Water-retention controlled-release ecological compound fertilizer and preparation method thereof

Abstract

The invention discloses a water-retention controlled-release ecological compound fertilizer which comprises the following components in parts by mass: 17-22 parts of a nitrogen source fertilizer, 6-10 parts of a phosphorus source fertilizer and 10-13 parts of a potassium source fertilizer, the water retention material comprises the following components in parts by mass: 2-3 parts of polyvinyl alcohol, 3-4 parts of humic acid and 2-4 parts of biochar, and the controlled release material comprises the following components in parts by mass: 2-4 parts of sulfur, 2-4 parts of calcium carbonate, 2-4 parts of calcium carbonate, 2-4 parts of calcium carbonate, 2-4 parts of calcium carbonate and 1-2 parts of a functional additive. The biological carbon is added into the decomposed organic matter, the nitrogen source fertilizer, the phosphorus source fertilizer and the potassium source fertilizer to be stirred and mixed, meanwhile, after raw materials are granulated, the polyvinyl alcohol film and the humic acid film are wrapped outside the raw materials, the multiple water absorption performance of the compound fertilizer is achieved, the water-retaining property is good, and after the compound fertilizer is applied to soil, the soil has the good water-retaining property. The soil water-retaining property can be improved, the irrigation frequency can be reduced, the irrigation period can be prolonged, and the drought resistance of crops can be enhanced.

Description

Technical Field

[0001] The present invention relates to the technical field of ecological compound fertilizers, and in particular to a water-retaining controlled-release ecological compound fertilizer and a preparation method thereof. Background Art

[0002] Compound fertilizer is a common fertilizer, typically containing two or more nutrients. Its advantages include high nutrient content, few by-products, and excellent physical properties. It plays a crucial role in balanced fertilization, improving fertilizer utilization, and promoting high and stable crop yields. Therefore, developing a multifunctional fertilizer that can achieve slow and controlled nutrient release, water absorption and retention, and drought resistance is an important approach to address this problem. Summary of the Invention

[0003] The purpose of the present invention is to overcome the above technical deficiencies, propose a water-retaining controlled-release ecological compound fertilizer and a preparation method, and solve the technical problems of poor water absorption and water retention and slow-release nutrient capacity of traditional compound fertilizers.

[0004] In order to achieve the above technical objectives, the technical solution of the present invention provides a water-retaining controlled-release ecological compound fertilizer, including the following components in parts by mass: 17-22 parts of nitrogen source fertilizer, 6-10 parts of phosphorus source fertilizer, 10-13 parts of potassium source fertilizer; 7-10 parts of water-retaining material, 8-11 parts of controlled-release material, 10-15 parts of organic matter, 1-3 parts of trace elements, and 1-2 parts of functional additives. The water-retaining material includes the following components in parts by mass: 2-3 parts of polyvinyl alcohol, 3-4 parts of humic acid, and 2-4 parts of biochar. The controlled-release material includes the following components in parts by mass: 2-4 parts of sulfur, 3-5 parts of polyolefin resin, 1-2 parts of zeolite, and 1-2 parts of bentonite.

[0005] Furthermore, the nitrogen fertilizer includes any one or more of urea, ammonium sulfate, and ammonium nitrate; the phosphorus source fertilizer includes any one or more of superphosphate, monoammonium phosphate, and diammonium phosphate; and the potassium source fertilizer includes any one or more of potassium chloride and potassium sulfate.

[0006] Furthermore, organic matter includes livestock and poultry manure and straw compost.

[0007] Furthermore, trace elements include calcium, magnesium, sulfur, iron, zinc, and boron.

[0008] Furthermore, the functional additives include microbial agents and adhesives.

