Slow-release compound fertilizer as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of fertilizers, and particularly relates to a slow-release compound fertilizer as well as a preparation method and application thereof. The slow-release compound fertilizer comprises the following raw materials: a fertilizer core: urea, monoammonium phosphate, potassium sulfate, zinc sulfate, modified sepiolite, sodium humate and sargassum fusiforme seaweed fine powder; the coating liquid is prepared from carboxymethyl chitosan, polyglutamic acid, nano-hydroxyapatite, tributyl citrate and water; the effective viable count of the compound microbial agent is greater than or equal to 2.0 * 10 < 9 > cfu / g. The fertilizer disclosed by the invention forms a gradient structure of'slow release core-multifunctional coating-active bacterium layer '. The structure ensures that nutrients are released according to needs, moisture is efficiently kept, and microorganisms are stored with high activity.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of fertilizers, and particularly relates to a slow-release compound fertilizer as well as a preparation method and application thereof. BACKGROUND

[0002] A compound fertilizer is a chemical fertilizer containing two or more than two nutrient elements such as nitrogen, phosphorus and potassium, and has the advantages of high nutrient content, few by-products and good physical properties. The main raw materials thereof include ammonium sulfate, ammonium nitrate, ammonium carbonate, ammonium chloride, urea, potassium sulfate, potassium chloride, ammonium phosphate and the like, and the nutrients are in the form of particles, which is convenient for storage and application, and is particularly suitable for mechanized fertilization. Traditional quick-acting fertilizers often have problems such as large nutrient loss and low utilization rate, while slow-release long-acting compound fertilizers can better solve these problems. The mechanism is to control the degradation rate of the coating material or the breaking speed of the chemical bond to regulate the release of nutrients, so that the fertilizer can continuously and stably supply the nutrients such as nitrogen, phosphorus and potassium required by crop growth, and realize slow release and continuous supply of nutrients. The advantage of this kind of fertilizer is that it can prolong the fertilizer effect period, reduce the fertilization frequency, improve the fertilizer utilization rate, and at the same time reduce the risk of environmental pollution.

[0003] At present, slow-release fertilizers are divided into two types: one is to block the contact between the fertilizer and the outside world by coating, thereby achieving the effect of slow release; and the other is to add urease inhibitors or nitrification inhibitors to inhibit the activity of enzymes and thereby achieve the effect of slow release. However, the use of coating forms is affected by the coating material, and some coating materials are difficult to degrade and pollute the soil; and the use of inhibitors belongs to drugs, such as dicyandiamide, which not only inhibits nitrite bacteria, but also affects other bacteria and destroys the microecological balance.

[0004] At present, slow-release fertilizers, water-retaining fertilizers and microbial fertilizers often have single or simple mixed functions, and have the following problems: (1) the high-salt environment in the fertilizer is easy to cause the inactivation of microorganisms; (2) different functional components inhibit each other, and it is difficult to achieve synergistic effect; (3) conventional coating materials (such as chitosan) have slow-release property, but have inhibitory effect on beneficial microorganisms.

[0005] Therefore, it is necessary to develop a compound fertilizer with slow-release, water-retaining and microbial activity protection. SUMMARY

[0006] In order to overcome the deficiencies of the prior art, the purpose of the present application is to provide a slow-release compound fertilizer as well as a preparation method and application thereof. The slow-release compound fertilizer introduces specific functional materials to achieve the multiple goals of efficient slow-release of nutrients, protection of microbial activity and water retention, and solve the antagonism problem between functional components.

[0007] The present application is realized by the following technical solutions: A slow-release compound fertilizer, wherein the raw material composition of the slow-release compound fertilizer includes: The core fertilizer ingredients are: urea, monoammonium phosphate, potassium sulfate, zinc sulfate, modified sepiolite, sodium humate, and Sargassum seaweed extract. Coating solution: carboxymethyl chitosan, polyglutamic acid, nano-hydroxyapatite, tributyl citrate, and water; Compound microbial inoculant: effective viable count ≥ 2.0 × 10⁻⁶ 9 cfu / g.

