Humic acid is a naturally occurring macromolecular organic compound that, when combined with chemical fertilizers, significantly enhances fertilizer efficiency: fixing nitrogen, reducing phosphorus and potassium loss, promoting root growth, and strengthening crop resistance. It also improves soil structure, activates microbial activity, and achieves overall synergistic effects between fertilizer, crops, and soil, making it a key component of green agriculture.
I. The Mechanism of Humic Acid Fertilizer Enhancement Due to its unique structure and composition, humic acid possesses natural advantages over ordinary chemical fertilizers when combined with them.
Firstly, some small-molecule humic acids can be absorbed and utilized by plants as organic matter, acting as nutrients or hormone-like substances to regulate plant metabolism. Because humic acid can undergo complexation and other chemical reactions with various substances, it allows for the slow release of nitrogen, phosphorus, and potassium fertilizers, thereby regulating nutrients in the soil and fertilizer and reducing fertilizer waste.
For nitrogen fertilizer, humic acid can utilize nitrogen rationally based on its structural characteristics. All oxygen-containing functional groups of humic acid can react chemically with urea to form relatively stable chemical bonds. Humic acid reacts with nitrogen to produce ammonium humate, which then fixes nitrogen in the urea solution, reducing ammonia volatilization and leaching.
Humic acid is also a potential urease inhibitor because it contains polyphenol oxides, which can inhibit urease activity, reduce the hydrolysis rate of urea, and improve the utilization rate of urea in the soil. Other studies have shown that in acidic soils, HA can act as an extracellular electron mediator in the metabolism of nitrogen-fixing microorganisms, providing electrons for them.
Phosphorus fertilizer promotes crop root growth, enhances crop resistance to cold and drought, and increases the number of ears and full grains. However, phosphorus is easily fixed in the soil by calcium and magnesium (alkaline soils) or iron and aluminum (acidic soils). In this case, humic acid can compete for phosphorus adsorption sites through ion exchange, reducing the fixation of available phosphorus by the soil.
Potassium fertilizer promotes robust crop growth, enhances crop stress resistance, and promotes the formation of sugars and starches. The acidic functional groups of humic acid have a storage and absorption function for potassium, preventing potassium fixation by clay minerals and improving potassium fertilizer fertility.
II. Effects of Humic Acid on Plants and Soil As a fertilizer, humic acid can be readily absorbed by crop roots, increasing seedling emergence and survival rates, promoting root growth and development, and enhancing the absorption of water and nutrients.
As an organic chemical aggregate with multiple acidic functional groups and a high specific surface area, humic acid's high molecular weight and aggregate form can be decomposed by organic acids secreted by plant roots, forming small molecules that plants can absorb. This increases the number of lateral root sprouting sites and promotes root growth.
Humic acid can also enhance the plant's resistance to stress. For example, humic acid can reduce the opening of stomata on leaves, thereby reducing water evaporation and allowing plants and soil to retain more moisture, thus enhancing the plant's resistance. Under stress conditions, the application of humic acid can activate and produce a large number of superoxide dismutase, catalase, and peroxidase. These enzymes regulate plant metabolism through the antioxidant enzyme system, reduce cell membrane permeability, thereby alleviating the damage caused by stress to plants and promoting plant growth.