[0009] The technical solution of the present invention also provides a method for preparing a water-retaining controlled-release ecological compound fertilizer, comprising the following steps: S1. Raw material pretreatment: compost the organic matter at high temperature to kill pathogens and weed seeds; grind the nitrogen, phosphorus and potassium fertilizers and pass through an 80-100 mesh sieve; grind the biochar and pass through a 70-80 mesh sieve; grind the bentonite and zeolite and pass through a 60-80 mesh sieve; S2. Mixing and stirring: Put the decomposed organic matter, crushed nitrogen fertilizer, phosphorus fertilizer, potassium fertilizer, biochar, zeolite, bentonite and trace elements into a mixer in proportion and mix and stir; S3, granulation: the mixed raw materials are granulated using a disc granulator or an extruder to form granules of uniform size; S4, drying: placing the granulated particles into a temperature-controlled box for low-temperature drying; S5. Coating treatment: The dried granules are fed into a coating machine, polyvinyl alcohol is dissolved in water and sprayed on the surface of the granulated granules. After spraying, the polyvinyl alcohol film is dried by hot air to solidify the polyvinyl alcohol film to form an inner water-retaining film; sulfur is melted and sprayed on the outside of the sprayed polyvinyl alcohol film. After spraying, the sulfur film is cooled by cold air to solidify the sulfur film to form an inner controlled-release film; polyolefin resin is dissolved in water and sprayed on the outside of the sulfur film. After spraying, the polyolefin resin film is dried and solidified by hot air to solidify the polyolefin resin film to form an outer controlled-release film; humic acid is mixed with water to form a humic acid suspension, which is sprayed on the outside of the polyolefin resin film. After spraying, the humic acid film is dried by hot air to solidify the humic acid film to form an outer water-retaining film.

[0010] Furthermore, in step S1, the composting time is 15-20 days, and the temperature is above 60°C; the composting method is: the ventilation rate is 0.02m3 / min / m3 of the pile material, and ventilation is 0.1h every 1h; and the pile is turned every 5 days.

[0011] Furthermore, in step S2, when the raw materials are mixed, the functional additive is dissolved in water and then sprayed into the mixture.

[0012] Furthermore, in step S3, the particle size of the granules after granulation is 2-3 mm.

[0013] Furthermore, in step S4, the temperature of low-temperature drying is 60-80°C.

[0014] The beneficial effects of the present invention include: 1. By adding biochar to decomposed organic matter, nitrogen source fertilizer, phosphorus source fertilizer, and potassium source fertilizer and stirring and mixing, and wrapping the raw materials with polyvinyl alcohol film and humic acid film on the outside after granulation, the compound fertilizer has multiple water absorption properties and good water retention. After the compound fertilizer is applied to the soil, it can improve the soil water retention, reduce the irrigation frequency, extend the irrigation cycle, enhance the drought resistance of crops, and is easily biodegradable, without causing secondary pollution to the soil; 2. By adding zeolite and bentonite to decomposed organic matter, nitrogen source fertilizer, phosphorus source fertilizer and potassium source fertilizer, stirring and mixing, and then wrapping the granules with sulfur film and polyolefin resin film on the outside, multiple controlled release of fertilizer effect can be achieved, providing nutrients that are adapted to the different nutrient requirements of crops at different growth stages, improving fertilizer utilization rate, ensuring sufficient nutrient supply throughout the entire growth cycle of crops, promoting increased production and harvest, and effectively reducing environmental pollution caused by nutrient loss. By precisely controlling nutrient release, excess nutrients are avoided from entering water bodies and soil, reducing the risks of eutrophication of water bodies and soil compaction and acidification, protecting the ecological balance, and promoting the development of agriculture in a green and sustainable direction, which has important ecological significance. DETAILED DESCRIPTION

[0015] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0016] An embodiment of the present invention provides a water-retaining controlled-release ecological compound fertilizer, comprising the following components in parts by mass: 17-22 parts of nitrogen source fertilizer, 6-10 parts of phosphorus source fertilizer, 10-13 parts of potassium source fertilizer; 7-10 parts of water-retaining material, 8-11 parts of controlled-release material, 10-15 parts of organic matter, 1-3 parts of medium and trace elements, and 1-2 parts of functional additives. The water-retaining material comprises the following components in parts by mass: 2-3 parts of polyvinyl alcohol, 3-4 parts of humic acid, and 2-4 parts of biochar. The controlled-release material comprises the following components in parts by mass: 2-4 parts of sulfur, 3-5 parts of polyolefin resin, 1-2 parts of zeolite, and 1-2 parts of bentonite.

