Compound soil conditioner based on plant-based organic acid and preparation method thereof

Abstract

The invention belongs to the technical field of soil improvement, and particularly relates to a compound soil conditioner based on plant-based organic acid and a preparation method thereof.The compound soil conditioner is prepared from, by weight, 30-50 parts of plant-based organic acid, 15-25 parts of mineral nutrient source, 20-30 parts of modified charcoal, 10-20 parts of humic acid, 5-15 parts of microbial agent and 3-8 parts of water-retaining agent. And 0.5 to 1.0 part of an adhesive. The composite soil conditioner disclosed by the invention has multiple effects of heavy metal passivation, phosphorus fixation, structure improvement and microorganism activation, and the comprehensive quality of soil and the growth environment of crops are remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of soil improvement technology, and specifically relates to a composite soil conditioner based on plant-based organic acids and its preparation method. Background Technology

[0002] Soil is the foundation of human agricultural production and the health of ecosystems. However, with the development of industrialization and intensive agriculture, soil degradation has become increasingly serious, mainly manifested as a complex problem including acidification, compaction, heavy metal pollution, and nutrient imbalance (especially phosphorus fixation and inactivation). These degradation problems are interconnected and severely restrict soil productivity, crop quality, and environmental safety. Utilizing plant-based organic acids to improve soil is an environmentally friendly strategy. Plant-based organic acids can activate fixed nutrients such as phosphorus and potassium through complexation and acidification, and regulate soil pH; however, when these active substances are directly applied to the soil, they face significant bottlenecks: small-molecule organic acids are easily and rapidly adsorbed and fixed by soil colloids, easily decomposed by microorganisms, or leached away by water, resulting in a very short effective time and low utilization rate.

[0003] Biochar, due to its porous structure, high specific surface area, and chemical stability, is used as an excellent soil conditioner, which can improve soil structure and increase carbon sequestration. However, biochar usually has limitations such as limited adsorption capacity, poor selectivity for specific pollutants (such as heavy metals), and may not have pH buffering or nutrient supply capabilities.

[0004] Therefore, developing a composite soil improvement system based on plant-based organic acids that releases in an orderly and long-term stable manner, thereby comprehensively and sustainably improving the soil's physical, chemical, and biological properties and its ability to retain fertilizer and water, has become an urgent problem to be solved. Summary of the Invention

[0005] To address the problems of existing technologies, this invention provides a composite soil conditioner based on plant-based organic acids and its preparation method. The composite soil conditioner of this invention has multiple functions, including heavy metal passivation, phosphorus fixation, structural improvement, and microbial activation, which significantly improves the overall soil quality and crop growth environment.

[0006] The technical solution of the present invention to solve the above problems is as follows: A composite soil conditioner based on plant-based organic acids, comprising, by weight, 30-50 parts of plant-based organic acids, 15-25 parts of mineral nutrient sources, 20-30 parts of modified biochar, 10-20 parts of humic acid, 5-15 parts of microbial inoculant, 3-8 parts of water-retaining agent, and 0.5-1.0 parts of binder. The modified biochar is prepared as follows: Step 1: Add biochar to deionized water, stir and mix to form a uniform suspension, add phosphoric acid solution, react at 60-90℃ for 1-4 hours, cool to room temperature, wash and dry to obtain activated biochar; Step 2: Add the activated biochar obtained in Step 1 to the solvent, mix well, then add hydroxyethylidene diphosphonic acid, adjust the pH of the reaction system to 4-5, stir the reaction at 50-60℃ for 2-3 hours, cool to <40℃ after the reaction is completed, add crosslinking agent dropwise, continue the reaction for 2-3 hours, cool after the reaction is completed, wash and dry to obtain functionalized biochar. Step 3: Add the functionalized biochar obtained in Step 2 to the aluminum magnesium hydrotalcite dispersion, and ultrasonically disperse it for 10-30 minutes at a power of 200-400W. Then adjust the pH to 9-10 and react at 40-70℃ for 1-3 hours. Centrifuge, filter and collect the solid product, and dry it to obtain nano-coated modified biochar.

[0007] Preferably, in step 1, the mass ratio of the biochar to deionized water and phosphoric acid solution is 1:1-2:0.5-0.8, and the concentration of the phosphoric acid solution is 15-20%.

[0008] Preferably, in step 2, the mass ratio of activated biochar, solvent, hydroxyethylidene diphosphonic acid, and crosslinking agent is 1:3-5:0.08-0.15:0.1-0.2, wherein the solvent is ethanol or deionized water, and the crosslinking agent is epichlorohydrin.