[0008] Preferably, the raw material composition of the slow-release compound fertilizer, by weight, includes: Fertilizer core: 20-30 parts urea, 15-25 parts monoammonium phosphate, 10-20 parts potassium sulfate, 1-2 parts zinc sulfate, 5-10 parts modified sepiolite, 3-6 parts sodium humate, and 1-5 parts Sargassum seaweed extract powder. Coating solution: 4-8 parts carboxymethyl chitosan, 2-5 parts polyglutamic acid, 1-3 parts nano hydroxyapatite, 1-2 parts tributyl citrate and 80-100 parts water; 2-4 parts of compound microbial inoculant.

[0009] Carboxymethyl chitosan (CMCS) is a film-forming matrix that is naturally biodegradable and biocompatible. It is also rich in nitrogen, and after degradation, it can be absorbed by crops as nutrients without secondary pollution.

[0010] Polyglutamic acid (γ-PGA) has a strong ability to absorb and retain water, which can significantly improve the soil environment around fertilizer particles and create favorable conditions for seed germination and root growth.

[0011] Nano-hydroxyapatite (n-HAP) allows both the fertilizer encapsulated within the coating and the coating itself to contain nutrients (N and P). Furthermore, the nanoparticles can fill the gaps in the polymer chains, improving the membrane's density and mechanical strength, thus enabling more precise control over the nutrient release rate. The solubility of hydroxyapatite increases in slightly acidic soil environments, giving the coating a potential pH-responsive release characteristic. This means that when roots secrete more acidic substances, the membrane dissolves more rapidly, releasing more nutrients to meet crop needs.

[0012] Tributyl citrate, as a plasticizer, improves the brittleness of carboxymethyl chitosan after film formation, making it flexible and less prone to breakage, thus ensuring the stability of the coating quality.

[0013] Preferably, the modified sepiolite is prepared by activating sepiolite with 0.5-1.0 mol / L citric acid solution at 50-70°C for 1-3 hours.

[0014] Preferably, the compound microbial agent is a mixture of Bacillus mucilaginosus and Bacillus megaterium, wherein the mass ratio of Bacillus mucilaginosus to Bacillus megaterium is 1:1-2.

[0015] Preferably, in the coating solution, the mass ratio of carboxymethyl chitosan to polyglutamic acid is 1.5-2:1.

[0016] This invention also relates to a method for preparing the above-mentioned slow-release compound fertilizer, comprising the following steps: (1) Crush, mix, and granulate each raw material in the core fertilizer component to obtain core fertilizer particles; (2) Dissolve carboxymethyl chitosan and polyglutamic acid in water, stir, add nano-hydroxyapatite and tributyl citrate, emulsify, and obtain coating solution; (3) The coating liquid is sprayed onto the surface of the preheated fertilizer core particles to form a coating layer, thus obtaining coated particles; (4) Spray the composite microbial agent suspension onto the surface of the coated particles obtained in step (3); (5) The granules obtained in step (4) are dried and sieved to obtain slow-release compound fertilizer.

[0017] Preferably, in step (1), the raw material is crushed to pass through an 80-100 mesh sieve, and the granulation includes: feeding the uniformly mixed powder into a dry roller press granulator and adjusting the roller press pressure to 4-6 MPa; the core fertilizer particles have a particle size of 2-3 mm and a particle strength ≥25 N.

[0018] Preferably, the stirring temperature in step (2) is 50-60℃, the stirring time is 1.5-2.5h, and the stirring speed is 200-300rpm; the emulsification temperature is 35-40℃, the emulsification speed is 5000-6000rpm, and the emulsification time is 20-30min.

