A slow-release compound fertilizer formulated by soil testing of sorghum in saline-alkali land improvement

Abstract

This invention relates to a slow-release compound fertilizer formulated by soil testing for sorghum in saline-alkali land improvement. The compound fertilizer comprises a core layer, an intermediate coating layer, and an outer protective layer. The core layer consists of sorghum-specific compound fertilizer granules determined by soil testing, with an N-P₂O₅-K₂O mass ratio of 18~22:8~12:10~15. The intermediate coating layer is an interpenetrating hydrogel formed by cross-linking polyaspartic acid, sodium alginate, and γ-polyglutamic acid with calcium gluconate, wherein salt-resistant microbial agents encapsulated in chitosan-calcium alginate composite wall microcapsules are dispersed. The outer protective layer is a chitosan-calcium gluconate composite membrane. This invention employs a three-layer structure, possessing multiple functions including high water absorption and retention, salt resistance, slow-release, microbial protection, biodegradability, and alkalinity neutralization.

Description

Technical Field

[0001] This invention relates to the field of saline-alkali land improvement and fertilizer preparation technology, and particularly to a slow-release compound fertilizer based on soil testing and formulation of sorghum for saline-alkali land improvement. Background Technology

[0002] Sorghum has a strong tolerance to saline-alkali soils and is one of the important crops for agriculture in saline-alkali lands. However, saline-alkali soils have problems such as high pH, ​​high salt content, poor soil structure, and weak water and fertilizer retention capacity, which seriously restrict the growth, development, and yield of sorghum.

[0003] Currently, fertilizers used for sorghum cultivation in saline-alkali land have many technical defects: When ordinary compound fertilizers are applied to saline-alkali land, nitrogen is easily lost through ammonia volatilization and denitrification; phosphorus is easily fixed by calcium and magnesium ions to form insoluble phosphates; and potassium is easily fixed by clay minerals, resulting in fertilizer utilization rates generally below 30%. Furthermore, sodium ions disrupt the aggregate structure of saline-alkali soils, significantly reducing their water and fertilizer retention capacity and making it difficult to alleviate drought stress. Ordinary fertilizers release nutrients quickly, which does not match the nutrient requirements of sorghum, easily causing early seedling burn and later seedling loss. Fertilizers: While existing saline-alkali land improvement fertilizers contain microbial agents, they employ direct mixing methods. The high temperatures and mechanical extrusion during granulation and drying lead to the death of a large number of microorganisms. Existing coating materials are mostly synthetic polymers such as polyacrylic acid and polyurethane, which have poor environmental compatibility in soil and are difficult to degrade, resulting in microplastic residues with long-term application. Current technologies use calcium chloride as a cross-linking agent to introduce chloride ions, but sorghum is a chloride-sensitive crop. Chloride ions inhibit root growth and damage leaf chlorophyll structure, further exacerbating salt damage in saline-alkali soils. Furthermore, the formulations have not been designed specifically for sorghum's salt tolerance mechanism and chloride sensitivity.

[0004] Therefore, developing a sorghum-specific slow-release compound fertilizer that combines water retention, slow release, biodegradability, high microbial activity, precise formulation, and the ability to neutralize soil alkalinity is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] To address the aforementioned problems in the existing technology, this invention provides a slow-release compound fertilizer based on soil testing and formulation of sorghum for saline-alkali land improvement. This fertilizer can improve fertilizer nutrient utilization efficiency and water retention performance, protect microbial activity, and achieve biodegradable coating material, chlorine-free and non-toxic properties, synergistic alkali reduction, and comprehensive improvement of saline-alkali land.

[0006] The objective of this invention can be achieved through the following technical solutions: Firstly, a slow-release compound fertilizer based on soil testing and formulation of sorghum for saline-alkali land improvement is provided, comprising: Core layer: composed of sorghum-specific compound fertilizer granules, wherein the N-P2O5-K2O mass ratio of the sorghum-specific compound fertilizer granules is 18~22:8~12:10~15; Intermediate coating layer: covering the outside of the core layer, the intermediate coating layer contains polyaspartic acid, sodium alginate, γ-polyglutamic acid, calcium gluconate, and salt-alkali resistant microbial agents encapsulated in chitosan-calcium alginate composite wall material microcapsules; Outer protective layer: covering the outside of the intermediate membrane layer, the outer protective layer contains a chitosan-calcium gluconate composite membrane.

[0007] Furthermore, the weight ratio of the intermediate cladding layer to the core layer is 1:5~10, and the weight ratio of the outer protective layer to the intermediate cladding layer is 1:3~5.

[0008] Further, by weight, the sorghum-specific compound fertilizer granules include: 18-30 parts urea, 10-20 parts diammonium phosphate, 14-22 parts potassium sulfate, 15-20 parts humic acid, 1-2 parts zinc sulfate, 0.5-1 parts EDTA iron sodium, 1-2 parts potassium silicate, and 3-41 parts bentonite.

[0009] Further, by weight, the raw materials for preparing the intermediate coating layer include 10-20 parts of polyaspartic acid, 5-10 parts of sodium alginate, 2-5 parts of γ-polyglutamic acid, and 1-3 parts of calcium gluconate.

[0010] Furthermore, the salt-tolerant microbial agent includes at least two of the following: halophilic bacteria, phosphate-solubilizing bacteria, and potassium-solubilizing bacteria.

[0011] Furthermore, the phosphate-solubilizing bacteria are Bacillus megaterium, the potassium-solubilizing bacteria are Paenibacillus mucilaginosus, and the halophilic bacteria are Halomonas sp.