[0009] Preferably, in step 3, the mass ratio of the functionalized biochar to the aluminum-magnesium hydrotalcite dispersion is 1:0.2-0.5, and the concentration of the aluminum-magnesium hydrotalcite is 0.02-0.08 g / mL.

[0010] Preferably, the plant-based organic acids and humic acid are pre-treated to form microcapsules.

[0011] Preferably, the microcapsules are prepared by: Step a: Add plant-based organic acids and humic acid to deionized water and stir at 45-55℃ for 1.5-2.5 hours to form a uniform slurry with a solid content of 20-35%. Step b: Add sodium alginate to deionized water and stir at 35-45℃ until completely dissolved to obtain a wall material solution; Step c: Add the slurry from step a to the wall material solution from step b, emulsify at 4000-6000 rpm in a high-speed shear emulsifier for 8-15 minutes to obtain an emulsion, then add the emulsion to a 1-3% calcium chloride solution, let it stand and solidify at room temperature for 15-30 minutes, then filter or centrifuge to collect, wash and dry to obtain the final product.

[0012] Preferably, in step a, the mass ratio of the plant-based organic acid to humic acid is 15-25:3-8, and the plant-based organic acid is citric acid.

[0013] Preferably, in step b, the mass ratio of sodium alginate to deionized water is 2-5:40-60.

[0014] Preferably, in step c, the mass ratio of the slurry in step a to the wall material solution and calcium chloride solution in step b is 1:1-3:0.8-2.0.

[0015] Preferably, the mineral nutrient source is composed of phosphate rock powder, potassium feldspar powder, and silicon-calcium-magnesium fertilizer in a mass ratio of 5-15:5-15:3-10, the binder is one or both of polyvinyl alcohol or sodium carboxymethyl cellulose, and the water-retaining agent is potassium polyacrylate.

[0016] The preparation method of the above-mentioned composite soil conditioner based on plant-based organic acids is as follows: Plant-based organic acids, mineral nutrient sources, composite modified biochar, humic acid, microbial agents, and water-retaining agents are mixed evenly to obtain a dry mixture. A binder solution with a concentration of 2-5% is prepared by adding water to the binder. The binder is sprayed into the dry mixture at 35-45℃, and then formed into 2-4mm particles. The particles are dried at a low temperature of 50-60℃ for 4-8 hours and cooled to room temperature to obtain the final product.

[0017] The present invention has the following beneficial effects: This invention's composite soil conditioner achieves a comprehensive improvement in soil improvement performance through the synergistic effect of multiple components. It uses plant-based organic acids as the core acidification and carbon source supply unit, combined with modified biochar (enhanced heavy metal passivation and phosphorus fixation) encapsulated in situ with phosphonate and aluminum-magnesium hydrotalcite, humic acid (improves soil structure and fertilizer retention), microbial agents (activates soil biological activity), mineral nutrient sources (slow-release phosphorus, potassium, silicon, calcium, and magnesium), and a water-retaining agent (enhances water retention). It is granulated using polyvinyl alcohol / sodium carboxymethyl cellulose binder. The modified biochar solves the problems of poor adsorption selectivity and insufficient nutrient fixation capacity of ordinary biochar; humic acid and microbial agents synergistically optimize the soil microecology; and the mineral nutrient source and water-retaining agent ensure long-term nutrient supply and water balance. The preparation method of this invention is simple and combines multiple functions such as heavy metal passivation, phosphorus fixation, structural improvement, and microbial activation, significantly improving overall soil quality and the crop growth environment.

