Preparation method of fly ash-based soil improvement type slow-release organic fertilizer

Abstract

The invention belongs to the technical field of organic fertilizers, and particularly relates to a preparation method of a fly ash-based soil improvement type slow-release organic fertilizer. The porosity and the specific surface area of the fly ash are remarkably improved by carrying out synergistic pretreatment of ball milling, acid pickling, plasma treatment and microwave treatment on the fly ash; pretreated fly ash and sodium alginate are subjected to cross-linking solidification to construct a porous gel particle system, a high-activity sporosarcina pasteurii bacterial solution and a chemical fertilizer are loaded into porous gel particles through a vacuum impregnation technology, and uniform and efficient immobilization of functional microorganisms and nutrients is achieved; a compact biomineral coating layer is constructed on the surface of a bacterial fertilizer compound and the inner walls of pores through a microorganism-induced mineralization process, so that a novel fertilizer integrating the functions of a slow-release fertilizer, a soil conditioner and an organic fertilizer is prepared; the fertilizer shows excellent synergistic effects in the aspects of slow release performance, soil pH improvement, lasting nutrient supply and crop growth promotion and quality improvement.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of organic fertilizer, and particularly relates to a preparation method of a fly ash-based soil improvement type slow-release organic fertilizer. BACKGROUND

[0002] Fly ash is a large industrial solid waste generated by coal-fired power plants, and its storage not only occupies land but also poses environmental risks. In the prior art, there are some explorations on the preparation of fertilizer from fly ash, but most of them have the problems of single function, high energy consumption of process, or failure to realize the synergistic effect of organic-inorganic-biological.

[0003] In the prior art, patent CN2021104679505 discloses a method for preparing silicon-potassium composite fertilizer by treating fly ash with KOH solution, but the product is mainly inorganic mineral, and the introduction of organic matter and comprehensive improvement of soil are not considered. Patent CN2023113554739 uses aluminum extraction residue to prepare silicon-containing composite fertilizer, removes heavy metals and improves silicon activity through washing and steam activation, but the core is still inorganic fertilizer, which lacks slow-release mechanism and organic components. Patent CN2018110632913 uses fly ash as a carrier to load ammonium magnesium phosphate to prepare slow-release fertilizer, but the activation process has high energy consumption, and mainly relies on chemical fertilizer loading, and does not fully utilize the structure of fly ash itself, organic matter and functional microorganisms to produce synergistic effect.

[0004] More importantly, the prior art all uses chemical / physical methods such as high temperature, strong alkali or strong acid to modify fly ash. The combination of modified fly ash and nutrients by these methods is mainly physical adsorption or simple chemical bonding, which has insufficient stability in complex soil environment and poor predictability of slow-release performance. The existing slow-release fertilizer generally lacks the ability to actively regulate the soil microecology, and cannot realize the release of nutrients. SUMMARY

[0005] In view of the above problems, the purpose of the present application is to provide a preparation method of a fly ash-based soil improvement type slow-release organic fertilizer.