[0012] Furthermore, the preparation method of the chitosan-calcium alginate composite wall material microcapsules is as follows: microbial agents are suspended in sodium alginate solution, calcium chloride solution is added dropwise to form calcium alginate microspheres, and then reacted with chitosan solution to form chitosan outer wall layer.

[0013] Secondly, a method for preparing a slow-release compound fertilizer based on soil testing and formulation of sorghum for saline-alkali land improvement, as described in the first aspect, is provided, comprising the following steps: (1) Mix urea, diammonium phosphate, potassium sulfate, humic acid, zinc sulfate, EDTA iron sodium, potassium silicate and bentonite in proportion and granulate. After granulation, the material is dried by hot air at 80~90℃ to a moisture content of 2%~3% to obtain sorghum-specific compound fertilizer granules. (2) Dissolve polyaspartic acid and sodium alginate in deionized water at 40-50℃, stir until completely transparent, cool to 25-30℃, add γ-polyglutamic acid, stir evenly to form a coating solution; add 3-8% of the microbial agent after being encapsulated in chitosan-calcium alginate composite wall material microcapsules, stir and disperse evenly, and coat the surface of the sorghum-specific compound fertilizer granules in step (1) by spraying, then spray calcium gluconate solution, and crosslink reaction at 40-50℃ for 15-30 minutes; (3) Dissolve chitosan in an aqueous solution of acetic acid with a volume concentration of 1-3%, add calcium gluconate, and coat the surface of the particles obtained in step (2) by fluidized bed spraying. After spraying, dry the particles under hot air at 40-50℃ until the moisture content is 3%-5%, and cool to room temperature to obtain a slow-release compound fertilizer.

[0014] Further, in step (2), the concentration of the calcium gluconate solution is 1~3wt%, and the weight ratio of the calcium gluconate solution to the coating solution is 1:5~1:10.

[0015] Further, the concentration of the chitosan solution in step (3) is 0.5~2wt%, and the amount of calcium gluconate added is 5~15% of the weight of chitosan.

[0016] The beneficial effects of this invention are as follows: (1) The core layer of this invention employs soil testing and formulation technology. Based on the nutrient status of the target saline-alkali soil and the fertilizer requirements of sorghum, the N-P2O5-K2O mass ratio is calculated to be 18~22:8~12:10~15. This ratio has the highest nitrogen content, meeting the nitrogen requirements of sorghum during its growth period; the phosphorus content is moderate, avoiding the waste caused by phosphorus being fixed by calcium and magnesium in saline-alkali soil; and the potassium content is relatively high, meeting the potassium requirements of sorghum. At the same time, potassium silicate, zinc sulfate, and EDTA iron sodium are added to the core layer to supplement elements such as silicon, zinc, and iron, promoting the growth and development of sorghum. In addition, the humic acid added to the core layer contains carboxyl groups, which can neutralize soil alkalinity and help reduce soil pH. The special compound fertilizer granules determined by soil testing and formulation achieve precise nutrient supply.

[0017] (2) The intermediate coating layer of this invention is an interpenetrating network hydrogel formed by cross-linking polyaspartic acid, sodium alginate, and γ-polyglutamic acid with calcium gluconate. Specifically, the Ca2+ released from calcium gluconate... 2+ It forms ionic crosslinks with the guluronic acid units on the sodium alginate molecular chain, while Ca 2+ It coordinates with the carboxyl groups on the polyaspartic acid molecular chain, and the side chain carboxyl groups on the γ-polyglutamic acid molecular chain also participate in Ca2+ coordination. 2+ Through coordination, the three components cross-link to form a three-dimensional interpenetrating network structure. This interpenetrating network has the following functions: Firstly, it has high water absorption and retention. The molecular chains of polyaspartic acid, sodium alginate, and γ-polyglutamic acid all contain hydrophilic groups such as carboxyl and hydroxyl groups. Upon contact with water, these groups bind to water molecules through hydrogen bonds, forming a three-dimensional interpenetrating network structure that traps free water within the network pores. During drought, the network slowly contracts, releasing water for sorghum to absorb, effectively alleviating drought stress in saline-alkali land.

[0018] Secondly, controlled nutrient release. Interpenetrating network structures facilitate the release of nutrient molecules (such as NH4+). + K + (etc.) have diffusion-blocking effects, which can slow down the diffusion rate of nitrogen, phosphorus and potassium, which is beneficial to matching the nutrient requirements of sorghum, reducing nutrient leaching loss and improving fertilizer utilization.

[0019] Third, it enhances salt resistance and water retention. The carboxyl group on the side chain of γ-polyglutamic acid can react with Na+ in salt solutions. + K + Complexation occurs, reducing the destructive effect of external salt ions on the network structure, allowing the hydrogel to maintain a high water absorption ratio in high-salt environments, thus enhancing the fertilizer's adaptability in saline-alkali land.

[0020] (3) In this invention, halophilic bacteria, phosphate-solubilizing bacteria, and potassium-solubilizing bacteria are pre-encapsulated in chitosan-calcium alginate composite wall material microcapsules, and then the microcapsules are dispersed in the intermediate coating layer. The microcapsule wall material has good mechanical strength and chemical stability, and can isolate the high temperature and mechanical extrusion during granulation, drying, and coating processes, thus protecting the internal microorganisms. After being applied to the soil, the microcapsules slowly swell upon contact with water, releasing microorganisms, activating soil nutrients, and improving the rhizosphere microenvironment.