[0019] Preferably, in step (3), the temperature of the fertilizer core particles is maintained at 40-45℃ during spraying, and the spraying continues until the coating weight gain reaches 5%-8% of the core particle mass; the preparation method of the composite microbial agent solution in step (4) includes: adding the composite microbial agent to 40-60 parts of sterile physiological saline, stirring at 100-150 rpm for 20-30 minutes at 25-30℃, re-dissolving and activating, and preparing a suspension; the temperature of the coated particles is maintained at 35-38℃ during spraying of the agent.

[0020] This invention also relates to the application of the above-mentioned slow-release compound fertilizer in promoting crop growth.

[0021] This invention employs a composite coating system of carboxymethyl chitosan (CMCS), polyglutamic acid (γ-PGA), and nano-hydroxyapatite (n-HAP). Polyglutamic acid acts as a water-retaining agent and complexing agent, and its numerous carboxyl groups can chelate calcium and phosphate ions released from n-HAP, as well as the nitrogen source (ammonium ions NH4+) generated after CMCS degradation. + Furthermore, it can effectively neutralize the remaining positive charge of carboxymethyl chitosan, completely eliminating the risk of inhibiting beneficial microorganisms. On the one hand, it slows down the release rate of nutrients and prolongs the fertilizer effect; on the other hand, it prevents nutrients such as phosphorus and ammonium from being fixed and ineffective in the soil, greatly improving fertilizer utilization.

[0022] In addition, CMCS and γ-PGA can pass through calcium ions (Ca) in solution. 2+ (Bridging from the slow dissolution of n-HAP) forms a denser and more stable polyelectrolyte complex membrane, which has far superior mechanical strength and water resistance compared to membranes formed from single components, and can more precisely control nutrient release.

[0023] Compared with the prior art, the present invention has the following beneficial effects: (1) In this invention, modified sepiolite, sodium humate, and Sargassum seaweed extract are used as the core of the fertilizer, which have a synergistic effect with the carboxymethyl chitosan-polyglutamic acid-nanohydroxyapatite coating system and specific microbial agents. The modified sepiolite, sodium humate, and Sargassum seaweed extract are combined to construct the initial slow-release structure of the fertilizer core and provide abundant adsorption sites and organic matter.

[0024] (2) The “carboxymethyl chitosan-polyglutamic acid-nanohydroxyapatite coating system” in this invention has a significant synergistic effect among the three components. The combination of the three components not only enables the degradation of the coating and the release of nutrients to respond to the rhizosphere microenvironment, but also activates nutrients, reduces fixation, retains water and resists drought, and ultimately achieves the goal of reducing fertilizer use and increasing efficiency and improving soil.

[0025] (3) The fertilizer in this invention forms a gradient structure of “slow-release core - multifunctional coating - active bacterial layer”. This structure ensures that nutrients are released on demand, water is efficiently retained, and microorganisms are stored with high activity. Detailed Implementation

[0026] The technical solutions in the embodiments of the present invention will be further described in detail below. The described embodiments are only a part of the present invention and are used to explain the present invention, but are not intended to limit the present invention. Therefore, other embodiments obtained by other people skilled in the art without creative labor are all within the protection scope of the present invention.

[0027] Sargassum seaweed extract was provided by Zhejiang Shiyuan Jinniu Biotechnology Co., Ltd.; nano-hydroxyapatite (200nm) and polyglutamic acid (item number jhhg03) were purchased from Xi'an Jinheng Chemical Co., Ltd. All other raw materials and reagents were obtained commercially.

[0028] Example 1 A slow-release compound fertilizer is made from the following raw materials in parts by weight: Fertilizer core: 25 parts urea, 20 parts monoammonium phosphate, 15 parts potassium sulfate, 1.5 parts zinc sulfate, 8 parts modified sepiolite, 4 parts sodium humate, and 3 parts Sargassum seaweed extract powder; Coating solution: 6 parts carboxymethyl chitosan, 3 parts polyglutamic acid, 2 parts nano-hydroxyapatite, 1.5 parts tributyl citrate, and 90 parts water; Microbial inoculant: 3 parts of a compound inoculant of Bacillus mucilaginosus and Bacillus megaterium (live count (cfu) ≥ 2.0 × 10⁻⁶) 9 / g), the mass ratio of Bacillus mucilaginosus to Bacillus megaterium is 1:1; The modified sepiolite is prepared by activating sepiolite with 0.8 mol / L citric acid solution at 60℃ for 2 hours.