[0006] The technical content of the present application is as follows: The present application provides a preparation method of a fly ash-based soil improvement type slow-release organic fertilizer, comprising the following steps: 1) Fly ash pretreatment: The fly ash with heavy metals removed is ball milled to a particle size of 100-200 mesh, then soaked in a 1-3 mol / L hydrochloric acid solution for 2 h (solid-liquid ratio 1:10) to dissolve part of the surface glass body, increase the specific surface area and reactivity, and then washed with water to neutral after soaking, dried, and then treated with a microwave at a power of 60-80 W and an oxygen flow rate of 30-50 L / min for 20-30 min, and then transferred to a microwave environment at a power of 360-450 W and a temperature of 80-100°C for 15-20 min to obtain pretreated fly ash; By plasma treatment, high-energy active particles are used to bombard and oxidize the surface of fly ash, effectively introducing oxygen-containing functional groups (such as -OH, -COOH), significantly improving its hydrophilicity and biocompatibility, and the rapid and selective heating characteristics of microwaves can rapidly vaporize the water in the fly ash, creating a rich microporous and mesoporous structure inside the particles; 2) Porous gel particles: The pretreated fly ash is mixed with a 2-4 wt% aqueous sodium alginate solution at a mass ratio of 1:(3-5), stirred at 50-60°C for 30-40 min to form a uniform slurry, then dropped into a 3-5 wt% calcium chloride solution for crosslinking and solidification. The sodium ions in the sodium alginate solution exchange with the calcium ions, and instantaneously form water-insoluble calcium alginate gel, which firmly wraps and fixes the fly ash, forming spherical porous gel particles with a particle size of 2-5 mm. After solidification for 20-30 min, the particles are removed and gently washed with deionized water to remove the residual calcium chloride on the surface; The volume ratio of the aqueous sodium alginate solution to the calcium chloride solution is 1:(2-3); 3) Fertilizer compound: The Paenibacillus pasteurii bacterial solution is mixed with urea and water-soluble fertilizer in deionized water to prepare a bacterial fertilizer mixed solution; The mass ratio of the urea-decomposing bacterial solution to urea and water-soluble fertilizer is 1:(2-3):(1-2); Preparation of the Paenibacillus pasteurii bacterial solution: Paenibacillus pasteurii is inoculated into LB medium and cultured at 30-32°C for 24-36 h with shaking, then centrifuged and concentrated to obtain a bacterial solution with an OD 600 value of 1.0-1.5, urease activity ≥800 U / mL, and viable bacterial count ≥1×10 8 CFU / mL; The water-soluble fertilizer includes one or more of ammonium dihydrogen phosphate, potassium nitrate, dipotassium hydrogen phosphate, ammonium sulfate, potassium chloride, and calcium ammonium nitrate, and can be flexibly compounded according to the nutrient requirements of the target crop to provide N, P, K, and medium elements; The porous gel particles are immersed in a bacterial fertilizer mixture, kept at a vacuum degree of -0.07 to -0.09 MPa for 15 to 25 min, and then slowly restored to normal pressure, so that the bacterial fertilizer mixture is forced into the internal pores of the gel particles by pressure difference, the saturated particles are taken out, the excess liquid on the surface is drained, and the bacterial fertilizer compound is obtained; 4) The bacterial fertilizer compound is immersed in a mineralization solution with a concentration of 0.3 to 0.7 mol / L, and 0.1 to 0.2% of potassium dihydrogen phosphate is added as a mineralization inducer, and the total mass of the mineralization solution is 0.1 to 0.2%, and the solution is kept at 20 to 35°C for 24 to 72 hours, so that a dense calcium carbonate / magnesium carbonate biological mineral coating is formed on the surface of the particles and in the internal pore channels; Finally, the mineralized fertilizer particles are taken out, washed with deionized water to remove loose crystals on the surface, and then dried at 35 to 40°C for 4 to 6 hours until the water content is less than or equal to 10%, and the final fly ash-based soil improvement type slow-release organic fertilizer is obtained; The mineralization solution is a mixed solution of one or both of calcium chloride and magnesium chloride, and the molar ratio of Ca 2+ :Mg 2+ The molar ratio can be 1:(0 to 1); During this period, the Bacillus pasteurii in the particles uses urea as energy to decompose CO3 2- and raises the pH of the surrounding microenvironment, inducing Ca 2+ / Mg 2+ to form calcium carbonate / magnesium carbonate composite biological minerals, and a dense and hard biological mineral coating is formed on the surface of the particles and the inner wall of the pores.

[0007] The beneficial effects of the present application are as follows: The preparation method of the fly ash-based soil improvement type slow-release organic fertilizer of the present application significantly improves the porosity and specific surface area of the fly ash through the synergistic pretreatment of ball milling, acid washing, plasma treatment and microwave treatment, which greatly improves the subsequent nutrient loading capacity on one hand, and provides abundant habitat for the colonization of microorganisms on the other hand; the porous gel particle system is constructed by cross-linking and solidifying the pretreated fly ash and sodium alginate, which not only firmly fixes the fly ash carrier, but also forms a slow-release matrix with ideal particle size and strength; the high-activity Bacillus pasteurii bacterial solution and chemical fertilizer are loaded into the internal pores of the porous gel particles through vacuum impregnation technology, realizing uniform and efficient fixation of functional microorganisms and nutrients; a dense biological mineral coating is constructed on the surface of the bacterial fertilizer compound and the inner wall of the pores through the process of microbial induced mineralization, which not only provides a key physical slow-release barrier, but also effectively protects the internal bacteria; and finally, the finished product is obtained through mild drying.

