Understanding the Role of Humic Acids on Crop Performance and Soil Health

Humic acids (HA) are organic molecules that play essential roles in improving soil properties, plant growth, and agronomic parameters. The sources of HA include coal, lignite, soils, and organic materials. Humic acid-based products have been used in crop production in recent years to ensure the sustainability of agriculture production. Reviewed literature shows that HA can positively affect soil physical, chemical, and biological characteristics, including texture, structure, water holding capacity, cation exchange capacity, pH, soil carbon, enzymes, nitrogen cycling, and nutrient availability. This review highlights the relevance of HA on crop growth, plant hormone production, nutrient uptake and assimilation, yield, and protein synthesis. The effect of HA on soil properties and crops is influenced by the HA type, HA application rate, HA application mode, soil type, solubility, molecular size, and functional group. This review also identifies some knowledge gaps in HA studies. HA and its application rate have not been tested in field experiments under different crops in rotation, nitrogen fertilizer forms, sites and climatic conditions. Furthermore, HA chemical and molecular structures, their water and alkaline soluble fractions have not been tested under field experiments to evaluate their effects on crop yield, quality, and soil health. The relationship between soil-plant nutrient availability and plant nutrient uptake following HA application should also be further studied.


Humic substances (HS) are remains of decomposed plant and animal materials such as lignin, tannins, cellulose, and cutins (Tan et al., 2000Billingham, 2012Hayes and Swift, 2020). High quantities of HS are present in the soil after incorporating harvested residues (Wiesler et al., 2016). Increased animal and biogas production have reduced the amount of harvested residues on most arable land, resulting in decreased HS in the soil. Over the past decades, researchers have attempted to replenish the decreased HS with external applications (Rose et al., 2014Gerke, 2018). The external sources of HS are mostly commercially produced from soils, coal, lignite, and organic materials (Gollenbeek and Van Der Weide, 2020Yang et al., 2021).

HS are classified as humic acids (HA), fulvic acids (FA), and humins based on their solubility in water, acidic or alkaline solutions (De Melo et al., 2016). Due to the non-degrading nature of the humin fraction in HS, researchers have focused on the HA and FA fractions because they are capable of improving soil fertility and health within short time frames. The HA and FA fractions of HS are chemically reactive and able to resist microbial reactions, thereby performing beneficial roles in soils and plants (Billingham, 2012). The ability of HA to withstand degradation for long periods and their amphiphilic properties enable them to form complex cations (Wood, 1996). HA fraction contains about 60% organic carbon (C), which plays an important role in the growth of soil microorganisms (Sible et al., 2021). In addition to C, HA also contain nitrogen (N), oxygen (O), hydrogen (H), and sulfur (S).

Humic acids play several important roles such as: increase soil physical and biochemical activities by improving structure, texture, water holding capacity (WHC), and microbial population (Nardi et al., 20172021Fuentes et al., 2018Shah et al., 2018); increase soil nutrients availability, especially micronutrients by chelating and co-transporting micronutrients to plants (Yang et al., 2021); reduce the transportation of toxic heavy metals by precipitating them, thus reducing toxic heavy metals intake by plants (Wu et al., 2017). Humic acids also increase crop growth by increasing plant growth promoting hormones such as auxin and cytokinin, which aid in stress resistance, nutrients metabolism, and photosynthesis (Billingham, 2012Rose et al., 2014Canellas et al., 2020Laskosky et al., 2020Nardi et al., 2021van Tol de Castro et al., 2021). Some studies have also reported no effects on crop growth and soil health following HA application (Albiach et al., 2001Bybordi and Ebrahimian, 2013El-Bassiouny et al., 2014Mukherjee et al., 2014Kelapa and Banyuasin, 2016). Although high HA doses are associated with enhanced soil physical characteristics (Gollenbeek and Van Der Weide, 2020), their effects on soil chemical characteristics and crops are still uncertain (Rose et al., 2014). In the review by Rose et al. (2014), among the factors analyzed in mostly greenhouse experiments, HA source had significant effects on both root and shoot growth while application rate only significantly affected shoot growth. A review by De Melo et al. (2016) highlighted carboxylic (COOH) and phenolic (OH) groups as predominant HA features that are largely responsible for their functions in the soil. A recent review by Nardi et al. (2021) showed that HA chemical and molecular structures, sources, and application rates are critical for determining their effects on crops and soil. Importantly, HA application can have inconsistent results on yield, possibly due to the different HA biological origins (Sible et al., 2021).

In view of inconsistent results of HA application on crop agronomic performance due to differences in HA sources and experimental conditions, and the paucity of literature on field experiments compared to laboratory trials, this review sought to understand HA application in agricultural production. The objectives of this review are to (1) identify the effects of HA on crop agronomic performance and soil health parameters in both laboratory and field experiments; (2) identify the factors that affect the efficiency of HA; (3) identify knowledge gaps in HA application on crop performance and soil health.

Relationship Between Humic Acids Structure and Functions

The functions of HA are associated with their structures, which are source dependent (Rupiasih, 2005Garciá et al., 2016García et al., 2019Nardi et al., 2021van Tol de Castro et al., 2021). Although HA structure contains many functional groups, the most predominant are phenolic (OH), and carboxylic (COOH) groups (Figure 1) (Nardi et al., 2021). The COOH and OH functional groups are mainly responsible for HA functions such as improving soil physical and chemical properties as well as plant growth (Figure 1) (De Melo et al., 2016Nardi et al., 2021). Dissociation of these functional groups creates polar and non-polar ends, which are the hydrophilic and hydrophobic parts, respectively (Mirza et al., 2011); both ends play roles in the mechanisms that confer useful HA functions (Figure 2A). The hydrophilic end is primarily involved in chelating functions, while the hydrophobic end is connected with repelling purposes (Billingham, 2012). Once the OH and COOH groups dissociate, the polar end of the anionic part forms complexes with cationic metals through electrostatic bonding in the soil, thus retaining these metals in the soil (Figure 2B). The hydrophilic part, which is also water-loving, forms micelle that increases soil WHC. On the other hand, the non-polar end repels water molecules reducing water infiltration and improving clay aggregate stability (Billingham, 2012). A recent study by van Tol de Castro et al. (2021) reports that the aromatic and aliphatic functional groups of HA were responsible for increasing N uptake and soluble sugars, which resulted in a corresponding yield increase in rice (Figure 1); meanwhile an earlier finding by Garciá et al. (2016) showed that HS aliphatic and aromatic functional groups stimulated root growth in rice seedlings.


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