[0021] (4) The outer protective layer of this invention is a chitosan-calcium gluconate composite membrane cross-linked with calcium gluconate. After being applied to the soil, the outer protective layer gradually degrades, and the calcium gluconate hydrolyzes to generate gluconic acid, which directly neutralizes the soil alkalinity and reduces the soil pH. The coating material of this invention is selected from bio-based polymers such as polyaspartic acid, sodium alginate, chitosan, and γ-polyglutamic acid. Microorganisms in the soil secrete specific enzymes (such as chitosanase and aspartic protease) to gradually degrade these polymers into small molecules such as aspartic acid, glutamic acid, and glucosamine. These small molecules can be directly absorbed and utilized by plants, or further decomposed into carbon dioxide and water in the soil, without causing microplastic residues and exhibiting good environmental compatibility. Detailed Implementation

[0022] To further illustrate the technical means and effects of the present invention in achieving the intended purpose, the following detailed description of the specific implementation methods, structures, features and effects of the present invention, in conjunction with preferred embodiments, is provided below.

[0023] Example 1 I. Soil testing and fertilizer recommendation A saline-alkali experimental field was selected in Xiaoxian County, Anhui Province. This area is saline-alkali soil formed by alluvial parent material from the Yellow River floodplain, commonly known as "alkaline soil." Soil samples were collected from the topsoil layer for testing. According to the NY / T 1121 series standards, the following were determined: organic matter 16.33 g / kg, available nitrogen 55.7 mg / kg, available phosphorus 20.5 mg / kg, available potassium 300.2 mg / kg, and the pH of the soil-water ratio 1:5 extract was 8.3, with an EC value of 10.5 dS / m.

[0024] According to the fertilizer requirements of sorghum, it needs to absorb 2.0-2.5 kg of N, 0.8-1.2 kg of P2O5, and 1.5-2.0 kg of K2O for every 100 kg of grain produced. Based on the soil test data mentioned above and in accordance with the "Technical Specification for Soil Testing and Fertilizer Recommendation", the N-P2O5-K2O ratio is determined to be 20:10:12 for the target yield.

[0025] II. Kernel Layer Preparation Weigh the following raw materials by weight: 25 parts urea, 16 parts diammonium phosphate, 17 parts potassium sulfate, 18 parts humic acid, 1.5 parts zinc sulfate, 0.8 parts sodium iron EDTA, 1.5 parts potassium silicate, and 20.2 parts bentonite.

[0026] The above raw materials are mixed evenly, and the mixture is fed into a drum granulator for granulation. After granulation, the material is dried with hot air at 85℃ until the moisture content is 2.5%, and then sieved to obtain sorghum-specific compound fertilizer granules. The weight ratio of the intermediate coating layer to the core layer is controlled at 1:7.5.

[0027] III. Preparation of Microencapsulated Microbial Agents Weigh 250 mL each of the fermentation broths of halophilic bacteria (Halomonas sp.), phosphate-solubilizing bacteria (Bacillus megaterium), and potassium-solubilizing bacteria (Paenibacillus mucilaginosus), mix thoroughly, centrifuge to collect the bacterial precipitate, wash with sterile physiological saline, and resuspend the precipitate in 500 mL of 2% sodium alginate solution, stirring until homogeneous to form a bacterial suspension. Add the bacterial suspension dropwise to a 2% calcium chloride solution to form calcium alginate microspheres, and allow to solidify. Filter and collect the microspheres, wash with deionized water, and transfer the microspheres to a 0.5% chitosan solution (chitosan dissolved in a 2% aqueous acetic acid solution), stirring to form a chitosan outer wall layer. Filter and dry to obtain chitosan-calcium alginate composite microcapsules.

[0028] IV. Preparation of the Intermediate Coating Layer Weigh the following raw materials by weight: 15 parts polyaspartic acid, 7.5 parts sodium alginate, 3.5 parts γ-polyglutamic acid, and 2 parts calcium gluconate.

[0029] Polyaspartic acid and sodium alginate were dissolved in deionized water at 45°C and stirred until completely transparent. The solution was then cooled to 28°C. γ-Polyglutamic acid was added and stirred until homogeneous to form a coating solution.

[0030] Add the microcapsule microbial agent prepared above at 5% of the volume of the encapsulation liquid and stir to disperse evenly.

[0031] The core layer particles were placed in a fluidized bed coating machine, and the coating solution was sprayed onto them. At the same time, a 2% calcium gluconate solution was sprayed on them for in-situ crosslinking. The weight ratio of calcium gluconate solution to coating solution was controlled at 1:7.5, the crosslinking temperature was 45℃, and the crosslinking time was 20 minutes to form a semi-permeable gel membrane.

[0032] V. Preparation of the outer protective layer Chitosan was dissolved in a 2% (v / v) aqueous solution of acetic acid, and calcium gluconate was added at a rate of 10% of the weight of chitosan.

[0033] The above solution was coated by fluidized bed spraying, with the weight ratio of the outer protective layer to the intermediate coating layer controlled at 1:4. After spraying, it was dried in hot air at 45°C until the moisture content was 3.5%, and then cooled to room temperature to obtain the slow-release compound fertilizer.

[0034] Example 2 I. Soil testing and fertilizer recommendation Soil samples were collected from a saline-alkali experimental field in Xiaoxian County, Anhui Province, for testing. According to the NY / T 1121 series standards, the following values ​​were determined: organic matter 16.33 g / kg, available nitrogen 55.7 mg / kg, available phosphorus 20.5 mg / kg, available potassium 300.2 mg / kg. The pH of the soil-water ratio 1:5 extract was 8.3, and the EC value was 10.5 dS / m.

