In the 1980s, Chinese researchers conducted a large number of agronomic experiments to summarize the five effects of humic acid. Foreign scholars also believe that humic acid plays a major role in stimulating plant growth, improving soil and enhancing fertilizer in the field of plant nutrition. Recent studies have found that humic acid has a greater passivation effect on the biological activity of heavy metals in soil. In this paper, this paper mainly summarizes the recent research and development and application of humic acid in soil improvement, and provides reference for the research and application of humic acid in agriculture and environmental protection.
1. Application of humic acid in soil water retention and crop drought resistance
The development of soil amendments with soil water retention and crop drought resistance is one of the main technical approaches to solve the problems of drought, water shortage and low yield in China. Humic acid is a kind of polybasic organic acid which exists in nature in a large amount. Its excellent soil water retention and crop drought resistance make the development and application of soil amendments such as humic acid and its derivatives develop rapidly. At present, domestic humic acid soil improvement products include humic acid water retention agent, humic acid multi-functional degradable liquid film and the like.
1.1 Humic acid water retention agent application
The soil water retention agent is known as the plant “micro reservoir†and is an organic high molecular polymer with a three-dimensional network structure. Rainwater or irrigation water can be quickly absorbed and slowly released in the soil to ensure sufficient water in the rhizosphere. Huang Zhanbin summarized the direct and indirect effects of soil water retention agents on soil improvement. The basic principles of action include: (1) self-water absorption, water retention and water release principles; (2) promotion of soil improvement and soil and water conservation principles; (3) improvement of fertilizers and pesticides The principle of agrochemical product utilization; (4) The principle of regulating plant physiological water saving effect.
Humic acid is a kind of colloidal substance with strong water absorption and water storage capacity. Its three-dimensional network cross-links with a large number of active hydrophilic groups such as carboxyl group, phenolic hydroxyl group, alcoholic hydroxyl group and carbonyl group, which can form hydrogen bonds with H2O and absorb a large amount of water. To form a hydrogel, the water absorption rate of humic acid can be as high as 500% to 600%, which is 10 times that of clay particles. The humic acid absorbs water and swells to fix the water molecules in the network space, which can reduce the evaporation rate, is not easy to be lost under pressure, and has excellent water retention and slow release water effects. The humic acid water retaining agent has repeated water absorption properties of water absorption-release water-drying-re-water absorption. The study found that the initial water absorption ratio of humic acid water retention agent was 615 g/g, and the water retention ratio decreased with the increase of the number of times of use. However, after repeated absorption of water for 5 times, the water absorption rate still reached the initial rate of 68% or more.
With the deep research and application of humic acid, it can be introduced into the water retaining agent, which can not only exert the effect of humic acid to improve soil, synergize fertilizer, stimulate growth, enhance stress resistance and improve quality, but also improve the water absorption of water retaining agent. Salt-tolerant performance, while significantly reducing the cost of preparation of water retention agent. The humic acid was prepared from weathered lignite as raw material, and sodium polyacrylate (PNa A) water-retaining agent was synthesized by solution polymerization. The surface cross-linking reaction was used to combine humic acid with sodium polyacrylate to obtain surface cross-linked poly. Sodium acrylate/humic acid water retention agent. The study found that the water absorption ratio of sodium polyacrylate to deionized water and tap water is 720 g/g and 180 g/g, respectively. When the water retaining agent contains 10% humic acid, the water absorption ratio of sodium polyacrylate to deionized water and tap water 750 g / g and 260 g / g, respectively, indicating that the addition of humic acid can improve the absorption capacity of water retaining agent for tap water. At the same time, the salt tolerance of the water retaining agent has also been significantly improved. Tests have shown that humic acid water retention agent can effectively promote plant growth, promote plant photosynthesis and improve plant water use efficiency; especially under high water conditions, humic acid water retention agent treated corn biomass and leaf area increased by 36% compared with the blank control. 24.8%.
