A soil nutrient and a method for preparing the same

Abstract

The present application relates to the technical field of soil conditioning material, in particular to a soil nutrient agent and a preparation method thereof.The soil nutrient agent is a core-shell structure particle, the inner core is a porous carrier loaded with functional microorganisms, and the shell layer is a composite coating layer with a gradient structure.The preparation method comprises the following steps: 1) preparing a microbial inner core through step-by-step loading;2) preparing a composite coating liquid from nitro-humic acid, activated diatomite, sodium alginate and chitosan;3) constructing a gradient shell layer with loose outer layer and dense inner layer by using fluidized bed gradient coating technology;4) cross-linking and solidifying by using calcium zinc salt;5) constructing micro-nano channels by etching with phosphate buffer and performing surface modification.The present application balances the contradiction between microorganism protection and release through gradient structure design and channel regulation, realizes environment-responsive controlled release, and simultaneously has multiple functions such as improving soil structure and providing nutrients, thereby solving the technical problems of low survival rate, uncontrollable release and single function of traditional microbial agents.

Description

Technical Field

[0001] This invention relates to the field of soil conditioning materials technology, specifically to a soil nutrient agent and its preparation method. Background Technology

[0002] Soil is the foundation of agricultural production and ecosystem health, and functional microorganisms play a crucial role in soil nutrient cycling, pollutant degradation, and crop growth promotion. To improve the survival and colonization of artificially applied microbial agents in complex soil environments, current technologies generally employ encapsulation or coating techniques, utilizing polymeric materials such as sodium alginate, chitosan, and starch to form physical barriers, thereby isolating the microorganisms from direct stress from adverse external environments.

[0003] However, these traditional coating methods have significant limitations. First, their release behavior depends on the overall swelling or degradation of the coating material, making it difficult to achieve controlled release that matches environmental needs, often resulting in slow initial release or premature failure of protection in the later stages. Second, to achieve sufficient protective strength, the coating layer is often too dense. While it can block external stresses, it also severely hinders the necessary exchange of water, gas, nutrients, and metabolic products, making it difficult for internal microorganisms to be effectively activated or released in a timely manner, thus greatly reducing its actual effectiveness. In addition, the functionality of existing coating systems is often relatively singular; most coating materials only provide physical isolation and lack synergistic functions such as actively improving soil structure, regulating pH, or providing organic matter.

[0004] Therefore, developing a composite coating system that can provide efficient protection for functional microorganisms, achieve controlled release, and also improve soil function has become a critical technical challenge in this field. The core challenge lies in how to precisely construct a shell with suitable permeability and structural gradient to fundamentally balance the contradictory relationship between "protection" and "release." Summary of the Invention

[0005] The purpose of this invention is to provide a soil nutrient and its preparation method to solve the technical problems of existing soil nutrient materials, such as difficulty in balancing the protection and release of functional microorganisms, low mass transfer efficiency, and limited functionality.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a soil nutrient agent, wherein the soil nutrient agent is a core-shell structured particle, the core being a porous carrier loaded with functional microorganisms, and the shell being a composite coating layer with a gradient structure; The composite coating layer consists of a loose and porous inner layer, a dense and nutrient-releasing middle layer, and a dense and protective outer layer from the inside out; micro-nano mass transfer channels are distributed within the shell.

[0007] Furthermore, the functional microorganisms include one or more of nitrogen-fixing bacteria, phosphate-solubilizing bacteria, potassium-solubilizing bacteria, photosynthetic bacteria, and Bacillus subtilis.

[0008] Furthermore, the porous carrier is a composite of porous starch and calcium sulfate.

[0009] Furthermore, the raw materials for preparing the composite coating layer include nitrohumic acid, activated diatomaceous earth, sodium alginate, and chitosan.

[0010] Furthermore, the nitrohumic acid is obtained by modifying weathered coal through nitric acid nitration.