[0035] According to the fertilizer requirements of sorghum, every 100 kg of grain produced requires the absorption of 2.0-2.5 kg of N, 0.8-1.2 kg of P2O5, and 1.5-2.0 kg of K2O. The calculated N-P2O5-K2O ratio is 18:8:10.

[0036] II. Kernel Layer Preparation Weigh the following raw materials by weight: 18 parts urea, 10 parts diammonium phosphate, 14 parts potassium sulfate, 15 parts humic acid, 1 part zinc sulfate, 0.5 parts sodium iron EDTA, 1 part potassium silicate, and 40.5 parts bentonite.

[0037] The above raw materials are mixed evenly, and the mixture is fed into a drum granulator for granulation. After granulation, the material is dried with hot air at 80℃ until the moisture content is 2%, and then sieved to obtain sorghum-specific compound fertilizer granules. The weight ratio of the intermediate coating layer to the core layer is controlled at 1:10.

[0038] III. Preparation of Microencapsulated Microbial Agents Weigh 250 mL each of the fermentation broths of halophilic bacteria (Halomonas sp.), phosphate-solubilizing bacteria (Bacillus megaterium), and potassium-solubilizing bacteria (Paenibacillus mucilaginosus), mix thoroughly, centrifuge to collect the bacterial precipitate, wash, and resuspend the precipitate in 500 mL of 2% sodium alginate solution, stirring until a bacterial suspension is formed. Add the bacterial suspension dropwise to a 2% calcium chloride solution to form calcium alginate microspheres, and allow to solidify. Filter and collect the microspheres, wash, and transfer to a 0.5% chitosan solution (chitosan dissolved in a 2% acetic acid aqueous solution), stirring to form a chitosan outer wall layer. Filter and dry to obtain chitosan-calcium alginate composite wall material microcapsules.

[0039] IV. Preparation of the Intermediate Coating Layer Weigh the following raw materials by weight: 10 parts polyaspartic acid, 5 parts sodium alginate, 2 parts γ-polyglutamic acid, and 1 part calcium gluconate.

[0040] Polyaspartic acid and sodium alginate were dissolved in deionized water at 40°C and stirred until completely transparent. The solution was then cooled to 25°C, and γ-polyglutamic acid was added and stirred until homogeneous to form a coating solution.

[0041] Add the above microcapsule microbial agent at 3% of the volume of the encapsulation liquid and stir to disperse evenly.

[0042] The core layer particles were placed in a fluidized bed coating machine, and the coating solution was sprayed onto them. At the same time, a 1% calcium gluconate solution was sprayed on them for in-situ crosslinking. The weight ratio of calcium gluconate solution to coating solution was controlled at 1:10, the crosslinking temperature was 40℃, and the crosslinking time was 16 minutes to form a semi-permeable gel membrane.

[0043] V. Preparation of the outer protective layer Chitosan was dissolved in a 1% (v / v) aqueous solution of acetic acid, and calcium gluconate was added at a rate of 5% of the weight of chitosan.

[0044] The above solution was coated by fluidized bed spraying, with the weight ratio of the outer protective layer to the intermediate coating layer controlled at 1:5. After spraying, it was dried in hot air at 40°C until the moisture content was 3%, and then cooled to room temperature to obtain the slow-release compound fertilizer.

[0045] Example 3 I. Soil testing and fertilizer recommendation Soil samples were collected from a saline-alkali experimental field in Xiaoxian County, Anhui Province, for testing. According to the NY / T 1121 series standards, the following values ​​were determined: organic matter 16.33 g / kg, available nitrogen 55.7 mg / kg, available phosphorus 20.5 mg / kg, available potassium 300.2 mg / kg, and the pH of the 1:5 soil-to-water extract was 8.3 with an EC value of 10.5 dS / m.

[0046] According to the fertilizer requirements of sorghum, every 100 kg of grain produced requires the absorption of 2.0-2.5 kg of N, 0.8-1.2 kg of P2O5, and 1.5-2.0 kg of K2O. The calculated N-P2O5-K2O ratio is 22:12:15.

[0047] II. Kernel Layer Preparation Weigh the following raw materials by weight: 30 parts urea, 20 parts diammonium phosphate, 22 parts potassium sulfate, 20 parts humic acid, 2 parts zinc sulfate, 1 part sodium iron EDTA, 2 parts potassium silicate, and 3 parts bentonite.

[0048] The above raw materials are mixed evenly, and the mixture is fed into a drum granulator for granulation. After granulation, the material is dried with hot air at 90℃ until the moisture content is 3%, and then sieved to obtain sorghum-specific compound fertilizer granules. The weight ratio of the intermediate coating layer to the core layer is controlled at 1:5.

[0049] III. Preparation of Microencapsulated Microbial Agents Weigh 250 mL each of the fermentation broths of halophilic bacteria (Halomonas sp.), phosphate-solubilizing bacteria (Bacillus megaterium), and potassium-solubilizing bacteria (Paenibacillus mucilaginosus), mix thoroughly, centrifuge to collect the bacterial precipitate, wash, and resuspend in 500 mL of 2% sodium alginate solution, stirring until homogeneous to form a bacterial suspension. Add the bacterial suspension dropwise to a 2% calcium chloride solution to form calcium alginate microspheres, and allow to solidify. Filter and collect the microspheres, wash, and transfer to a 0.5% chitosan solution (chitosan dissolved in a 2% aqueous acetic acid solution), stirring to form a chitosan outer wall layer. Filter and dry to obtain chitosan-calcium alginate composite wall material microcapsules.

[0050] IV. Preparation of the Intermediate Coating Layer Weigh the following raw materials by weight: 20 parts polyaspartic acid, 10 parts sodium alginate, 5 parts γ-polyglutamic acid, and 3 parts calcium gluconate.

