of abiotic stress on crop and forage species.

    Ⅲ. SOURCES AND CHARACTERISTICS OF HUMIC ACID

    Humic acid is produced during long-term microbial decomposition of organic matter in varied environments such as soils, peatlands, composts, freshwater sediments, and fossil organic deposits (Fig. 1A). HA is not a single compound but rather a supramolecular assembly of diverse macromolecules with molecular weights typically ranging from 10,000 to more than 100,000 Da (Akimbekov et al., 2020a). Its dynamic structure changes throughout humification, influenced by environmental conditions and interactions with soil minerals and plant roots (Boguta and Sokołowska, 2022). Natural raw materials rich in HA include peat, lignite, leonardite, and compost (Antu et al., 2025) (Fig. 1B). However, the use of peat is increasingly restricted due to ecological concerns, including carbon release and biodiversity loss. Consequently, lignite and leonardite— particularly oxidized lignite with up to 85% humic substances— have become preferred sources due to their abundant HA content and environmentally acceptable extraction (Ciavatta et al., 1995). Compost-derived HA, although more variable, often shows higher biological activity due to its diverse molecular composition (de Melo et al., 2016). HA extraction typically involves alkaline extraction followed by acid precipitation, producing HA in powder, granular, or liquid forms (Canellas and Olivares, 2014;Antu et al., 2025). Structurally, HA contains both aromatic and aliphatic components, with aliphatic fractions comprising roughly 65% of its carbon content (Al-Faiyz, 2017). Functional groups such as carboxyl, phenolic, methoxyl, and carbonyl groups provide the molecular basis for HA’s high cation exchange capacity (CEC) and strong binding affinity for essential mineral nutrients (Antu et al., 2025).

    HA also contains quinone–hydroquinone redox-active centers capable of participating in electron-transfer reactions. These functional groups contribute to HA’s antioxidant capacity by modulating reactive oxygen species (ROS) dynamics, which may play a role in protecting plant cells from oxidative stress (MacCarthy, 2001;de Melo et al., 2016). Together, these physicochemical characteristics explain the unique multifunctionality of HA in soil systems and plant biology.

    Ⅳ. PHYSICOCHEMICAL AND BIOLOGICAL MECHANISMS BY HUMIC ACID

    HA improves crop yields and productivity through complex mechanisms, with its synergetic interplay of physicochemical, biochemical, and molecular mechanisms as shown in Fig. 2 (Canellas et al., 2023;Canellas and Olivares, 2014).

    1. Soil physicochemical improvements

    HA contains functional groups such as the carboxyl group (COOH) and the phenolic group (OH). These functional groups will be separate into polar and non-polar ends with hydrophilic and hydrophobic portions. The hydrophilic ends participate in chelation by forming a bridge between the metal ion and the soil surface, thereby increasing soil water-holding capacity. As a result, the non-polar ends increase clay aggregate stability by repelling water molecules (Ampong et al., 2022). Higher water-holding capacity and reduced soil compaction enable roots to penetrate deeper soil layers and access water and nutrients under limit conditions (Jing et al., 2022;Pitann et al., 2024). The enhanced water-holding capacity of HA, combined with improved soil pores, increases soil water availability, providing moisture supply for plants (Ibrahim et al., 2024;Pitann et al., 2024).

    2. Nutrient availability and chelation

    HA contains negatively charged functional groups that chelate essential micronutrients such as Fe, Zn, and Cu, keeping them soluble and bioavailable—especially in alkaline soils where micronutrient precipitation is common (Nikoosefat et al., 2023). The calcareous conditions of HA sources further promoted Fe2+ precipitation as Fe(OH)3 or FeCO3. Although phenolic-mediated Fe3+ reduction was responsible for Fe mobilization in the soil, HA’s high aromaticity promoted insoluble Fe-HA complexes. Furthermore, due to the large active surface for covalent binding, Cu2+ can be reduced in availability by forming stable Cu-HA-clay complexes, which could mitigate Cu toxicity in the soil (Barzgar et al., 2025). The chelation thus reduces nutrient loss through leaching and improves nutrient use efficiency (NUE), also reducing heavy metal stress effects across crop and forage species (Ibrahim et al., 2024;Barzgar et al., 2025).