[0011] Furthermore, the activated diatomaceous earth is obtained by activating diatomaceous earth through high-temperature calcination.

[0012] A method for preparing a soil nutrient agent includes the following steps: S1, Kernel Preparation The porous carrier is combined with a functional microbial agent solution, and then granulated, solidified, and dried at low temperature to obtain dried core particles; S2, Preparation of composite coating solution Nitrohumic acid powder, activated diatomaceous earth, trisodium citrate, sodium alginate solution and chitosan solution are mixed, and the mixture is subjected to high-speed stirring, high-pressure homogenization, and adjustment of viscosity and pH to obtain a composite coating solution. S3, Gradient Coating Molding S1 dried core particles are placed in a fluidized bed and composite coating solutions with different dilution ratios are sprayed sequentially to form a multi-layer coating layer with a gradient structure of loose inner and dense outer layers. S4, Crosslinking Curing In a fluidized state, calcium chloride solution and a mixed solution of calcium chloride and zinc sulfate are sprayed sequentially to chemically crosslink and solidify the multi-layer coating to obtain coated particles. S5, Channel Construction and Post-processing The S4 coated particles were immersed in phosphate buffer solution for etching to construct micro-nano mass transfer channels; after surface modification, drying and sieving, the soil nutrient of the present invention was obtained.

[0013] Furthermore, in S3, the gradient coating process includes: first spraying a mixture of composite coating liquid and water diluted at a ratio of 1:0.8 to form the bottom layer; then spraying a mixture of composite coating liquid and water diluted at a ratio of 1:0.5 to form the middle layer; and finally spraying a mixture of composite coating liquid and water diluted at a ratio of 1:0.3 to form the outer layer.

[0014] Furthermore, in S5, the surface modification is achieved by spraying a polyethylene glycol solution.

[0015] The beneficial effects of this invention are: 1. This invention constructs a gradient shell with a loose inner layer and a dense outer layer and controllable micro-nano channels through core-shell structure design, gradient coating process and pore control technology. It effectively balances the contradiction between "protection" and "release", providing efficient protection for functional microorganisms and ensuring efficient mass transfer of water, nutrients and metabolites, realizing environmentally responsive and controllable release of microorganisms and nutrients.

[0016] 2. The composite coating layer of the present invention integrates multiple functional components such as nitrohumic acid and activated diatomaceous earth, which not only provides physical protection, but also has the synergistic functions of improving soil structure, regulating pH, providing organic matter and slow-release nutrients, thus overcoming the problem of the single function of traditional coating systems.

[0017] 3. The preparation method of the present invention has a close connection between processes and complementary principles. Through the synergy of key technologies such as porous carrier immobilization, polyelectrolyte composite film formation, gradient coating, chemical crosslinking and etching pore formation, a complete technical system of "protection-slow release-mass transfer-enhancement" is formed. The final product has stable performance and can achieve long-term slow release of microorganisms and nutrients and targeted improvement of soil fertility. Detailed Implementation

[0018] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention.

[0019] This invention utilizes a core-shell structure design, gradient coating process, and pore control technology to prepare a multifunctional soil nutrient agent that combines microbial activity protection, controlled nutrient release, and efficient mass transfer. The overall process revolves around "constructing a microbial protective core → preparing a nutrient-rich composite coating solution → fluidized bed gradient coating → chemical cross-linking and solidification → micro / nano pore construction and surface modification." The core technologies utilize porous carriers to immobilize microorganisms, polyelectrolyte composite film formation, gradient structures for controlled release, and etching methods to construct mass transfer pores. These technologies address the technical problems of traditional soil nutrients, such as easy loss of microbial activity, rapid nutrient release, and the contradiction between protection and mass transfer. Ultimately, this invention achieves the technical goals of long-term controlled release of microorganisms and nutrients, and targeted improvement of soil fertility. The various process steps are closely linked and complementary, forming a complete technical system of "protection-controlled release-mass transfer-enhancement."