[0017] Phosphorus source fertilizers include any one or more of superphosphate, monoammonium phosphate, and diammonium phosphate; potassium source fertilizers include any one or more of potassium chloride and potassium sulfate.

[0018] Organic matter includes livestock and poultry manure and straw compost.

[0019] Trace elements include calcium, magnesium, sulfur, iron, zinc and boron.

[0020] Functional additives include microbial agents and adhesives.

[0021] The preparation method of the above-mentioned water-retaining controlled-release ecological compound fertilizer comprises the following steps: Step S1, raw material pretreatment: composting organic matter at high temperature to kill pathogens and weed seeds; crushing nitrogen fertilizer, phosphorus fertilizer, and potassium fertilizer, and passing through an 80-100 mesh sieve; crushing biochar, and passing through a 70-80 mesh sieve; crushing bentonite and zeolite, and passing through a 60-80 mesh sieve; Step S2, mixing and stirring: adding decomposed organic matter, crushed nitrogen source fertilizer, phosphorus source fertilizer, potassium source fertilizer, biochar, zeolite, bentonite and trace elements into a blender in proportion, and mixing and stirring; Step S3, granulation: the mixed raw materials are granulated using a disc granulator or an extruder to form granules of uniform size; Step S4, drying: placing the granulated particles into a temperature-controlled box for low-temperature drying; Step S5, coating treatment: the dried particles are fed into a coating machine, polyvinyl alcohol is dissolved in water and sprayed on the surface of the granulated particles, and then dried with hot air to solidify the polyvinyl alcohol film to form an inner water-retaining film; sulfur is melted and sprayed on the outside of the sprayed polyvinyl alcohol film, and then cooled with cold air to solidify the sulfur film to form an inner controlled-release film; polyolefin resin is dissolved in water and sprayed on the outside of the sulfur film, and then dried and solidified with hot air to solidify the polyolefin resin film to form an outer controlled-release film; humic acid is mixed with water to form a humic acid suspension, and then sprayed on the outside of the polyolefin resin film, and then dried with hot air to solidify the humic acid film to form an outer water-retaining film.

[0022] In step S1, the composting time is 15-20 days, and the temperature is above 60°C; the composting method is: the ventilation rate is 0.02m3 / min / m3 of the pile material, and the ventilation is 0.1h every 1h; and the pile is turned every 5 days.

[0023] In step S2, when the raw materials are mixed, the microbial agent is dissolved in water and then sprayed into the mixture.

[0024] In step S3, the particle size of the granules after granulation is 2-3 mm.

[0025] In step S4, the low-temperature drying temperature is 60-80°C.

[0026] The following is an introduction with reference to specific embodiments: Example 1: This embodiment provides a water-retaining controlled-release ecological compound fertilizer, comprising the following components in parts by mass: 17 parts of nitrogen source fertilizer; 6 parts of phosphorus fertilizer; 10 parts potassium source fertilizer; 7 parts water-retaining material, 2 parts polyvinyl alcohol, 3 parts humic acid, 2 parts biochar; 8 parts of controlled release material, 2 parts of sulfur, 3 parts of polyolefin resin, 1 part of zeolite, 2 parts of bentonite; 10 parts organic matter, 1 part of trace elements, 1 part functional additive.