    3. Enhancement of microbial activity

    HA promotes diverse microbial communities by serving as a carbon source and modifying the rhizosphere environment (Antu et al., 2025). Beneficial microbes such as Mycobacterium and Crossiella—known to stimulate IAA production and enhance root development—are enriched following HA application (Ren et al., 2022). The microarray analysis examining the impact of HA indicates that large populations of nitrate-reducing bacterial communities are phylogenetically diverse. These communities include members of the Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria (Van Trump et al., 2006). In line with this finding, a previous study reported a predominance of proteobacteria in soil following HA treatment in potato plants (Akimbekov et al., 2020b). In cucumber plants, the phyla bacteria such as Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidetes are detected from sedimentary humic acid (De Hita et al., 2020). HA also supports microbial metabolic processes that accelerate nutrient cycling, including nitrogen mineralization and phosphorus solubilization (Lumactud et al., 2022). Collectively, these microbial shifts enhance plant–microbe interactions critical for plant health.

    4. Direct effects on plant physiology

    HA promotes primary root elongation, lateral root formation, and root hair proliferation. These effects are partly attributed to hormone-like activity that mimics auxin and cytokinin pathways (Nardi et al., 2021;Rathor et al., 2024). HA also activates plasma membrane H⁺-ATPase, generating proton gradients that drive nutrient uptake and regulate membrane permeability (Canellas et al., 2002;Jing et al., 2022). Additionally, it has been reported that HA can transiently change the pH of the cell wall, leading to wall loosening. This process involves the promotion of ZmEXPA4 and ZmEXPA9 in Zea mays, which require ZmPIN1b and ZmLAX3 as auxin transporters for the transcriptional induction of branching roots. The RBOH/ROS/auxin/H+-ATPase pathways are also believed to play an important role in root development by modulating oxidative stress (Zandonadi et al., 2025). Furthermore, artificial HA has been shown to upregulate genes associated with chlorophyll synthesis and the light energy capture in maize. The equation models also indicates that this effect will impact C4 plants (Guo et al., 2022). Overall, HA has the potential to influence plant physiology by enhancing root growth, loosening cell wall, improving nutrient uptake through plasma membrane modulation, and promoting overall growth by facilitating photosynthesis.

    5. Molecular mechanisms

    At the molecular level, HA upregulates genes involved in nitrogen assimilation pathways (e.g., NR, GS), antioxidant enzymes (SOD, CAT, APX), stress-responsive factors, and heat-shock proteins (HSPs) that protect plant cells from environmental stresses (Cha et al., 2020;Abbas et al., 2022;Aulia et al., 2025). HA structure has quinone dan phenolic sites, which allow it to participate in redox reaction. This can reduce oxidative burst by regulating the antioxidant system, counterbalancing ROS production, and protecting cells (Canellas et al., 2024). The transcriptomic studies reveal that HA upregulates redox-related genes such as OXS3, SEN1, ROXY3, and ROXY10 in the absence of external stress. This upregulation promotes oxidative stress tolerance by conferring priming effects in Arabidopsis. Additionally, the study found that HA application enhances stress tolerance in Brassica napus (Aulia et al., 2025). These molecular responses enhance physiological efficiency and resilience, ultimately improving crop growth and yield. (Zandonadi et al., 2025). Together, the synergistic effects of soil improvements, microbial enhancement, and plant-level responses explain the significant yield and quality benefits observed across diverse crop and forage species following HA application (Canellas et al., 2024;Zhang et al., 2024b). However, the underlying mechanism HA-promoted growth and stress tolerance need further investigation.

    Ⅴ. THE ROLE OF HUMIC ACID IN ENHANCING PLANT STRESS TOLERANCE

    Excessive accumulation of ROS under abiotic stress disrupts cellular homeostasis, damages membranes and proteins, and ultimately reduces crop productivity (Hasanuzzaman et al., 2020). Numerous studies indicate that HA mitigates this stressinduced damage by regulating ROS levels, enhancing antioxidant capacity, stabilizing ion balance, and priming multiple stress-responsive signaling pathways (Antu et al., 2025;Aulia et al., 2025). These coordinated responses strengthen the plant’s intrinsic defense capacity and improve growth under otherwise inhibitory conditions.

    Under drought stress, which severely limits yield in major crops such as maize and sorghum (Safian et al., 2022), HA enhances tolerance through multiple interacting mechanisms. HA improves soil moisture availability by promoting macroaggregate formation and enhancing water retention, while simultaneously increasing nutrient accessibility in the rhizosphere (Chen et al., 2022;Sun et al., 2024). Within the plant, HA elevates auxin content, stimulates osmolyte accumulation, and activates drought-responsive genes (Arslan et al., 2021). These physiological and molecular adjustments help maintain biomass accumulation and sustain metabolism under water-deficit conditions.