1.2 Humic acid degradable black liquid film application
The use of plastic mulching cultivation technology to meet the crop insulation, fertilizer, frost protection, to achieve the purpose of increasing production and income, has become one of the world's agricultural application technology. The plastic mulch is polyethylene or polyvinyl chloride, which is difficult to recycle and difficult to recycle, producing "white pollution" in the soil. Tian Yuanyu and others use brown coal, weathered coal or peat to modify papermaking black liquor, seaweed waste liquid, molasses waste liquid, brewing waste liquid or starch waste liquid, and black liquor is used as an extractant of humic acid to produce soil. Organic fertilizer; lignin, cellulose and polysaccharide form a polymer under the action of a crosslinking agent, and then mixed with various additives, silicon fertilizer, trace elements, pesticides and herbicides to prepare a multifunctional degradable black liquid mulch film. The application results show that spraying the liquid mulch on the soil surface can increase the ground temperature by 1~4 °C, the evaporation inhibition rate is over 30%, the soil water content is increased by more than 20%, the soil bulk density is reduced by 6%~10%, and the number of water stable aggregates in the soil ( >0.25 mm) can increase by more than 10%, the growth period of crops is 3~10 days ahead of schedule, the salt content of 0~50 cm soil is reduced by about 50%, and the crop yield is increased by more than 20%. Qiao Yingyun and other research results show that the effect of liquid film on crops has obvious drought resistance, warming effect and early maturity increase effect compared with no application of mulch film, which is equivalent to plastic film. It is gradually degraded 40 to 60 days after spraying, and degraded to humic acid (organic fertilizer) in 2 to 3 months without pollution.
2. Application of humic acid in the improvement of saline-alkali land
China's saline-alkali soil is more than 33 million hectares, mainly distributed in the northeast, northwest and north China, with secondary saline-alkali soils. The salt-alkali soil is lightly saline-alkali, moderately saline-alkali and heavy-saline. The pH of the light saline-alkali soil is 7.1-8.5, the salt content is below 0.3%, the p H value of the moderate saline-alkali soil is above 8.5-9.5, and the p H value of the heavy saline-alkali soil is at Above 9.5, the salt content is above 0.6%. The high concentration of salt in the saline-alkali soil will cause the soil water potential to decrease, which will make the plant difficult to absorb water, and the phenomenon of “physiological drought†may occur. Or the concentration of certain ions in the soil will cause the plants to reduce the absorption of other ions, resulting in “single salt poisoningâ€. The application of soil amendments to reduce or eliminate soil salinity and alkali content is one of the main methods for improving the application of saline-alkali soil. Among them, humic acid is a kind of saline-alkali improver with broad application prospects.
Humic acid is a kind of colloid with negative electricity. When combined with soil, it can increase the amount of cation adsorption, and it can separate salt and salt, thus reducing the content of topsoil. At the same time, humic acid is a weak acid, which can combine with various cations in the soil to form humate, forming a humic acid-humate mutual conversion buffer system, which can well adjust soil pH. The exchange rate of humic acid to cation is 5~100 times that of soil clay mineral. In addition, humic acid has excellent water retention effect, so humic acid can effectively reduce the salinity of saline soil and improve crop yield. Wei Kunfeng et al applied humic acid and alkali to the alkaline soil, the salt rejection rate was 18.76%~29.7%, the N a of the cultivated layer soil decreased significantly, the plant survival rate reached 84.5%~96.5%, and the survival rate of the poplar tree without the application of humic acid and alkali Only 16%. Sun Zaijin et al. used desulfurization gypsum and humic acid as modifiers for the coastal saline-alkali soil in the Yellow River Delta. Both desulfurization gypsum and humic acid treatment significantly reduced soil p H value, substitution sodium ion content and sodium adsorption ratio (SAR). ). Zhang Xiaoming and other environmental materials (humic acid HA, high molecular SAP, organic nutrition OF) on the growth of cypress, soil physical and chemical properties and soil microbial research shows that snow melting agent will cause soil salinization, combined materials (SAP+ HA+OF) significantly improved its improvement effect, increased the number of soil bacteria, fungi and actinomycetes, improved soil pH and electrical conductivity, decreased sodium adsorption ratio, and promoted the growth of new shoots.
3. Application of humic acid in the treatment of heavy metal pollution in soil
Heavy metal pollution in soil refers to the introduction of heavy metals into the soil due to human activities, resulting in the accumulation of heavy metals in the soil is significantly higher than the background value of the soil environment, and the soil environmental quality and ecological deterioration. With the rapid development of industry, agriculture and production, heavy metals have been continuously expanded through the “three wastes†of mining and mineral processing, sewage irrigation and Phosphate fertilizers and other agricultural products. In 2014, the Ministry of Environmental Protection and the Ministry of Land and Resources jointly issued the “National Soil Pollution Status Survey Bulletinâ€. The total over-standard rate of heavy metals in the country was 16.1%, and the highest rate of over-standard points was cultivated land, reaching 19.4%. Due to the concealed, long-term and irreversible nature of soil heavy metal pollution, the accumulation of harmful heavy metals in the soil will not only lead to soil degradation, but also can contaminate surface water and groundwater through runoff and leaching, and may directly poison plants. Or endanger human health through the food chain. The Action Plan for Soil Pollution Prevention and Control promulgated by the State Council in May 2016 pointed out that by 2020, the safe utilization rate of contaminated cultivated land will reach 90%, and the safe utilization rate of contaminated land will reach over 90%. By 2030, the safe utilization rate of contaminated cultivated land will reach over 95%, and the safe utilization rate of contaminated land will reach over 95%.