[0027] The preparation method of the above-mentioned water-retaining controlled-release ecological compound fertilizer comprises the following steps: Step S1, raw material pretreatment: composting organic matter at high temperature to kill pathogens and weed seeds; crushing nitrogen fertilizer, phosphorus fertilizer, and potassium fertilizer, and passing through an 80-mesh sieve; crushing biochar, and passing through an 80-mesh sieve; crushing bentonite and zeolite, and passing through a 60-mesh sieve; Step S2, mixing and stirring: adding decomposed organic matter, crushed nitrogen source fertilizer, phosphorus source fertilizer, potassium source fertilizer, biochar, zeolite, bentonite and trace elements into a blender in proportion, and mixing and stirring; Step S3, granulation: the mixed raw materials are granulated using a disc granulator or an extruder to form granules of uniform size; Step S4, drying: placing the granulated particles into a temperature-controlled box for low-temperature drying; Step S5, coating treatment: the dried particles are fed into a coating machine, polyvinyl alcohol is dissolved in water and sprayed on the surface of the granulated particles, and then dried with hot air to solidify the polyvinyl alcohol film to form an inner water-retaining film; sulfur is melted and sprayed on the outside of the sprayed polyvinyl alcohol film, and then cooled with cold air to solidify the sulfur film to form an inner controlled-release film; polyolefin resin is dissolved in water and sprayed on the outside of the sulfur film, and then dried and solidified with hot air to solidify the polyolefin resin film to form an outer controlled-release film; humic acid is mixed with water to form a humic acid suspension, and then sprayed on the outside of the polyolefin resin film, and then dried with hot air to solidify the humic acid film to form an outer water-retaining film.

[0028] In step S1, the composting time is 15 days, the temperature is above 60°C; the composting method is: the ventilation rate is 0.02m3 / min / m3 of the pile material, ventilation is 0.1h every 1h; and the pile is turned every 5 days.

[0029] In step S2, when the raw materials are mixed, the microbial agent is dissolved in water and then sprayed into the mixture.

[0030] In step S3, the particle size of the granules after granulation is 2 mm.

[0031] In step S4, the temperature of low-temperature drying is 60°C.

[0032] Example 2 20 parts nitrogen fertilizer; 8 parts of phosphorus fertilizer; 11 parts potassium source fertilizer; 8 parts water-retaining material, 2 parts polyvinyl alcohol, 4 parts humic acid, 2 parts biochar; 10 parts of controlled release material, 2 parts of sulfur, 4 parts of polyolefin resin, 2 parts of zeolite, and 2 parts of bentonite; 13 parts organic matter, 2 parts of trace elements, 2 parts functional additives.

[0033] The preparation method of the above-mentioned water-retaining controlled-release ecological compound fertilizer comprises the following steps: Step S1, raw material pretreatment: composting organic matter at high temperature to kill pathogens and weed seeds; crushing nitrogen fertilizer, phosphorus fertilizer, and potassium fertilizer, and passing through an 80-mesh sieve; crushing biochar, and passing through a 70-mesh sieve; crushing bentonite and zeolite, and passing through a 60-mesh sieve; Step S2, mixing and stirring: adding decomposed organic matter, crushed nitrogen source fertilizer, phosphorus source fertilizer, potassium source fertilizer, biochar, zeolite, bentonite and trace elements into a blender in proportion, and mixing and stirring; Step S3, granulation: the mixed raw materials are granulated using a disc granulator or an extruder to form granules of uniform size; Step S4, drying: placing the granulated particles into a temperature-controlled box for low-temperature drying; Step S5, coating treatment: the dried particles are fed into a coating machine, polyvinyl alcohol is dissolved in water and sprayed on the surface of the granulated particles, and then dried with hot air to solidify the polyvinyl alcohol film to form an inner water-retaining film; sulfur is melted and sprayed on the outside of the sprayed polyvinyl alcohol film, and then cooled with cold air to solidify the sulfur film to form an inner controlled-release film; polyolefin resin is dissolved in water and sprayed on the outside of the sulfur film, and then dried and solidified with hot air to solidify the polyolefin resin film to form an outer controlled-release film; humic acid is mixed with water to form a humic acid suspension, and then sprayed on the outside of the polyolefin resin film, and then dried with hot air to solidify the humic acid film to form an outer water-retaining film.

