Preparation method of multi-nutrient long-acting slow-release fertilizer with soil improvement

Abstract

The application relates to the technical field of fertilizer preparation, and discloses a preparation method of multi-element nutrient long-acting slow-release fertilizer with soil improvement. The method comprises the following steps: compounding agricultural waste and humic acid through carbonization pyrolysis treatment, grinding mineral powder and surface modification, preparing quick-acting nutrient core particles through wet granulation, coating slow-release fertilizer particles, and finally spraying a double-layer controlled-release shell to obtain slow-release fertilizer. The application improves the fertilizer coating material, adopts a double-layer controlled-release coating technology of a bio-based polyester material and a modified starch composite material, solves the deficiencies of the existing fertilizer coating material in terms of biodegradability, water permeability and stability, and realizes accurate regulation and control of the nutrient of slow-release fertilizer.

Description

Technical Field

[0001] This application relates to the field of fertilizer preparation technology, and in particular to a method for preparing a multi-nutrient, long-acting, slow-release fertilizer that also improves soil quality. Background Technology

[0002] In existing technologies, traditional fertilizers often use fast-acting fertilizers that can quickly release nutrients. However, the rapid release of nutrients by these fertilizers often leads to plants absorbing too much nutrient in a short period, resulting in nutrient waste and environmental pollution. For example, nitrogen fertilizer loss can cause eutrophication in water bodies. In addition, existing fertilizers often cannot precisely regulate the release of nutrients, making it difficult to meet the long-term nutrient needs of different crops, and they cannot continuously release nutrients into the soil, resulting in a lack of necessary nutrient supply in the later stages of crop growth.

[0003] However, while existing slow-release fertilizers can slow down the nutrient release rate, the selection and application of their coating materials still have shortcomings. For example, traditional slow-release fertilizers often rely on single synthetic materials for coating, which have poor biodegradability or still exhibit uneven slow-release effects, leading to unstable fertilizer release times. Particularly in controlling the coating process, traditional production methods often struggle to precisely control the film thickness and uniformity, affecting the fertilizer's effectiveness and reliability. Summary of the Invention

[0004] This application provides a method for preparing a long-lasting slow-release fertilizer with multiple nutrients that also improves soil quality. By improving the fertilizer coating material, a double-layer controlled-release coating technology using bio-based polyester material and modified starch composite material is adopted, which solves the shortcomings of existing fertilizer coating materials in terms of biodegradability, water permeability and stability, and at the same time realizes the precise regulation of nutrients in the slow-release fertilizer.

[0005] This application provides a method for preparing a multi-nutrient, long-acting, slow-release fertilizer that also improves soil quality. The method for preparing the multi-nutrient, long-acting, slow-release fertilizer that also improves soil quality includes: Step S1: Agricultural waste is carbonized and pyrolyzed and then mixed with humic acid powder in a mass ratio. The biochar-humic acid composite modifier is obtained by extrusion granulation process. Step S2: Silicate mineral powder and carbonate mineral powder are mixed in a mass ratio and then ultra-fine ground. After surface modification treatment with a coupling agent, silicon-calcium-magnesium composite mineral powder is obtained. Step S3: The fast-acting nitrogen and phosphorus nutrients, the biochar-humic acid composite modifier, the silicon-calcium-magnesium composite mineral powder and the chelated trace element solution are subjected to wet granulation to obtain fast-acting nutrient core particles. Step S4: Coat the surface of the fast-acting nutrient core particles with slow-release nitrogen fertilizer granules and slow-release potassium fertilizer powder using a binder solution to obtain slow-release nutrient middle layer particles; Step S5: The bio-based polyester coating liquid is sprayed onto the surface of the slow-release nutrient middle layer particles through a fluidized bed, and a modified starch film liquid is sprayed onto the outer layer to form a double-layer composite controlled-release shell, thus obtaining a multi-nutrient long-acting slow-release fertilizer.

[0006] The technical solution provided in this application employs a combination of carbonization and pyrolysis of agricultural waste and humic acid powder, which not only improves the nutrient release effect of fertilizer but also enhances soil amendment functions. The introduction of biochar, with its excellent pore structure and specific surface area, enhances the soil's ability to adsorb water and nutrients, while humic acid, through its active functional groups, forms stable complexes with metal ions in the soil, effectively preventing nutrients from being fixed by the soil. This composite amendment not only promotes the continuous absorption of nutrients such as nitrogen, phosphorus, and potassium by plants but also improves soil fertility, achieving an integrated effect of nutrient supply and soil improvement.

[0007] Secondly, through ultrafine grinding of silicate and carbonate mineral powders and surface modification with coupling agents, the mineral elements in the fertilizer can be released into the soil more efficiently. This modified mineral powder not only slowly releases nutrients such as potassium, magnesium, and silicon, but also neutralizes soil acidity, further improving the soil environment. Combined with wet granulation technology, the fast-acting nutrient core particles can stably release nutrients, ensuring plants receive sufficient nutrients at different growth stages. The coating of slow-release nitrogen fertilizer granules and slow-release potassium fertilizer powder further enhances the fertilizer's persistence, preventing rapid loss, reducing fertilization frequency, and minimizing waste. Finally, the double-layer composite controlled-release shell design allows for more precise nutrient release and dynamic adjustment based on soil moisture and temperature changes, effectively improving fertilizer utilization efficiency. Attached Figure Description

[0008] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0009] Figure 1 This is a schematic diagram of an embodiment of the preparation method of a multi-nutrient slow-release fertilizer that also improves soil quality, as described in this application. Detailed Implementation

[0010] This application provides a method for preparing a long-acting, slow-release fertilizer with soil-improving properties. The terms "first," "second," "third," "fourth," etc. (if present)," in the specification, claims, and accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments described herein can be implemented in a sequence other than that illustrated or described herein. Furthermore, the terms "comprising" or "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0011] For ease of understanding, the specific process of the embodiments of this application is described below. Please refer to [link / reference]. Figure 1 One embodiment of the preparation method of the multi-nutrient slow-release fertilizer that also has soil improvement functions in this application includes: Step S1: Agricultural waste is carbonized and pyrolyzed and then mixed with humic acid powder in a mass ratio. The biochar-humic acid composite modifier is obtained by extrusion granulation process. Step S2: Silicate mineral powder and carbonate mineral powder are mixed in a mass ratio and then ultra-fine ground. After surface modification treatment with a coupling agent, silicon-calcium-magnesium composite mineral powder is obtained. Step S3: The fast-acting nitrogen and phosphorus nutrients, the biochar-humic acid composite modifier, the silicon-calcium-magnesium composite mineral powder and the chelated trace element solution are subjected to wet granulation to obtain fast-acting nutrient core particles. Step S4: Coat the surface of the fast-acting nutrient core particles with slow-release nitrogen fertilizer granules and slow-release potassium fertilizer powder using a binder solution to obtain slow-release nutrient middle layer particles; Step S5: The bio-based polyester coating liquid is sprayed onto the surface of the slow-release nutrient middle layer particles through a fluidized bed, and a modified starch film liquid is sprayed onto the outer layer to form a double-layer composite controlled-release shell, thus obtaining a multi-nutrient long-acting slow-release fertilizer.

[0012] It is understood that the executing entity of this application can be a preparation system for a multi-nutrient slow-release fertilizer that also improves soil quality, or it can be a terminal or a server; no specific limitation is made here. This application's embodiments use a server as an example for illustration.

[0013] Specifically, in the process of obtaining a biochar-humic acid composite modifier by carbonizing and pyrolyzing agricultural waste and then compounding it with humic acid powder, the agricultural waste includes crop residues such as straw, rice husks, and corn cobs. After being crushed, the waste is fed into a closed pyrolysis furnace and heated under a nitrogen protective atmosphere. The nitrogen acts to isolate oxygen and prevent combustion, not carbonization. By controlling the heating rate and the isothermal time, the cellulose, hemicellulose, and lignin in the waste undergo pyrolysis. After the volatile components escape, the remaining solid carbon skeleton forms biochar. The pyrolysis temperature directly affects the pore structure and specific surface area of ​​the biochar. Specific surface area refers to the area per unit mass of material... The total surface area is a key indicator of a material's overall pore size. A larger value indicates a richer pore structure and a stronger ability to adsorb nutrients and moisture. When the temperature is too low, the organic matter decomposes incompletely, resulting in fewer pores. When the temperature is too high, the carbon skeleton shrinks and collapses, reducing the number of pores. The pore size distribution includes three scales: micropores, mesopores, and macropores. Micropores mainly adsorb small molecule nutrients such as ammonium and potassium ions, mesopores accommodate medium molecules such as phosphate ions, and macropores serve as channels for the transport of moisture and nutrients. After pyrolysis, the biochar is cooled and pulverized to a specific mesh size. The mesh size refers to the number of holes per inch of the sieve. A larger mesh size indicates finer particles. The purpose of pulverization is to increase the contact interface between the biochar and humic acid. Humic acid is a large organic molecule extracted from carbonaceous materials such as peat, lignite, and weathered coal. Its molecular structure contains active functional groups such as carboxyl, phenolic hydroxyl, and carbonyl groups, which can form complexes with metal ions. Humic acid is extracted using an alkaline extraction method. The humic acid-containing raw material is mixed with sodium hydroxide solution at a specific material-to-liquid ratio. Under heating and stirring, the humic acid dissolves in the alkaline solution. After filtration to remove insoluble residues, hydrochloric acid is added to the filtrate to adjust the pH. Humic acid's solubility decreases under acidic conditions, causing it to precipitate. If the pH is too high... High pH levels can lead to incomplete precipitation and reduced extraction rates. Excessive acid at low pH levels can damage the molecular structure of humic acid and reduce its active functional groups. The precipitated humic acid is separated into solid and liquid by centrifugation, and then repeatedly washed with deionized water to remove residual inorganic salts. The conductivity of the washing solution is used to determine whether the washing is sufficient. Conductivity indicates the concentration of ions in the solution; the lower the value, the less inorganic salt remains. After washing, the solution is dried in a vacuum oven to a low moisture content. The vacuum environment lowers the boiling point of water, so the drying temperature does not need to be too high to avoid thermal degradation of humic acid. The compounding of biochar powder and humic acid powder is carried out in a specific mass ratio. The ratio is determined based on the synergistic function of the two. Biochar provides physical adsorption by adsorbing nutrients and water through its pore structure, while humic acid provides chemical complexation by forming stable complexes with metal ions through its active functional groups to prevent nutrients from being fixed by the soil. At the optimal mass ratio, the adsorption capacity of the compound amendment for ammonium nitrogen is significantly improved. The adsorption capacity is determined by batch adsorption experiments. A certain mass of the compound amendment is added to an ammonium nitrogen solution with a known initial concentration. After adsorption reaches equilibrium, the equilibrium concentration of the solution is measured. The difference between the initial concentration and the equilibrium concentration is multiplied by the solution volume and then divided by the mass of the adsorbent to obtain the adsorption amount per unit mass of adsorbent.During compounding, the two powders are added to a high-speed mixer and mixed for a certain time at a set speed. A small amount of deionized water is sprayed in as a wetting agent during the mixing process. The addition of water makes the powder surface slightly moist, generating capillary adsorption force to promote close contact between biochar and humic acid. After mixing, the powder is transferred to an extrusion granulator. Extrusion granulation is the process of mechanically pressing the powder through a die with a specific aperture to form cylindrical particles. During the extrusion process, the porous structure of the biochar is partially filled with humic acid. Scanning electron microscopy shows that humic acid forms a uniform coating on the surface of the biochar and penetrates into both mesopores and macropores. This composite structure of biochar skeleton and humic acid coating enables the synergistic effect of physical adsorption and chemical complexation. The granulated particles are dried to a low moisture content in a forced-air drying oven. The dried particles have a certain mechanical strength. The crushing force of a single particle is measured using a hardness tester to evaluate the particle strength. Crushing force refers to the minimum force required to break a particle. Particles with sufficient strength are not easily broken during subsequent mixing and granulation processes and maintain their integrity.