Heavy metal contaminated soils have physical, chemical, biological and engineering techniques. Among them, chemical remediation is currently the most technical choice for the treatment of heavy metal pollution in soil. It is to add modifiers to contaminated soil, increase soil organic matter, cation exchange capacity and clay content, and change soil physical and chemical properties to make heavy metals adsorb. Precipitation, inhibition, redox and other effects to reduce the bioavailability of heavy metals in the soil.
Humic acid has hydrophilicity, adsorption, ion exchange, complexation and redox. Humic acid is applied to the soil to react with water-soluble and bioavailable heavy metal ions, so that heavy metal ions are complexed, chelated or adsorbed, reducing the bioavailability of heavy metals and reducing the absorption of plants. The purification mechanism of humic acid on soil heavy metal pollution mainly includes: (1) reduction of heavy metal ions to form chelate compounds, thereby inactivating heavy metal ions; (2) formation of soil organic-inorganic composites by ion exchange and complexation reactions, Heavy metal ions in the soil are adsorbed and fixed to prevent them from entering the biological cycle. It is generally believed that van der Waals forces, hydrogen bonds, electrostatic adsorption, cation bond bridges, etc. are the main mechanisms of soil organic-inorganic composite bonding; (3) Stabilizing soil structure, providing matrix and energy for soil microbial activities, thereby indirectly affecting soil heavy metal ions Activity ability. Li Chun and others believe that the effects of humic acid on heavy metal pollution in soil mainly include planting and promoting the adsorption of heavy metals; supporting physical restoration, ensuring soil fertility, coordinating chemical repair, reducing secondary pollution and saving energy and reducing consumption.
It should be noted that the effect of humic acid on soil heavy metals is closely related to its species and composition. Some reports that humic acid has an activating effect on soil heavy metals, most of which are reported as passivation effects. The reason is found to be closely related to the humic acid materials used in the test. The conclusion of regularity requires a lot of related research. Wu Ruiping test found that weathered coal humic acid can reduce the concentration of lead in soil leachate, inhibit the transfer of lead to plants in soil, and reduce the accumulation of lead in rape. Peng Licheng and other experiments found that the combination of humic acid material, water retention agent and zeolitic material on Pb-Cd complex contaminated soil can inhibit the migration of Pb to the aerial part of maize, and the effect on P b fixation is significant. The leaching and adsorption desorption experiments of soil column by Shan Ruijuan found that humic acid had significant adsorption to Cd under acidic conditions. When p H was 11, the adsorption amount of Cd was the largest and stable. Hou Yueqing and other studies on the effect of passivation materials on the heavy metal form of pig manure compost showed that the humic acid had a significant effect on the passivation of Cu, Pb, Zn and Cd heavy metals, and the passivation effect on Zn was 64.94%. It is found that the ineffective oxidizing humic acid is better for the passivation of heavy metals in soil. Bai Lingyu and other researches believe that the fulvic acid in the soil mainly acts as a metal cation. The fulvic acid releases metal cations from inorganic minerals and forms water-soluble complexes, thereby improving the effectiveness of metals. Wei Shiqiang and other studies have shown that the complex capacity of fulvic acid and the abundant holding sites can transform the refractory cadmium into a highly active soluble organic complex state.
4. Suggestions for research and development of humic acid agricultural soil improvement application
(1) In order to solve the problem of large difference in the effect of humic acid products in soil improvement applications, it is necessary to strengthen the research and development of humic acid products for different application approaches and ranges such as saline-alkali improvement, cationic heavy metal passivation, degraded soil structure improvement and organic-inorganic compound fertilizer. In particular, the research and development of processing technology and product application technology of related products, and the establishment of corresponding application specifications or standards through industry organizations such as China Humic Acid Industry Association.
(2) For the basic types of coal humic acid basic products, the effective nutrient difference of humic acid basic products produced by different regions and processes is large, and the characteristics of humic acid products produced by alkali, acid and oxidation methods are analyzed. It is necessary to strengthen the processing technology of coal humic acid, the physical and chemical characteristics of products, the effects on soil and biological effects, and strengthen the study of the existence forms of humic acid at different interfaces such as soil, water and atmosphere, humic acid and NPK. The chemical combination of heavy metal elements, the mechanism and regulation of humic acid microbial degradation, and the environmental and economical nature of humic acid products.
(3) It is recommended that the state set up relevant research projects and funds to encourage associations and enterprises to actively invest in humic acid-related applied basic research.
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