[0020] First, weathered coal is pulverized and dried for pretreatment. Humic acid is modified through nitrification to prepare nitrohumic acid powder with higher water solubility and bioavailability, providing the core nutrient component for subsequent coating solutions. Second, porous carriers and microbial agents are combined through stepwise permeation loading. The porous carriers are used to immobilize microorganisms and reduce damage to their activity. Granulation and low-temperature drying induce dormancy in the microorganisms, constructing a stable microbial active core. Then, activated diatomaceous earth is compounded with nitrohumic acid, and a composite coating solution is prepared using polyelectrolyte composite film formation and high-pressure homogenization technology. Adsorption and electrostatic interactions are used to fuse the nutrient and film-forming systems, providing a uniform and stable film-forming medium for gradient coating. Furthermore, a fluidized bed gradient control process is employed to construct a loose inner and dense outer gradient shell by adjusting coating parameters. This is combined with calcium-zinc salt cross-linking and solidification. The gradient structure balances protection and mass transfer efficiency, and the complexation reaction enhances shell stability, forming a core-shell structure with both protective and controlled-release functions. Finally, pores are created by etching with phosphate buffer solution, and PO4... 3- Competing with the carboxyl group of sodium alginate to bind Ca 2+ This process leads to the disintegration of local cross-linking points and the relaxation of polymer chain segments, thereby forming micro-nano channels. Subsequently, PEG surface modification is performed, combined with graded drying and sieving, to construct micro-nano mass transfer channels and optimize particle performance, improve material transport efficiency, ensure product storage and application stability, and ultimately obtain a multifunctional soil nutrient.

[0021] Example 1 In this embodiment, weathered coal is used as raw material to prepare nitrohumic acid.

[0022] The weathered coal (C=76.42%) raw material was crushed and passed through an 80-mesh sieve to obtain weathered coal powder; the weathered coal powder was dried at 105±2℃ for 2~4h to constant weight to obtain weathered coal powder material.

[0023] Take the above-mentioned weathered coal powder, add water and 20% nitric acid solution, the mass ratio of water to weathered coal is 3:1, and the mass ratio of 20% nitric acid solution to weathered coal is 1:1; keep the reaction at 400 rpm and 80℃; continue stirring until no reddish-brown smoke is produced (2.5~4h), stop heating, and let it cool naturally to room temperature to obtain the reaction mixture.

[0024] Filter the above reaction mixture to separate the solid product; wash the solid product with distilled water until the pH of the filtrate stabilizes at 3.0~3.5 (about 3~5 washes), and then vacuum dry at 60℃ for 4~6 hours until the moisture content is ≤15%. Crush the dried product to 80 mesh to obtain nitrohumic acid powder (crude product).

[0025] Example 2 In this embodiment, the soil nutrient agent of the present invention is prepared.

[0026] 1. Kernel Preparation Porous starch (particle size 50~100μm) was mixed with food-grade calcium sulfate dihydrate at a mass ratio of 6:1. The mixture was then dried at 105±2℃ until the moisture content was ≤5%, and cooled to room temperature to obtain a composite carrier, which was then placed in a sterile container for later use.

[0027] Commercially available microbial agents, including nitrogen-fixing bacteria, phosphate-solubilizing bacteria, and Bacillus subtilis, were prepared into a microbial agent solution at a ratio of 2:1:1 (effective viable count ≥1×10⁻⁶). 10 (CFU / mL) (Note: In other embodiments, one or more microbial inoculants, including nitrogen-fixing bacteria, phosphate-solubilizing bacteria, potassium-solubilizing bacteria, photosynthetic bacteria, and Bacillus subtilis, can be selected according to the actual soil conditions to prepare the microbial inoculant solution); Under the conditions of 25±1℃ and relative humidity≤50%, the microbial inoculant solution is divided into 3 equal parts and slowly added to the above composite carrier. The volume-to-mass ratio of microbial inoculant to porous starch is 2:1 (L / kg). After each addition, the mixture is thoroughly stirred to allow the microbial inoculant solution to fully penetrate and obtain the wet-loaded material.

