The synergistic improvement effects of humic acid on fertilizers and soil

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I. Synergistic effect on nitrogen fertilizers: Nitrogen fertilizers are mainly in the form of carbonates and urea. Ammonium nitrogen is unstable and highly volatile; when mixed with humic acid fertilizers, the presence of functional groups such as carboxyl groups and phenolic carboxyl groups in humic acid gives it strong ion-exchange and adsorption capabilities, thereby reducing the loss of ammonium nitrogen. Urea is an amide-based nitrogen fertilizer; the nitrogen it contains must be broken down into ammonium carbonate by urease secreted by urea bacteria before it can be absorbed by plants. If urea is applied to calcareous soil, the ammonium carbonate produced by the decomposition of urea will also be lost through volatilization due to the alkaline conditions, which is one of the reasons for the low nitrogen utilization efficiency of urea fertilizers. Adding humic acid to urea has a significant synergistic effect on it. First, it inhibits the activity of urease, slowing down the decomposition of urea and reducing volatilization. Second, humic acid can form complexes with urea, which are gradually broken down to release nitrogen, thereby extending the fertilizer efficacy of urea. At the same time, the biological activity of humic acid promotes the development of plant roots and nitrogen metabolism within the plants, facilitating nitrogen absorption. II. Synergistic effect on phosphate fertilizers: When quick-acting phosphate fertilizers are applied to the soil, the soluble phosphorus is easily fixed by the soil. In acidic soils, phosphorus is primarily fixed by free iron and aluminum ions; in alkaline soils, it is mainly fixed by calcium. As a result, quick-acting phosphorus is converted into slow-acting or even ineffective phosphorus, with only 10–20% of the phosphorus being available for use during that growing season. The application of humic acid fertilizers can suppress the soil’s fixation of water-soluble phosphorus, slowing down the conversion of readily available phosphorus into less available or ineffective forms. The hydrolyzed nitrohumic acid increases the distance that phosphorus can travel within the soil, thereby facilitating its absorption by the roots. III. Synergistic effect on potassium fertilizers: Humic acid contains functional groups that can absorb and store potassium ions, thereby preventing their loss in water in sandy and leachable soils, as well as preventing the fixation of potassium in clayey soils. Certain components of humic acid, such as fulvic acid and other low-molecular-weight humic acids, have a dissolving effect on minerals like potassium silicates and potash feldspar; they enable these minerals to decompose slowly, increasing the release of potassium and raising the level of readily available potassium. IV. The synergistic effect on micronutrient fertilizers: There is a considerable amount of trace elements stored in the soil, but only a small fraction of them is available for absorption by plants. Humic acid can form complex compounds with insoluble trace elements, resulting in humic acid-trace element complexes that are highly soluble and easily absorbed by plants. These complexes facilitate absorption through the roots or via the leaves, and help plants transport the absorbed trace elements from the roots to the above-ground parts as well as from one set of leaves to another. V. Indirect effects of humic acid on soil 1. Physical effects 1) Improving soil structure 2) Preventing soil cracking and erosion. 3) Increase soil water retention and enhance drought resistance. 4) Darken the soil color, which facilitates the absorption of solar energy. 2. Chemical effects: 1) Regulate the soil’s pH value. 2) Improve and optimize the plant’s absorption of nutrients and water _ 3) Act as a natural chelator in alkaline conditions to facilitate the absorption of metal ions by the roots. 4) Rich in organic substances and minerals essential for plant growth. 5) Maintain soluble inorganic fertilizers in the root zone to reduce their loss. 6) Increase soil cation exchange capacity. 7) Convert nutrient elements into a form that is easily absorbed by plants. 8) Enhance plants’ absorption of nitrogen. 9) Reduce the chances of phosphorus reacting with Ca, Fe, Mg, and Al, so that phosphorus remains in a form beneficial for plant growth. 10) It helps release CO2 from soil calcium carbonate, which can be utilized in photosynthesis. 11) It helps reduce plant chlorosis caused by trace elements. 12) Reduce the availability of toxic substances in the soil. 3. Biological effects 1) Stimulates the growth and reproduction of beneficial microorganisms in the soil. 2) Enhance the plant’s natural resistance and ability to fight pests. VI. Overview and Prospects of the Application of Humic Substances in Agricultural Production. The humic acids contained in peat, lignite, and weathered coal are referred to as coal humic acids. They possess similar structures and properties to soil humic acids found in soil organic matter, and they also have many similar effects on soil and crops. Therefore, over the years abroad, considerable attention has been paid to the use of these humic substances from peat, lignite, and weathered coal in forestry and agricultural production.