[0034] In step S1, the composting time is 18 days, the temperature is above 60°C; the composting method is: the ventilation rate is 0.02m3 / min / m3 of the pile material, ventilation is 0.1h every 1h; and the pile is turned every 5 days.

[0035] In step S2, when the raw materials are mixed, the microbial agent is dissolved in water and then sprayed into the mixture.

[0036] In step S3, the particle size of the granules after granulation is 2 mm.

[0037] In step S4, the temperature of low-temperature drying is 60°C.

[0038] Example 3 22 parts nitrogen fertilizer; 10 parts of phosphorus fertilizer; 13 parts potassium source fertilizer; 10 parts water-retaining material, 3 parts polyvinyl alcohol, 3 parts humic acid, 4 parts biochar; 11 parts of controlled release material, 3 parts of sulfur, 4 parts of polyolefin resin, 2 parts of zeolite, and 2 parts of bentonite; 15 parts organic matter, 3 parts of trace elements, 2 parts functional additives.

[0039] The preparation method of the above-mentioned water-retaining controlled-release ecological compound fertilizer comprises the following steps: Step S1, raw material pretreatment: composting organic matter at high temperature to kill pathogens and weed seeds; crushing nitrogen fertilizer, phosphorus fertilizer, and potassium fertilizer, and passing through an 80-mesh sieve; crushing biochar, and passing through a 70-mesh sieve; crushing bentonite and zeolite, and passing through a 60-mesh sieve; Step S2, mixing and stirring: adding decomposed organic matter, crushed nitrogen source fertilizer, phosphorus source fertilizer, potassium source fertilizer, biochar, zeolite, bentonite and trace elements into a blender in proportion, and mixing and stirring; Step S3, granulation: the mixed raw materials are granulated using a disc granulator or an extruder to form granules of uniform size; Step S4, drying: placing the granulated particles into a temperature-controlled box for low-temperature drying; Step S5, coating treatment: the dried particles are fed into a coating machine, polyvinyl alcohol is dissolved in water and sprayed on the surface of the granulated particles, and then dried with hot air to solidify the polyvinyl alcohol film to form an inner water-retaining film; sulfur is melted and sprayed on the outside of the sprayed polyvinyl alcohol film, and then cooled with cold air to solidify the sulfur film to form an inner controlled-release film; polyolefin resin is dissolved in water and sprayed on the outside of the sulfur film, and then dried and solidified with hot air to solidify the polyolefin resin film to form an outer controlled-release film; humic acid is mixed with water to form a humic acid suspension, and then sprayed on the outside of the polyolefin resin film, and then dried with hot air to solidify the humic acid film to form an outer water-retaining film.

[0040] In step S1, the composting time is 20 days, the temperature is above 60°C; the composting method is: the ventilation rate is 0.02m3 / min / m3 of the pile material, ventilation is 0.1h every 1h; and the pile is turned every 5 days.

[0041] In step S2, when the raw materials are mixed, the microbial agent is dissolved in water and then sprayed into the mixture.

[0042] In step S3, the particle size of the granules after granulation is 3 mm.

[0043] In step S4, the temperature of low-temperature drying is 80°C.

[0044] Comparative Example 1: The difference from Example 1 is that the compound fertilizer does not contain water-retaining materials or controlled-release materials, and no biochar, zeolite, or bentonite is added in step S2, and step 5 is omitted.

[0045] Comparative Example 2: The difference from Example 1 is that the compound fertilizer does not contain polyvinyl alcohol, humic acid, sulfur, and polyolefin resin, and step 5 is omitted.

[0046] Comparative Example 3: The difference from Example 1 is that the compound fertilizer does not contain biochar, zeolite, or bentonite, and no biochar, zeolite, or bentonite is added in step S2.