[0014] In the process of obtaining silicon-calcium-magnesium composite mineral powder by mixing silicate mineral powder and carbonate mineral powder in a mass ratio, ultrafine grinding, and surface modification with a coupling agent, potassium feldspar or serpentine are selected as the silicate minerals. The chemical composition of potassium feldspar is mainly composed of oxides of potassium, aluminum, and silicon, while that of serpentine is mainly composed of oxides of magnesium and silicon. These minerals have a dense crystal structure in their natural state and release nutrients very slowly, requiring mechanical activation to improve their reactivity. After crushing the ore, it is fed into a planetary ball mill for ultrafine grinding. The working principle of the planetary ball mill is that the grinding jar rotates on its own axis while revolving around a planetary frame. The grinding balls inside the jar generate strong impact and grinding on the material under the action of centrifugal force and Coriolis force. Zirconia balls are selected as the grinding media because of their high hardness and non-contamination of the sample. During the high-energy ball milling process, the mineral crystals... The body is subjected to strong mechanical forces, resulting in lattice distortion, increased dislocations, and grain breakage. The significant reduction in particle size leads to a substantial increase in particle surface area. Particle size distribution is determined using a laser particle size analyzer. The instrument calculates the particle size and distribution based on the scattering angle and intensity of the laser light by the particles. The median particle size (D50) represents the particle size corresponding to 50% of the cumulative distribution. Specific surface area is determined using nitrogen adsorption. The sample adsorbs nitrogen molecules at liquid nitrogen temperature, and the total surface area of ​​the sample is calculated based on the adsorption amount and the cross-sectional area of ​​the nitrogen molecules. X-ray diffraction analysis is used to characterize changes in the mineral's crystal structure. The broadening and decreased intensity of the diffraction peaks after grinding indicate increased lattice distortion and amorphization. These structural defects increase the chemical activity of the mineral, accelerating its weathering release rate in the soil. Dolomite, a calcium-magnesium carbonate complex, is selected as the carbonate mineral. In the soil, dolomite reacts with hydrogen ions to release calcium and magnesium ions while simultaneously consuming acidity and neutralizing soil acidity. The dolomite ore is also crushed and ground into an ultrafine powder using a ball mill; the smaller the particle size, the faster the reaction rate. The key to successful blending is the optimal mass ratio of silicate and carbonate mineral powders. This ratio is determined based on a balance between nutrient release rates and soil conditioning functions. Silicate minerals primarily provide potassium, magnesium, and silicon, with a slower release rate, exhibiting slow-release characteristics. Carbonate minerals mainly neutralize soil acidity and provide calcium and magnesium, with a faster reaction rate, exhibiting rapid-acting characteristics. Under the optimal mass ratio, the content of each element in the composite mineral powder reaches the designed value. The release of these nutrients in the soil follows a first-order kinetic equation, with the release rate directly proportional to the remaining unreleased amount. The release rate constant k reflects the rate of release. The cumulative release at different time points is measured through soil incubation experiments, and the experimental data are fitted to the kinetic equation to obtain the release rate constant. The silicate and carbonate mineral powders are added to a mixer according to the mass ratio and mixed until homogeneous. The homogeneity of the mixture is evaluated by analyzing the coefficient of variation of each component content through sampling.The mixed composite mineral powder undergoes surface modification treatment to improve its activity and dispersibility. Silane coupling agents or titanate coupling agents are selected as surface modifiers. Coupling agents are molecules with two different reactive groups: an alkoxy group at one end reacts with the hydroxyl groups on the mineral surface to form a covalent bond, and an organic group at the other end imparts a certain organic affinity to the mineral surface. The coupling agent is dissolved in a solvent to prepare a modified solution of a certain concentration. The modified solution is then evenly sprayed onto the surface of the composite mineral powder using a sprayer, while the mixer is simultaneously turned on to continue mixing, ensuring the coupling agent is evenly dispersed on the powder surface. After the alkoxy end of the agent molecule is hydrolyzed, it condenses with the hydroxyl groups on the mineral surface to form a chemical bond, anchoring the coupling agent to the surface. The organic group ends extend outward, changing the wettability of the mineral surface. The increased contact angle of the surface-modified composite mineral powder indicates that the surface changes from completely hydrophilic to partially hydrophobic. This change in surface properties improves the compatibility and dispersibility of the mineral powder with the organic components in the subsequent granulation process, preventing excessive agglomeration and sedimentation of mineral particles in the aqueous system. After the modification treatment is completed, the composite mineral powder is dried to remove residual solvent and moisture. After drying, it is sieved to remove a small amount of large particle agglomerates.