[0018] In the modified biochar process, step 1 involves the reaction of phosphoric acid with the disordered carbon structure in the biochar under heating conditions. This process etches and expands the existing pores, significantly increasing the specific surface area and pore volume, providing more space for subsequent loading. Simultaneously, the phosphorophosphoric acid groups in the phosphoric acid molecules bond to the carbon framework through esterification and cross-linking reactions. This treatment endows the biochar with excellent initial adsorption capacity and surface reactivity, laying the foundation for subsequent functional grafting. In step 2, the phosphonic acid groups of hydroxyethylidene diphosphonic acid undergo a dehydration condensation reaction with the oxygen-containing functional groups on the surface of the activated biochar from step 1 under acidic conditions, or are covalently fixed to the biochar surface through the bridging effect of an epichlorohydrin cross-linking agent. The introduction of hydroxyethylidene diphosphonic acid gives the material an extremely strong and specific chelating ability for heavy metal ions (such as cadmium and lead ions). Its phosphonic acid groups can form extremely stable cyclic chelates with heavy metals, converting them from an active state to a stable state, achieving efficient passivation, and preventing re-release due to changes in soil pH. Step 3: Aluminum-magnesium hydrotalcite, a layered bimetallic hydroxide, is grown in situ on the surface of functionalized biochar under alkaline conditions and then coated. This nanolayered structure brings multiple benefits: (a) Physical barrier effect: The nanosheets cover part of the macropores, forming a more complex mesoporous and microporous structure, further regulating the pore size distribution, which is beneficial for the confinement and slow release of organic acid molecules. (b) pH buffering and nutrient slow release: Anions (such as carbonate ions) between the hydrotalcite layers can reversibly exchange with acid radicals in the soil, thereby effectively buffering soil pH fluctuations. At the same time, the magnesium element it is rich in can be slowly released as a medium-quantity nutrient. (c) Enhanced structural stability: The coating layer acts like "armor," reducing the physical wear and chemical oxidation of biochar particles in the soil and extending its service life. In summary, the modified biochar is a high-performance composite carrier integrating "physical adsorption (porous structure), chemical chelation, pH buffering and nutrient supply (hydrotalcite coating layer)," and its stability and functional diversity far exceed those of the original biochar.

[0019] Furthermore, the microcapsules containing plant-based organic acids and humic acids have core materials of citric acid (a small-molecule organic acid, used for acidification and carbon source) and humic acid (a large-molecule organic matter). These materials can pre-form intermolecular hydrogen bonds or coordination bonds within the microcapsules, producing a synergistic complexation effect, which allows for more efficient activation of soil nutrients after release. Simultaneously, the hydrophobic framework of humic acid reduces the local polarity of citric acid, decreasing its direct contact with soil colloids (such as montmorillonite) and reducing initial strong adsorption (resulting in a lower fixation rate compared to pure citric acid). The microcapsule shell effectively blocks direct attacks on the core material (especially organic acids) from strong oxidizing substances, ultraviolet light, and some microorganisms in the soil, significantly reducing non-targeted degradation during storage and initial application. The release of the microcapsules depends on water penetration, shell swelling, and gradual degradation. This slow release mode allows the organic acids and humic acids to act continuously and stably on the soil, avoiding the rapid leaching caused by traditional direct application and significantly extending the effective action time.

[0020] The modified biochar, combined with plant-based organic acids and humic acid, forms microcapsules in the soil, creating an "adsorption-slow release" dynamic equilibrium. The porous structure and layered bimetallic hydroxide coating of the modified biochar preferentially adsorb free heavy metals and excess phosphorus in the soil, reducing their "ineffective competitive adsorption" of the microcapsules. The microcapsules, protected by humic acid and blocked by the calcium alginate shell, only slowly release citric acid, avoiding strong one-time adsorption by the modified biochar and extending the effective action time. The phosphorus fixed by the modified biochar and the slowly released citric acid from the microcapsules synergistically improve phosphorus utilization. The porous structure of the biochar provides a habitat for microbial agents, while the citric acid and humic acid released by the microcapsules can serve as carbon sources, promoting agent proliferation and further enhancing soil biological activity. The layered bimetallic hydroxide coating of the modified biochar delays nutrient release, while the calcium alginate shell of the microcapsules controls organic acid release. Together with the mineral nutrient source (slow-release phosphorus and potassium), they form a three-stage nutrient supply system, ensuring the sustainability of soil improvement effects. The two work together to solve the problems of "excessive adsorption leading to functional failure" or "excessive release leading to resource waste" of single materials, significantly improving the overall performance of composite soil conditioners. Attached Figure Description

[0021] Figure 1 This is a comparison of the Cd content in soil treated with the composite soil conditioner in Examples 1-6 and Comparative Examples 1-4. Figure 2 This study compares the cadmium content in lettuce after soil was treated with a composite soil conditioner in Examples 1-6 and Comparative Examples 1-4. Detailed Implementation

[0022] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0023] The following raw materials are all commercially available. Biochar, 97% content, 20 mesh particle size, 0.15% porosity, Shijiazhuang Huijin Mineral Products Co., Ltd.; Humic acid, mineral extract, powder, 42.59% humic acid content, Shandong Yishunfa Chemical Co., Ltd.; Microbial inoculant, effective live bacteria count >500 million / g, Hebei Ruinong Fertilizer Co., Ltd.; Phosphate rock powder, 99% effective ingredient content, Shandong Xuguang Chemical Co., Ltd.; Potassium feldspar powder, potassium oxide content >8%, iron oxide content <0.1%, Guangzhou Yifeng Chemical Technology Co., Ltd.; Silicon-calcium-magnesium fertilizer, 30% effective ingredient content, magnesium oxide ≥5%, Inner Mongolia Jinwoyuan Fertilizer Co., Ltd.; Sodium carboxymethyl cellulose, 99% effective ingredient content, Shandong Fangchang Biotechnology Co., Ltd.; Polyvinyl alcohol, 99% effective ingredient content, Hubei Xinghengye Technology Co., Ltd.; Potassium polyacrylate, 99% effective ingredient content, Wuhan Smike Biotechnology Co., Ltd.; Aluminum-magnesium hydrotalcite, 99% effective ingredient content, Wuhan Kanos Technology Co., Ltd.

