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Application of humic substances

Updated: May 8

Medicinal application

Due to their antiviral , anticarcinogenic , antibacterial, antioxidant, anti-inflammatory, and antiseptic properties, the medicinal usage of HS has been practiced for centuries . The antioxidant properties of lignin-derived materials have also been reported due to the availability of phenolic and acidic (aliphatic and aromatic) groups, which have chelating and radical scavenging properties . On the other hand, low molecular weight (i.e., 1500 g/mol) fractions of HS show inhibiting effects against HIV-1 in vitro . The anticarcinogenic properties of FA fractions were also reported earlier. In addition, an earlier study reported that the oral consumption of HA by domestic animals could reduce the cholesterol, lipids, and glucose content and increase the red blood cells and hemoglobin in the animal bodies. One recent study also reported the potential antiviral effects of natural HS against the recent COVID-19 virus .

Wastewater treatment

Wastewater treatment by HA has been studied extensively. Similar to its action in soil, it can develop complexes with heavy metal ions in solution systems, reducing the toxicity of drinking water, industrial wastewater, and surface water. The mechanisms of HS for wastewater treatment depend on factors, such as the nature of the HS (particularly the fulvic and humic acid content), soil chemistry, and water's chemical properties, such as acidic or alkaline.

Foliar application of HSs to plants

The stimulating action of HSs is well recognized in the scientific literature. The bioactivity that HSs exerts on plants is highly dependent on the HS structural characteristics and initially occurs through chemical-physical interactions with the plant root system (Asli and Neumann 2010). Such HS-root interactions promote pore clogging and modify their functioning, creating a perception of mild stress called “eustress” in plants. Under this physiological condition, plants regulate the levels of reactive oxygen species (ROS) through the synthesis of redox enzymes. This mechanism of action promotes root growth in plants and protects against stress (García et al., 2016, Castro et al., 2021, Castro et al., 2022). Studies conducted by de Hita et al. (2020) showed that the beneficial effects of HSs when applied to both foliar and root tissues were due to adaptation to mild stress that is regulated mainly by the action of jasmonic acid.

Thus, the effect of HSs on plants when applied via foliar application has been established. Foliar application is a fertilization method widely used as an alternative to soil application of fertilizer, thus contributing to more environmentally sustainable agriculture. This practice has been used to apply macro- and micronutrients, as well as biostimulants and humic fertilizers, favoring the assimilation and use of nutrients by plants and increasing crop yield and quality (Manuel-Tejada et al., 2018). The use of HS-enriched compost extracts is an economically important tool for foliar spraying, especially when soil nutrient absorption is impaired, such as under calcareous conditions due to nutrient precipitation. However, this type of fertilization is limited to certain climatic conditions since high temperatures, rainfall, and wind reduce its efficiency. Similarly, high application rates can damage plants, such as through leaf burns due to the concentration of salts after water evaporates (Jindo et al., 2020).



HSs have been shown to have beneficial effects on various plant groups, such as vegetables, grasses, legumes, fruit, oilseeds, and medicinal and ornamental plants. The effects are diverse and include changes at the biochemical, morphological, and stress-protection levels (Table 1). Due to all the effects of HSs in promoting plant growth previously reported, these substances are widely used as biostimulants for several crops of agronomic interest. Although most studies address the application of these humic materials to plant roots, another way of supplying HSs is through its direct application to leaves (Olaetxea et al., 2018). Unlike the effects of HSs on roots (H+-ATPase activation, ion transport in the plasma membrane, hormonal responses, among others), the effects on leaves have been minimally explored, and there are reports that foliar application of humified compounds increases chlorophyll levels and acts on photosynthesis. In addition, foliar application also influences transpiration, although the mechanisms are still uncertain, with increases and decreases in water loss and gas exchange in leaves (Rose et al., 2014).





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What are humic substances

Humic substances were first defined in 1761 by Wallerius as a decomposed organic matter . In 1786, Achard extracted a brown substance from soil and peat using a KOH solution and named it humic acid .

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