[0008] The formed soil improvement type slow-release organic fertilizer has a structure of gel skeleton-bacterial fertilizer inner core-mineral shell, and achieves the following synergistic effects: the biological mineral coating layer gives the product excellent slow-release performance, the cumulative release rate of nutrients is ≤45% in 28 days, meeting the standard of slow-release fertilizer; the functional microorganisms in the fertilizer maintain a high survival rate under the protection of mineralization, can effectively colonize and continuously exert urease activity after being applied to soil, and realize biological regulation of nutrients; the product has multiple functions such as acid soil improvement, soil structure optimization and persistent nutrient supply, and makes the crop yield increase by more than 80% in a potting test, while significantly improving the product quality. DETAILED DESCRIPTION

[0009] The application will be further described in detail through specific implementation examples, and it should be understood that these examples are only used to illustrate the application and not to limit the protection scope of the application, and various equivalent modifications of the application made by those skilled in the art after reading the application all fall within the scope defined by the claims attached hereto.

[0010] Unless otherwise specified, all raw materials and reagents of the application are conventional market raw materials and reagents.

[0011] Raw material preparation: The fly ash is pre-treated by washing and removing; Reagents: hydrochloric acid (analytical pure), sodium alginate (food grade, viscosity 250 mPa・s), calcium chloride (analytical pure), magnesium chloride (analytical pure), urea (agricultural grade), water-soluble chemical fertilizer (ammonium dihydrogen phosphate, potassium nitrate, dipotassium hydrogen phosphate, ammonium sulfate, potassium chloride, calcium ammonium nitrate, agricultural grade), potassium dihydrogen phosphate (analytical pure); Strain: Pasteur's Sporosarcina (Sporosarcina pasteurii, preservation number CGMCC 1.3687); Culture medium: LB medium (10 g / L of proteose peptone, 5 g / L of yeast extract, 10 g / L of NaCl, pH 7.0-7.2).

[0012] Example 1 A preparation method of a fly ash-based soil improvement type slow-release organic fertilizer 1) Fly ash pretreatment: the fly ash is ball milled to 100 mesh, 100 kg of undersize is mixed with 1000 L of 2 mol / L hydrochloric acid solution, soaked at 35℃ for 2 h, stirred every 30 min during the period, washed to neutral (between 6.5~7), and dried at 65℃; transferred into a plasma processor, treated at 70 W power and 40 L / min oxygen flow for 25 min; then transferred into a microwave reactor, treated at 40 0W power and 90℃ for 18 min, to obtain pretreated fly ash (porosity 57%, specific surface area 158 m 2 / g).

[0013] 2) Porous gel particles: Prepare 350 kg of 3 wt% sodium alginate aqueous solution, add 100 kg of pretreated fly ash, stir at 55℃ for 35 min to form a uniform slurry; slowly drip 700 L of 4 wt% calcium chloride solution through a peristaltic pump, crosslink and solidify for 25 min, remove and rinse with deionized water to remove unreacted calcium chloride, and obtain porous gel particles with a particle size of 3-4 mm. 3) Bacterial fertilizer complex: Bacillus pasteurellii was inoculated onto LB medium and cultured at 31℃-30-32℃ with shaking for 30 hours. The bacterial solution was then concentrated by centrifugation to obtain the bacterial solution (OD). 600 =1.0, urease activity 850 U / mL, viable count 1.2×10 8 CFU / mL); Mix 20 kg of bacterial solution, 50 kg of urea, and 30 kg of water-soluble fertilizer (20 kg of ammonium dihydrogen phosphate + 10 kg of potassium nitrate) in a mass ratio of 1:2.5:1.5, add 100 kg of deionized water to dissolve and prepare a bacterial fertilizer mixture. 100 kg of porous gel particles were immersed in a bacterial fertilizer mixture and kept under a vacuum of -0.08 MPa for 20 min. The pressure was then slowly restored to normal, and the particles were removed and drained to obtain the bacterial fertilizer complex. 4) Organic fertilizer: Prepare a mineralization solution of 0.5 mol / L calcium chloride + 0.5 mol / L magnesium chloride (Ca 2+ :Mg 2+ =1:1) 800L, add 0.8kg potassium dihydrogen phosphate; immerse 400kg of microbial fertilizer compound in mineralization solution, and incubate at 28℃ for 48h; take it out, rinse the surface to loosen the crystals, and dry it with hot air at 38℃ for 5h until the moisture content is ≤10%, and the final fly ash-based soil-improving slow-release organic fertilizer is obtained.

[0014] Example 2 A method for preparing a fly ash-based soil-modified slow-release organic fertilizer 1) Fly ash pretreatment: Fly ash was ball-milled to 120 mesh. 100 kg of the sieve undersize was mixed with 1000 L of 1.5 mol / L hydrochloric acid solution and soaked at 35℃ for 2 h. The mixture was then washed with water until neutral and dried at 65℃. The ash was then transferred to a plasma processor and treated at 70 W power and 35 L / min oxygen flow rate for 22 min. Finally, it was transferred to a microwave reactor and treated at 420 W power and 85℃ for 16 min to obtain pretreated fly ash (porosity 55%, specific surface area 152 m²). 2 / g).