[0024] Example 1 A composite soil conditioner based on plant-based organic acids comprises, by weight, 40 parts plant-based organic acids, 20 parts mineral nutrient source, 25 parts modified biochar, 15 parts humic acid, 10 parts microbial inoculant, 6 parts water-retaining agent, and 0.8 parts binder; the mineral nutrient source is composed of phosphate rock powder, potassium feldspar powder, and silicon-calcium-magnesium fertilizer in a mass ratio of 10:10:8; the binder is a mixture of polyvinyl alcohol and sodium carboxymethyl cellulose in a mass ratio of 1:1; and the water-retaining agent is potassium polyacrylate.

[0025] The modified biochar is prepared as follows: Step 1: Add biochar to deionized water and stir to form a uniform suspension. Add phosphoric acid solution and react at 75°C for 2 hours. Cool to room temperature and wash repeatedly with deionized water until the pH of the filtrate is 6.5-7.5. Then dry at 100°C for 9 hours to obtain activated biochar. The mass ratio of biochar to deionized water and phosphoric acid solution is 1:1.5:0.7, and the concentration of phosphoric acid solution is 18%. Step 2: Add the activated biochar obtained in Step 1 to the solvent, mix well, then add hydroxyethylidene diphosphonic acid, adjust the pH of the reaction system to 4-5 with acetic acid, stir the reaction at 55℃ for 2.5h, cool to <40℃ after the reaction is complete, add the crosslinking agent dropwise at a rate of 1.4mL / min, continue the reaction for 2.5h, cool after the reaction is complete, wash three times alternately with water and ethanol, and dry at 80℃ for 6h to obtain functionalized biochar; wherein, the mass ratio of activated biochar, solvent, hydroxyethylidene diphosphonic acid, and crosslinking agent is 1:4:0.12:0.15, the solvent is ethanol, and the crosslinking agent is epichlorohydrin; Step 3: Add the functionalized biochar obtained in Step 2 to the aluminum-magnesium hydrotalcite dispersion, and sonicate for 20 min at 300 W. Then, add sodium carbonate solution (concentration 0.2 mol / L) dropwise until the pH is 9-10. React at 55℃ for 2 h, centrifuge, filter to collect the solid product, wash with water 3 times, and dry at 75℃ for 9 h to obtain nano-coated modified biochar. The mass ratio of the functionalized biochar to the aluminum-magnesium hydrotalcite dispersion is 1:0.4, and the concentration of the aluminum-magnesium hydrotalcite is 0.05 g / mL.

[0026] The preparation method of the above-mentioned composite soil conditioner based on plant-based organic acids is as follows: Plant-based organic acids, mineral nutrient sources, composite modified biochar, humic acid, microbial agents, and water-retaining agents are mixed to obtain a dry mixture. A binder solution with a concentration of 4% is prepared by adding water to the binder. The binder is sprayed into the dry mixture at 40°C and then formed into 2-4 mm particles. The particles are dried at 55°C for 6 hours and then cooled to room temperature to obtain the final product.

[0027] Example 2 A composite soil conditioner based on plant-based organic acids comprises, by weight, 30 parts plant-based organic acids, 25 parts mineral nutrient source, 20 parts modified biochar, 20 parts humic acid, 5 parts microbial inoculant, 8 parts water-retaining agent, and 0.5 parts binder; the mineral nutrient source is composed of phosphate rock powder, potassium feldspar powder, and silicon-calcium-magnesium fertilizer in a mass ratio of 15:5:10; the binder is a mixture of polyvinyl alcohol and sodium carboxymethyl cellulose in a mass ratio of 1:1; and the water-retaining agent is potassium polyacrylate.