[0051] Polyaspartic acid and sodium alginate were dissolved in deionized water at 50°C and stirred until completely transparent. The solution was then cooled to 30°C. γ-Polyglutamic acid was added and stirred until homogeneous to form a coating solution. The above microcapsule microbial agent was added at 8% of the volume of the coating solution and stirred until evenly dispersed.

[0052] The core layer particles were placed in a fluidized bed coating machine, and the coating solution was sprayed onto them. At the same time, a 3% calcium gluconate solution was sprayed on them for in-situ crosslinking. The weight ratio of calcium gluconate solution to coating solution was controlled at 1:5, the crosslinking temperature was 50℃, and the crosslinking time was 30 minutes to form a semi-permeable gel membrane.

[0053] V. Preparation of the outer protective layer Chitosan was dissolved in a 3% (v / v) aqueous solution of acetic acid, and calcium gluconate was added at a rate of 15% of the weight of chitosan.

[0054] The above solution was coated by fluidized bed spraying, with the weight ratio of the outer protective layer to the intermediate coating layer controlled at 1:3. After spraying, it was dried under hot air at 50°C until the moisture content was 5%, and then cooled to room temperature to obtain the slow-release compound fertilizer.

[0055] Comparative Example 1 Based on Example 1, without an intermediate membrane layer, after granulation of the core layer, chitosan-calcium gluconate solution is directly sprayed to form an outer protective layer, and other conditions are the same as in Example 1.

[0056] Comparative Example 2 Based on Example 1, γ-polyglutamic acid was not added to the intermediate membrane layer, and other conditions were the same as in Example 1.

[0057] Comparative Example 3 Based on Example 1, the microbial agent in the intermediate coating layer is not encapsulated in microcapsules, but is directly dispersed in the coating solution in the form of bacterial powder, and other conditions are the same as in Example 1.

[0058] Comparative Example 4 Based on Example 1, calcium chloride was used to replace calcium gluconate in the intermediate coating layer crosslinking agent and the outer protective layer, while other conditions remained the same as in Example 1.

[0059] Comparative Example 5 Based on Example 1, an equal amount of polyacrylic acid was used to replace polyaspartic acid in the intermediate coating layer, while other conditions remained the same as in Example 1.

[0060] Comparative Example 6 Based on Example 1, no humic acid was added to the kernel layer, and other conditions were the same as in Example 1.

[0061] Comparative Example 7 Based on Example 1, there is no chitosan outer protective layer, only the core layer and intermediate membrane layer, and other conditions are the same as in Example 1.

[0062] Effect verification The common compound fertilizer used in this experiment was a commercially available potassium sulfate compound fertilizer (N-P2O5-K2O=15-15-15), with urea, diammonium phosphate, and potassium sulfate as raw materials. It was prepared using a drum granulation process without a coating layer, water-retaining agent, or microbial agent.

[0063] The blank control group was the treatment that did not receive any fertilizer.

[0064] I. Soil Moisture Retention Rate Referring to the water retention rate determination principle in NY / T 886-2022 Agricultural and Forestry Water Retention Agents, 2g of fertilizer sample was weighed and placed in a 100-mesh nylon mesh bag, then immersed in deionized water until fully saturated. After draining the surface moisture, it was thoroughly mixed with 200g of air-dried saline-alkali soil (soil-to-water ratio 1:5, extract EC=10.5 dS / m, pH=8.3), placed in a plastic cup, and weighed, recorded as G0. The plastic cup was placed in a constant temperature and humidity chamber at 25℃ and 60% relative humidity, and weighed after 5, 10, and 15 days, recorded as G. t .

[0065] Soil moisture retention rate (%) = (G t / G0) × 100% In the formula: G0 is the initial weight (soil + fertilizer + cup weight), G t The weight after t days (soil + fertilizer + cup weight).

[0066] The test results are shown in Table 1. Table 1. Soil moisture retention rate test results for each embodiment and comparative example.

[0067] The 15-day water retention rates of Examples 1-3 were 24.6%, 23.8%, and 25.3%, respectively, all significantly higher than the blank control and ordinary compound fertilizer. A higher water retention rate indicates better drought resistance and water retention performance of the fertilizer. Comparative Example 1 shows that the intermediate coating layer is a key structure for water retention; Comparative Example 2 shows that γ-polyglutamic acid can improve the water retention rate; and Comparative Example 5 demonstrates that polyaspartic acid has better water retention performance than polyacrylic acid.

[0068] II. Nutrient Release Curve Refer to "HG / T 4216-2011 Rapid Detection Method for Nutrient Release Period and Release Rate of Slow-Release / Controlled-Release Fertilizers". Weigh 10g of fertilizer sample, place it in a 100-mesh nylon mesh bag, put it in an Erlenmeyer flask, add 200mL of 25℃ deionized water, seal, and place in a 25℃ constant temperature incubator. Take samples at 1, 3, 7, 14, 28, and 60 days, and take 10mL of the release solution. Determine the nitrogen content using the Kjeldahl method. Replace all deionized water after each sampling.

[0069] Cumulative release rate (%) = (Cumulative nitrogen released / Total nitrogen in fertilizer) × 100% The test results are shown in Table 2. Table 2. Test results of cumulative nutrient release rate for each embodiment and comparative example.