[0015] 2) Porous gel particles: Prepare 300 kg of 2.5 wt% sodium alginate aqueous solution, add 100 kg of pretreated fly ash, stir at 53℃ for 32 min to form a uniform slurry; slowly drip 600 L of 3 wt% calcium chloride solution through a peristaltic pump, crosslink and solidify for 23 min, remove and rinse with deionized water to remove unreacted calcium chloride, and obtain porous gel particles with a particle size of 2-3 mm; 3) Bacterial fertilizer complex: Bacillus pasteurellii was inoculated onto LB medium and cultured at 31°C with shaking for 30 hours. The bacterial solution was then concentrated by centrifugation to obtain the bacterial solution (OD). 600 =0.9, urease activity 820 U / mL, viable count 1.0×10 8 CFU / mL); Mix 15 kg of bacterial solution, 30 kg of urea, and 15 kg of water-soluble fertilizer (10 kg of dipotassium hydrogen phosphate + 5 kg of ammonium sulfate) in a mass ratio of 1:2:1, add 80 kg of deionized water to dissolve and prepare a bacterial fertilizer mixture. 100 kg of porous gel particles were immersed in a bacterial fertilizer mixture and kept under a vacuum of -0.07 MPa for 18 min. The pressure was then slowly restored to normal, and the particles were removed and drained to obtain the bacterial fertilizer complex. 4) Organic fertilizer: Prepare 700L of 0.6 mol / L calcium chloride mineralization solution and add 0.7 kg of potassium dihydrogen phosphate; immerse 350 kg of microbial fertilizer compound in the mineralization solution and let it stand at 30℃ for 60 h; take it out, rinse the surface to loosen the crystals, and dry it with hot air at 36℃ for 4.5 h until the moisture content is ≤10%, thus obtaining the final fly ash-based soil-improving slow-release organic fertilizer.

[0016] Example 3 A method for preparing a fly ash-based soil-modified slow-release organic fertilizer 1) Fly ash pretreatment: Fly ash was ball-milled to 180 mesh. 100 kg of the sieve undersize was mixed with 1000 L of 2.5 mol / L hydrochloric acid solution and soaked at 38℃ for 2 h. After soaking, it was washed with water until pH=6.8 and dried at 65℃. It was then transferred to a plasma processor and treated at 60 W power and 45 L / min oxygen flow rate for 28 min. Finally, it was transferred to a microwave reactor and treated at 380 W power and 95℃ for 20 min to obtain pretreated fly ash (porosity 59%, specific surface area 165 m²). 2 / g).

[0017] 2) Porous gel particles: Prepare 400 kg of 3.5 wt% sodium alginate aqueous solution, add 100 kg of pretreated fly ash, stir at 58℃ for 38 min to form a uniform slurry; slowly drip in 800 L of 5 wt% calcium chloride solution through a peristaltic pump, crosslink and solidify for 28 min, remove and rinse with deionized water to remove unreacted calcium chloride, and obtain porous gel particles with a particle size of 4-5 mm; 3) Bacterial fertilizer complex: Bacillus pasteurellii was inoculated onto LB medium, cultured at 30°C with shaking for 36 hours, and then concentrated by centrifugation to obtain the bacterial solution (OD). 600 =1.2, urease activity 850 U / mL, viable count 1.2×10 8 CFU / mL); Mix 20 kg of bacterial solution, 60 kg of urea, and 40 kg of water-soluble fertilizer (20 kg of potassium nitrate + 20 kg of diammonium hydrogen phosphate) in a mass ratio of 1:3:2, add 100 kg of deionized water to dissolve and prepare a bacterial fertilizer mixture. 100 kg of porous gel particles were immersed in a bacterial fertilizer mixture and kept under a vacuum of -0.08 MPa for 20 min. The pressure was then slowly restored to normal, and the particles were removed and drained to obtain the bacterial fertilizer complex. 4) Organic fertilizer: Prepare 900L of 0.4 mol / L magnesium chloride mineralization solution and add 0.9 kg of potassium dihydrogen phosphate; immerse 450 kg of microbial fertilizer compound in the mineralization solution and let it stand at 25℃ for 72 h; take it out, rinse the surface to loosen the crystals, and dry it with hot air at 39℃ for 4 h until the moisture content is ≤10%, thus obtaining the final fly ash-based soil-improving slow-release organic fertilizer.