[0028] The modified biochar is prepared as follows: Step 1: Add biochar to deionized water and stir to form a uniform suspension. Add phosphoric acid solution and react at 60°C for 4 hours. Cool to room temperature and wash repeatedly with deionized water until the pH of the filtrate is 6.5-7.5. Then dry at 120°C for 6 hours to obtain activated biochar. The mass ratio of biochar to deionized water and phosphoric acid solution is 1:2:0.8, and the concentration of phosphoric acid solution is 15%. Step 2: Add the activated biochar obtained in Step 1 to the solvent, mix well, then add hydroxyethylidene diphosphonic acid, adjust the pH of the reaction system to 4-5 with acetic acid, stir the reaction at 60℃ for 2 hours, cool to <40℃ after the reaction is complete, add the crosslinking agent dropwise at a rate of 2.0 mL / min, continue the reaction for 3 hours, cool after the reaction is complete, wash three times alternately with water and ethanol, and dry at 100℃ for 4 hours to obtain functionalized biochar; wherein, the mass ratio of activated biochar, solvent, hydroxyethylidene diphosphonic acid, and crosslinking agent is 1:5:0.15:0.1, the solvent is ethanol, and the crosslinking agent is epichlorohydrin; Step 3: Add the functionalized biochar obtained in Step 2 to the aluminum-magnesium hydrotalcite dispersion, and sonicate for 30 min at 200 W. Then, add sodium carbonate solution (concentration 0.2 mol / L) dropwise until the pH is 9-10. React at 40℃ for 3 h, centrifuge, filter to collect the solid product, wash with water 3 times, and dry at 90℃ for 6 h to obtain nano-coated modified biochar. The mass ratio of the functionalized biochar to the aluminum-magnesium hydrotalcite dispersion is 1:0.2, and the concentration of the aluminum-magnesium hydrotalcite is 0.08 g / mL.

[0029] The preparation method of the above-mentioned plant-based organic acid-based composite soil conditioner is the same as that in Example 1.

[0030] Example 3 A composite soil conditioner based on plant-based organic acids comprises, by weight, 50 parts plant-based organic acids, 15 parts mineral nutrient source, 30 parts modified biochar, 10 parts humic acid, 15 parts microbial inoculant, 3 parts water-retaining agent, and 1.0 part binder; the mineral nutrient source is composed of phosphate rock powder, potassium feldspar powder, and silicon-calcium-magnesium fertilizer in a mass ratio of 5:15:3; the binder is a mixture of polyvinyl alcohol and sodium carboxymethyl cellulose in a mass ratio of 1:1; and the water-retaining agent is potassium polyacrylate.

[0031] The modified biochar is prepared as follows: Step 1: Add biochar to deionized water and stir to form a uniform suspension. Add phosphoric acid solution and react at 90°C for 1 hour. Cool to room temperature and wash repeatedly with deionized water until the pH of the filtrate is 6.5-7.5. Then dry at 80°C for 12 hours to obtain activated biochar. The mass ratio of biochar to deionized water and phosphoric acid solution is 1:1:0.5, and the concentration of phosphoric acid solution is 20%. Step 2: Add the activated biochar obtained in Step 1 to the solvent, mix well, then add hydroxyethylidene diphosphonic acid, adjust the pH of the reaction system to 4-5 with acetic acid, stir the reaction at 50℃ for 3 hours, cool to <40℃ after the reaction is complete, add the crosslinking agent dropwise at a rate of 0.5 mL / min, continue the reaction for 2 hours, cool after the reaction is complete, wash twice alternately with water and ethanol, and dry at 70℃ for 10 hours to obtain functionalized biochar; wherein, the mass ratio of activated biochar, solvent, hydroxyethylidene diphosphonic acid, and crosslinking agent is 1:3:0.08:0.2, the solvent is ethanol, and the crosslinking agent is epichlorohydrin; Step 3: Add the functionalized biochar obtained in Step 2 to the aluminum-magnesium hydrotalcite dispersion, and sonicate for 10 min at 400 W. Then, add sodium carbonate solution (concentration 0.1 mol / L) dropwise until the pH is 9-10. React at 70℃ for 1 h, centrifuge, filter to collect the solid product, wash twice with water, and dry at 60℃ for 12 h to obtain nano-coated modified biochar. The mass ratio of the functionalized biochar to the aluminum-magnesium hydrotalcite dispersion is 1:0.5, and the concentration of the aluminum-magnesium hydrotalcite is 0.02 g / mL.

[0032] The preparation method of the above-mentioned plant-based organic acid-based composite soil conditioner is the same as that in Example 1.