[0015] In the process of wet granulation of readily available nitrogen and phosphorus nutrients, biochar-humic acid composite modifier, silicon-calcium-magnesium composite mineral powder and chelated trace element solution to obtain readily available nutrient core particles, readily available nitrogen and phosphorus nutrients are first prepared. Urea is selected as the readily available nitrogen source because of its high nitrogen content and high solubility in water. Diammonium phosphate is selected as the readily available phosphorus source because it contains both nitrogen and phosphorus and is in readily available form. Urea and diammonium phosphate are pulverized into fine powder. The purpose of pulverization is to reduce the particle size to facilitate uniform mixing in the subsequent process and increase the reaction contact area during granulation.The preparation of chelated micronutrient solutions is a key technology. Micronutrients include essential nutrients for crops such as calcium, magnesium, zinc, and boron. In traditional fertilization, these elements are added in the form of inorganic salts, such as calcium sulfate, magnesium sulfate, and zinc sulfate. However, inorganic salts readily react with anions such as phosphate and carbonate in the soil to form insoluble precipitates, significantly reducing their effectiveness. Furthermore, antagonistic effects exist between different cations, such as calcium antagonizing magnesium and zinc antagonizing iron, affecting crop absorption of these elements. Chelation technology utilizes organic chelating agents to form stable chelates with metal ions. Multiple coordinating atoms in the chelating agent molecule, such as nitrogen and oxygen, simultaneously coordinate with the central metal ion to form a ring structure called a chelate ring. The formation of chelate rings significantly improves the absorption of these elements. The stability of metal ions makes them less susceptible to fixation by soil. Amino acids, as chelating agents, contain two coordination groups: amino and carboxyl. When glycine or glutamic acid reacts with calcium or magnesium ions, the nitrogen atom of the amino group and the oxygen atom of the carboxyl group simultaneously coordinate with the metal ion to form a five-membered chelate ring. EDTA, as a chelating agent, contains six coordination sites: two nitrogen atoms and four carboxyl groups. When it coordinates with zinc ions, it forms an octahedral coordination structure, resulting in extremely high chelate stability. In preparing chelated calcium solutions, calcium chloride and glycine are mixed in a molar ratio. The molar ratio is determined based on coordination chemistry principles. Glycine is dissolved in deionized water, and calcium chloride is slowly added while stirring. During the addition process, the pH value is adjusted using sodium hydroxide solution. The control of the chelation reaction is based on the dissociation characteristics of amino acids. Glycine exhibits amphoteric dissociation properties, existing in different forms at different pH values. Under acidic conditions, glycine exists as a cation with no coordination ability, while under alkaline conditions, it exists as an anion with the strongest coordination ability. Therefore, the pH value is adjusted to the alkaline range to allow glycine to coordinate with calcium ions in an anionic form. The reaction is carried out at a certain temperature for a certain time until the chelation reaction is complete. After the reaction, the volume is adjusted to a certain level to obtain a chelated calcium solution of known concentration. The chelation rate is determined by ultraviolet spectrophotometry. The absorption characteristics of chelated metals and free metals in the ultraviolet region are different. The proportion of chelated metals in the total metal is calculated based on the absorbance value at the characteristic wavelength. The percentage of chelated magnesium and zinc solutions were prepared using the same method. Boron was not chelated but rather boric acid solution was prepared by dissolving borax in deionized water. Boron mainly exists in the soil as boric acid and is not easily fixed, so no chelation treatment is required. The prepared chelated calcium, magnesium, and zinc solutions and boric acid solutions were mixed according to the designed content ratio of each element in the target fertilizer. The mixed solution was concentrated under reduced pressure. Reduced pressure concentration is the evaporation of solvent under pressure lower than normal pressure. Reduced pressure lowers the boiling point of water, thereby concentrating at a lower temperature to avoid damage to the chelates by high temperature. When the solution volume is concentrated to a certain proportion, a high-concentration chelated trace element solution is obtained.Wet granulation integrates readily available nitrogen and phosphorus nutrients, biochar-humic acid composite modifier, silicon-calcium-magnesium composite mineral powder, and chelated trace element solution into granules. Urea powder, diammonium phosphate powder, biochar-humic acid composite modifier granules, and silicon-calcium-magnesium composite mineral powder are fed into a granulator. Granulator types include disc granulators and rotary drum granulators. The working principle of a disc granulator is that the tilted rotating disc causes the material to roll along the disc surface due to the combined force of centrifugal force and gravity. During the rolling process, a liquid binder is sprayed in to gradually agglomerate the powder into spheres. The chelated trace element solution is then evenly sprayed onto the surface of the rolling material through nozzles. The spraying speed of the solution... It is necessary to control the spraying process to avoid excessively rapid wetting in certain areas. During spraying, the moisture content of the material gradually increases. Moisture content is the ratio of the mass of water in the material to the total mass of the material. When the moisture content reaches a suitable range, capillary adsorption forces are generated on the powder surface, causing the particles to adhere to each other. Under the mechanical tumbling action of the granulator, small particles gradually aggregate into larger particles. During granulation, urea and diammonium phosphate powders act as nucleating agents, first forming micro-cores. The particles of the biochar-humic acid composite modifier act as a framework, embedded in the growing particles to form a supporting structure. The ultrafine powder of the silicon-calcium-magnesium composite mineral powder fills the pores of the particles. The chelated trace element solution acts as... The liquid binder firmly bonds the components together while uniformly distributing chelated calcium, magnesium, zinc, and boron within the granules. By controlling the spray volume and granulation time, the particle size is brought to the target size. Spraying is stopped when the particles reach the target size. The resulting wet granules have a high moisture content and require immediate drying to prevent clumping. Drying is performed using hot air drying or fluidized bed drying. Hot air drying involves placing the wet granules in a drying device and introducing hot air at a specific temperature. The hot air contacts the particle surface, causing moisture to evaporate. The drying temperature must be strictly controlled to avoid excessively high temperatures that could decompose urea. Urea decomposes at high temperatures to produce biuret, and if the biuret content exceeds a certain proportion... It can be toxic to crops. During the drying process, the temperature of the particles is monitored in real time by an infrared thermometer to ensure that it does not exceed the safe temperature. When the moisture content drops to the set value, the drying is completed. The dried particles are cooled to near room temperature and then screened. The screening is done by a vibrating screen. The vibrating screen moves the particles on the screen surface by vibration. Particles smaller than the screen holes pass through the screen, while particles larger than the screen holes remain on the screen surface. The particles are divided into different sizes using screens with different apertures. Powder and broken particles that are too small are returned to the granulator for re-granulation. Oversized particles that are too large are crushed and returned to the granulator. Qualified particles with a particle size within the target range are collected as core particles for fast-acting nutrients.

[0016] In the process of coating slow-release nitrogen fertilizer granules and slow-release potassium fertilizer powder onto the surface of fast-acting nutrient core granules using a binder solution to obtain slow-release nutrient middle-layer granules, the slow-release nitrogen fertilizer granules are first prepared. The slow-release nitrogen fertilizer is prepared using coated urea technology. Coated urea involves coating ordinary urea granules with a membrane material to control the nitrogen release rate. The coating material is a composite material of polycaprolactone and poly(N-isopropylacrylamide). Polycaprolactone is a biodegradable aliphatic polyester that can be decomposed into carbon dioxide and water by microorganisms in soil without causing environmental pollution. Poly(N-isopropylacrylamide) is a temperature-sensitive polymer with a low critical solubility temperature in aqueous solution. When the temperature is below the critical temperature, the polymer molecular chains exhibit an extended hydrophilic conformation; when the temperature is above the critical temperature, a hydrophobic phase transition occurs, and the molecular chains coil and shrink. The sensitivity of the coating material causes its permeability to change with temperature. At low temperatures, the coating is dense, resulting in slow nutrient release; at high temperatures, the micropores expand, accelerating nutrient release. Urea granules are fed into a fluidized bed coating machine. The working principle of the fluidized bed coating machine is to introduce hot air from the bottom, causing the granules to be fluidized and suspended under the action of airflow. Polycaprolactone and polyN-isopropylacrylamide are dissolved in an organic solvent according to a certain mass ratio to prepare a coating solution with a certain solid content. The coating solution is sprayed into the fluidized bed through a two-fluid spray gun. The two-fluid spray gun uses compressed air to atomize the liquid into tiny droplets. After the atomized coating solution droplets come into contact with the surface of the urea granules, the solvent evaporates rapidly, and the polymer is deposited on the surface to form a thin film. The coating thickness is controlled by controlling the spraying speed and spraying time. After coating, hot air continues to be introduced for drying to completely evaporate the residual solvent. Slow-release potassium fertilizer uses potassium sulfate. The potassium sulfate is crushed to a set particle size and then sieved to obtain slow-release potassium fertilizer powder. Slow-release nitrogen fertilizer granules and slow-release potassium fertilizer powder are mixed according to the specified ratio and then fed into a rotary drum coating machine. Fast-acting nutrient core granules are also fed into the machine. Simultaneously, a polyvinyl alcohol aqueous solution is sprayed in as a binder solution through a spray system. Polyvinyl alcohol is a water-soluble polymer with good adhesion and film-forming properties. Under the tumbling action of the drum, the polyvinyl alcohol solution wets the surface of the core granules, and the slow-release nitrogen fertilizer granules and slow-release potassium fertilizer powder gradually adhere to the surface of the core granules. By controlling the amount of slow-release nutrients added and the coating time, the intermediate layer reaches the set thickness. After coating, the mixture continues to tumble and dry in the drum to solidify the polyvinyl alcohol binder, firmly bonding the intermediate layer to the core granules. To prevent moisture absorption and clumping, the coated particles undergo pre-coating treatment. An acrylic copolymer is formulated into an emulsion with a solid content within a set range to obtain a pre-coated emulsion. The coated particles are kept in a tumbling state in a rotary drum coating machine. The pre-coated emulsion is sprayed onto the surface of the coated particles at a set spraying speed through a spraying system. The spraying time is controlled to ensure that the coating thickness reaches the set range. The pre-coated particles are dried in the rotary drum coating machine with drying hot air at a set temperature to ensure that the pre-coated emulsion forms a complete film. After the film-formed particles are cooled to a set temperature, the crushing force of a single particle is measured using a hardness tester. Particles with a crushing force reaching a set threshold are used as the intermediate particles for slow-release nutrients.

[0017] In the process of obtaining a long-lasting slow-release fertilizer with multiple nutrients, the bio-based polyester coating solution is first prepared by spraying a bio-based polyester coating solution onto the outer layer of slow-release nutrient intermediate particles through a fluidized bed, followed by spraying a modified starch film solution to form a double-layer composite controlled-release shell. Polyhydroxy fatty acid ester is a bio-based polyester produced by microbial fermentation and has good biodegradability and mechanical properties. The role of nano-silica is to form uniformly distributed nanoscale pores in the polymer matrix. The dispersion of silica nanoparticles in the polymer creates localized nanoscale defects, which are interconnected to form a pore network. The role of starch microparticles is as a biodegradation promoter. Starch is easily decomposed by microorganisms in soil, and after degradation, it leaves micron-sized pores in the coating, increasing the water permeability and degradation rate of the coating. The bio-based polyester coating solution is then prepared by formulating a coating solution with a set solid content. The slow-release nutrient intermediate particles are fed into a fluidized bed coating machine, and hot air at a set temperature is introduced to fluidize the particles. The operating temperature is controlled at a low temperature, significantly lower than that of traditional fluidized beds. The advantage of low-temperature coating is that it avoids the damage of the temperature-sensitive coating in the coated urea to high temperatures, protects the temperature-sensitive function, prevents the thermal decomposition of urea in the core particles, and avoids the formation of biuret. The bio-based polyester coating liquid is sprayed into the fluidized bed through a dual-fluid spray gun for atomization coating to the set thickness. Under low-temperature conditions, the solvent evaporates more slowly, which is conducive to the uniform spread of the coating liquid on the particle surface to form a dense and uniform coating layer. After coating, low-temperature drying air is continued to dry the particles to evaporate the solvent and obtain inner-layer coated particles. Modified starch was prepared into an aqueous solution of a set concentration to obtain a modified starch film solution. Hydroxypropyl starch or acetate starch was selected as the modified starch, as these modified starches have significantly lower solubility in water than natural starch, and their degradation rate in soil is controllable. The modified starch film solution was sprayed onto the surface of the inner coating particles to a set thickness in a fluidized bed. After drying, a double-layer composite controlled-release shell was formed. The mechanism of action of the modified starch film is that in the initial stage of fertilizer application to the soil, the modified starch film absorbs water and swells but does not dissolve, forming a hydrogel layer that blocks water penetration into the inner layer. Subsequently, under the action of soil microorganisms, the modified starch film gradually degrades, water begins to penetrate into the inner coating, and the nutrient release rate gradually increases. After the modified starch film is completely degraded, most of the starch particles in the inner coating are also degraded, increasing the porosity of the coating and allowing nutrient release to enter a rapid phase. The inner material is degraded by microbial activity, the coating gradually breaks down, and residual nutrients are released rapidly. This double-layer composite coating structure of modified starch outer layer and bio-based polyester-nano silica-starch inner layer achieves multi-stage precise control of nutrient release. After cooling the controlled-release coating particles, they are sieved. Particles with particle size within the set range are sprayed with anti-caking agent and packaged to obtain multi-nutrient long-acting slow-release fertilizer. The anti-caking agent is selected from talc, diatomaceous earth or hydrophobic silica to prevent particle adhesion caused by moisture absorption during storage.