[0018] Example 4 A method for preparing a fly ash-based soil-modified slow-release organic fertilizer 1) Fly ash pretreatment: Fly ash was ball-milled to 200 mesh. 100 kg of the sieve undersize was mixed with 1000 L of 1 mol / L hydrochloric acid solution and soaked at 30℃ for 2 h. It was then washed with water until neutral and dried at 65℃. The mixture was then transferred to a plasma processor and treated at 80 W power and 30 L / min oxygen flow rate for 20 min. Finally, it was transferred to a microwave reactor and treated at 450 W power and 80℃ for 15 min to obtain pretreated fly ash (porosity 54%, specific surface area 150 m²). 2 / g).

[0019] 2) Porous gel particles: Prepare 500 kg of 4 wt% sodium alginate aqueous solution, add 100 kg of pretreated fly ash, stir at 60℃ for 40 min to form a uniform slurry; slowly drip 1000 L of 3.5 wt% calcium chloride solution into the slurry using a peristaltic pump, crosslink and solidify for 30 min, remove the slurry and rinse with deionized water to remove unreacted calcium chloride, and obtain porous gel particles with a particle size of 3-5 mm. 3) Bacterial fertilizer complex: Bacillus pasteurellii was inoculated onto LB medium, cultured at 32°C with shaking for 24 hours, and then concentrated by centrifugation to obtain the bacterial solution (OD). 600 =1.3, urease activity 860 U / mL, viable count 1.25 × 10⁻⁶ 8CFU / mL); Mix 20 kg of bacterial solution, 44 kg of urea, and 36 kg of water-soluble fertilizer (26 kg of calcium ammonium nitrate + 10 kg of dipotassium hydrogen phosphate) in a mass ratio of 1:2.2:1.8, add 120 kg of deionized water to dissolve and prepare a bacterial fertilizer mixture. 120 kg of porous gel particles were immersed in a bacterial fertilizer mixture and kept under a vacuum of -0.08 MPa for 25 min. The pressure was then slowly restored to normal, and the particles were removed and drained to obtain the bacterial fertilizer complex. 4) Organic fertilizer: Prepare a 0.7 mol / L mineralization solution (0.4 mol / L calcium chloride + 0.3 mol / L magnesium chloride, Ca... 2+ :Mg 2+ =4:3) 850L, add 0.85kg potassium dihydrogen phosphate; immerse 420 kg of microbial fertilizer compound in mineralization solution, and incubate at 35℃ for 24h; take it out, rinse the surface to loosen the crystals, and dry it with hot air at 40℃ for 4h until the moisture content is ≤10%, thus obtaining the final fly ash-based soil-improving slow-release organic fertilizer.

[0020] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that plasma + microwave treatment was not used in the pretreatment of fly ash for subsequent preparation; other aspects remained the same. The resulting fly ash had a porosity of 32% and a specific surface area of ​​78 m². 2 / g.

[0021] Comparative Example 2 The difference from Example 1 is that Comparative Example 2 does not prepare porous gel particles, i.e., skips step 2), and directly mixes the pretreated fly ash and microbial fertilizer mixture from step 1) for subsequent preparation, while other aspects remain unchanged.

[0022] Comparative Example 3 The difference from Example 1 is that, in Comparative Example 3, after the microbial fertilizer mixture was prepared, the porous gel particles were mixed and soaked with the microbial fertilizer mixture under normal pressure for 2 hours, then drained to obtain the microbial fertilizer complex, while other aspects remained unchanged.

[0023] Comparative Example 4 The difference from Example 1 is that, after obtaining the microbial fertilizer compound, Comparative Example 3 does not use mineralization liquid for mineralization treatment, but directly dries to obtain organic fertilizer product, while other aspects remain unchanged.

[0024] Comparative Example 5 The difference from Example 1 is that in Comparative Example 5, potassium dihydrogen phosphate was not added in the mineralization treatment in step 4), while everything else remained the same.

[0025] Comparative Example 6 The difference from Example 1 is that the organic fertilizer of Comparative Example 6 was prepared as follows: 100 kg of fly ash was ball-milled and activated, then mixed with 20 kg of magnesium ammonium phosphate and 15 kg of humic acid. After granulation, 0.6 kg of polyvinyl alcohol was dissolved in 5 kg of deionized water (heated to dissolve at 60°C), and sprayed onto the surface of the granules with a sprayer. The granules were then dried at 50°C for 3 hours as a coating layer to obtain a slow-release organic fertilizer.