[0033] Example 4 A composite soil conditioner based on plant-based organic acids, wherein the plant-based organic acids and humic acid are pre-treated to form microcapsules, and the preparation method of the microcapsules is as follows: Step a: Add plant-based organic acid and humic acid to deionized water and stir at 50°C for 2 hours to form a uniform slurry with a solid content of 28%; In step a, the mass ratio of plant-based organic acid to humic acid is 20:5, and the plant-based organic acid is citric acid. Step b: Add sodium alginate to deionized water and stir at 40°C until completely dissolved to obtain a wall material solution; in step b, the mass ratio of sodium alginate to deionized water is 3:50. Step c: Add the slurry from step a to the wall material solution from step b, emulsify at 5000 rpm in a high-speed shear emulsifier for 12 minutes to obtain an emulsion. Then, add the emulsion to a 2% calcium chloride solution, allow it to stand and solidify at room temperature for 20 minutes to form microcapsule shells, then collect by centrifugation at 3500 rpm, wash three times with deionized water to remove unreacted calcium ions and free sodium alginate, dry at 65°C for 7 hours, and pass the dried particles through a 0.4 mm sieve to obtain the final product. In step c, the mass ratio of the slurry from step a to the wall material solution and calcium chloride solution from step b is 1:2:1.4. The rest is the same as in Example 1.

[0034] The preparation method of the above-mentioned composite soil conditioner based on plant-based organic acids is as follows: the microcapsules, mineral nutrient source, composite modified biochar, microbial agent and water-retaining agent are mixed evenly to obtain a dry mixture. The binder is added to water to prepare a binder solution with a concentration of 4%. The binder is sprayed into the dry mixture at 40°C and then made into 2-4 mm particles. The particles are dried at 55°C for 6 hours and cooled to room temperature to obtain the final product.

[0035] Example 5 A composite soil conditioner based on plant-based organic acids, wherein the plant-based organic acids and humic acid are pre-treated to form microcapsules, and the preparation method of the microcapsules is as follows: Step a: Add plant-based organic acid and humic acid to deionized water and stir at 45°C for 2.5 hours to form a uniform slurry with a solid content of 20%; in step a, the mass ratio of plant-based organic acid to humic acid is 15:8, and the plant-based organic acid is citric acid. Step b: Add sodium alginate to deionized water and stir at 35°C until completely dissolved to obtain a wall material solution; in step b, the mass ratio of sodium alginate to deionized water is 5:60. Step c: Add the slurry from step a to the wall material solution from step b, emulsify at 4000 rpm in a high-speed shear emulsifier for 15 minutes to obtain an emulsion. Then, add the emulsion to a 2% calcium chloride solution, allow it to stand and solidify at room temperature for 30 minutes to form microcapsule shells, collect by centrifugation at 3500 rpm, wash three times with deionized water to remove unreacted calcium ions and free sodium alginate, dry at 60°C for 8 hours, and pass the dried particles through a 0.4 mm sieve to obtain the final product. In step c, the mass ratio of the slurry from step a to the wall material solution and calcium chloride solution from step b is 1:1:2.0. The rest is the same as in Example 1.

[0036] The preparation method of the above-mentioned plant-based organic acid-based composite soil conditioner is the same as that in Example 4.

[0037] Example 6 A composite soil conditioner based on plant-based organic acids, wherein the plant-based organic acids and humic acid are pre-treated to form microcapsules, and the preparation method of the microcapsules is as follows: Step a: Add plant-based organic acid and humic acid to deionized water and stir at 55°C for 1.5 hours to form a uniform slurry with a solid content of 35%; in step a, the mass ratio of plant-based organic acid to humic acid is 25:3, and the plant-based organic acid is citric acid. Step b: Add sodium alginate to deionized water and stir at 45°C until completely dissolved to obtain a wall material solution; in step b, the mass ratio of sodium alginate to deionized water is 2:40. Step c: Add the slurry from step a to the wall material solution from step b, emulsify at 6000 rpm for 8 minutes in a high-speed shear emulsifier to obtain an emulsion. Then, add the emulsion to a 3% calcium chloride solution, allow it to stand and solidify at room temperature for 15 minutes to form microcapsule shells, then collect by centrifugation at 3500 rpm, wash three times with deionized water to remove unreacted calcium ions and free sodium alginate, dry at 70°C for 5 hours, and pass the dried particles through a 0.4 mm sieve to obtain the final product. In step c, the mass ratio of the slurry from step a to the wall material solution and calcium chloride solution from step b is 1:3:0.8. The rest is the same as in Example 1.

[0038] The preparation method of the above-mentioned plant-based organic acid-based composite soil conditioner is the same as that in Example 4.

[0039] Comparative Example 1 A composite soil conditioner based on plant-based organic acids, wherein the modified biochar is replaced with ordinary commercially available biochar, and the rest is the same as in Example 1.