[0018] In one specific embodiment, step S1 includes: The agricultural waste is crushed and placed in a pyrolysis furnace. Under a nitrogen protective atmosphere, the temperature is raised to the preset pyrolysis temperature at a set heating rate and maintained at a constant temperature to obtain biochar. The biochar is cooled and then pulverized and sieved to a set mesh size to obtain biochar powder. Weathered coal or peat is extracted with alkaline solution at a material-to-liquid ratio. After filtration, the pH value is adjusted with acid solution to precipitate humic acid. After centrifugation, washing, drying, pulverizing and sieving, the humic acid powder is obtained. The biochar powder and the humic acid powder are mixed in a mixer according to the mass ratio and a wetting agent is sprayed in. After mixing, the mixture is transferred to an extrusion granulator, extruded and shaped under a set pressure, and dried to a set moisture content to obtain the biochar-humic acid composite modifier.

[0019] Specifically, agricultural waste is pulverized and placed in a pyrolysis furnace. Under a nitrogen protective atmosphere, the temperature is raised to a preset pyrolysis temperature at a set heating rate and maintained at a constant temperature to obtain biochar. Agricultural waste includes crop residues such as straw, rice husks, and corn cobs. Pulverization refers to reducing the particle size of the waste to less than 5 mm through mechanical crushing. Excessively large particle sizes can lead to temperature differences between the inside and outside during pyrolysis, affecting the uniformity of carbonization. The sealed pyrolysis furnace is isolated from the outside air to prevent oxygen from entering. The nitrogen protective atmosphere involves continuously introducing nitrogen into the furnace to replace oxygen in the air. Nitrogen, as an inert gas, does not participate in chemical reactions, preventing the waste from burning at high temperatures rather than carbonizing. Combustion is an exothermic reaction under aerobic conditions that completely oxidizes organic matter into carbon dioxide and water, while carbonization is a thermal decomposition reaction under anaerobic or hypoxic conditions, retaining a solid carbon skeleton. The heating rate refers to the rate of temperature change over time. The rapid heating rate can cause the surface temperature of waste to be much higher than its interior, resulting in uneven pyrolysis. The preset pyrolysis temperature is selected between 450℃ and 550℃. The basis for temperature selection is that the pore structure and specific surface area of ​​biochar are closely related to the pyrolysis temperature. When the temperature is below 400℃, the cellulose, hemicellulose and lignin in the waste are not fully pyrolyzed, leaving a large amount of undecomposed organic matter, resulting in low carbon content and underdeveloped pores in the biochar. When the temperature is above 600℃, the carbon skeleton structure that has been formed shrinks and collapses at high temperatures, reducing pore size and specific surface area. Constant temperature holding refers to maintaining the temperature for a certain period of time after the temperature rises to the preset value. The purpose of constant temperature holding time is to ensure that the interior of the waste also reaches the same temperature, so that the entire material is fully pyrolyzed. The mass of the waste gradually decreases during the pyrolysis process because of the escape of volatile components. After cooling, the biochar is pulverized and sieved to a set mesh size to obtain biochar powder. Cooling is carried out under nitrogen protection to prevent the high-temperature biochar from oxidizing upon contact with air. After cooling to below 80°C, the activity of the biochar decreases and it is less prone to spontaneous combustion. Pulverization breaks the blocky biochar into fine particles. The pulverized biochar is then sieved through a sieve. The mesh size of the sieve indicates the number of holes per inch. A 100-mesh sieve has a pore size of 150μm, and a 200-mesh sieve has a pore size of 75μm. Passing the pulverized biochar through a 100-mesh to 200-mesh sieve means that the particle size of the biochar powder is controlled between 75μm and 150μm. The necessity of particle size control is that if the particle size is too large, the contact area between the biochar and humic acid is insufficient, resulting in poor compounding effect. If the particle size is too small, the powder is prone to dust loss during subsequent granulation. Weathered coal or peat is extracted with alkaline solution according to a specific material-to-liquid ratio. After filtration, the pH is adjusted with acid to precipitate humic acid. Following centrifugation, washing, drying, and pulverization, humic acid powder is obtained. Weathered coal is a loose coal formed through weathering after long-term exposure to air. Peat is an organic matter deposit formed from the incomplete decomposition of plant remains in wetland environments. Both contain abundant humic acid, a complex high-molecular-weight organic compound containing aromatic rings, fatty chains, and various functional groups such as carboxyl, phenolic, hydroxyl, and carbonyl groups. The alkaline extraction method utilizes the property that humic acid's solubility increases under alkaline conditions.Crush weathered coal or peat to a finer mesh (less than 75 μm) to increase the contact area with the alkaline solution. A material-to-liquid ratio of 1:8 to 1:10 means adding 8 to 10 L of alkaline solution per 1 kg of solid raw material. The alkaline solution should be a sodium hydroxide solution with a mass concentration of 5 to 8. Extract at 85°C to 95°C with stirring for 180 to 240 minutes. Heating increases the dissolution rate of humic acid and shortens the extraction time, while stirring ensures thorough contact between the solid raw material and the alkaline solution. During extraction, the carboxyl and phenolic hydroxyl groups on the humic acid molecules lose protons under alkaline conditions to form carboxylate ions and phenoxy anions. These negatively charged ions... Electrostatic repulsion between functional groups causes humic acid molecules to extend and dissolve into the alkaline solution. After extraction, the solid residue and alkaline extract containing humic acid are separated by filtration. Hydrochloric acid with a concentration of 25-30 is added to the extract to adjust the pH. The addition of hydrochloric acid increases the hydrogen ion concentration and decreases the pH. When the pH drops to 1.5-2.0, humic acid molecules regain protonated carboxylate ions and phenolic anions, transforming into carboxyl and phenolic hydroxyl groups. Under acidic conditions, the solubility of humic acid molecules decreases, causing them to precipitate from the solution as a dark brown precipitate. At pH values ​​above 2.5, humic acid still retains some solubility, leading to incomplete precipitation and a lower extraction rate. When the hydrogen ion concentration (H) is below 1.0, excess hydrogen ions can damage the molecular structure of humic acid, particularly the carboxyl and phenolic hydroxyl groups, causing decarboxylation reactions that reduce the content of active functional groups and lower the molecular weight. The precipitated humic acid is then separated from the liquid phase by centrifugation. Centrifugation utilizes centrifugal force to cause denser solid particles to aggregate towards the outside of the centrifuge tube, while less dense liquid particles remain on the inside. The centrifugation speed is set to 3500 rpm for 15 minutes. After centrifugation, the solid humic acid concentrates at the bottom of the centrifuge tube, and the supernatant is poured off. The humic acid is then repeatedly washed with deionized water to remove residual inorganic salts such as sodium chloride. Sodium chloride is a salt... The product of the neutralization reaction of acid and sodium hydroxide is washed by redispersing humic acid in deionized water, stirring, and then centrifuging again. The number of washes is 3 to 5. The conductivity of the washing solution is measured to determine whether the washing is sufficient. The conductivity reflects the total concentration of ions in the solution. When the conductivity of the washing solution is less than 200 μS / cm, it indicates that the residual amount of inorganic salts is very small. The washed humic acid is dried in a vacuum oven at 60℃ to 70℃. The pressure in the vacuum oven is lower than the atmospheric pressure, which lowers the boiling point of water and thus dries at a lower temperature to avoid thermal degradation of humic acid. After drying to a moisture content of less than 8%, it is pulverized and passed through a 200-mesh sieve to obtain humic acid powder. Biochar powder and humic acid powder are mixed in a mixer according to a specific mass ratio, and a wetting agent is sprayed in. After mixing, the mixture is transferred to an extrusion granulator, extruded and shaped under a set pressure, and dried to a set moisture content to obtain a biochar-humic acid composite modifier. The mass ratio of biochar to humic acid is selected as 1:2 to 1:3. The ratio is determined based on the synergistic function of the two. Biochar provides physical adsorption by adsorbing nutrients and moisture through its pore structure.Humic acid provides chemical complexation, forming stable complexes with metal ions through its active functional groups to prevent nutrient fixation by the soil. At the optimal mass ratio, the adsorption capacity of the composite amendment for ammonium nitrogen is significantly improved. Adsorption capacity was determined through batch adsorption experiments. A certain mass of the composite amendment was added to an ammonium nitrogen solution with a known initial concentration. After adsorption reached equilibrium, the equilibrium concentration of the solution was measured. The difference between the initial concentration and the equilibrium concentration, multiplied by the solution volume and divided by the mass of the adsorbent, yielded the adsorption capacity per unit mass of adsorbent. The two powders were then mixed in a high-speed mixer at a set speed for a certain time. During mixing, a small amount of deionized water was sprayed in as a wetting agent. The mass of the wetting agent was 5 to 8 times the total mass of biochar and humic acid. The addition of water slightly moistened the powder surface, creating capillary action. Adsorption promotes close contact between biochar and humic acid. After mixing, the mixture is transferred to an extrusion granulator. Extrusion granulation uses mechanical pressure to force the powder through a die with a specific aperture to form cylindrical particles. The extrusion pressure is set to 8 MPa to 12 MPa. During the extrusion process, the porous structure of the biochar is partially filled with humic acid. Scanning electron microscopy shows that humic acid forms a uniform coating on the surface of the biochar and penetrates into both mesopores and macropores. The granulated particles are dried at 70°C to 80°C for 180 to 240 minutes until the moisture content is below 6%, yielding a biochar-humic acid composite modifier. The dried particles possess a certain mechanical strength. The particle strength is evaluated by measuring the crushing force of a single particle using a hardness tester. Crushing force refers to the minimum force required to break a particle.

