Humic acid is a polymer organic mixture formed by microbial decomposition and geological polymerization of animal and plant residues for tens of thousands of years, dark brown to black in color, and is widely found in weathered coal, peat and fertile black soil. According to molecular weight and solubility, it can be subdivided into fulvic acid, Hymatomelanic acid and black vic acid. Among them, fulvic acid has a small molecular weight and high activity, which is the core form of modern agricultural applications.

1. Structural reshaping
   Humic acid is rich in carboxyl groups, phenolic hydroxyl groups and other functional groups, which can form a "humic acid-metal-mucil" bridge with multivalent cations such as calcium, magnesium and iron, and promote the evolution of microaggregates into macroaggregates. Studies have shown that the addition of 0.2 % humic acid can increase the content of soil water-stable macroaggregates by 18%–25%, thereby significantly reducing the risk of compaction.

2. Water and fertilizer retention
   The colloidal properties of humic acid allow it to have a specific surface area of up to 800 m²/g and can adsorb water and nutrients up to 5–20 times its own weight. After the application of sandy soil, the water holding capacity in the field can be increased by 1.5–2 percentage points. The air permeability of clay soil is increased by 25% due to the increase in porosity.
3. Acid-base buffering
   Humic acid is a combination of weak acids and weak bases, and can be buffered in both directions in the pH range of 4–9. After 8 years of continuous cropping of tomatoes in Shouguang greenhouse, Shandong Province, the soil pH dropped to 4.6, and the pH rose to 6.2 after 3 consecutive seasons of potassium fulvic acid, and the salt damage index decreased by 35%.

Humic acid can form soluble chelates with trace elements such as Fe²⁺, Zn²⁺, Cu²⁺, etc., avoiding nutrient fixation by the soil. Heilongjiang corn experiments showed that compared with EDTA-Fe, foliar spraying of fulvic acid-Fe increased the iron content of leaves by 42%, and the yellowing rate decreased from 31% to 7%.

Humic acid inhibited the activities of soil urease and nitrifying bacteria, and reduced urea hydrolysis and nitrate nitrogen leaching. The nitrogen utilization rate increased from 35% to 48% in Hebei wheat-based application of humic acid urea, and the yield increased by 9.8% under the condition of 20% nitrogen reduction.

Humic acid dissolves insoluble calcium phosphate, iron and aluminum phosphate, and reduces potassium ions immobilization by 2:1 clay minerals. After the addition of fulvic acid to Jiangxi paddy soil, the available phosphorus increased by 27 mg/kg and the available potassium increased by 42 mg/kg.

Humic acid lowers rhizosphere pH and enhances cell elongation by stimulating plasma membrane H⁺-ATPase activity. Cucumber hydroponic experiments showed that 50 mg/L fulvic acid increased root length and root surface area by 36% and 29%, respectively.

Humic acid provides a carbon source for beneficial bacteria while inhibiting pathogenic fungi. High-throughput sequencing showed that the abundance of probiotic bacteria such as Bacillus and Pseudomonas increased by 1–2 orders of magnitude after humic acid treatment, while the relative abundance of Fusarium decreased by 55%.

Humic acid can induce the production of antioxidant enzymes (SOD, POD, CAT) in plants and reduce malondialdehyde content. Under drought stress, the relative water content of tomato leaves increased by 15%, and the wilting index decreased by 40%.

1. The sugar-acid ratio is increased
   When potassium fulvic acid was applied during the fruit swelling period of grapes in Xinjiang, the soluble solids were increased from 18.2% to 21.5%, the titratable acid was reduced by 0.12%, and the purchase price was 0.6 yuan higher per kilogram.
2. Coloring and hardness
   The foliar spraying of calcium fulvic acid on the foliar of Shaanxi Fuji apples increased the anthocyanin content of the peel by 30%, the hardness increased by 1.2 kg/cm², and the shelf life was extended by 7 days.
3. Comprehensive benefits
   The demonstration of Shouguang tomatoes showed that the comprehensive humic acid scheme (base fertilizer + flushing + foliar) invested 220 yuan per mu, increased the income by 2 952 yuan per mu, and the input-output ratio was 1:13.4.

Humic acid is extracted from natural weathered coal, and the production process has no high temperature and pressure, and low energy consumption. After application, chemical fertilizers can be reduced by 10%–30%, and greenhouse gas emissions can be reduced by 0.3–0.8 t CO₂-e/ha/yr, which meets the needs of green agriculture under the background of "carbon peaking and carbon neutrality".

From improving soil physical structure, to improving nutrient utilization, to activating roots and microbial networks, humic acid plays multiple roles in agriculture as a "soil doctor, nutrition steward, and stress coach". With the advancement of green and efficient agriculture, humic acid, the "black gold", will surely release greater ecological and economic value.