[0029] The preparation method of the above-mentioned slow-release compound fertilizer is as follows: (1) Preparation of fertilizer core particles: Urea, monoammonium phosphate, potassium sulfate, zinc sulfate, modified sepiolite, sodium humate, and Sargassum seaweed powder were pulverized separately and passed through a 100-mesh sieve. They were then mixed at 25 rpm for 30 minutes. The uniformly mixed powder was fed into a dry roller press granulator, and the roller pressure was adjusted to 5 MPa to prepare fertilizer core granules with a particle size of 2.5 mm and a particle strength of 26 N.

[0030] (2) Preparation of coating solution: Water was added to a stirred reactor, and stirring was started (250 rpm). Carboxymethyl chitosan and polyglutamic acid (γ-PGA) were added. After the addition was complete, the temperature was raised to 55°C, and stirring was continued for 2 hours to induce swelling and obtain a homogeneous colloid. The reactor temperature was then lowered to 38°C, and nano-hydroxyapatite and tributyl citrate were added. The stirring speed was increased to 900 rpm, and a high-speed shear emulsifier was started. Emulsification was carried out at 5500 rpm for 25 minutes to obtain a homogeneous and stable milky white coating solution.

[0031] (3) Fluidized bed coating: Add the fertilizer core granules obtained in step (1) to a fluidized bed coating machine, turn on the blower to fluidize the bed material, adjust the inlet air temperature to 63°C, preheat the material to 42°C (material temperature), and spray using a bottom spray gun. Place the coating liquid obtained in step (2) in a tank, spray at a rate of 12 mL / min, atomize at a pressure of 0.3 MPa, and maintain the material temperature at 42°C. Spray until the coating weight gain reaches 7% of the core granule mass. Then continue fluidized drying at 42°C for 10 min.

[0032] (4) Microbial agent immobilization: Preparation of bacterial suspension: The composite microbial agent was added to 50 parts of sterile physiological saline and stirred at 130 rpm for 25 min at 28℃ to re-dissolve and activate, thus preparing a bacterial suspension. The inlet air temperature of the fluidized bed was reduced to 32℃ to stabilize the temperature of the coated particles obtained in step (3) at 36℃. The bacterial suspension was sprayed through a top spray gun at a spray rate of 6 mL / min and an atomization pressure of 0.3 MPa (lower pressure to reduce mechanical damage to the bacteria). After spraying, fluidization was continued for 8 min at an inlet air temperature of 32℃ to ensure that the bacteria were fully and uniformly adsorbed onto the surface of the coating layer.

[0033] (5) Low-temperature drying and sieving: Set the inlet air temperature of the coating machine to 45℃ and continue fluidized drying for 25 minutes. After drying, sieve the granules through a vibrating screen, take qualified products with a particle size of 2-4 mm, and seal them for packaging.

[0034] Example 2 A slow-release compound fertilizer is made from the following raw materials in parts by weight: Fertilizer core components: 20 parts urea, 15 parts monoammonium phosphate, 10 parts potassium sulfate, 1 part zinc sulfate, 5 parts modified sepiolite, 3 parts sodium humate, and 1 part Sargassum seaweed extract powder; The coating solution consists of: 4 parts carboxymethyl chitosan, 2 parts polyglutamic acid, 1 part nano-hydroxyapatite, 1 part tributyl citrate, and 80 parts water. Compound microbial inoculant: 2 parts of a compound inoculant of Bacillus mucilaginosus and Bacillus megaterium (live count (cfu) ≥ 2.0 × 10⁻⁶) 9 / g), the mass ratio of Bacillus mucilaginosus to Bacillus megaterium is 1:1; The modified sepiolite is prepared by activating sepiolite with 0.5 mol / L citric acid solution at 50℃ for 3 hours.