[0020] In one specific embodiment, step S2 includes: Potassium feldspar ore or serpentine ore is crushed and then fed into a planetary ball mill for ultrafine grinding. The median particle size of the ground silicate mineral powder is controlled within a set range to obtain activated silicate mineral powder. After crushing the dolomite ore, it is fed into a planetary ball mill for ultrafine grinding to a set median particle size to obtain the carbonate mineral powder. The activated silicate mineral powder and the carbonate mineral powder are added to a mixer according to the mass ratio and mixed until uniform to obtain mixed mineral powder; The coupling agent is dissolved in a solvent to prepare a modified solution of a set concentration, which is then sprayed onto the surface of the mixed mineral powder and mixed further. After drying, the solution is sieved to a set mesh size to obtain the silicon-calcium-magnesium composite mineral powder.

[0021] Specifically, potassium feldspar or serpentine ore is crushed and then fed into a planetary ball mill for ultrafine grinding to obtain activated silicate mineral powder. The chemical composition of potassium feldspar is mainly potassium aluminum silicon oxide, while serpentine is mainly magnesium silicon oxide. These minerals have a dense crystal structure in their natural state, resulting in extremely slow nutrient release; therefore, mechanical activation is necessary to improve their reactivity. Ore crushing refers to crushing large pieces of ore to a particle size of less than 10mm using a jaw crusher or hammer crusher. The crushed ore is then fed into a planetary ball mill for ultrafine grinding. The working principle of the planetary ball mill is that the grinding jar rotates on its own axis while revolving around a planetary frame. The grinding balls inside the jar generate strong impact and grinding on the material under the action of centrifugal force and Coriolis force. Zirconia balls are selected as the grinding media because of their high hardness and non-contamination properties. The material-to-ball ratio (the ratio of raw material mass to grinding ball mass) is set to 1:10 to 1:15, and the grinding speed is 400 rpm to 500 rpm. At 0 rpm and a grinding time of 120 to 180 minutes, the mineral crystals are subjected to strong mechanical forces during high-energy ball milling, leading to lattice distortion, increased dislocations, and grain breakage. The particle size is significantly reduced, resulting in a significant increase in particle surface area. The particle size distribution is determined by a laser particle size analyzer. The instrument calculates the particle size and distribution based on the scattering angle and intensity of the laser light by the particles. The median particle size D50 represents the particle size corresponding to the cumulative distribution reaching 50%. The median particle size of the ground silicate mineral powder is controlled between 8 μm and 12 μm, and the specific surface area increases from 0.5 m² / g to 1.0 m² / g before grinding to 8 m² / g to 15 m² / g. X-ray diffraction analysis is used to characterize the changes in the crystal structure of the minerals. The broadening of the diffraction peaks and the decrease in intensity of the ground mineral powder indicate an increase in lattice distortion and amorphization. These structural defects increase the chemical activity of the minerals, accelerating their weathering release rate in the soil. After crushing, dolomite ore is fed into a planetary ball mill for ultrafine grinding to a set median particle size to obtain carbonate mineral powder. The chemical composition of dolomite is a calcium-magnesium carbonate complex salt. In soil, dolomite reacts with hydrogen ions to release calcium and magnesium ions while consuming acidity to neutralize soil acidity. Dolomite ore is also crushed and ground into ultrafine powder using a ball mill, with grinding conditions similar to those for silicate minerals. The resulting dolomite powder has a median particle size of 10 μm to 15 μm and a specific surface area of ​​6 m² / g to 12 m² / g. The neutralization reaction rate of dolomite powder in soil is closely related to particle size and specific surface area. The neutralization rate increases significantly when the particle size decreases from coarse to fine particles.Activated silicate mineral powder and carbonate mineral powder are added to a mixer in a specific mass ratio and mixed until homogeneous to obtain a mixed mineral powder. The mass ratio is selected as 2:1 to 3:1. The ratio is determined based on the balance between nutrient release rate and soil conditioning function. Silicate minerals mainly provide nutrients such as potassium, magnesium, and silicon, and their release rate is relatively slow, exhibiting slow-release characteristics. Carbonate minerals mainly neutralize soil acidity and provide calcium and magnesium, and their reaction rate is relatively fast, exhibiting rapid-acting characteristics. Under the optimal mass ratio, the content of each element in the composite mineral powder reaches the design value. The release of these nutrients in the soil follows a first-order kinetic equation, and the release rate is directly proportional to the remaining unreleased amount. The release rate constant k reflects the release... The release rate was determined by measuring the cumulative release at different time points through soil incubation experiments. The experimental data were fitted to the kinetic equation to obtain the release rate constant. The potassium release rate constant of potassium feldspar was less than that of calcium and magnesium release rate constant of dolomite, indicating that the former releases more slowly and the latter releases more quickly. The two mineral powders were added to a mixer according to the mass ratio. The mixer type was selected as a V-type mixer or a conical mixer. The mixing speed was 60 to 80 rpm, and the mixing time was 30 to 45 minutes. The mixing uniformity was evaluated by sampling and analyzing the coefficient of variation of each component content. The coefficient of variation is the ratio of the standard deviation to the mean. The smaller the value, the more uniform the mixing.The coupling agent is dissolved in a solvent to prepare a modified solution of a set concentration. This solution is sprayed onto the surface of the mixed mineral powder, and after further mixing and drying, it is sieved to a set mesh size to obtain a silicon-calcium-magnesium composite mineral powder. The coupling agent used is either silane coupling agent KH-550 or titanate coupling agent. Coupling agents are molecules with two different reactive groups. The alkoxy group at one end reacts with the hydroxyl groups on the mineral surface to form a covalent bond, while the organic group at the other end imparts a certain organic affinity to the mineral surface. The coupling agent is dissolved in anhydrous ethanol to prepare a 5% concentration solution to obtain the modified solution. The amount of modified solution added is 0.3 to 0.8 times the mass of the composite mineral powder. The modified solution is evenly sprayed onto the surface of the composite mineral powder using a sprayer, while the mixer is turned on and mixed for 20 to 30 minutes. After the alkoxy end of the coupling agent molecule hydrolyzes, it condenses with the hydroxyl groups on the mineral surface to form a chemical bond, anchoring the coupling agent to the surface. The organic group extends outward, changing the wettability of the mineral surface. The contact angle of the surface-modified composite mineral powder is increased from that before treatment. The contact angle is increased from 0° to 35° to 50°. The contact angle is the contact angle between the droplet and the solid surface. A contact angle of 0° indicates complete hydrophilicity. An increase in the contact angle indicates that the surface changes from completely hydrophilic to partially hydrophobic. This change in surface properties improves the compatibility and dispersibility of the mineral powder with the organic components during subsequent granulation and prevents excessive agglomeration and sedimentation of mineral particles in the aqueous system. After modification, the composite mineral powder is dried in a forced-air drying oven at 80°C to 90°C for 120 to 180 minutes to remove residual ethanol and water. After drying until the moisture content is less than 2%, it is passed through a 200-mesh sieve (particle size less than 75μm) to remove a small amount of large particle agglomerates, thus obtaining silicon-calcium-magnesium composite mineral powder. The obtained composite mineral powder has certain chemical composition and physical properties. The content of elements such as silicon, potassium, calcium, and magnesium is determined by chemical analysis. The pH value is characterized by measuring the pH value of the suspension after mixing the powder with water at a ratio of 1:5. The pH value of the composite mineral powder is alkaline, reflecting the alkaline characteristics of the carbonate components.

[0022] In one specific embodiment, step S3 includes: Urea and diammonium phosphate are pulverized to a set particle size and then mixed in a mixer to obtain the fast-acting nitrogen and phosphorus nutrients. Calcium chloride and glycine were reacted at a set pH and temperature in a set molar ratio and then diluted to volume to obtain a chelated calcium solution; magnesium sulfate and glycine were reacted at a set pH and temperature in a set molar ratio and then diluted to volume to obtain a chelated magnesium solution; zinc sulfate and disodium EDTA were reacted at a set pH and temperature in a set molar ratio and then diluted to volume to obtain a chelated zinc solution; borax was dissolved in deionized water to obtain a boric acid solution; the chelated calcium solution, the chelated magnesium solution, the chelated zinc solution, and the boric acid solution were mixed in a specified ratio and then concentrated under reduced pressure to obtain the chelated trace element solution. The fast-acting nitrogen and phosphorus nutrients, the biochar-humic acid composite modifier and the silicon-calcium-magnesium composite mineral powder are put into a granulator. The chelated trace element solution is sprayed onto the surface of the tumbling material through a nozzle to perform wet granulation to the set particle size, and wet granules are obtained. The wet granules are dried to a set moisture content at a set temperature and then sieved. Particles with a particle size within a set range are used as the core particles of the fast-acting nutrients.