[0040] Comparative Example 2 A composite soil conditioner based on plant-based organic acids, wherein the modified biochar is replaced with ordinary commercially available biochar, and the rest is the same as in Example 4.

[0041] Comparative Example 3 A composite soil conditioner based on plant-based organic acids, wherein the modified biochar is replaced with ordinary commercially available biochar, and the rest is the same as in Example 5.

[0042] Comparative Example 4 A composite soil conditioner based on plant-based organic acids, wherein the modified biochar is prepared as follows: Step 1: Add biochar to deionized water and stir to form a uniform suspension. Add phosphoric acid solution and react at 75°C for 2 hours. Cool to room temperature and wash repeatedly with deionized water until the pH of the filtrate is 6.5-7.5. Then dry at 100°C for 9 hours to obtain activated biochar. The mass ratio of biochar to deionized water and phosphoric acid solution is 1:1.5:0.7, and the concentration of phosphoric acid solution is 18%. Step 2: Add the activated biochar obtained in Step 1 to the solvent, mix well, then add hydroxyethylidene diphosphonic acid, adjust the pH of the reaction system to 4-5 with acetic acid, stir the reaction at 55℃ for 2.5h, cool to <40℃ after the reaction is complete, add the crosslinking agent dropwise at a rate of 1.4mL / min, continue the reaction for 2.5h, cool after the reaction is complete, wash three times alternately with water and ethanol, and dry at 80℃ for 6h to obtain modified biochar; wherein, the mass ratio of the activated biochar, solvent and crosslinking agent is 1:4:0.15, the solvent is ethanol and the crosslinking agent is epichlorohydrin.

[0043] The rest is the same as in Example 1.

[0044] Performance testing Acidified soil (pH≈4.8, low available phosphorus, Cd 0.85mg / kg) was taken and soil conditioners from Examples 1-6 and Comparative Examples 1-4 were added respectively. 20g of conditioner (i.e., 2%) was added per 1kg of dry soil. The improved soil was then tested: soil pH, organic matter content, available phosphorus, and heavy metal content (Cd) of each group were determined according to "Soil Agrochemical Analysis". Lettuce was planted in the acidified soils improved by the soil conditioners from Examples 1-6 and Comparative Examples 1-4 respectively, and the yield and heavy metal content were tested. The acidified soil served as a blank control group.

[0045] Table 1. Measurement Results From Table 1 and Figure 1 , Figure 2It can be seen that the soil conditioners in all examples can increase soil pH, organic matter and available phosphorus content, reduce soil Cd content, and improve lettuce yield and safety, but the effects vary significantly among different formulations. The blank control soil was severely acidified, with low organic matter (8.5 g / kg), scarce available phosphorus, and high Cd content, resulting in a lettuce yield of only 1.50 kg / m² and a lettuce Cd content as high as 0.425 mg / kg. In contrast, Examples 1-3 introduced modified biochar and mineral nutrients, raising the pH to 6.15-6.25, increasing organic matter to 19.8-21.2 g / kg, increasing available phosphorus to 64-71 mg / kg, reducing soil Cd to 0.36-0.40 mg / kg, increasing lettuce yield to 2.96-3.15 kg / m², and reducing lettuce Cd to 0.039-0.043 mg / kg. Examples 4-6 employed plant-based organic acids and humic acid microencapsulation technology. The slow-release effect further optimized the pH to 6.38-6.45, achieving organic matter and available phosphorus levels of 23.5-24.6 g / kg and 81-87 mg / kg, respectively. Soil Cd was reduced to 0.27-0.29 mg / kg, and lettuce yield increased to 3.46-3.60 kg / m². Lettuce Cd was stabilized at 0.030-0.032 mg / kg. The effects were comprehensively superior to those of Examples 1-3, demonstrating the advantages of microencapsulation in improving nutrient utilization and reducing the bioavailability of heavy metals. Comparative Examples 1-3 used commercially available biochar, which lacked sufficient pore structure and surface functional groups. The pH only increased to 5.85-5.92, with limited improvement in organic matter and available phosphorus. Soil Cd removal rate was low (0.52-0.55 mg / kg), and lettuce yield was only 2.62-2.74 kg / m². Lettuce Cd levels remained high (0.063-0.068 mg / kg), posing a safety risk. Comparative Example 4 lacked the nano-encapsulation step of modified biochar, resulting in less pH and nutrient improvement compared to a complete modification process. Yield and Cd fixation were also poor. In conclusion, modified biochar combined with microencapsulated organic acids / humic acids can significantly improve the physicochemical properties and biosafety of acidified soils, with Examples 4-6 showing the best performance.