[0028] The wet-loaded material was made into particles with a particle size of 0.4±0.1mm, and a 2% sodium alginate solution was sprayed by atomization (spraying amount 1mL / 100g material). The treated particles were allowed to stand and solidify at 30±1℃ and 30% relative humidity to obtain core wet particles. The core wet particles were spread flat in a tray and dried at 38±2℃ until the particle moisture content was 18±2% to obtain dried core particles.

[0029] 2. Preparation of composite coating solution Diatomaceous earth was calcined in a muffle furnace at 550℃ for 2 hours with a heating rate of 5℃ / min. After calcination, it was naturally cooled to room temperature to obtain activated diatomaceous earth.

[0030] The nitrohumic acid powder prepared in Example 1 was thoroughly dry-mixed with activated diatomaceous earth at a mass ratio of 2:1. 5% (by weight) of trisodium citrate and water were added, with a volume ratio of 2:1 (L / kg) between the water and the nitrohumic acid powder. The mixture was stirred at 2000 rpm until homogeneous. A 1.2% sodium alginate solution was slowly added, with a volume ratio of 10:1 (L / kg) between the sodium alginate solution and the nitrohumic acid powder. The mixture was stirred until homogeneous. A 0.5% chitosan solution (dissolved in 1% acetic acid) was added, with a volume ratio of 2:1 (L / kg) between the chitosan solution and the nitrohumic acid powder. The mixture was homogenized under high pressure at 40 MPa for 3 cycles. Water was slowly added dropwise to adjust the viscosity to 400 ± 50 mPa·s (25°C) and the pH to 7.0–7.2 to obtain the composite coating solution.

[0031] 3. Preparation of coated particles The dried core particles were placed into a fluidized bed coating machine, and the inlet air temperature was controlled at 40±1℃ and the material temperature at 30±1℃. A mixture of composite coating liquid and deionized water diluted at a ratio of 1:0.8 was sprayed at a rate of 1.5mL / min to form a bottom layer of 10~20μm.

[0032] Adjust the air inlet temperature to 42±1℃ and the material temperature to 32±1℃; spray a mixture of composite coating liquid and deionized water diluted at a rate of 1:0.5 at a rate of 2.5mL / min, with the volume-to-mass ratio of the diluted mixture to the dried core particles being 1.0~1.2:1 (L / kg), to form a middle layer of 30~40μm.

[0033] Adjust the inlet air temperature to 45±1℃ and the material temperature to 34±1℃; spray a mixture of composite coating liquid and deionized water diluted at a rate of 1:0.3 at a rate of 3.0mL / min, with the volume-to-mass ratio of the diluted mixture to the dried core particles being 0.6~0.8:1 (L / kg), to form an outer layer of 20~30μm.

[0034] Maintaining the fluidized state, a 2.5% calcium chloride solution was sprayed at a rate of 4 mL / min, with a volume-to-mass ratio of 2.5% calcium chloride solution to dried core particles of 2:1. The material was cured for 20 min at a temperature of 35±1℃. Then, a mixture of 1% calcium chloride and 0.5% zinc sulfate was sprayed at a rate of 6 mL / min, with a volume-to-mass ratio of the mixture to dried core particles of 1:1 (L / kg). The material was cured for 15 min at a temperature of 38±1℃ to obtain coated particles.

[0035] 4. Construction and post-processing of controlled-release channels The coated particles were immersed in 0.1M phosphate buffer (pH 6.5), with a volume-to-mass ratio of 0.1M phosphate buffer to coated particles of 10:1 (L / kg), and treated at room temperature and 60 rpm for 15 min. The particles were then removed and drained.