[0023] Specifically, urea and diammonium phosphate are pulverized to a set particle size and then mixed in a mixer to obtain readily available nitrogen and phosphorus nutrients. Urea is a solid nitrogen fertilizer with the highest nitrogen content, with the chemical formula CO(NH2)2 and a nitrogen content of 46%. Diammonium phosphate is a high-concentration compound fertilizer with the chemical formula (NH4)2HPO4, containing both 18% nitrogen and 46% phosphorus. The two fertilizers exist in granular form with uneven particle size distribution and different densities. The density of urea is 1.33 g / cm³, and the density of diammonium phosphate is 1.62 g / cm³. Direct mixing will cause stratification due to the density and particle size difference, affecting the uniformity of subsequent granulation. Pulverization reduces the particle size through mechanical crushing. The particle size of pulverized urea and diammonium phosphate is controlled to be less than 1 mm, i.e., passing through a 20-mesh sieve. After pulverization, the particle sizes of the two powders are similar, reducing the tendency for density stratification. The pulverized urea powder and diammonium phosphate powder are put into a forced mixer with a mixing speed of 120 rpm to 150 rpm and a mixing time of 10 to 15 minutes. Calcium chloride and glycine were reacted in a molar ratio at a set pH and temperature, and the mixture was brought to volume to obtain a chelated calcium solution. Chelation refers to the formation of a cyclic structure by a metal ion and an organic ligand through multiple coordinating atoms. Chelates are more stable than simple ionic complexes. Glycine is the simplest amino acid with the chemical formula NH₂CH₂COOH, containing two coordinating groups: an amino group and a carboxyl group. Calcium chloride (CaCl₂·2H₂O) and glycine were weighed and mixed according to a molar ratio of calcium ions to glycine of 1:2. This 1:2 molar ratio is determined based on the principle of coordination chemistry, that is, one calcium ion requires two glycine molecules to provide sufficient coordination sites to form a stable chelate. Glycine was dissolved in deionized water, and calcium chloride was slowly added while continuously stirring. During the addition, the pH was adjusted to 8.0 to 9.0 using sodium hydroxide solution. The pH adjustment was based on the amphoteric dissociation characteristics of glycine. The isoelectric point of glycine is 5.97. Under acidic conditions, glycine exists as a cation, NH₃. + CH2COOH contains protonated amino groups and cannot provide coordination; under alkaline conditions, glycine exists as an anion, NH2CH2COO. -The carboxyl group loses a proton to form a carboxylate ion, while the amino group remains neutral. Within the pH range of 8.0 to 9.0, glycine mainly exists in anionic form. The oxygen atom of the carboxyl group and the nitrogen atom of the amino group coordinate with calcium ions to form a five-membered chelate ring. The pH and temperature are maintained at 60℃ for 120 to 180 minutes until the chelation reaction is complete. After the reaction is completed, it is cooled to room temperature and diluted to a set volume with deionized water to obtain a chelated calcium solution of known concentration. The chelation rate is determined by ultraviolet spectrophotometry. The absorption spectra of chelated calcium and free calcium in the ultraviolet region are different. The percentage of chelated calcium in the total calcium is calculated based on the absorbance at the characteristic wavelength. Magnesium sulfate and glycine react at a set pH and temperature in a molar ratio to obtain a chelated magnesium solution. Magnesium sulfate hexahydrate (MgSO4·6H2O) and glycine are weighed and mixed at a magnesium ion to glycine molar ratio of 1:2. The reaction conditions are similar to those for calcium chelation, but the optimal pH is 8.5 to 9.5, the optimal reaction temperature is 65℃ to 75℃, and the optimal reaction time is 150 to 210 minutes. Since the ionic radius of magnesium ions (0.072 nm) is smaller than that of calcium ions (0.100 nm), the steric hindrance during coordination is greater, thus requiring a slightly higher pH and temperature to promote the chelation reaction. Zinc sulfate and disodium EDTA react at a set pH and temperature in a molar ratio to obtain a chelated zinc solution. EDTA, also known as ethylenediaminetetraacetic acid, has the chemical formula C1. 10 H 16 N2O8 is a strong chelating agent containing six coordination sites: two nitrogen atoms and four carboxyl groups. Weigh out zinc sulfate heptahydrate (ZnSO4·7H2O) and disodium EDTA (C). 10 H 14Following a 1:1 molar ratio of zinc ions to EDTA, disodium EDTA was dissolved in deionized water, and the pH was adjusted to 5.0-6.0. The optimal pH range for the zinc chelation reaction is slightly acidic to neutral. Upon continuous stirring, the zinc sulfate solution immediately changed from colorless to a slightly yellow clear solution, indicating rapid formation of the Zn-EDTA chelate. The six coordination sites of EDTA coordinate with zinc ions to form an octahedral coordination structure, resulting in extremely high chelation stability. The reaction was carried out at 50°C for 60-90 minutes, followed by cooling and volume adjustment. Borax was dissolved in deionized water to obtain a boric acid solution. Borax, with the chemical formula Na₂B₄O₇·10H₂O, hydrolyzes in water to produce boric acid (H₃BO₃) and sodium hydroxide. Boron in soil mainly exists in the form of boric acid and is not easily fixed, therefore no chelation treatment is required. Borax was weighed and directly dissolved in deionized water to a predetermined volume. Chelated calcium, magnesium, zinc, and boric acid solutions were mixed in a specific ratio and then concentrated under reduced pressure to obtain a chelated trace element solution. The mixing ratio was calculated based on the designed content of each element in the target fertilizer, determining the volume proportion of each solution. The resulting solution had a large volume, making it inconvenient to add during granulation. Reduced pressure concentration involved evaporating the solvent under pressure below atmospheric pressure. The reduced pressure lowered the boiling point of water, allowing for concentration at a lower temperature and avoiding damage to the chelates from high temperatures. The vacuum degree was controlled at -0.08 MPa to -0.09 MPa, and the concentration temperature was 60°C to 70°C. Concentration was stopped when the solution volume reached 15% to 20% of the original volume. After concentration, the concentration of each element was significantly increased.Fast-acting nitrogen and phosphorus nutrients, biochar-humic acid composite modifier, and silicon-calcium-magnesium composite mineral powder are fed into a granulator. The chelated trace element solution is sprayed onto the surface of the tumbling material through nozzles for wet granulation to a set particle size, resulting in wet granules. A disc granulator or a rotary drum granulator is selected. The disc granulator has a disc inclination angle of 45° to 55° and a rotation speed of 18 to 25 rpm. Urea powder, diammonium phosphate powder, biochar-humic acid composite modifier granules, and silicon-calcium-magnesium composite mineral powder are fed into the granulator at a set mass ratio. The chelated trace element solution is sprayed evenly onto the surface of the tumbling material through nozzles at a spray rate of 1.5 L / min to 2.5 L / min. During the spraying process, the moisture content of the material gradually increases. Moisture content is the water content of the material. The ratio of the amount of powder to the total mass of the material. When the moisture content reaches 22% to 26%, capillary adsorption force is generated on the surface of the powder, causing the particles to stick together. Under the mechanical tumbling action of the granulator, small particles gradually aggregate into large particles. Urea and diammonium phosphate powders act as nucleating agents to first form tiny cores with a diameter of 0.5 mm to 1 mm. The particles of biochar-humic acid composite modifier act as a skeleton embedded in the growing particles to form a skeleton support structure. The ultrafine powder of silicon-calcium-magnesium composite mineral powder fills the pores of the particles. The chelated trace element solution acts as a liquid binder to firmly bind the components together and at the same time evenly distribute the chelated calcium, magnesium, zinc and boron inside the particles. The moisture content is monitored in real time by a near-infrared moisture meter. When the particle size reaches 3 mm to 5 mm, the spraying is stopped. After being dried to a set moisture content at a set temperature, the wet granules are sieved. Granules within the set size range are used as the core particles for readily available nutrients. High moisture content in the wet granules necessitates immediate drying to prevent clumping. Drying is performed using a fluidized bed dryer or belt dryer, with the drying temperature strictly controlled between 75℃ and 85℃. This temperature control is based on the thermal stability of urea. Urea has a melting point of 132.7℃, but begins to slowly decompose above 90℃, forming biuret. A biuret content exceeding 1.5% can be toxic to crops. The drying hot air velocity is 1.2 m / s to 1.8 m / s, and the drying time is 45 to 65 minutes. The surface temperature of the granules is monitored in real-time using an infrared thermometer to ensure it does not exceed 80℃. Drying is stopped when the moisture content is 3% to 5%. The dried granules are cooled to below 40°C in the cooling section and then screened. The screening uses a vibrating screen with mesh sizes of 2mm, 3mm, 5mm, and 6mm. Powder and broken particles with a particle size of less than 2mm are returned to the granulator for re-granulation. Qualified particles with a particle size of 3mm to 5mm account for 75% to 85% of the total and are collected as core particles for readily available nutrients. Extra-large particles with a particle size of more than 6mm are crushed and returned to the granulator. The core particles after screening have a certain content of readily available nitrogen, readily available phosphorus, and chelated calcium, magnesium, zinc, and boron. The particle strength is evaluated by measuring the crushing force of a single particle. The crushing force must be greater than 50N to ensure that it is not easily broken during the subsequent coating process.

[0024] In one specific embodiment, step S4 includes: Urea granules are fed into a fluidized bed coating machine, and preheated hot air is introduced to make the urea granules fluidized. Polycaprolactone and polyN-isopropylacrylamide are dissolved in a solvent according to the mass ratio to prepare a coating solution. The solution is sprayed into the fluidized bed through a two-fluid spray gun for atomized coating. After coating, hot air is continued to dry to obtain the slow-release nitrogen fertilizer granules. Potassium sulfate is pulverized to a set particle size and then sieved to obtain the slow-release potassium fertilizer powder; The slow-release nitrogen fertilizer granules and the slow-release potassium fertilizer powder are mixed in a certain proportion and then fed into a rotary drum coating machine. The fast-acting nutrient core granules are also fed into the rotary drum coating machine. At the same time, a polyvinyl alcohol aqueous solution is sprayed in through a spray system as the binder solution. Under the tumbling action of the rotary drum, the slow-release nitrogen fertilizer granules and the slow-release potassium fertilizer powder adhere to the surface of the fast-acting nutrient core granules to form an intermediate layer, thus obtaining coated granules. An acrylic copolymer emulsion is sprayed onto the surface of the coated particles to form a pre-coating layer and then dried to obtain the slow-release nutrient middle layer particles.