[0026] The organic fertilizers prepared in the above examples and comparative examples, and their performance in soil, were tested. 1. Survival rate of bacterial cells before and after mineralization Sample preparation: Before mineralization: Take the microbial fertilizer compound prepared in step 3) (i.e., the particles before mineralization).

[0027] After mineralization: Take the final fertilizer product prepared in step 4) (i.e., the mineralized granules).

[0028] Viable bacteria count: Plate count method was used. 1.0 g of sample was accurately weighed, aseptically ground, serially diluted with physiological saline, spread on urea medium plates, and incubated at 30°C for 48-72 h before counting. The number of viable bacteria per gram of fertilizer (CFU / g) was calculated.

[0029] Table 1 Comparison of viable bacteria count inside particles before and after mineralization

[0030] As shown in Table 1, the products of this invention (Examples 1-4) maintained a cell survival rate of approximately 60% after undergoing the mineralization process. This demonstrates that the process of this invention (especially the suitable phosphorus source and buffer system provided by potassium dihydrogen phosphate) creates a relatively mild mineralization environment for microorganisms. The formed mineral layer provides a slow-release function while also serving as a protective shell for the internal cells. Comparative Example 5, lacking a mineralization inducer, underwent a crude mineralization process, resulting in a significantly reduced cell survival rate, demonstrating the important role of potassium dihydrogen phosphate in maintaining cell activity.

[0031] 2. Physicochemical properties of organic fertilizer products Total nutrient content: The total nitrogen (N), available phosphorus (P2O5), and potassium (K2O) content were determined in accordance with the standard GB / T 17767.1-2008 Organic-Inorganic Compound Fertilizer.

[0032] pH value: Samples were prepared according to GB / T 8571-2008 Laboratory Sample Preparation of Compound Fertilizers, and the pH value of the saturated aqueous solution of fertilizer (fertilizer:water = 1:5) was measured using a pH meter.

[0033] Particle strength: 30 whole particles were randomly selected and their crushing resistance was measured using a particle strength tester. The average value was taken.

[0034] Heavy metal content: Refer to "GB / T 23349-2020 Determination of Arsenic, Cadmium, Chromium, Lead and Mercury Content in Fertilizers" to ensure the environmental safety of the product.

[0035] Table 2 Basic Physicochemical Properties of Organic Fertilizers

[0036] Heavy metal testing results showed that none of the samples contained heavy metals, and the content was far below the national standard limit.

[0037] As shown in Table 2, the pH values ​​of the organic fertilizer products in the embodiments of this invention are all above 8.0, indicating alkalinity and suggesting their potential to improve acidic soils. Furthermore, the particle strength of the organic fertilizer products in the embodiments is all >20N, far exceeding that of ordinary granulated fertilizers (typically around 10N), which may be attributed to the effect of the biomineral coating layer formed by the mineralization process. The carrier in Comparative Example 1 was not fully activated, resulting in low strength; Comparative Example 4 lacked mineralization and a coating layer, resulting in low strength; while Comparative Example 6, due to its polyvinyl alcohol coating layer, had high strength, but its nutrient content was relatively low.

[0038] 3. Sustained-release performance test Referencing the water dissolution rate method in the standard GB / T 23348-2009 Slow-Release Fertilizers, accurately weigh each fertilizer sample (based on phosphorus isotopes), place them in a plastic bottle, add a certain amount of pure water, and incubate at a constant temperature of 25℃. Take water samples at specific time points, determine the phosphorus content in the water, and calculate the cumulative nutrient release rate.

[0039] Table 3. Cumulative release rate of nutrients (as P2O5) in water (%)

[0040] As shown in Table 3, the initial dissolution rate (day 1) and differential dissolution rate (day 7) of the products of this invention (Examples 1-4) are extremely low, and the cumulative release rate over 28 days does not exceed 45%, fully complying with the national standards for slow-release fertilizers. Their release curves are smooth, demonstrating excellent long-term slow-release performance.

[0041] All comparative products (Comparative Examples 1-6) showed release rates well exceeding 75% on day 28, indicating poor immediate or controlled release effects. This demonstrates that the three core steps—plasma-microwave activation to construct a highly porous carrier, vacuum impregnation to achieve high loading, and microbial induction to form a dense biomineralized coating—are indispensable for achieving efficient sustained release. While Comparative Example 6 (traditional physical coating) exhibited some sustained-release properties, its effectiveness was far inferior to that of this invention.