[0036] The drained coated granules were placed back into a fluidized bed and sprayed with a 0.1% polyethylene glycol (PEG-4000) solution via atomization at 40°C. The volume-to-mass ratio of the 0.1% polyethylene glycol solution to the dried core granules was 0.5:1 (L / kg) for surface modification. Then, the granules were graded and dried: first at 35°C and 40% relative humidity for 2 hours, and then at 30°C and 25% relative humidity for 1.5 hours. The final product moisture content was controlled to be ≤8.5%, thus obtaining the dried product. The dried product was passed through a 30-40 mesh sieve, and particles with a diameter of approximately 0.5 mm were collected, which is the soil nutrient of this invention.

[0037] This invention is not limited to the preferred embodiments described above. Anyone can derive other forms of products under the guidance of this invention. However, regardless of any changes made in their shape or structure, any technical solution that is the same as or similar to this application falls within the protection scope of this invention.

Claims

1. A soil nutrient agent, characterized in that: The soil nutrient is a core-shell structured particle, with the core being a porous carrier loaded with functional microorganisms and the shell being a composite coating layer with a gradient structure. The composite coating layer consists of a loose and porous inner layer, a dense and nutrient-releasing middle layer, and a dense and protective outer layer from the inside out; micro-nano mass transfer channels are distributed within the shell.

2. The soil nutrient agent according to claim 1, characterized in that: The functional microorganisms include one or more of nitrogen-fixing bacteria, phosphate-solubilizing bacteria, potassium-solubilizing bacteria, photosynthetic bacteria, and Bacillus subtilis.

3. The soil nutrient agent according to claim 1, characterized in that: The porous carrier is a composite of porous starch and calcium sulfate.

4. A soil nutrient agent according to claim 1, characterized in that: The raw materials for preparing the composite coating layer include nitrohumic acid, activated diatomaceous earth, sodium alginate, and chitosan.

5. A soil nutrient agent according to claim 4, characterized in that: The nitrohumic acid is obtained by modifying weathered coal through nitric acid nitration.

6. A soil nutrient agent according to claim 4, characterized in that: The activated diatomaceous earth is obtained by calcining and activating diatomaceous earth at high temperature.

7. The method for preparing the soil nutrient agent according to any one of claims 1-6, characterized in that, Includes the following steps: S1, Kernel Preparation The porous carrier is combined with a functional microbial agent solution, and then granulated, solidified, and dried at low temperature to obtain dried core particles; S2, Preparation of composite coating solution Nitrohumic acid powder, activated diatomaceous earth, trisodium citrate, sodium alginate solution and chitosan solution are mixed, and the mixture is subjected to high-speed stirring, high-pressure homogenization, and adjustment of viscosity and pH to obtain a composite coating solution. S3, Gradient Coating Molding S1 dried core particles are placed in a fluidized bed and composite coating solutions with different dilution ratios are sprayed sequentially to form a multi-layer coating layer with a gradient structure of loose inner and dense outer layers. S4, Crosslinking Curing In a fluidized state, calcium chloride solution and a mixed solution of calcium chloride and zinc sulfate are sprayed sequentially to chemically crosslink and solidify the multi-layer coating to obtain coated particles. S5, Channel Construction and Post-processing The S4 coated particles were immersed in phosphate buffer solution for etching to construct micro-nano mass transfer channels; after surface modification, drying and sieving, the soil nutrient of the present invention was obtained.

8. The method for preparing the soil nutrient agent according to claim 7, characterized in that: In S3, the gradient coating process includes: first, spraying a mixture of composite coating liquid and water diluted at a ratio of 1:0.8 to form the bottom layer; then, spraying a mixture of composite coating liquid and water diluted at a ratio of 1:0.5 to form the middle layer; and finally, spraying a mixture of composite coating liquid and water diluted at a ratio of 1:0.3 to form the outer layer.

9. The method for preparing the soil nutrient agent according to claim 7, characterized in that: In S5, the surface modification is achieved by spraying a polyethylene glycol solution.