[0025] Specifically, urea granules are fed into a fluidized bed coating machine, and preheated hot air is introduced to fluidize the urea granules. This process involves the flow of hot air within the fluidized bed coating machine suspending the urea granules and placing them in a fluidized state, meaning the granules are uniformly distributed in the airflow like a fluid. At this time, the surface of each urea granule is uniformly heated by the hot air, ensuring that the coating material is evenly coated on the granule surface, thus laying the foundation for subsequent coating steps. Fluidization not only ensures uniform heating of the granule surface but also enhances the contact between the granules and the coating solution, ensuring consistent coating results. Polycaprolactone and poly(N-isopropylacrylamide) are dissolved in a solvent according to a specific mass ratio to prepare the coating solution. Polycaprolactone is a biodegradable aliphatic polyester material suitable for slow-release fertilizers, while poly(N-isopropylacrylamide) is a temperature-sensitive polymer that can regulate the membrane's permeability, helping to control the nutrient release rate. The dissolved coating solution is sprayed into the fluidized bed using a two-fluid spray gun. The atomized solution is then evenly sprayed onto the surface of the fluidized urea granules, forming a thin film. This spray atomization coating process is crucial because it ensures the uniformity of the coating layer, preventing uneven or excessively thick films that could affect the slow-release effect of the fertilizer.

[0026] After coating, hot air continues to be introduced for drying, helping to evaporate the solvent and allowing the coating layer to completely solidify and fix onto the surface of the urea granules. The hot air not only removes the solvent but also ensures the coating layer adheres stably to the granule surface, preventing it from peeling off during use. By controlling the drying temperature and time, the quality of the coating layer is ensured, giving it excellent controlled-release performance. The resulting slow-release nitrogen fertilizer granules can gradually release nutrients into the soil, maintaining a continuous supply of nutrients and preventing excessive release in a short period. Potassium sulfate is pulverized to a set particle size and then sieved to ensure uniform particle size and compliance with subsequent coating operations. Pulverization and sieving allow for precise control of the potassium fertilizer particle size, ensuring consistent particle size for more uniform mixing with other components, thus ensuring the slow-release effect of the fertilizer. This step aims to optimize the coating performance of the potassium fertilizer and enhance its effectiveness.

[0027] Slow-release nitrogen fertilizer granules and slow-release potassium fertilizer powder are mixed in a specific ratio and then fed into a rotary drum coating machine. The machine's rotation and tumbling action ensures that the slow-release nitrogen fertilizer granules and potassium fertilizer powder adhere evenly to the surface of the fast-acting nutrient core granules. To ensure adhesion, a spray system applies a polyvinyl alcohol aqueous solution as a binder to the granule surface. Polyvinyl alcohol has excellent adhesive properties, helping the slow-release nitrogen fertilizer granules and potassium fertilizer powder adhere firmly to the surface of the fast-acting nutrient core granules, forming a uniform intermediate layer. This coating intermediate layer not only enhances granule stability but also further regulates the nutrient release rate. An acrylic copolymer emulsion is sprayed onto the surface of the coated granules, forming a pre-coating layer. This film further regulates the fertilizer release rate, helping to control nutrient release in the soil. After spraying, a drying process completely solidifies the acrylic copolymer emulsion, ensuring the stability of the film layer and ultimately forming the slow-release nutrient intermediate layer granules. These granules have excellent controlled nutrient release properties, which can continuously supply the nutrients needed by crops, prevent rapid nutrient loss, and improve fertilizer utilization and crop growth.

[0028] In one specific embodiment, an acrylic copolymer emulsion is sprayed onto the surface of the coated particles to form a pre-coating layer and then dried to obtain the slow-release nutrient middle layer particles, comprising: Acrylic copolymers are formulated into emulsions with a solid content within a set range to obtain pre-coated emulsions; The coated particles are kept in a tumbling state in the rotary drum coating machine, and the pre-coated emulsion is sprayed onto the surface of the coated particles at a set spraying speed through the spraying system. The spraying time is controlled so that the coating thickness reaches the set range to obtain pre-coated particles. The pre-coated particles are dried by passing hot air at a set temperature through the rotary drum coating machine, so that the pre-coated emulsion is completely film-formed to obtain film-formed particles. After the film-forming particles are cooled to a set temperature, the crushing force of a single particle is measured by a hardness tester. Particles whose crushing force reaches a set threshold are used as the intermediate particles of the slow-release nutrients.

[0029] Specifically, acrylic copolymers are formulated into emulsions with a solids content controlled within a set range. These acrylic copolymer emulsions are used to form a sustained-release film. The solids content of the emulsion refers to the proportion of active ingredients in the emulsion. The set solids content ensures that the formed film layer is neither too thin nor too thick, achieving an ideal controlled release effect. The coated particles are kept in a tumbling state in a rotary drum coating machine. The rotary drum coating machine, through rotation, causes the particles to continuously tumble within the drum, ensuring that the emulsion evenly covers the particle surface. The prepared pre-coated emulsion is sprayed onto the particle surface using a spraying system. The spraying speed (spray velocity) and spraying time need to be precisely controlled to ensure that a uniform film layer is formed on the surface of each particle, while simultaneously controlling the film thickness to meet the set requirements. The purpose of this step is to coat the particle surface with the emulsion, forming a pre-coated layer.

[0030] The pre-coated granules continue to undergo drying with hot air at a set temperature in a rotary drum coating machine. The purpose of the hot air is to rapidly evaporate the solvent in the pre-coated emulsion, ensuring that the solid components in the emulsion form a stable film layer on the granule surface. This drying process not only ensures the integrity and stability of the film layer but also prevents it from peeling off or breaking during subsequent use. After drying, a robust film layer forms on the granule surface; these granules are called film-forming granules. The film-forming granules are cooled to a set temperature and then subjected to a crushing force test using a hardness tester. Crushing force refers to the minimum force required to break a granule; this test assesses whether the granule strength is sufficient. Only granules with a crushing force reaching a set threshold are considered qualified slow-release nutrient middle-layer granules, and these granules will continue to be used in the next stage of fertilizer preparation. Ensuring the hardness and strength of the granules is crucial for the stability of the fertilizer; overly soft granules may break during application, leading to uneven nutrient release.

[0031] In one specific embodiment, step S5 includes: Polyhydroxy fatty acid esters were dissolved in a solvent and then nano-silica and starch particles were added to prepare a coating solution with a set solid content, thus obtaining the bio-based polyester coating solution. The slow-release nutrient intermediate particles are fed into a fluidized bed coating machine, and hot air at a set temperature is introduced to make the particles fluidized. The bio-based polyester coating liquid is sprayed into the fluidized bed through a dual-fluid spray gun to atomize and coat to a set thickness. Low-temperature drying air is then introduced to dry the particles and evaporate the solvent, thus obtaining inner-coated particles. The modified starch is prepared into an aqueous solution of a set concentration to obtain the modified starch film liquid. The modified starch film liquid is sprayed onto the surface of the inner coating particles to a set thickness in a fluidized bed. After drying, the double-layer composite controlled-release shell is formed, and controlled-release coated particles are obtained. The controlled-release coated particles are cooled and then sieved. Particles with a particle size within a set range are sprayed with an anti-caking agent and packaged to obtain the multi-nutrient long-acting slow-release fertilizer.

[0032] Specifically, polyhydroxyalkanoates are dissolved in a solvent to form a basic coating solution. Next, nano-silica and starch microparticles are added to enhance the coating layer's performance. Nano-silica forms uniformly distributed nanoscale channels within the polymer matrix, improving the coating's permeability and mechanical properties, while starch microparticles promote the biodegradability of the coating material, enhancing its environmental friendliness. Finally, all components are mixed according to a preset solids content ratio to obtain the desired bio-based polyester coating solution, which will be used for subsequent particle coating operations. The slow-release nutrient intermediate particles are fed into a fluidized bed coating machine. In the fluidized bed, hot air at a set temperature is introduced, causing the particles to enter a fluidized state. The particles are suspended and tumbled under the airflow, ensuring that each particle is uniformly contacted with the coating solution. Subsequently, the prepared bio-based polyester coating solution is sprayed into the fluidized bed using a two-fluid spray gun. The dual-fluid spray gun uses compressed air to atomize the coating solution, ensuring a uniform coating coverage on the fluidized particle surface. By adjusting the spraying time and liquid volume, the coating thickness can be controlled to ensure it meets the set requirements. After spraying, low-temperature drying air continues to flow for drying. This process aims to evaporate the solvent, allowing the coating solution to solidify on the particle surface, forming an inner coated particle. This coating layer is an important component of slow-release fertilizers, ensuring a controllable fertilizer release rate and preventing rapid nutrient loss from the soil.

[0033] After completing the inner coating, the next step is to prepare the modified starch film solution. The modified starch is prepared into an aqueous solution of a set concentration. The modified starch enhances the controlled-release function of the coating and regulates the nutrient release rate through biodegradability. In a fluidized bed coating machine, the modified starch film solution is sprayed onto the surface of the particles that have already formed the inner coating. The spraying process also uses a two-fluid spray gun to ensure that the modified starch film evenly covers the surface of the inner-coated particles, forming a double-layer composite controlled-release shell. This shell structure is composed of two materials with different properties, which can provide longer-lasting nutrient release control in the soil. After spraying, the particles continue to dry in the fluidized bed to ensure that the shell is completely solidified, forming a stable controlled-release coating. The resulting controlled-release coated particles are then cooled to ensure that their temperature is reduced to a safe range. The cooled particles are then sieved to select particles that meet the required particle size. After sieving, particles that meet the size standard are sprayed with an anti-caking agent to prevent clumping due to moisture during storage or transportation. Commonly used anti-caking agents include talc, diatomaceous earth, or hydrophobic silica, which effectively prevent granules from absorbing moisture and clumping. After treatment with anti-caking agents, the granules are packaged and ultimately become a multi-nutrient, long-lasting, slow-release fertilizer. This fertilizer has the characteristic of long-term slow release, providing crops with a continuous supply of nutrients and avoiding the nutrient waste and environmental pollution caused by the rapid release of traditional fertilizers.

