Slow and controlled release fertilizer for saline-alkali soil and preparation method thereof

Abstract

The invention relates to the technical field of agricultural fertilizers, in particular to a slow / controlled-release fertilizer for saline-alkali soil and a preparation method of the slow / controlled-release fertilizer. The slow / controlled-release fertilizer sequentially comprises a core layer, an inner coating, a shell layer and an outer coating from inside to outside, the core layer comprises a functional aid, diammonium hydrogen phosphate, urea, isobutylidene diurea, potassium nitrate and biochar; a shell layer contains bagasse, a compound strain and fulvic acid; the coating material is an adhesive containing montmorillonite powder; the functional additive is loaded with an organic acid and polymethacrylamide network through modified diatomite, so that the synergistic effect of sodium ion adsorption, acid-base regulation and nutrient slow release is realized; through the multi-layer structure design and the component synergistic effect, it is guaranteed that the slow and controlled release fertilizer can improve the saline-alkali soil structure in a long-acting mode, accurately regulate nutrient release and improve the stress resistance of crops, and the slow and controlled release fertilizer is suitable for efficient utilization of saline-alkali soil.

Description

Technical Field

[0001] The present invention relates to the technical field of agricultural fertilizers, in particular to a slow-release fertilizer for saline-alkali land and a preparation method thereof. Background Art

[0002] Saline-alkali land refers to land where excessive soluble salts or alkaline substances (such as sodium salts) accumulate on the soil surface, leading to deterioration of soil structure and decreased fertility, which in turn affects the normal growth of crops. Saline-alkali land can be divided into light saline-alkali land, moderate saline-alkali land, and heavy saline-alkali land. The essence of its formation is mainly the horizontal and vertical redistribution of various soluble salts on the ground, which causes salt to gradually accumulate on the soil surface in salt-collecting areas. Traditional fertilizers are prone to nutrient fixation or loss in saline-alkali environments and lack the effect of improving soil microecology. Although slow-release fertilizers can prolong the nutrient release cycle, existing products generally have the following defects: 1. Lack of efficient soil sodium ion adsorption and pH adjustment function; 2. The effect of improving microbial activity is insufficient, making it difficult to build a salt-tolerant microecology; 3. The nutrient release rhythm does not match the crop demand, which can easily lead to excessive growth in the early stage or lack of nutrients in the later stage.

[0003] Therefore, the development of a slow-release fertilizer that integrates salt-alkali improvement, microbial activation and precise fertilizer release has important application value. Summary of the Invention

[0004] The purpose of the present invention is to provide a slow-release fertilizer for saline-alkali land and a preparation method thereof. The present invention ensures that the slow-release fertilizer can improve the saline-alkali soil structure in a long-term manner, accurately regulate nutrient release, and enhance the stress resistance of crops through multi-layer structure design and synergistic effect of ingredients, and is suitable for the efficient utilization of saline-alkali land.

[0005] To achieve the above object, the present invention provides the following technical solutions: A slow-release fertilizer for saline-alkali land, comprising a core layer, an inner coating, a shell layer and an outer coating from the inside to the outside; The core layer is composed of the following raw materials in parts by weight: 30-40 parts of functional additives, 20-30 parts of diammonium hydrogen phosphate, 10-15 parts of urea, 6-10 parts of isobutylene diurea, 15-25 parts of potassium nitrate and 20-30 parts of biochar; The shell layer is composed of the following raw materials in parts by weight: 10-15 parts of bagasse, 0.05-0.1 parts of composite bacteria and 4-6 parts of fulvic acid; The coating materials used for the inner coating and the outer coating have the same composition, both containing 0.3-0.6 parts by weight of montmorillonite powder and 1.5-2.5 parts by weight of adhesive.

[0006] Furthermore, the functional additive includes the following steps: ultrasonically dispersing the modified diatomaceous earth in a 10-20wt% organic acid aqueous solution at a dosage ratio of 20-50g / L, and then adding methacrylamide with a mass of 1-2 times that of the organic acid and an emulsifier with a mass of 0.1-0.2 times that of the organic acid; after uniform dispersion, adding cyclohexane with a mass of 3-5 times that of the organic acid aqueous solution, high-speed shearing at a rate of 5000-8000r / min for 20-30min, and then adding benzoyl peroxide with a mass of 0.2-0.5 times that of the emulsifier under nitrogen protection, reacting at a temperature of 50-60°C for 5-8h, and then filtering and drying.