[0047] Effect verification 1. Water retention determination: Compound fertilizer and water were mixed to prepare a fertilizer-soil mixture (the mass ratio of compound fertilizer to water was 1:10). The mixture was loaded into a standard ring cutter (volume 100 cm³, diameter 5 cm × height 5.09 cm) in three layers. Each layer was tapped 10 times to a bulk density of 1.3±0.05 g / cm³. The sample ring cutter was immersed in deionized water for 24 h, with the water level 2 cm above the upper edge of the ring cutter. The saturated sample was transferred to a drainage device covered with filter paper (Whatman No.1) and drained under a negative pressure of -10 kPa (equivalent to 1 m of water column) for 24, 48, 72, and 96 h. The mass water content was calculated by weighing. The mass water content was calculated as follows: mass water content (%) = (wet soil mass (g) - dry soil mass (g) / dry soil mass (g) × 100%).

[0048] Table 1: Mass moisture content test data table

[0049] As shown in Table 1, the water retention rate of the soil added with the water-retaining controlled-release compound fertilizers prepared in Examples 1-3 of the present invention is better than that of the soil added with the compound fertilizers prepared in Comparative Examples 1-3, indicating that the water-retaining controlled-release compound fertilizers prepared by the present invention have good water retention capacity after being applied to the soil and can slow down the evaporation of water in the soil. On rainy days, the water-retaining slow-release fertilizer stores excess water. When it is dry, the water absorbed in the water-retaining slow-release fertilizer can be adsorbed by the capillaries of the plant roots and then transported into the plant body, acting as a "micro-reservoir", thereby reducing the number of irrigation times and improving water utilization efficiency. At the same time, by comparing the data of Example 1, Comparative Example 2, and Comparative Example 3, it can be seen that simply mixing biochar into the compound fertilizer or coating the granules after granulation with a polyvinyl alcohol film or a humic acid film cannot achieve the best water retention performance. It is necessary to mix biochar into the compound fertilizer and coat the granules after granulation with a polyvinyl alcohol film or a humic acid film to achieve the best water retention performance.

[0050] 2. Controlled release effect test: Weigh 5 g of sample and place it in a 250 mL conical flask. Add 100 mL of deionized water and place the conical flask in a constant temperature shaking water bath with the temperature set at (25 ± 1) °C and the shaking speed at 120 r / min.

[0051] Sampling was performed on the 1st, 7th, 28th and 90th day, respectively. Before each sampling, the shaking was stopped and the mixture was allowed to stand for 30 minutes. Then, the mixture was filtered through filter paper, the filtrate was collected and diluted to an appropriate volume (e.g., 250 mL). Nitrogen was determined by Kjeldahl method or UV spectrophotometry, phosphorus by molybdenum antimony spectrophotometry; potassium by flame photometry or atomic absorption spectrometry. Cumulative release rate (%) = total nutrient content in the sample (mg / g) / nutrient content determined at each time point (mg / g) × 100%.

[0052] Table 2: Nitrogen release data

[0053] Table 3: Phosphorus release data

[0054] Table 4: Potassium release data

[0055] As shown in Tables 2-4, the water-retaining controlled-release compound fertilizers prepared in Examples 1-3 of the present invention have good sustained-release effects. Comparison of the data of Example 1, Comparative Example 2, and Comparative Example 3 shows that the optimal controlled-release performance cannot be achieved by simply mixing zeolite or bentonite into the compound fertilizer or coating the granules with a sulfur film or a polyolefin resin film after granulation. Optimal controlled-release performance requires mixing zeolite or bentonite into the compound fertilizer and coating the granules with a sulfur film or a polyolefin resin film after granulation.

[0056] The above specific embodiments of the present invention do not constitute a limitation on the scope of protection of the present invention. Any other corresponding changes and modifications made based on the technical concept of the present invention should be included in the scope of protection of the claims of the present invention.