[0034] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a multi-nutrient, long-acting, slow-release fertilizer that also improves soil quality, characterized in that, The method includes: Step S1: Agricultural waste is carbonized and pyrolyzed and then mixed with humic acid powder in a mass ratio. The biochar-humic acid composite modifier is obtained by extrusion granulation process. Step S2: Silicate mineral powder and carbonate mineral powder are mixed in a mass ratio and then ultra-fine ground. After surface modification treatment with a coupling agent, silicon-calcium-magnesium composite mineral powder is obtained. Step S3: The fast-acting nitrogen and phosphorus nutrients, the biochar-humic acid composite modifier, the silicon-calcium-magnesium composite mineral powder and the chelated trace element solution are subjected to wet granulation to obtain fast-acting nutrient core particles. Step S4: Coat the surface of the fast-acting nutrient core particles with slow-release nitrogen fertilizer granules and slow-release potassium fertilizer powder using a binder solution to obtain slow-release nutrient middle layer particles; Step S5: The bio-based polyester coating liquid is sprayed onto the surface of the slow-release nutrient middle layer particles through a fluidized bed, and a modified starch film liquid is sprayed onto the outer layer to form a double-layer composite controlled-release shell, thus obtaining a multi-nutrient long-acting slow-release fertilizer.

2. The method for preparing a multi-nutrient, slow-release fertilizer with soil-improving properties according to claim 1, characterized in that, Step S1 includes: The agricultural waste is crushed and placed in a pyrolysis furnace. Under a nitrogen protective atmosphere, the temperature is raised to the preset pyrolysis temperature at a set heating rate and maintained at a constant temperature to obtain biochar. The biochar is cooled and then pulverized and sieved to a set mesh size to obtain biochar powder. Weathered coal or peat is extracted with alkaline solution at a material-to-liquid ratio. After filtration, the pH value is adjusted with acid solution to precipitate humic acid. After centrifugation, washing, drying, pulverizing and sieving, the humic acid powder is obtained. The biochar powder and the humic acid powder are mixed in a mixer according to the mass ratio and a wetting agent is sprayed in. After mixing, the mixture is transferred to an extrusion granulator, extruded and shaped under a set pressure, and dried to a set moisture content to obtain the biochar-humic acid composite modifier.

3. The method for preparing a multi-nutrient, slow-release fertilizer with soil-improving properties according to claim 1, characterized in that, Step S2 includes: Potassium feldspar ore or serpentine ore is crushed and then fed into a planetary ball mill for ultrafine grinding. The median particle size of the ground silicate mineral powder is controlled within a set range to obtain activated silicate mineral powder. After crushing the dolomite ore, it is fed into a planetary ball mill for ultrafine grinding to a set median particle size to obtain the carbonate mineral powder. The activated silicate mineral powder and the carbonate mineral powder are added to a mixer according to the mass ratio and mixed until uniform to obtain mixed mineral powder; The coupling agent is dissolved in a solvent to prepare a modified solution of a set concentration, which is then sprayed onto the surface of the mixed mineral powder and mixed further. After drying, the solution is sieved to a set mesh size to obtain the silicon-calcium-magnesium composite mineral powder.

4. The method for preparing a multi-nutrient, slow-release fertilizer with soil-improving properties according to claim 1, characterized in that, Step S3 includes: Urea and diammonium phosphate are pulverized to a set particle size and then mixed in a mixer to obtain the fast-acting nitrogen and phosphorus nutrients. Calcium chloride and glycine were reacted at a set pH and temperature in a set molar ratio and then diluted to volume to obtain a chelated calcium solution; magnesium sulfate and glycine were reacted at a set pH and temperature in a set molar ratio and then diluted to volume to obtain a chelated magnesium solution; zinc sulfate and disodium EDTA were reacted at a set pH and temperature in a set molar ratio and then diluted to volume to obtain a chelated zinc solution; borax was dissolved in deionized water to obtain a boric acid solution; the chelated calcium solution, the chelated magnesium solution, the chelated zinc solution, and the boric acid solution were mixed in a specified ratio and then concentrated under reduced pressure to obtain the chelated trace element solution. The fast-acting nitrogen and phosphorus nutrients, the biochar-humic acid composite modifier and the silicon-calcium-magnesium composite mineral powder are put into a granulator. The chelated trace element solution is sprayed onto the surface of the tumbling material through a nozzle to perform wet granulation to the set particle size, and wet granules are obtained. The wet granules are dried to a set moisture content at a set temperature and then sieved. Particles with a particle size within a set range are used as the core particles of the fast-acting nutrients.

5. The method for preparing a long-acting, slow-release fertilizer with soil-improving properties according to claim 1, characterized in that, Step S4 includes: Urea granules are fed into a fluidized bed coating machine, and preheated hot air is introduced to make the urea granules fluidized. Polycaprolactone and polyN-isopropylacrylamide are dissolved in a solvent according to the mass ratio to prepare a coating solution. The solution is sprayed into the fluidized bed through a two-fluid spray gun for atomized coating. After coating, hot air is continued to dry to obtain the slow-release nitrogen fertilizer granules. Potassium sulfate is pulverized to a set particle size and then sieved to obtain the slow-release potassium fertilizer powder; The slow-release nitrogen fertilizer granules and the slow-release potassium fertilizer powder are mixed in a certain proportion and then fed into a rotary drum coating machine. The fast-acting nutrient core granules are also fed into the rotary drum coating machine. At the same time, a polyvinyl alcohol aqueous solution is sprayed in through a spray system as the binder solution. Under the tumbling action of the rotary drum, the slow-release nitrogen fertilizer granules and the slow-release potassium fertilizer powder adhere to the surface of the fast-acting nutrient core granules to form an intermediate layer, thus obtaining coated granules. An acrylic copolymer emulsion is sprayed onto the surface of the coated particles to form a pre-coating layer and then dried to obtain the slow-release nutrient middle layer particles.

6. The method for preparing a long-acting, slow-release fertilizer with soil-improving properties according to claim 5, characterized in that, The step of spraying an acrylic copolymer emulsion onto the surface of the coated particles to form a pre-coating layer and then drying it yields the slow-release nutrient middle layer particles, comprising: Acrylic copolymers are formulated into emulsions with a solid content within a set range to obtain pre-coated emulsions; The coated particles are kept in a tumbling state in the rotary drum coating machine, and the pre-coated emulsion is sprayed onto the surface of the coated particles at a set spraying speed through the spraying system. The spraying time is controlled so that the coating thickness reaches the set range to obtain pre-coated particles. The pre-coated particles are dried by passing hot air at a set temperature through the rotary drum coating machine, so that the pre-coated emulsion is completely film-formed to obtain film-formed particles. After the film-forming particles are cooled to a set temperature, the crushing force of a single particle is measured by a hardness tester. Particles whose crushing force reaches a set threshold are used as the intermediate particles of the slow-release nutrients.

7. The method for preparing a long-acting, slow-release fertilizer with soil-improving properties according to claim 1, characterized in that, Step S5 includes: Polyhydroxy fatty acid esters were dissolved in a solvent and then nano-silica and starch particles were added to prepare a coating solution with a set solid content, thus obtaining the bio-based polyester coating solution. The slow-release nutrient intermediate particles are fed into a fluidized bed coating machine, and hot air at a set temperature is introduced to make the particles fluidized. The bio-based polyester coating liquid is sprayed into the fluidized bed through a dual-fluid spray gun to atomize and coat to a set thickness. Low-temperature drying air is then introduced to dry the particles and evaporate the solvent, thus obtaining inner-coated particles. The modified starch is prepared into an aqueous solution of a set concentration to obtain the modified starch film liquid. The modified starch film liquid is sprayed onto the surface of the inner coating particles to a set thickness in a fluidized bed. After drying, the double-layer composite controlled-release shell is formed, and controlled-release coated particles are obtained. The controlled-release coated particles are cooled and then sieved. Particles with a particle size within a set range are sprayed with an anti-caking agent and packaged to obtain the multi-nutrient long-acting slow-release fertilizer.

8. The method for preparing a multi-nutrient, slow-release fertilizer with soil-improving properties according to claim 7, characterized in that, The modified starch is prepared into an aqueous solution of a set concentration to obtain the modified starch film solution. The modified starch film solution is then sprayed onto the surface of the inner coated particles to a set thickness in a fluidized bed. After drying, the double-layer composite controlled-release shell is formed, resulting in controlled-release coated particles, comprising: Polyhydroxy fatty acid esters were dissolved in a solvent and then nano-silica and starch particles were added to prepare a coating solution with a set solid content, thus obtaining the bio-based polyester coating solution. The slow-release nutrient intermediate particles are fed into a fluidized bed coating machine, and hot air at a set temperature is introduced to make the particles fluidized. The bio-based polyester coating liquid is sprayed into the fluidized bed through a dual-fluid spray gun to atomize and coat to a set thickness. Low-temperature drying air is then introduced to dry the particles and evaporate the solvent, thus obtaining inner-coated particles. The modified starch is prepared into an aqueous solution of a set concentration to obtain the modified starch film liquid. The modified starch film liquid is sprayed onto the surface of the inner coating particles to a set thickness in a fluidized bed. After drying, the double-layer composite controlled-release shell is formed, and controlled-release coated particles are obtained. The controlled-release coated particles are cooled and then sieved. Particles with a particle size within a set range are sprayed with an anti-caking agent and packaged to obtain the multi-nutrient long-acting slow-release fertilizer.

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