[0007] Furthermore, the preparation method of the modified diatomaceous earth is: dichlorodimethylsilane and nitrogen benzene are mixed in a volume ratio of 1:15 to 20, and then diatomaceous earth with a mass of 1.5 to 2 times that of dichlorodimethylsilane is added, and after mixing, the mixture is stirred and reacted at a temperature of 30 to 40°C for 3 to 5 hours; after the reaction is completed, witch hazel tannin with a mass of 0.2 to 0.3 times that of the diatomaceous earth is added while stirring, and the mixture is stirred and reacted for 2 to 3 hours, and then filtered, washed and dried.

[0008] Furthermore, the diatomaceous earth needs to be cleaned before preparing the modified diatomaceous earth. The specific cleaning steps are: calcining diatomaceous earth with a particle size of 100-150 mesh at a temperature of 500-600°C for 2-3 hours, and then soaking it in a 5-10wt% hydrochloric acid solution for 3-5 hours; after soaking, filtering out the diatomaceous earth, washing it with water until it is neutral, and then drying it.

[0009] Furthermore, the organic acid is selected from any one of malic acid, tartaric acid and citric acid.

[0010] Furthermore, the emulsifier is selected from any one of nonylphenol polyoxyethylene ether and alkylphenol polyoxyethylene ether.

[0011] Furthermore, the preparation method of the biochar is: crushing soybean straw into particles with a particle size of less than 5 mm, heating to 500-600°C at a rate of 3-5°C / min under anaerobic conditions, pyrolyzing for 2-3 hours, and then cooling and crushing in sequence to obtain biochar with a particle size of 0.1-0.2 mm.

[0012] Furthermore, the composite bacterial strain is composed of Daban Halobacillus (the preservation number of Daban Halobacillus NBL-BS214 is CCTCC NO: 20221370) and 0.5 to 0.8 times the mass of Daban Halobacillus (Bacillus subtilis LVLE14, the preservation number is CGMCC No. 12639); and the total effective viable count is ≥ 3 × 10 9 CFU / g.

[0013] Furthermore, the adhesive is composed of polyvinyl alcohol and 0.3 to 0.5 times the mass of oxidized starch.

[0014] A method for preparing a slow-release fertilizer for saline-alkali land comprises the following steps: Step 1: Mix the raw materials of the core layer and granulate them to obtain a core layer powder with a particle size of 2 to 4 mm; Step 2: Mixing montmorillonite powder and adhesive to prepare a coating material; Step 3: Put the core layer powder into the coating machine, add the coating material and coat it to obtain a coated core material with an inner coating thickness of 25 to 35 μm; Step 4: The coating core material and the shell layer raw materials are mixed and granulated, and then a coating material is used for secondary coating to obtain a slow-release fertilizer with an outer coating thickness of 35 to 45 μm.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. Cleaned diatomaceous earth, which has a large specific surface area, is reacted sequentially with dichlorodimethylsilane and witch hazel tannins. Finally, witch hazel tannins are grafted onto the diatomaceous earth surface using dichlorodimethylsilane as a "bridge," producing modified diatomaceous earth. The modified diatomaceous earth is ultrasonically dispersed in an aqueous organic acid solution, allowing the organic acid to be uniformly adsorbed and loaded onto the surface and internal pores of the diatomaceous earth. Then, under the action of benzoyl peroxide, methacrylamide undergoes polymerization to produce a functional additive. Ultimately, a dense three-dimensional network coating forms on the surface and within the porous structure of the diatomaceous earth, effectively "binding" the witch hazel tannins and organic acids on the diatomaceous earth and reducing their migration. Witch hazel tannins absorb sodium ions from the soil through ion exchange and coordination chelation, reducing the damage of free sodium ions to soil structure and plant roots. They also lower soil pH, regulate the soil's acid-base balance, improve the soil's physical structure, enhance its air permeability and water retention, and effectively alleviate soil salinization. Organic acids can continuously and effectively neutralize alkaline substances in the soil. Working in conjunction with witch hazel tannins, they effectively regulate soil pH and provide a favorable living environment for the compound bacterial strains. Furthermore, the polymer network formed by methacrylamide polymerization absorbs water and swells when water infiltrates the soil, enlarging the pores and allowing nutrients to diffuse through the pores for a slow release. Furthermore, the polymer network encapsulates nutrients, further delaying their release, enabling precise regulation based on soil moisture and crop needs.