[0035] The preparation method of the above-mentioned slow-release compound fertilizer is the same as that in Example 1.

[0036] Example 3 A slow-release compound fertilizer is made from the following raw materials in parts by weight: Fertilizer core components: 30 parts urea, 25 parts monoammonium phosphate, 20 parts potassium sulfate, 2 parts zinc sulfate, 10 parts modified sepiolite, 6 parts sodium humate, and 5 parts Sargassum seaweed extract powder; The coating solution consists of: 8 parts carboxymethyl chitosan, 5 parts polyglutamic acid, 3 parts nano-hydroxyapatite, 2 parts tributyl citrate, and 100 parts water. Compound microbial inoculant: 4 parts of a compound inoculant of Bacillus mucilaginosus and Bacillus megaterium (viable count (cfu) ≥ 2.0 × 10⁻⁶). 9 / g), the mass ratio of Bacillus mucilaginosus to Bacillus megaterium is 1:1; The modified sepiolite is prepared by activating sepiolite with 1.0 mol / L citric acid solution at 70℃ for 1 h.

[0037] The preparation method of the above-mentioned slow-release compound fertilizer is the same as that in Example 1.

[0038] Comparative Example 1 The only difference between this comparative example and Example 1 is the composition of the raw materials in the coating solution, specifically: 11 parts carboxymethyl chitosan, 0 parts polyglutamic acid, 0 parts nano hydroxyapatite, 1.5 parts tributyl citrate, and 90 parts water.

[0039] Comparative Example 2 The only difference between this comparative example and Example 1 is the composition of the raw materials in the coating solution, specifically: Carboxymethyl chitosan 0 parts, polyglutamic acid 6.6 parts, nano hydroxyapatite 4.4 parts, tributyl citrate 1.5 parts, and water 90 parts.

[0040] Comparative Example 3 The only difference between this comparative example and Example 1 is the composition of the raw materials in the coating solution, specifically: 6 parts carboxymethyl chitosan, 5 parts polyglutamic acid, 0 parts nano hydroxyapatite, 1.5 parts tributyl citrate, and 90 parts water.

[0041] Comparative Example 4 The only difference between this comparative example and Example 1 is the composition of the raw materials in the coating solution, specifically: 6 parts carboxymethyl chitosan, 0 parts polyglutamic acid, 5 parts nano-hydroxyapatite, 1.5 parts tributyl citrate, and 90 parts water.

[0042] Comparative Example 5 The only difference between this comparative example and Example 1 is the preparation method: Before granulation in step (1), the compound microbial agent powder is directly dry-mixed with other raw materials of the fertilizer core, and then granulated and coated. The remaining steps are exactly the same as in Example 1.

[0043] Comparative Example 6 This comparative example uses the raw materials from Example 1 separately, specifically: The uncoated fertilizer core granules, coating liquid powder (dried and pulverized), and microbial inoculant are mixed in the same proportion as in the product of Example 1 and then applied to the soil simultaneously after simple physical mixing during fertilization.

[0044] Comparative Example 7 Conventional fertilizer: Commercially available ordinary compound fertilizer (N-P2O5-K2O = 15-15-15), without coating, without microbial agents.

[0045] Effect test Test Example 1: Sustained-release effect test Test method: Referring to the national standard GB / T 23348-2009, the soil column leaching method was adopted. The specific steps are as follows: a. Column packing: The bottom of the soil column is sealed with quantitative filter paper and a layer of quartz sand. Weigh 500g of air-dried soil that has passed through a 2mm sieve, mix it with 5.0g of the fertilizer sample to be tested, and then pack it evenly into the soil column. Cover the top of the soil column with a layer of quartz sand to prevent erosion.