[0070] The ideal release curve of slow-release fertilizer is "S"-shaped. A low release rate in the early stage avoids burning seedlings, a moderate release rate in the middle stage meets crop needs, and continuous release in the later stage prevents nutrient deficiency. The release curves of Examples 1-3 all exhibit a typical "S" shape. The release rates in the early stage (1 day) were 8.2%, 9.5%, and 7.1%, effectively preventing seedling burn; the release rates in the middle stage (7 days) were 28.5%, 32.0%, and 25.8%, meeting the needs of sorghum seedlings to jointing stages; and the release rates in the later stage (60 days) were 88.5%, 92.0%, and 85.0%, with some nutrients still being released. The lower the release rate and the longer the release period, the better the slow-release effect. Comparative Example 1, without an intermediate coating layer, had a release rate of 28.3% in 1 day and 78.2% in 7 days, indicating excessively rapid release; the release rate of Comparative Example 5 was slightly faster than that of Example 1, demonstrating that the slow-release performance of polyaspartic acid is superior to that of polyacrylic acid.

[0071] III. Fertilizer Utilization Rate Measurement Nitrogen use efficiency was determined using the difference method, with a no-fertilizer control area and a fertilized treatment area, each area replicated three times. Sorghum was harvested 90 days after planting, and the total nitrogen content of the aboveground parts of the plants in both the fertilized and no-fertilizer areas was measured. Total nitrogen determination followed GB / T17767.1-2008 Determination of Organic-Inorganic Compound Fertilizers Part 1: Total Nitrogen Content, and plant sample digestion followed NY / T 2017-2011 Determination of Nitrogen, Phosphorus, and Potassium in Plants.

[0072] Nitrogen use efficiency (%) = [(Nitrogen uptake by plants in fertilized areas - Nitrogen uptake by plants in unfertilized areas) / Nitrogen application rate] × 100% The determination of total phosphorus in plants was carried out in accordance with GB / T 17767.2-2010 Determination of Organic-Inorganic Compound Fertilizers Part 2: Total Phosphorus Content, using the molybdenum antimony colorimetric method.

[0073] Phosphorus utilization rate (%) = [(Phosphorus uptake by plants in fertilized areas - Phosphorus uptake by plants in unfertilized areas) / Phosphorus application rate] × 100% The total potassium content of the plant was determined according to the method specified in GB / T 17767.3-2010 "Determination of Organic-Inorganic Compound Fertilizers - Part 3: Total Potassium Content", using the flame photometry method.

[0074] Potassium utilization rate (%) = [(Kernel uptake by plants in fertilized areas - Potassium uptake by plants in unfertilized areas) / Potassium application rate] × 100% The test results are shown in Table 3. Table 3. Fertilizer utilization rate test results for each embodiment and comparative example.

[0075] Higher fertilizer utilization rate indicates a higher proportion of nutrients absorbed by crops and less waste. In Examples 1-3, nitrogen utilization rates were 55.8%, 52.1%, and 58.2%, respectively, representing increases of 23.3, 19.6, and 25.7 percentage points compared to ordinary compound fertilizers; phosphorus utilization rates were 30.5%, 28.2%, and 32.1%, respectively, increases of 15.3, 13.0, and 16.9 percentage points compared to ordinary compound fertilizers; and potassium utilization rates were 60.2%, 57.5%, and 62.8%, respectively, increases of 21.7, 19.0, and 24.3 percentage points compared to ordinary compound fertilizers. Comparative Example 1 demonstrates that the intermediate coating layer can improve nitrogen utilization rate. Comparative Example 2 demonstrates that γ-polyglutamic acid can improve nitrogen utilization rate.

[0076] IV. Soil Improvement Effects After the sorghum harvest, topsoil samples were collected from each treatment plot, air-dried, sieved, and the following indicators were measured: The pH value was determined according to "NY / T 1377-2007 Determination of Soil pH", using a soil-to-water ratio of 1:2.5 for extraction and pH measurement.

[0077] EC values ​​were determined according to the "HJ 802-2016 Determination of Soil Electrical Conductivity - Electrode Method", using a soil-to-water ratio of 1:5 extraction and electrical conductivity meter measurement. The results are expressed as dS / m.

[0078] Organic matter was determined according to "NY / T 1121.6-2006 Soil Testing Part 6: Determination of Soil Organic Matter", using the potassium dichromate oxidation-ferrous sulfate titration method.

[0079] Alkaline hydrolysis nitrogen was determined according to the standard LY / T 1228-2015 "Determination of Nitrogen in Forest Soils" using the alkaline hydrolysis-diffusion method.

[0080] Available phosphorus was determined according to NY / T 1121.7-2014 Soil Testing Part 7: Determination of Available Phosphorus in Soil, using the sodium bicarbonate extraction-molybdenum antimony colorimetric method.

[0081] Available potassium was determined according to the standard NY / T 889-2004 "Determination of Available and Slow-release Potassium Content in Soil" using the ammonium acetate extraction-flame photometric method.

[0082] The test results are shown in Table 4. Table 4. Soil improvement effect tests of each embodiment and comparative example.

[0083] Evaluation criteria for soil amendment effects: the closer the pH value is to neutral (7.0), the better; the lower the EC value (lower salinity), the better; and the higher the content of organic matter and available nutrients, the better. In Examples 1-3, the pH values ​​decreased to 7.2, 7.4, and 7.1, respectively, with decreases of 1.1, 0.9, and 1.2; the EC values ​​decreased to 0.62, 0.70, and 0.58 dS / m, respectively, with decreases of 0.76, 0.68, and 0.80 dS / m; and the organic matter content increased to 18.72, 18.35, and 18.95 g / kg, respectively, with increases of 2.39, 2.02, and 2.62 g / kg. Comparative Example 2 demonstrates that γ-PGA can enhance the alkalinity reduction effect; Comparative Example 6 demonstrates that humic acid makes an important contribution to alkalinity reduction; and Comparative Example 7 demonstrates that the outer protective layer can enhance the alkalinity reduction effect.