[0046] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0047] Although embodiments of this application have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of this application, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A composite soil conditioner based on plant-based organic acids, characterized in that, The ingredients, by weight, include 30-50 parts of plant-based organic acid, 15-25 parts of mineral nutrient source, 20-30 parts of modified biochar, 10-20 parts of humic acid, 5-15 parts of microbial inoculant, 3-8 parts of water-retaining agent, and 0.5-1.0 parts of binder. The method for preparing the modified biochar is as follows: Step 1: Add biochar to deionized water, stir and mix to form a uniform suspension, add phosphoric acid solution, react at 60-90℃ for 1-4 hours, cool to room temperature, wash and dry to obtain activated biochar; Step 2: Add the activated biochar obtained in Step 1 to the solvent, mix well, then add hydroxyethylidene diphosphonic acid, adjust the pH of the reaction system to 4-5, stir the reaction at 50-60℃ for 2-3 hours, cool to <40℃ after the reaction is completed, add crosslinking agent dropwise, continue the reaction for 2-3 hours, cool after the reaction is completed, wash and dry to obtain functionalized biochar. Step 3: Add the functionalized biochar obtained in Step 2 to the aluminum magnesium hydrotalcite dispersion, and ultrasonically disperse it for 10-30 minutes at a power of 200-400W. Then adjust the pH to 9-10 and react at 40-70℃ for 1-3 hours. Centrifuge, filter and collect the solid product, and dry it to obtain nano-coated modified biochar.

2. The composite soil conditioner based on plant-based organic acids according to claim 1, characterized in that, In step 1, the mass ratio of biochar to deionized water and phosphoric acid solution is 1:1-2:0.5-0.8, and the concentration of phosphoric acid solution is 15-20%.

3. The composite soil conditioner based on plant-based organic acids according to claim 1, characterized in that, In step 2, the mass ratio of activated biochar, solvent, hydroxyethylidene diphosphonic acid, and crosslinking agent is 1:3-5:0.08-0.15:0.1-0.2, the solvent is ethanol or deionized water, and the crosslinking agent is epichlorohydrin.

4. The composite soil conditioner based on plant-based organic acids according to claim 1, characterized in that, Step 3, the mass ratio of the functionalized biochar and aluminum-magnesium hydrotalcite dispersion is 1:0.2-0.5, and the concentration of the aluminum-magnesium hydrotalcite is 0.02-0.08 g / mL.

5. The composite soil conditioner based on plant-based organic acids according to claim 1, characterized in that, The plant-based organic acids and humic acid are pre-treated to form microcapsules.

6. The composite soil conditioner based on plant-based organic acids according to claim 5, characterized in that, The microcapsules are prepared by: Step a: Add plant-based organic acids and humic acid to deionized water and stir at 45-55℃ for 1.5-2.5 hours to form a uniform slurry with a solid content of 20-35%. Step b: Add sodium alginate to deionized water and stir at 35-45℃ until completely dissolved to obtain a wall material solution; Step c: Add the slurry from step a to the wall material solution from step b, and shear emulsify at 4000-6000 rpm for 8-15 minutes to obtain an emulsion. Then add the emulsion to a 1-3% calcium chloride solution, let it stand and solidify at room temperature for 15-30 minutes, and then filter or centrifuge to collect, wash, and dry to obtain the final product.

7. The composite soil conditioner based on plant-based organic acids according to claim 6, characterized in that, In step a, the mass ratio of the plant-based organic acid to humic acid is 15-25:3-8, and the plant-based organic acid is citric acid.

8. The composite soil conditioner based on plant-based organic acids according to claim 6, characterized in that, In step b, the mass ratio of sodium alginate to deionized water is 2-5:40-60.

9. The composite soil conditioner based on plant-based organic acids according to claim 6, characterized in that, In step c, the mass ratio of the slurry from step a to the wall material solution and calcium chloride solution from step b is 1:1-3:0.8-2.

0.

10. The method for preparing the composite soil conditioner based on plant-based organic acids as described in any one of claims 1-9, characterized in that, Specifically, the following steps are taken: Plant-based organic acids, mineral nutrient sources, composite modified biochar, humic acid, microbial agents, and water-retaining agents are mixed to obtain a dry mixture. The binder is added to water to prepare a binder solution with a concentration of 2-5%. The binder is sprayed into the dry mixture at 35-45℃, and then formed into 2-4mm particles. The particles are dried at a low temperature of 50-60℃ for 4-8 hours and cooled to room temperature to obtain the final product.

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