[0016] 2. The synergistic effect of the various ingredients in the core layer effectively enhances the efficiency of the slow-release fertilizer. The modified diatomaceous earth has a large specific surface area and abundant surface active sites, which can fully adsorb and chelate sodium ions in the soil. The polymer network structure on the surface of the functional additives regulates the rate of nutrient release. Isobutylene diurea, under the action of urease secreted by soil microorganisms, slowly releases nitrogen through a complex decomposition process, meeting the needs of crops in the later stages of growth. Potassium nitrate dissolves into potassium and nitrate ions, providing potassium and nitrogen nutrients. The functional additives not only provide long-term and effective regulation of soil pH, but also provide excellent water retention and air permeability. The different particle sizes of the ingredients match the nutrient release rate to the crop growth stage. The small particle size of diammonium hydrogen phosphate dissolves quickly, meeting the crop's initial phosphorus needs and promoting root development. The large particle size and unique structure of isobutylene diurea ensure a slow and sustained release of nitrogen, preventing excessive growth in the early stages and nutrient starvation in the later stages.

[0017] 3. Sugarcane bagasse absorbs water and expands in the soil, effectively improving its air and water permeability, creating a suitable living environment for soil microorganisms. The synergistic interaction between the bacterium Halobacillus dasaka and Bacillus subtilis in the blend allows for rapid colonization in the soil, performing nitrogen fixation, phosphorus and potassium solubilization, and enhancing crop resistance. Furthermore, biochar and sugarcane bagasse provide nutrients for the blended bacteria to grow and reproduce.

[0018] In summary, the present invention ensures that the slow-release fertilizer can improve the saline-alkali soil structure in a long-term manner, accurately regulate nutrient release, and enhance the stress resistance of crops through the synergistic effect of multi-layer structure design and ingredients, and is suitable for the efficient utilization of saline-alkali land. DETAILED DESCRIPTION

[0019] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention. Example 1

[0020] A slow-release fertilizer for saline-alkali land, comprising a core layer, an inner coating, a shell layer and an outer coating from the inside to the outside; The core layer is composed of the following raw materials in parts by weight: 30 parts of functional additives, 20 parts of diammonium hydrogen phosphate, 10 parts of urea, 6 parts of isobutylene diurea, 15 parts of potassium nitrate and 20 parts of biochar; The biochar preparation method is as follows: soybean straw is crushed into particles with a particle size of 3 mm, heated to 500°C at a rate of 5°C / min under anaerobic conditions, pyrolyzed for 3 hours, and then cooled and crushed to obtain biochar with a particle size of 0.1 mm. The shell layer is composed of the following raw materials in parts by weight: 10 parts of sugarcane bagasse, 0.05 parts of a composite bacterial strain, and 4 parts of fulvic acid; the composite bacterial strain is composed of Daban halobacterium and 0.5 times the mass of Bacillus subtilis; and the total effective viable bacteria count is 4×10 9 CFU / g; The coating materials used for the inner coating and the outer coating have the same composition, both containing 0.3 parts by weight of montmorillonite powder and 1.5 parts by weight of an adhesive; the adhesive is composed of polyvinyl alcohol and 0.3 times the weight of oxidized starch.

[0021] The functional additive comprises the following steps: ultrasonically dispersing the modified diatomaceous earth in a 10wt% malic acid aqueous solution at a dosage ratio of 20g / L, then adding methacrylamide with a mass of 1 times that of the malic acid and nonylphenol polyoxyethylene ether with a mass of 0.1 times that of the malic acid respectively; after uniform dispersion, adding cyclohexane with a mass of 3 times that of the malic acid aqueous solution, high-speed shearing at a rate of 5000r / min for 30 minutes, and then adding benzoyl peroxide with a mass of 0.2 times that of the nonylphenol polyoxyethylene ether under nitrogen protection, reacting at a temperature of 50°C for 8 hours, and then filtering and drying.

[0022] The preparation method of modified diatomaceous earth is as follows: dichlorodimethylsilane and nitrogen benzene are mixed in a volume ratio of 1:15, and then diatomaceous earth with a mass 1.5 times that of dichlorodimethylsilane is added, and after mixing, the mixture is stirred and reacted at a temperature of 30°C for 5 hours; after the reaction is completed, witch hazel tannin with a mass 0.2 times that of diatomaceous earth is added while stirring, and the mixture is stirred and reacted for 2 hours, and then filtered, washed and dried.

