Fertilizers are the basis of grain production, but the massive use of traditional chemical fertilizers not only leads to waste of resources, but also causes soil quality decline and environmental pollution, which seriously affects the realization of the concept of green, safe and pollution-free development in modern agriculture. As a natural organic matter, humic acid has unique physical and chemical properties and biological activities, with high commercial value and good development prospects. The use of humic acid in combination with other fertilizers can not only reduce costs and improve economic benefits, but also enhance fertilizer effectiveness, promote crop growth, and improve soil characteristics and ecological environment.

Chemical fertilizers are the food of grain and an important means of production in contemporary agriculture. Chemical fertilizers are the largest source of carbon emissions in the production process of planting, accounting for about 60% of the total carbon emissions in the input link, and humic acids have the effects of improving soil physical and chemical properties, stimulating crop growth and development, promoting the absorption and utilization of nutrients by crops, improving crop stress resistance and yield, improving crop quality, energy conservation and emission reduction, etc., and are widely used in important fields such as agricultural production and environmental protection.

1. Humic Acid Retention and Release of Nutrients from Fertilizers

   Traditional chemical fertilizers release nutrients rapidly. Most nutrients in the fertilizers are prone to leaching, volatilization, and fixation, resulting in low utilization efficiency and causing environmental pollution. The molecular structure of humic acid contains abundant functional groups such as hydroxyl and phenolic hydroxyl groups. These can react with the amide groups in urea to form a humic acid-urea complex. This complex exhibits high stability, inhibits urea decomposition, improves nitrogen utilization efficiency, and achieves slow-release and long-acting properties for urea.

   Humic acid is a special organic colloid substance. It can form a protective film on the surfaces of substances like aluminum oxide and iron oxide, helping to slow down the adsorption of phosphorus by iron and aluminum hydroxides, thereby enhancing phosphorus availability.

   The acidic functional groups in humic acid can adsorb and retain potassium ions, preventing the loss of soil potassium and reducing the fixation of potassium by clay minerals. Additionally, humic acid possesses excellent chelating ability. It can combine with cations in fertilizers (such as calcium, manganese, zinc ions, etc.) to form chelates. This reduces the loss and leaching of fertilizer nutrients and slows down the release rate of nutrients, allowing them to be gradually supplied to plants and improving nutrient utilization efficiency.

2. Humic Acid Transformation and Utilization of Fertilizer Nutrients

   Due to its abundant acidic functional groups and high cation exchange capacity (CEC), humic acid can act as a soil conditioner to improve soil physicochemical properties and enhance the soil's ability to retain nutrient ions. It retains and mobilizes nutrients in fertilizers through binding with NPK nutrients, competing with phosphates, and adsorbing potassium. This increases fertilizer effectiveness and slow-release properties, thereby improving fertilizer utilization efficiency.

3. Regulation of Soil Microbial Activity by Humic Acid

   Humic acid is an organic macromolecule that provides abundant carbon sources for soil microorganisms. Soil microbial nutrition primarily includes elements such as carbon (C), oxygen (O), nitrogen (N), and essential mineral nutrients. With its functions of adsorption, complexation, chelation, ion exchange, and redox reactions, humic acid can convert fixed nutrients into available forms, thereby enhancing the availability of previously inaccessible soil nutrients. This process supplies more adequate nutrition to soil microorganisms.

   Field trials were conducted to investigate the effects of humic acid fertilizers on the abundance and diversity of rhizosphere soil microorganisms in pepper crops. The results demonstrate that humic acid activates the soil, increases microbial abundance and diversity, and optimizes soil microbial community structure. Among the treatments, the application of 6% humic-acid compound fertilizer yielded the optimal effects.

   

4. Combined Application of Humic Acid with Inorganic Fertilizers

   Humic acid serves as the optimal green synergist for enhancing fertilizer efficiency. Combined application of humic acid with reduced fertilizer dosages (10%-30% reduction) improves soil quality, increases crop yield and quality, and demonstrates universal efficacy for both single-nutrient and compound fertilizers. This approach significantly boosts fertilizer effectiveness while reducing nutrient loss and fixation, and ameliorates soil physicochemical properties.

   Effect on nitrogen fertilizer

   The combination of nitrogen fertilizer with humic acid forms slow-release nitrogen fertilizer, which can promote plant absorption of ammonia, reduce nitrogen fertilizer loss and the resulting ammonia pollution, inhibit nitrification, and thus increase nitrogen fertilizer utilization rate.

   Effect on phosphorus fertilizer

   The active oxygen-containing functional groups of humic acid effectively combine with phosphorus fertilizer to form humic acid-phosphate complexes, transforming some water-soluble phosphorus into available phosphorus. On one hand, this can reduce the fixation of phosphorus in the soil, greatly improving the seasonal utilization rate of phosphorus fertilizer; on the other hand, humic acid can activate the phosphorus that is fixed in the soil, increasing the soil's phosphorus supply capacity.

   Effect on potassium fertilizer

   Humic acid reacts with potassium hydroxide to form a new type of potassium humic acid fertilizer, which combines with K to facilitate crop absorption and longer fertilizer effect. The complex reaction of humic acid and nitrogen, phosphorus and potassium to produce humic acid compound fertilizer is conducive to the "combined effect" of multiple nutrients, improve the utilization efficiency of nitrogen, phosphorus and potassium, enhance crop stress resistance, promote crop growth, and thus increase crop yield.

5. Application of humic acid in organic fertilizer

   The application of humic acid in organic fertilizer is not simply to use it as a carrier or filler, but to let it participate in the whole process of making organic fertilizer, humic acid participates in the decomposition and fermentation of organic matter such as livestock and poultry, can adjust the carbon-ammonia ratio in the fermentation material, and make the fermentation process tend to be in an ideal state.

   Added to chicken manure can accelerate the decomposition process of compost, shorten the time required for composting, thereby improving production efficiency, maintaining more available nitrogen, and further enhancing the fertilizer efficiency of chicken manure. Moreover, ammonia in the decomposition process forms ammonium humate. Its own peptides and proteins are also degraded into low molecular weight amino acids, thus facilitating the uptake of microorganisms and crops in the soil.

Brief summary

Humic acid fertilizers have broad application prospects in agricultural production. The use of humic acid fertilizers can bring a series of comprehensive benefits, including improving soil fertility, improving fertilizer utilization, enhancing crop resistance, increasing crop yield, improving crop quality, enhancing soil vitality, reducing environmental pollution, carbon sequestration, energy conservation and emission reduction, etc., but there are still some problems in the application of humic acid fertilizers.

For example, humic acid fertilizers may produce some adverse reactions or interactions when used with other fertilizers, affecting the effectiveness of fertilizer. For example, acidic humic acid fertilizers are mixed with high-calcium magnesium fertilizers, and acid-base neutralization reactions occur. When using together, attention should be paid to the compatibility and ratio relationship between fertilizers, and the nature of the soil should be paid attention to to ensure a balanced supply of nutrients.