[0042] 4. Soil culture experiment Acidic red soil (initial pH=5.3) was divided into pots. Each fertilizer sample was applied with the same amount of fertilizer (calculated as P2O5) and placed in a greenhouse (25℃, maintaining 60% field capacity) for 60 days. Samples were taken before cultivation, and after 30 and 60 days of cultivation for analysis. Soil pH value: determined by potentiometry; Available phosphorus content in soil: determined by sodium bicarbonate extraction-molybdenum antimony colorimetric method.

[0043] Table 4 Results of Soil Culture Experiment

[0044] *Note: Available phosphorus retention rate = (available phosphorus content in soil after 60 days / available phosphorus content in soil on the 3rd day after fertilization) × 100%.

[0045] As shown in Table 4, in terms of soil improvement effect, the products of this invention (Examples 1-4) can continuously and stably increase soil pH, improving acidic soil to near neutral after 60 days. This is due to the slow dissolution of its biomineral coating layer (calcium carbonate / magnesium). In contrast, the comparative products, especially Comparative Example 4 (without coating layer) and Comparative Example 2 (without carrier structure), showed negligible improvement effects.

[0046] In terms of nutrient retention capacity, Examples 1-4 retained over 50% of available phosphorus in the soil after 60 days, demonstrating extremely strong nutrient retention and continuous supply capabilities. This is not only due to the slow release of nutrients, but may also be due to the adsorption and fixation of phosphorus by the fly ash carrier and biochar, reducing the amount fixed by the soil. In contrast, Comparative Example 4 (without a coating layer) showed greater nutrient loss.

[0047] 5. Detection of persistent bacterial colonization and urease activity in soil This was conducted concurrently with the above soil culture experiments. Samples were taken and measured at 0, 30, and 60 days of culture. Specific bacterial populations in soil: The maximum probability number (MPN) method was used. Soil samples were serially diluted and cultured in a selective medium with urea as the sole nitrogen source. The number of Bacillus pasteurellii per gram of soil was calculated based on urease production characteristics (the phenol red indicator turns red as the medium pH increases).

[0048] Soil urease activity: The sodium phenolate-sodium hypochlorite colorimetric method was used. The amount of NH3-N produced by the enzymatic hydrolysis of urea per unit weight of soil per unit time was determined to characterize the overall urease activity in the soil.

[0049] Table 5. Changes in soil microbial community quantity and urease activity

[0050] As shown in Table 5, in terms of the soil colonization capacity of the microorganisms, the soil treated with the products of this invention (Examples 2 and 4) still showed detectable levels of 10 microorganisms after 60 days. 5 The presence of *Bacillus pasteurellii* at the CFU / g level indicates that the microbial agent in the fertilizer successfully colonized the soil. In contrast, the bacteria in Comparative Example 4 (without a mineralization coating) almost completely perished after 60 days due to a lack of protection. The number of surviving bacteria in Comparative Example 5 (with poor mineralization) was also far lower than in this invention.

[0051] Regarding the persistence of urease activity, the soil treated with the product of this invention maintained a high level of urease activity even after 60 days, significantly higher than the control (CK) and control example 4. This demonstrates that the fertilizer of this invention not only provides physicochemical nutrients but also continuously injects biological activity into the soil by introducing and protecting functional microorganisms. This activity continues to play a role in the later stages of crop growth, potentially further converting applied or existing urea in the soil to provide a continuous nitrogen source and possibly producing a sustained, weak mineralization effect, further optimizing the nutrient release pattern.

[0052] 6. Potted plant growth promotion experiment A pot experiment was conducted in acidic red soil using lettuce as an indicator crop. A control group (CK) without fertilizer and treatment groups with equal nutrient levels were established. After 45 days of growth, plant biomass (fresh weight) and vitamin C content were measured.

[0053] Table 6. Results of pot experiment (lettuce)

[0054] As shown in Table 6, the lettuce treated with the products of this invention (Examples 1-4) exhibited significantly higher biomass and quality (vitamin C content) than the comparative example and control (CK). This comprehensively demonstrates that the fertilizer of this invention can supply nutrients more efficiently and stably, and its synergistic growth-promoting effect with soil improvement ultimately achieves a dual improvement in crop yield and quality.

[0055] In summary, the method for preparing fly ash-based soil-modified slow-release organic fertilizer provided by this invention constructs a high-performance porous carrier through "plasma-microwave synergistic activation," achieves efficient loading of microbial fertilizer through "vacuum impregnation," and finally forms a dense and robust slow-release coating layer through "microbial-induced mineralization." This successfully prepares a novel fertilizer integrating the functions of slow-release fertilizer, soil conditioner, and organic fertilizer, exhibiting excellent synergistic effects in slow-release performance, soil pH improvement, sustained nutrient supply, and crop growth promotion and quality improvement.