[0023] Before preparing modified diatomaceous earth, the diatomaceous earth needs to be cleaned. The specific cleaning steps are: calcining diatomaceous earth with a particle size of 100 mesh at a temperature of 500°C for 3 hours, and then soaking it in a 5wt% hydrochloric acid solution for 5 hours; after soaking, the diatomaceous earth is filtered out, washed with water until neutral, and then dried.

[0024] A method for preparing a slow-release fertilizer for saline-alkali land comprises the following steps: Step 1: Mix the raw materials of the core layer and granulate them to obtain a core layer powder with a particle size of 2 mm; Step 2: Mixing montmorillonite powder and adhesive to prepare a coating material; Step 3: Put the core layer powder into the coating machine, add the coating material and coat it to obtain a coated core material with an inner coating thickness of 25 μm; Step 4: The coating core material and the shell layer raw materials are mixed and granulated, and then a coating material is used for secondary coating to obtain a slow-release fertilizer with an outer coating thickness of 35 μm. Example 2

[0025] The preparation method of a slow-release fertilizer for saline-alkali land provided in this embodiment is basically the same as that in Example 1, except that the specific composition of the slow-release fertilizer and the preparation method of the functional additives are not exactly the same. The specific composition of the slow-release fertilizer and the preparation method of the functional additives in this embodiment are as follows: A slow-release fertilizer for saline-alkali land, comprising a core layer, an inner coating, a shell layer and an outer coating from the inside to the outside; The core layer is composed of the following raw materials in parts by weight: 35 parts of functional additives, 25 parts of diammonium hydrogen phosphate, 10 parts of urea, 8 parts of isobutylene diurea, 20 parts of potassium nitrate and 25 parts of biochar; The shell layer is composed of the following raw materials in parts by weight: 15 parts of bagasse, 0.08 parts of composite bacteria and 5 parts of fulvic acid; The coating materials used for the inner coating and the outer coating have the same composition, both containing 0.4 parts by weight of montmorillonite powder and 2 parts by weight of adhesive.

[0026] The functional additive comprises the following steps: ultrasonically dispersing the modified diatomaceous earth in a 15wt% malic acid aqueous solution at a dosage ratio of 30g / L, then adding methacrylamide with a mass of 1.5 times that of the malic acid and nonylphenol polyoxyethylene ether with a mass of 0.2 times that of the malic acid respectively; after uniform dispersion, adding cyclohexane with a mass of 4 times that of the malic acid aqueous solution, high-speed shearing at a rate of 6000r / min for 30 minutes, and then adding benzoyl peroxide with a mass of 0.3 times that of the nonylphenol polyoxyethylene ether under nitrogen protection, reacting at a temperature of 60°C for 6 hours, and then filtering and drying.

[0027] The preparation method of modified diatomaceous earth is as follows: dichlorodimethylsilane and nitrogen benzene are mixed in a volume ratio of 1:20, and then diatomaceous earth with a mass twice that of dichlorodimethylsilane is added, and after mixing, the mixture is stirred and reacted at a temperature of 35°C for 4 hours; after the reaction is completed, witch hazel tannin with a mass of 0.25 times that of diatomaceous earth is added while stirring, and the mixture is stirred and reacted for 3 hours, and then filtered, washed and dried.

[0028] Before preparing modified diatomaceous earth, the diatomaceous earth needs to be cleaned. The specific cleaning steps are: calcining diatomaceous earth with a particle size of 150 mesh at 550°C for 3 hours, and then soaking it in a 10wt% hydrochloric acid solution for 4 hours; after soaking, filter out the diatomaceous earth, wash it with water until it is neutral, and then dry it. Example 3

[0029] The preparation method of a slow-release fertilizer for saline-alkali land provided in this embodiment is basically the same as that in Example 1, except that the specific composition of the slow-release fertilizer and the preparation method of the functional additives are not exactly the same. The specific composition of the slow-release fertilizer and the preparation method of the functional additives in this embodiment are as follows: A slow-release fertilizer for saline-alkali land, comprising a core layer, an inner coating, a shell layer and an outer coating from the inside to the outside; The core layer is composed of the following raw materials in parts by weight: 40 parts of functional additives, 30 parts of diammonium hydrogen phosphate, 15 parts of urea, 10 parts of isobutylene diurea, 25 parts of potassium nitrate and 30 parts of biochar; The shell layer is composed of the following raw materials in parts by weight: 15 parts of bagasse, 0.1 parts of composite bacteria and 6 parts of fulvic acid; The coating materials used for the inner coating and the outer coating have the same composition, both containing 0.6 parts by weight of montmorillonite powder and 2.5 parts by weight of adhesive.