[0084] V. Sorghum Growth and Yield Fifteen days after sowing, the number of seedlings in each treatment plot was counted. The seedling emergence rate (%) = (number of seedlings / number of seeds) × 100%.

[0085] Before harvest, 10 plants are randomly selected from each plot, and the height is measured from the base of the stem to the top of the ear. The average value is taken as the plant height.

[0086] 1000 seeds were randomly selected from each plot and weighed. The weight was repeated three times and the average value was taken as the weight of 1000 seeds.

[0087] Grain yield was measured according to the "DB1411 / T 65-2024 Field Yield Measurement Operation Procedures for Sorghum in Dryland Areas". After harvest, each plot was threshed, dried, and weighed separately, and the yield was calculated per acre.

[0088] Yield increase rate (%) = [(Treatment group yield - Blank control yield) / Blank control yield] × 100% The test results are shown in Table 5. Table 5. Test results on sorghum growth and yield of each embodiment and comparative example.

[0089] Evaluation criteria for sorghum growth and yield: a higher emergence rate indicates better fertilizer safety; higher plant height indicates sufficient nutrient supply; a higher thousand-grain weight indicates better grain plumpness; and a higher yield indicates better overall effect. The emergence rates in Examples 1-3 were 82.5%, 79.5%, and 84.2%, respectively, representing increases of 30.0, 27.0, and 31.7 percentage points compared to the blank control; the grain yields were 412, 398, and 425 kg / mu, respectively, representing increases of 44.6%, 39.6%, and 49.1% compared to the blank control. Comparative Example 4 demonstrates that calcium gluconate avoids chlorine toxicity and increases the emergence rate; Comparative Example 5 demonstrates that polyaspartic acid increases yield compared to polyacrylic acid; Comparative Example 6 demonstrates that humic acid increases yield; and Comparative Example 7 demonstrates that the outer protective layer increases yield.

[0090] VI. Survival rate of microbial inoculants The determination of viable bacteria content after coating was performed according to the method for determining the effective viable bacteria count in GB 20287-2006 Agricultural Microbial Agents. 1g of the coated fertilizer sample was weighed and placed in 9mL of sterile physiological saline. The microcapsules were shaken to rupture and release the microorganisms. The supernatant was serially diluted 10-fold, and 100μL of each dilution was spread onto different culture media: Bacillus megaterium was spread on nutrient agar; Bacillus spp. gelatinosa was spread on nitrogen-free agar; and Halomonas halophila was spread on TSA medium containing 4% NaCl. The cultures were incubated at 28-30℃ for 3-5 days, and the colony count was determined.

[0091] Viable bacteria content (CFU / g) = (colony count × dilution factor) / sample mass Survival rate (%) = (Viable bacteria content after coating / Viable bacteria content of the pre-coating agent) × 100% The viable bacteria content of the pre-coating bacterial agent was 2.5 × 10⁻⁶. 8 CFU / g.

[0092] Rhizosphere viable bacteria count determination 30 days after soil application: The sorghum plants were dug up along with their roots, the loose soil was shaken off, and the rhizosphere soil close to the root surface was collected. 1 g of rhizosphere soil was weighed, placed in 9 mL of sterile physiological saline, shaken, serially diluted, and then spread for culture. The number of colonies was counted and converted to the number of viable bacteria per gram of rhizosphere soil (CFU / g soil).

[0093] The test results are shown in Table 6. Table 6. Survival rate test results of microbial agents in each example and comparative example.

[0094] A higher survival rate of microbial inoculants indicates a better protective effect on microorganisms during fertilizer preparation; a higher number of viable rhizosphere bacteria indicates a stronger colonization ability of microorganisms in the soil. The viable bacteria content after coating in Examples 1-3 was 1.8 × 10⁻⁶. 8 1.7×108 1.9×10 8 The CFU / g survival rates were 72.0%, 68.0%, and 76.0%, respectively. Comparative Example 3 had a survival rate of only 2.5%, demonstrating that microencapsulation improves survival. Comparative Example 5 had a survival rate of 48.0%, demonstrating that polyaspartic acid is more beneficial to microbial survival than polyacrylic acid. Comparative Example 2 had a survival rate of 64.0%, slightly lower than Example 1, demonstrating that γ-PGA also has a certain promoting effect on microbial survival. After 30 days of application to the soil, the rhizosphere viable bacteria counts for Examples 1-3 were 2.5 × 10⁻⁶, respectively. 6 2.3×10 6 2.7×10 6 CFU / g soil, Comparative Example 3 was undetectable (<1.0×10⁻⁶). 3 (CFU / g) demonstrates that microcapsule encapsulation can achieve continuous colonization of microorganisms.

[0095] VII. Degradation performance of coating materials Referring to GB / T 19276.1-2003 "Determination of the Final Aerobic Biodegradation Capacity of Materials in Aqueous Culture Media by Determination of Carbon Dioxide Release", 10g of the coating material sample was weighed and buried in the soil at a depth of 10cm, with a soil moisture content of 60% and a temperature of 25℃. The sample was removed after 30 days and 60 days, washed, dried, and weighed. Residual mass percentage (%) = (mass of degraded sample / initial sample mass) × 100% The results are shown in Table 7. Table 7. Degradation performance results of the coating materials in each embodiment and comparative example.

