Introduction

Humic substances (HS) are remains of decomposed plant and animal materials such as lignin, tannins, cellulose, and cutins (Tan et al., 2000; Billingham, 2012; Hayes 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., 2014; Gerke, 2018). The external sources of HS are mostly commercially produced from soils, coal, lignite, and organic materials (Gollenbeek and Van Der Weide, 2020; Yang 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., 2017, 2021; Fuentes et al., 2018; Shah 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, 2012; Rose et al., 2014; Canellas et al., 2020; Laskosky et al., 2020; Nardi et al., 2021; van 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., 2001; Bybordi and Ebrahimian, 2013; El-Bassiouny et al., 2014; Mukherjee et al., 2014; Kelapa 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, 2005; Garciá et al., 2016; García et al., 2019; Nardi et al., 2021; van 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., 2016; Nardi 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.