[0030] The functional additive comprises the following steps: ultrasonically dispersing the modified diatomaceous earth in a 20wt% malic acid aqueous solution at a dosage ratio of 50g / L, then adding methacrylamide with a mass of 2 times that of the malic acid and nonylphenol polyoxyethylene ether with a mass of 0.2 times that of the malic acid respectively; after uniform dispersion, adding cyclohexane with a mass of 5 times that of the malic acid aqueous solution, high-speed shearing at a rate of 8000r / min for 20 minutes, and then adding benzoyl peroxide with a mass of 0.5 times that of the nonylphenol polyoxyethylene ether under nitrogen protection, reacting at a temperature of 60°C for 5 hours, and then filtering and drying.

[0031] The preparation method of modified diatomaceous earth is as follows: dichlorodimethylsilane and nitrogen benzene are mixed in a volume ratio of 1:20, and then diatomaceous earth with a mass twice that of dichlorodimethylsilane is added, and after mixing, the mixture is stirred and reacted at a temperature of 40°C for 3 hours; after the reaction is completed, witch hazel tannin with a mass of 0.3 times that of diatomaceous earth is added while stirring, and the mixture is stirred and reacted for 3 hours, and then filtered, washed and dried.

[0032] Before preparing modified diatomaceous earth, the diatomaceous earth needs to be cleaned. The specific cleaning steps are: calcining diatomaceous earth with a particle size of 150 mesh at a temperature of 600°C for 2 hours, and then soaking it in a 10wt% hydrochloric acid solution for 3 hours; after soaking, the diatomaceous earth is filtered out, washed with water until neutral, and then dried.

[0033] Comparative Example 1: This comparative example differs from Example 1 in that modified diatomaceous earth is used to replace an equal amount of the functional additive in this comparative example.

[0034] Comparative Example 2: The difference between this comparative example and Example 1 is that in this comparative example, cleaned diatomaceous earth is used instead of an equal amount of functional additives. Comparative Example 3: This comparative example differs from Example 1 in that an equal amount of Bacillus subtilis is replaced by Halobacillus dasakaensis in this comparative example.

[0035] Comparative Example 4: This comparative example differs from Example 1 in that an equal amount of Halobacillus dasakani was replaced by Bacillus subtilis in this comparative example.

[0036] Comparative Example 5: This comparative example differs from Example 1 in that urea is used to replace an equal amount of isobutylene diurea in this comparative example.

[0037] Experimental verification 1. Experimental Design 1. Experimental Materials Test soil: taken from saline-alkali land in the Hetao Plain of Inner Mongolia, with initial pH=8.5, salt content 0.35%, EC=4.5mS / cm, and ESP=32.5%.

[0038] Test crop: corn (Zhengdan 958), potted experiment, each pot contains 5 kg of soil, each group is repeated 5 times.

[0039] 2. Testing indicators and methods 1. Soil physical and chemical properties pH value: glass electrode method (GB / T 33469-2016), water-soil ratio 1:2.5; Salt content: measured by electrical conductivity (EC meter), water-soil ratio 5:1, converted to NaCl equivalent; Exchangeable sodium content (ESP): extraction with ammonium acetate-ammonium hydroxide buffer, determination of Na by atomic absorption spectrometry + concentration (LY / T1243-1999).

[0040] 2. Microbial activity Effective viable bacteria count: Testing time: 7th, 30th and 60th day after fertilization; Methods: The dilution spread plate method was used. 7% NaCl beef extract medium was used for the culture of Halobacillus dabanensis, and conventional beef extract medium (ISO 11290-1:2017) was used for the culture of Bacillus subtilis.

[0041] 3. Determination of sustained-release performance The static water extraction method (25±1℃) was used, and each group was measured 5 times in parallel. Testing indicators and methods: Nitrogen: Kjeldahl method (GB / T 8572) Phosphorus: Vanadium molybdenum yellow colorimetric method (NY / T 2542) Potassium: Flame photometry (NY / T 2541) Test period: 15 days and 30 days.