[0096] A lower residual mass indicates better degradation performance of the material in soil. The residual mass rates of the polyaspartic acid coating materials in Examples 1-3 after 60 days were 15.0%, 18.5%, and 12.0%, respectively, all significantly lower than that of the polyacrylic acid material in Comparative Example 5, which had a residual mass rate as high as 82%. This demonstrates that the polyaspartic acid used in this invention has excellent biodegradability and will not cause microplastic pollution in the soil.

[0097] The controlled-release compound fertilizer of this invention has a wide range of raw material sources and a mature preparation process, and can be produced on a large scale using existing compound fertilizer production lines. The product is suitable for sorghum planting in saline-alkali areas of northern my country (including northern Anhui, Shandong, Hebei, Henan and other Yellow River flood plains), and has significant water retention, slow release, soil improvement, and yield-increasing effects, resulting in good economic and social benefits.

[0098] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A slow-release compound fertilizer formulated by soil testing of sorghum for saline-alkali land improvement, characterized in that, include: Core layer: composed of sorghum-specific compound fertilizer granules, wherein the N-P2O5-K2O mass ratio of the sorghum-specific compound fertilizer granules is 18~22:8~12:10~15; Intermediate coating layer: covering the outside of the core layer, the intermediate coating layer contains polyaspartic acid, sodium alginate, γ-polyglutamic acid, calcium gluconate, and salt-alkali resistant microbial agents encapsulated in chitosan-calcium alginate composite wall material microcapsules; Outer protective layer: covering the outside of the intermediate coating layer, the outer protective layer contains a chitosan-calcium gluconate composite membrane.

2. The sorghum-based soil testing and controlled-release compound fertilizer for saline-alkali land improvement according to claim 1, characterized in that, The weight ratio of the intermediate cladding layer to the core layer is 1:5~10, and the weight ratio of the outer protective layer to the intermediate cladding layer is 1:3~5.

3. The sorghum soil testing and formulation controlled-release compound fertilizer based on saline-alkali land improvement according to claim 1, characterized in that, By weight, the sorghum-specific compound fertilizer granules include: 18-30 parts urea, 10-20 parts diammonium phosphate, 14-22 parts potassium sulfate, 15-20 parts humic acid, 1-2 parts zinc sulfate, 0.5-1 parts EDTA iron sodium, 1-2 parts potassium silicate, and 3-41 parts bentonite.

4. The sorghum soil testing and formulation controlled-release compound fertilizer based on saline-alkali land improvement according to claim 1, characterized in that, The raw materials for preparing the intermediate coating layer, by weight, include 10-20 parts of polyaspartic acid, 5-10 parts of sodium alginate, 2-5 parts of γ-polyglutamic acid, and 1-3 parts of calcium gluconate.

5. The sorghum soil testing and formulation controlled-release compound fertilizer based on saline-alkali land improvement according to claim 1, characterized in that, The salt-tolerant microbial agent includes at least two of the following: halophilic bacteria, phosphate-solubilizing bacteria, and potassium-solubilizing bacteria.

6. The sorghum soil testing and formulation controlled-release compound fertilizer based on saline-alkali land improvement according to claim 5, characterized in that, The phosphate-solubilizing bacteria are Bacillus megaterium, the potassium-solubilizing bacteria are Paenibacillus mucilaginosus, and the halophilic bacteria are Halomonas sp.

7. The sorghum-based soil testing and controlled-release compound fertilizer for saline-alkali land improvement according to claim 1, characterized in that, The preparation method of the chitosan-calcium alginate composite wall material microcapsules is as follows: microbial agents are suspended in sodium alginate solution, calcium chloride solution is added dropwise to form calcium alginate microspheres, and then reacted with chitosan solution to form chitosan outer wall layer.

8. A method for preparing a slow-release compound fertilizer based on soil testing and formulation of sorghum for saline-alkali land improvement, as described in any one of claims 1 to 7, characterized in that, Includes the following steps: (1) Mix urea, diammonium phosphate, potassium sulfate, humic acid, zinc sulfate, EDTA iron sodium, potassium silicate and bentonite in proportion and granulate. After granulation, the material is dried by hot air at 80~90℃ to a moisture content of 2%~3% to obtain sorghum-specific compound fertilizer granules. (2) Dissolve polyaspartic acid and sodium alginate in deionized water at 40~50℃, stir until completely transparent, cool to 25~30℃, add γ-polyglutamic acid, stir evenly to form a coating solution; add 3-8% of the microbial agent after being encapsulated in chitosan-calcium alginate composite wall material microcapsules, stir and disperse evenly, and coat the surface of the sorghum-specific compound fertilizer granules in step (1) by spraying, then spray calcium gluconate solution, and crosslink reaction at 40~50℃ for 15~30 minutes; (3) Dissolve chitosan in an aqueous solution of acetic acid with a volume concentration of 1-3%, add calcium gluconate, and coat the surface of the particles obtained in step (2) by fluidized bed spraying. After spraying, dry the particles under hot air at 40-50℃ until the moisture content is 3%-5%, and cool to room temperature to obtain a slow-release compound fertilizer.

9. The method for preparing sorghum soil testing and formula controlled-release compound fertilizer based on saline-alkali land improvement according to claim 8, characterized in that, In step (2), the concentration of the calcium gluconate solution is 1~3wt%, and the weight ratio of the calcium gluconate solution to the coating solution is 1:5~1:

10.

10. The method for preparing sorghum soil testing and formula controlled-release compound fertilizer based on saline-alkali land improvement according to claim 8, characterized in that, The concentration of the chitosan solution in step (3) is 0.5~2wt%, and the amount of calcium gluconate added is 5~15% of the weight of chitosan.

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