Claims

1. A water-retaining controlled-release ecological compound fertilizer, characterized in that: It includes the following components in parts by mass: 17-22 parts of nitrogen source fertilizer, 6-10 parts of phosphorus source fertilizer, 10-13 parts of potassium source fertilizer; 7-10 parts of water-retaining material, 8-11 parts of controlled-release material, 10-15 parts of organic matter, 1-3 parts of trace elements, 1-2 parts of functional additives, the water-retaining material includes the following components in parts by mass: 2-3 parts of polyvinyl alcohol, 3-4 parts of humic acid, 2-4 parts of biochar, the controlled-release material includes the following components in parts by mass: 2-4 parts of sulfur, 3-5 parts of polyolefin resin, 1-2 parts of zeolite, and 1-2 parts of bentonite.

2. The water-retaining controlled-release ecological compound fertilizer according to claim 1, characterized in that The nitrogen fertilizer includes any one or more of urea, ammonium sulfate, and ammonium nitrate; the phosphorus source fertilizer includes any one or more of superphosphate, monoammonium phosphate, and diammonium phosphate; and the potassium source fertilizer includes any one or more of potassium chloride and potassium sulfate.

3. The water-retaining controlled-release ecological compound fertilizer according to claim 1, characterized in that The organic matter includes livestock and poultry manure and straw compost.

4. The water-retaining controlled-release ecological compound fertilizer according to claim 1, characterized in that The trace elements include calcium, magnesium, sulfur, iron, zinc and boron.

5. The water-retaining controlled-release ecological compound fertilizer according to claim 1, characterized in that The functional additives include microbial agents and adhesives.

6. The method for preparing the water-retaining controlled-release ecological compound fertilizer according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1. Raw material pretreatment: compost the organic matter at high temperature to kill pathogens and weed seeds; grind the nitrogen, phosphorus and potassium fertilizers and pass through an 80-100 mesh sieve; grind the biochar and pass through a 70-80 mesh sieve; grind the bentonite and zeolite and pass through a 60-80 mesh sieve; S2. Mixing and stirring: Put the decomposed organic matter, crushed nitrogen fertilizer, phosphorus fertilizer, potassium fertilizer, biochar, zeolite, bentonite and trace elements into a mixer in proportion and mix and stir; S3, granulation: the mixed raw materials are granulated using a disc granulator or an extruder to form granules of uniform size; S4, drying: placing the granulated particles into a temperature-controlled box for low-temperature drying; S5. Coating treatment: The dried granules are fed into a coating machine, polyvinyl alcohol is dissolved in water and sprayed on the surface of the granulated granules. After spraying, the polyvinyl alcohol film is dried by hot air to solidify the polyvinyl alcohol film to form an inner water-retaining film; sulfur is melted and sprayed on the outside of the sprayed polyvinyl alcohol film. After spraying, the sulfur film is cooled by cold air to solidify the sulfur film to form an inner controlled-release film; polyolefin resin is dissolved in water and sprayed on the outside of the sulfur film. After spraying, the polyolefin resin film is dried and solidified by hot air to solidify the polyolefin resin film to form an outer controlled-release film; humic acid is mixed with water to form a humic acid suspension, which is sprayed on the outside of the polyolefin resin film. After spraying, the humic acid film is dried by hot air to solidify the humic acid film to form an outer water-retaining film.

7. The method for preparing the water-retaining controlled-release ecological compound fertilizer according to claim 6, characterized in that: In step S1, the composting time is 15-20 days, and the temperature is above 60°C; the composting method is: the ventilation rate is 0.02m3 / min / m3 of the pile material, and the ventilation is 0.1h every 1h; and the pile is turned every 5 days.

8. The method for preparing the water-retaining controlled-release ecological compound fertilizer according to claim 6, characterized in that: In step S2, when the raw materials are mixed, the functional additive is dissolved in water and then sprayed into the mixture.

9. The method for preparing the water-retaining controlled-release ecological compound fertilizer according to claim 6, characterized in that: In step S3, the particle size of the granules after granulation is 2-3 mm.

10. The method for preparing the water-retaining controlled-release ecological compound fertilizer according to claim 6, characterized in that: In step S4, the low-temperature drying temperature is 60-80°C.

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