[0046] b. Leaching: The soil column was placed in a constant temperature environment of 25°C. For the first leaching, 200 mL of deionized water was added, and after standing for 24 hours, all the leachate was collected using a vacuum device, and the volume was recorded. The leaching operation was repeated every 7 days thereafter, for a total of 8 times (56 days in total).

[0047] c. Measurement: The cumulative release rate of nitrogen from the fertilizer was determined using the Kjeldahl method.

[0048] The test results are shown in Table 1.

[0049]

[0050] The nutrient release in Examples 1-3 was more gradual compared to Comparative Examples 1-6, with only about 80% of nitrogen released within 56 days, and the release period exceeding 56 days.

[0051] Test Example 2: Microbial Survival Rate Test Fertilizer samples from Examples 1-3 and Comparative Examples 1-6 were stored at 25°C under sealed, light-protected conditions for 0 days (initial) and 90 days. The number of viable bacteria in the microbial compound fertilizer was tested according to the national standard GB / T 20287-2006. All samples were stored at 25°C under sealed, light-protected conditions for 90 days to simulate a conventional storage environment. The test results are shown in Table 2.

[0052]

[0053] Test Example 3: Water Retention Performance Test Water absorption ratio test: Weigh 0.5g of fertilizer sample (m1), put it into a 100-mesh nylon mesh bag (m0) and seal it. Immerse the bag in deionized water and soak it at room temperature for 1 hour. Then take it out and hang it until no water drips down. Weigh the total mass (m2).

[0054] Water retention rate (room temperature evaporation) test: The water-saturated sample was placed in a constant temperature chamber at 25℃ and 50% relative humidity, and weighed at regular intervals (m). t ).

[0055] The formulas for calculating water absorption ratio and water retention rate are as follows: Water absorption rate (%) = (m2 - m0 - m1) / m1 × 100%; Water retention rate (%) = [(m t -m0-m1) / (m2-m0-m1)]×100%.

[0056] The test results are shown in Table 3.

[0057]

[0058] Test Example 4: Potted Plant Experiment Results (Corn, Growth Cycle 60 Days) Soil samples with mild to moderate heavy metal (Cd) contamination were collected from farmland, air-dried, and sieved (2 mm). 10 kg of the air-dried soil was accurately weighed and placed into each pot. Fertilizers prepared in Example 1 and Comparative Examples 1-6 were thoroughly mixed with the soil in each pot. Each treatment group was replicated 5 times (i.e., 5 pots), for a total of 35 pots. All fertilization treatments were based on an isonitrogen level (0.2 g N / kg soil).

[0059] Sow 5 uniformly sized, pre-germinated corn seeds ("Zhengdan 958" variety) at a depth of 3cm in each pot. When the seedlings have 3-4 true leaves, thin each pot to 2 healthy seedlings of uniform growth. Water the corn regularly and in measured amounts. Do not apply any compound fertilizer or pesticides during the corn's growth process. Thirty days after sowing (mid-growth stage), the length from the soil surface to the highest point of the plant was measured using a ruler. Using a chlorophyll meter, the SPAD value of the newly fully expanded leaves was uniformly measured, with three measurements taken for each leaf and the average value recorded. Sixty days after sowing (maturity stage), the plants were cut at soil level, washed, and placed in a 105℃ oven for 30 minutes, then dried at 75℃ until constant weight. The dry weight was measured using an electronic balance. All soil was poured out of the pots, mixed thoroughly, and approximately 500g of fresh soil was taken, air-dried, and sieved (1 mm) for determining soil pH and available heavy metal content. The test results are shown in Table 4.

[0060]

[0061] The above detailed description is a specific description of one of the feasible embodiments of the present invention. This embodiment is not intended to limit the patent scope of the present invention. All equivalent implementations or modifications that do not depart from the present invention should be included within the scope of the technical solution of the present invention.