Claims

1. A method for preparing a fly ash-based soil-modified slow-release organic fertilizer, characterized in that, Includes the following steps: 1) Fly ash pretreatment: The fly ash after heavy metal removal is ball-milled, then soaked in 1-3 mol / L hydrochloric acid solution, washed with water until neutral, dried, and then subjected to plasma treatment, followed by microwave treatment to obtain pretreated fly ash. 2) Porous gel particles: Pretreated fly ash is mixed with 2-4wt% sodium alginate aqueous solution at a mass ratio of 1:(3-5), and stirred at 50-60℃ for 30-40 minutes to form a uniform slurry. Then, 3-5wt% calcium chloride solution is added dropwise for cross-linking and curing to form spherical porous gel particles with a particle size of 2-5mm. After curing for 20-30 minutes, the particles are taken out and rinsed lightly with deionized water to remove residual calcium chloride on the surface. 3) Microbial fertilizer complex: Bacillus pasteurellus bacterial solution is mixed with urea and water-soluble fertilizer and dissolved in deionized water to prepare microbial fertilizer mixture; Porous gel particles were immersed in a mixture of microbial fertilizer and kept under a vacuum of -0.07 to -0.09 MPa for 15-25 minutes, and then slowly restored to normal pressure to obtain the microbial fertilizer complex. 4) Immerse the microbial fertilizer compound in the mineralization solution, add potassium dihydrogen phosphate as a mineralization inducer, and incubate at 20-35℃ for 24-72h to form a dense calcium carbonate / magnesium carbonate biomineral coating layer on the surface of the particles and in the internal pore channels. Finally, the mineralized fertilizer granules are removed and lightly rinsed with deionized water to remove loose crystals on the surface. Then, they are dried with hot air at 35-40℃ for 4-6 hours until the moisture content is ≤10%, thus obtaining the final fly ash-based soil-modified slow-release organic fertilizer.

2. The method for preparing fly ash-based soil-modified slow-release organic fertilizer according to claim 1, characterized in that, The plasma treatment is carried out at a power of 60-80W and an oxygen flow rate of 30-50L / min for 20-30 minutes.

3. The method for preparing fly ash-based soil-modified slow-release organic fertilizer according to claim 1, characterized in that, The microwave treatment is performed for 15-20 minutes in a microwave environment with a power of 360-450W and a temperature of 80-100℃.

4. The method for preparing fly ash-based soil-modified slow-release organic fertilizer according to claim 1, characterized in that, The volume ratio of sodium alginate aqueous solution to calcium chloride solution is 1:(2-3).

5. The method for preparing fly ash-based soil-modified slow-release organic fertilizer according to claim 1, characterized in that, The mass ratio of the urea-decomposing bacterial solution to urea and water-soluble fertilizer is 1:(2~3):(1~2).

6. The method for preparing fly ash-based soil-modified slow-release organic fertilizer according to claim 1, characterized in that, Preparation of the *Sporosarcina pasteurii* bacterial suspension: *Sporosarcina pasteurii* was inoculated into LB medium and cultured with shaking at 30-32℃ for 24-36 h. The suspension was then concentrated by centrifugation. OD 600 Values ​​between 1.0 and 1.5, urease activity ≥ 800 U / mL, viable cell count ≥ 1 × 10⁻⁶ 8 CFU / mL.

7. The method for preparing fly ash-based soil-modified slow-release organic fertilizer according to claim 1, characterized in that, The water-soluble fertilizer includes one or more of the following: ammonium dihydrogen phosphate, potassium nitrate, dipotassium hydrogen phosphate, ammonium sulfate, potassium chloride, and calcium ammonium nitrate.

8. The method for preparing fly ash-based soil-modified slow-release organic fertilizer according to claim 1, characterized in that, The concentration of the mineralization solution is 0.3-0.7 mol / L.

9. The method for preparing fly ash-based soil-modified slow-release organic fertilizer according to claim 1, characterized in that, The mineralizing solution is one or a mixture of two of calcium chloride and magnesium chloride, wherein the Ca in the mixed solution... 2+ :Mg 2+ The molar ratio can be 1:(0~1).

10. The method for preparing fly ash-based soil-modified slow-release organic fertilizer according to claim 1, characterized in that, The potassium dihydrogen phosphate accounts for 0.1-0.2% of the total mass of the mineralization solution.

Images