[0042] 4. Crop growth indicators Plant height: measured with a ruler (cm), once at the jointing stage (30th day) and once at the filling stage (60th day); Chlorophyll content (SPAD value): measured at the middle of the second leaf with a chlorophyll meter during the grain filling period (60th day); Root activity: Root dehydrogenase activity (μmol / g·h) was determined by the TTC method, and samples were collected during the grain filling period (day 60).

[0043] 3. Experimental Results and Analysis 1. Changes in soil pH

[0044] By analyzing the relevant data in the table, it can be seen that the pH drop in the Example group is significant, which indicates that the sustained release of organic acid in the functional additive and the synergistic effect of witch hazel tannin play a key role, and also confirms from the side that the three-dimensional network structure of the functional additive fixes the organic acid and witch hazel tannin. Comparative Examples 1-2 lack sustained release of organic acid or modifying groups, so the pH drop is relatively small. The single strain in Comparative Examples 3-4 has weak acid production ability, resulting in a relatively weak pH drop.

[0045] 2. Soil salinity and ESP changes

[0046] By analyzing the relevant data in the table, it can be seen that the salt content and ESP of the embodiment group decreased significantly. The porous structure and functional groups (-OH, -COOH) of the modified diatomite efficiently adsorbed Na + , the metabolites of the composite bacteria promote salt dissolution. Comparative Example 1 and Comparative Example 2 lack the protection of the polymer network for organic acids, Na + The adsorption efficiency is low. Comparative Examples 3 and 5 have relatively limited salt removal effects due to insufficient microbial (bacterial) synergy or a single nitrogen source release pattern.

[0047] 3. Dynamics of effective living bacteria count in soil

[0048] Analysis of the relevant data in the table shows that the effective viable cell count in the Example group was significantly higher than that in the Comparative Example group. This is due to the symbiotic relationship between the composite bacterial strains (the salt-tolerant bacteria provide a microenvironment, and the growth-promoting bacteria secrete growth factors) and the biochar carbon source supply being key. Comparative Examples 3-4, due to a lack of synergy between the single bacterial strains, had relatively low effective viable cell counts. Comparative Examples 1-2 lacked the carbon source and microenvironment required for bacterial colonization, resulting in relatively slow viable cell count growth.

[0049] Table 4: Cumulative release rate of nitrogen, phosphorus and potassium (n=5, x̄±s)

[0050] Analysis of the relevant data in the table shows that the nitrogen release rate of the Example group is significantly lower than that of the Comparative Example group. The data in Comparative Example 5 also indirectly confirms that the microbial decomposition and slow-release mechanism of isobutylene diurea plays an important role in this process. This also shows that the slow-release fertilizer for saline-alkali land prepared by the present invention releases nitrogen, phosphorus, and potassium more slowly, has a good slow-release effect, and meets the growth requirements of crops.

[0051] 5. Corn Growth Indicators (Day 60)

[0052] Analysis of the relevant data in the table shows that the plant height, SPAD value, and root activity of the Example group were significantly superior to those of the Comparative Example. The water-retention and air-permeability of the functional additives synergistically promoted growth with the slow-release nutrients. In Comparative Example 5, rapid urea release led to late-stage nutrient depletion and decreased root activity. In Comparative Examples 1-2, soil salinity and alkali stress inhibited crop growth. In Comparative Example 3, the effect of a single bacterial strain was relatively limited, resulting in a relatively small impact on corn root activity.

[0053] The above comparison and analysis demonstrate that the present invention, through its multi-layered structural design and synergistic composition, ensures that the slow-release fertilizer can effectively improve saline-alkali soil structure over the long term, precisely regulate nutrient release, and enhance crop resistance, making it suitable for efficient utilization of saline-alkali land. This demonstrates that the slow-release fertilizer for saline-alkali land and its preparation method provided by the present invention have broader market prospects and are more suitable for promotion.

[0054] Throughout this specification, references to terms such as "one embodiment," "example," or "specific example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0055] The preferred embodiments of the present invention disclosed above are intended only to help illustrate the present invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the present invention to the specific embodiments described. Obviously, many modifications and variations are possible based on the content of this specification. These embodiments are selected and described in detail in this specification to better explain the principles and practical applications of the present invention, thereby enabling those skilled in the art to better understand and utilize the present invention. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A slow-release fertilizer for saline-alkali land, characterized by: From the inside to the outside, it includes the core layer, inner coating, shell layer and outer coating; The core layer is composed of the following raw materials in parts by weight: 30-40 parts of functional additives, 20-30 parts of diammonium hydrogen phosphate, 10-15 parts of urea, 6-10 parts of isobutylene diurea, 15-25 parts of potassium nitrate and 20-30 parts of biochar; The shell layer is composed of the following raw materials in parts by weight: 10-15 parts of bagasse, 0.05-0.1 parts of composite bacteria and 4-6 parts of fulvic acid; The coating materials used for the inner coating and the outer coating have the same composition, both containing 0.3-0.6 parts by weight of montmorillonite powder and 1.5-2.5 parts by weight of adhesive.