Claims

1. A slow-release compound fertilizer, characterized in that, The raw material composition of the slow-release compound fertilizer includes: The core fertilizer ingredients are: urea, monoammonium phosphate, potassium sulfate, zinc sulfate, modified sepiolite, sodium humate, and Sargassum seaweed extract. Coating solution: carboxymethyl chitosan, polyglutamic acid, nano-hydroxyapatite, tributyl citrate, and water; Compound microbial inoculant: effective viable count ≥ 2.0 × 10⁻⁶ 9 cfu / g.

2. The slow-release compound fertilizer according to claim 1, characterized in that, The raw material composition of the slow-release compound fertilizer, by weight, includes: Fertilizer core: 20-30 parts urea, 15-25 parts monoammonium phosphate, 10-20 parts potassium sulfate, 1-2 parts zinc sulfate, 5-10 parts modified sepiolite, 3-6 parts sodium humate, and 1-5 parts Sargassum seaweed extract powder. Coating solution: 4-8 parts carboxymethyl chitosan, 2-5 parts polyglutamic acid, 1-3 parts nano hydroxyapatite, 1-2 parts tributyl citrate and 80-100 parts water; 2-4 parts of compound microbial inoculant.

3. The slow-release compound fertilizer according to any one of claims 1-2, characterized in that, The modified sepiolite is prepared by activating sepiolite with 0.5-1.0 mol / L citric acid solution at 50-70℃ for 1-3 hours.

4. The slow-release compound fertilizer according to any one of claims 1-2, characterized in that, The compound microbial agent is a mixture of Bacillus mucilaginosus and Bacillus megaterium, with a mass ratio of Bacillus mucilaginosus to Bacillus megaterium of 1:1-2.

5. The slow-release compound fertilizer according to any one of claims 1-2, characterized in that, In the coating solution, the mass ratio of carboxymethyl chitosan to polyglutamic acid is 1.5-2:

1.

6. A method for preparing the slow-release compound fertilizer according to any one of claims 1-5, characterized in that, Includes the following steps: (1) Crush, mix, and granulate each raw material in the core fertilizer component to obtain core fertilizer particles; (2) Dissolve carboxymethyl chitosan and polyglutamic acid in water, stir, add nano-hydroxyapatite and tributyl citrate, emulsify, and obtain coating solution; (3) The coating liquid is sprayed onto the surface of the preheated fertilizer core particles to form a coating layer, thus obtaining coated particles; (4) Spray the composite microbial agent suspension onto the surface of the coated particles obtained in step (3); (5) The granules obtained in step (4) are dried and sieved to obtain slow-release compound fertilizer.

7. The preparation method according to claim 6, characterized in that, In step (1), the raw materials are crushed to pass through an 80-100 mesh sieve. The granulation includes feeding the uniformly mixed powder into a dry roller press granulator and adjusting the roller press pressure to 4-6 MPa. The core fertilizer particles have a particle size of 2-3 mm and a particle strength of ≥25 N.

8. The preparation method according to claim 6, characterized in that, The stirring temperature in step (2) is 50-60℃, the stirring time is 1.5-2.5h, and the stirring speed is 200-300rpm; the emulsification temperature is 35-40℃, the emulsification speed is 5000-6000rpm, and the emulsification time is 20-30min.

9. The preparation method according to claim 6, characterized in that, In step (3), the temperature of the fertilizer core particles is maintained at 40-45℃ during spraying, and the coating is sprayed until the weight gain reaches 5%-8% of the core particle mass; the preparation method of the composite microbial agent solution in step (4) includes: adding the composite microbial agent to 40-60 parts of sterile physiological saline, stirring at 100-150 rpm for 20-30 minutes at 25-30℃, re-dissolving and activating, and preparing a suspension; the temperature of the coated particles is maintained at 35-38℃ during spraying of the agent.

10. The application of the slow-release compound fertilizer according to any one of claims 1-5 in promoting crop growth.

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