    HA also exerts strong protective effects under salinity stress, where excessive Na⁺ and Cl⁻ accumulation induces ionic imbalance and oxidative damage. HA reduces the uptake of toxic ions while promoting the retention of essential nutrients such as K⁺, Ca²⁺, and Mg²⁺, thereby improving ionic homeostasis (Adil Aydin, 2012;Abbas et al., 2022). In addition, HA enhances antioxidant defense systems, increases levels of compatible solutes including proline (Amerian et al., 2024), and primes signaling pathways such as the Salt-Overly Sensitive (SOS) network that regulate ion transport. These effects collectively enable plants—including maize, wheat, sorghum, barley, cucumber, bean, and spinach—to maintain metabolic stability and sustain growth under saline environments, as summarized in Table 1.

    Together, these findings demonstrate that HA improves stress tolerance through a multifaceted strategy involving soillevel improvements, enhanced nutrient dynamics, and activation of stress-mitigation pathways at the physiological and molecular levels.

    Ⅵ. HUMIC ACID IN FORAGE CROP AND GRASSLAND MANAGEMENT

    HA has been shown to enhance stress tolerance in various crop species through both physiological and molecular mechanisms. Importantly, these mechanisms are not limited to crops but are widely observed across higher plants, including perennial grasses (Meng et al., 2023). Recent studies focusing on grassland species, such as switchgrass (Panicum virgatum) and perennial ryegrass (Lolium perenne L.), have indicated that HA improves photosynthetic activity, enhances antioxidant enzyme activity, and balances hormones under salt stress. This suggests a common mechanistic response between crop and grassland systems (Fidanza et al., 2023). Additionally, research shown that the foliar application of HA on forage crop species like Italian ryegrass (Lolium multiflorum Lam.) promotes both shoot and root growth (Khaleda et al., 2017).

    In the context of grassland management, the functional outcomes of HA-induced stress tolerance not only promote short-term biomass but also play important roles in maintaining ecosystem sustainability. HA application in sown grass pastures has been reported to increase overall nitrogen, phosphorus, and potassium uptake across six different field experiments (Verlinden et al., 2010). Unlike annual crops, which prioritize yield optimization within the growing season, grassland management focuses on system persistence and long-term stability. In a two-year field experiment on maize, the application of HA resulted in a reduction of nitrogen dioxide (N2O) emissions by 29%. This reduction is attributed to the promotion of genes that are involved in nitrate absorption and assimilation. Additionally, during of the experiment periods, carbon dioxide (CO2) assimilation decreased significantly, which helped maintain carbon storage in the soil (Guo et al., 2022). In this context, HA not only acts as a growth stimulant for plants productivity but also as a regulator of soil-plant feedback mechanisms that are essential for ecosystem sustainability.

    Ⅶ. FUTURE PRESPECTIVES

    The pronounced differences between HA-treated and untreated plants under stress highlight the value of HA as a versatile biostimulant for sustainable agriculture (Fig. 3). Continued progress in this field will require the development of optimized HA formulations that integrate HA with beneficial microbial inoculants or organic amendments to maximize synergistic effects. Molecular studies using transcriptomics, proteomics, and metabolomics will be essential for elucidating how HA influences stress-responsive regulatory networks and for identifying specific signaling components that mediate its effects. Equally important is expanding field-scale validation across contrasting soil types, climatic conditions, and crop management systems to determine how controlled-environment findings translate to agricultural practice. Standardizing HA quality—particularly regarding source materials, extraction processes, and functional characterization— will be crucial for reducing product variability and ensuring consistent performance. Overall, HA functions through integrated physicochemical, physiological, and molecular mechanisms that collectively enhance crop resilience and productivity, positioning HA as a promising tool for future sustainable agricultural systems.

    Ⅷ. ACKNOWLEDGEMENTS

    This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MOE-2022R1I1A1A01070257), Republic of Korea.

    Figure

    KGFS-45-4-275_F1.jpg
    Sources and characteristic of humic acid. (A) Diagram the humification process by decomposition of organic matter including plant and animal residues into humus. (B) Representative natural sources of humic acid, including plant and animal residues, coal, compost, bentonite, lignite, and peat, which became sources of organic macromolecules.
    KGFS-45-4-275_F2.jpg
    Physicochemical and biological mechanisms HA promote crop productivity. Humic acid contributes to soil properties and biological mechanism further promotes crop productivity.
    KGFS-45-4-275_F3.jpg
    Summary of comparative effects HA treatment on plant under stress condition. HA-treated plants enhanced stress tolerance by several mechanisms. In contrast non-treated plants exposed with stress impaired plant growth further reduce yield productivity.

    Table

    Effects of humic acid on plant responses to abiotic stress

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