The Formation of Humic Acid and Its Effects on Crops

　　Humic acid is a class of organic substances formed through the decomposition and transformation of animal and plant remains—primarily plant residues—by microorganisms, as well as through a series of complex geochemical processes and subsequent accumulation. It exhibits favorable physiological activity and functions such as absorption, complexation, and ion exchange. The principal elemental composition of humic acid includes carbon, hydrogen, oxygen, nitrogen, and sulfur, and it is widely distributed in low-rank coals, soils, aquatic sediments, animal manures, organic fertilizers, and the remains of animals and plants.

　　The soil we often see is dark in color because it contains humic acid; soils that lack or contain only small amounts of humic acid are yellow. In lower‑rank coals—such as weathered coal, lignite, and peat—the humic acid content ranges from 30% to 80%, followed by organic composts at about 5% to 20%. Coal can be converted into regenerated humic acid through artificial oxidation, for example by treatment with air, ozone, or nitric acid.

　　The Effects of Humic Acid on Crops

　　It stimulates physiological metabolism: humic acid contains a variety of bioactive functional groups that can enhance the activities of catalase and polyphenol oxidase within plants, thereby promoting metabolic processes and accelerating growth and development. For example, humic acid can induce earlier seed germination and increase emergence rates, with particularly pronounced effects under low-temperature conditions. It also stimulates cell division and expansion in the apical meristematic tissues of roots, resulting in rapid root initiation, abundant root formation, increased root biomass, and extended root length—thus enhancing the plant’s capacity to absorb water and nutrients. Consequently, with an ample supply of nutrients, stems become robust, foliage lush, photosynthesis is intensified, and the translocation of assimilates to reproductive organs is accelerated, leading to earlier fruit coloring and maturation.

　　By modifying fertilizer properties, humic acid—containing functional groups such as carboxyl and phenolic hydroxyl groups—exhibits strong ion-exchange and adsorption capacities. It helps reduce the loss of ammonium‑nitrogen from ammonium carbonate, thereby enhancing the utilization efficiency of nitrogen fertilizers. Oxidatively degraded nitro‑humic acid can inhibit urease activity, thus minimizing urea volatilization. The degraded nitro‑humic acid also increases the mobility of phosphorus in the soil, suppresses its fixation by the soil, converts readily soluble phosphorus into slow‑release forms, and promotes root uptake of phosphorus. Humic acid’s functional groups can adsorb and store potassium ions, enabling a gradual release of potassium fertilizer, increasing its availability, and elevating its content. Furthermore, humic acid can form chelates with poorly soluble micronutrients, producing highly soluble humic‑micronutrient complexes that are readily absorbed by crops, thereby facilitating both root and foliar uptake of these essential elements.

　　Improving soil structure, humic acid promotes the formation of soil aggregate structures, regulates soil pH, and balances soil water, nutrient, air, and temperature conditions. It enhances soil cation exchange capacity, achieves acid–base equilibrium, boosts the soil’s ability to retain water and nutrients, and stimulates microbial activity, increasing the populations of aerobic bacteria, actinomycetes, and cellulose-degrading microorganisms. This accelerates the decomposition and transformation of organic matter, facilitates the release of essential nutrients, and makes them more readily available for crop uptake.

　　Enhancing stress resistance, humic acid can reduce the degree of stomatal opening in plant leaves and decrease transpiration, thereby lowering water consumption and improving the plant’s internal water balance. This ensures normal growth and development under drought conditions, bolstering drought tolerance. Humic acid is predominantly an amphoteric colloid with high surface activity; it exerts inhibitory effects on fungi, enhances cold tolerance, readily adsorbs onto cell membranes, alters membrane permeability, and promotes the uptake of inorganic nutrients, while also helping to prevent rot diseases and root rot and reducing pest and disease incidence.

　　To improve fruit quality, humic acid can form complexes or chelates with micronutrients, enhancing the translocation of these micronutrients from the roots to the leaves and other plant parts. It helps regulate the ratio and balance between macronutrients and micronutrients, while also boosting enzymatic activity involved in the synthesis and transport of sugars, starches, proteins, fats, and various vitamins. Furthermore, it promotes enzyme activity, facilitating the conversion of polysaccharides into soluble monosaccharides, increasing the biosynthesis and accumulation of starches, proteins, and lipids, and accelerating the translocation of primary metabolic products from stems and leaves to fruits and seeds, resulting in plumper, more robust fruits.

　　Amino acids serve as promoters and catalysts for the synthesis of various enzymes within plants, playing a crucial role in plant metabolism. However, amino acids are readily assimilated and decomposed by soil bacteria; therefore, they are not suitable for application as a base fertilizer in the soil. Instead, they are formulated into foliar fertilizers and sprayed onto leaves, allowing plants to directly absorb amino acids and other nutrients through their leaf surfaces.