2. The slow-release fertilizer for saline-alkali land according to claim 1, characterized in that: The functional additive comprises the following steps: ultrasonically dispersing modified diatomaceous earth in a 10-20wt% organic acid aqueous solution at a dosage ratio of 20-50g / L, then adding methacrylamide with a mass of 1-2 times that of the organic acid and an emulsifier with a mass of 0.1-0.2 times that of the organic acid respectively; after uniform dispersion, adding cyclohexane with a mass of 3-5 times that of the organic acid aqueous solution, high-speed shearing at a rate of 5000-8000 r / min for 20-30 minutes, and then adding benzoyl peroxide with a mass of 0.2-0.5 times that of the emulsifier under nitrogen protection, reacting at a temperature of 50-60°C for 5-8 hours, and finally filtering and drying.

3. The slow-release fertilizer for saline-alkali land according to claim 2, characterized in that: The preparation method of the modified diatomaceous earth comprises the following steps: uniformly mixing dichlorodimethylsilane and nitrogen benzene at a volume ratio of 1:15-20, then adding diatomaceous earth in an amount 1.5-2 times the mass of dichlorodimethylsilane, stirring and reacting at a temperature of 30-40°C for 3-5 hours; and adding witch hazel tannin in an amount 0.2-0.3 times the mass of the diatomaceous earth while stirring after the reaction is completed, stirring and reacting for 2-3 hours, and then filtering, washing and drying the mixture.

4. The slow-release fertilizer for saline-alkali land according to claim 3, characterized in that: Before preparing the modified diatomaceous earth, the diatomaceous earth needs to be cleaned. The specific cleaning steps are as follows: calcining diatomaceous earth with a particle size of 100-150 mesh at a temperature of 500-600°C for 2-3 hours, and then soaking it in a 5-10wt% hydrochloric acid solution for 3-5 hours; After soaking is completed, filter out the diatomaceous earth, wash it with water until it is neutral, and then dry it.

5. The slow-release fertilizer for saline-alkali land according to claim 2, characterized in that: The organic acid is selected from any one of malic acid, tartaric acid and citric acid.

6. The slow-release fertilizer for saline-alkali land according to claim 2, characterized in that: The emulsifier is selected from any one of nonylphenol polyoxyethylene ether and alkylphenol polyoxyethylene ether.

7. The slow-release fertilizer for saline-alkali land according to claim 1, characterized in that: The biochar preparation method comprises the following steps: crushing soybean straw into particles with a particle size of less than 5 mm, heating the mixture to 500-600° C. at a rate of 3-5° C. / min under anaerobic conditions, pyrolyzing the mixture for 2-3 hours, and then cooling and crushing the mixture to obtain biochar with a particle size of 0.1-0.2 mm.

8. The slow-release fertilizer for saline-alkali land according to claim 1, characterized in that: The composite bacteria consists of Daban Halobacillus and Bacillus subtilis with a mass of 0.5 to 0.8 times that of Daban Halobacillus; and the total effective viable bacteria count is ≥3×10 9 CFU / g.

9. The slow-release fertilizer for saline-alkali land according to claim 1, characterized in that: The adhesive consists of polyvinyl alcohol and oxidized starch with a mass of 0.3 to 0.5 times that of polyvinyl alcohol.

10. A method for preparing a slow-release fertilizer for saline-alkali land according to any one of claims 1 to 9, characterized in that: The following steps are involved: Step 1: Mix the raw materials of the core layer and granulate them to obtain a core layer powder with a particle size of 2 to 4 mm; Step 2: Mixing montmorillonite powder and adhesive to prepare a coating material; Step 3: Put the core layer powder into the coating machine, add the coating material and coat it to obtain a coated core material with an inner coating thickness of 25 to 35 μm; Step 4: The coating core material and the shell layer raw materials are mixed and granulated, and then a coating material is used for secondary coating to obtain a slow-release fertilizer with an outer coating thickness of 35 to 45 μm.

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