Hydrogel slow-release fertilizer as well as preparation method and application thereof

Abstract

The invention discloses a hydrogel slow-release fertilizer as well as a preparation method and application thereof. According to the hydrogel, sodium carboxymethyl cellulose, dialdehyde starch and gelatin are crosslinked to form a three-dimensional network structure, after the hydrogel is loaded with urea and other fertilizers, slow release of nutrients can be achieved (the release rate in 25 days is larger than or equal to 95%), and the hydrogel has high water absorption rate (35-45 times) and biodegradability (the soil degradation rate in 7 days is larger than or equal to 60%). The product is suitable for crop cultivation, and can significantly improve the fertilizer utilization rate and reduce environmental pollution.

Description

Technical Field

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

[0002] Traditional chemical fertilizers have low nutrient utilization rates due to their high solubility and rapid release. They are easily lost through leaching, volatilization, and other pathways, resulting in resource waste and environmental pollution. Existing slow-release fertilizers mostly use inorganic minerals or synthetic polymer materials, which have problems such as high cost, poor biodegradability, and complex preparation processes. In the context of sustainable development, the development of green and efficient fertilizer carrier materials has become a new trend. Carboxymethyl cellulose (CMC), as a natural polymer material, has high water absorption, biocompatibility, and degradability, but its slow-release performance is limited when used alone.

[0003] In the prior art, carboxymethyl cellulose (CMC) is a water-soluble anionic cellulose derivative produced by partially replacing the 2nd, 3rd, and 6th hydroxyl groups of the cellulose backbone with carboxymethyl groups. CMC is often compounded with other materials to improve performance, but suffers from issues such as low cross-linking efficiency and unstable sustained-release effects. Because CMC combines the advantages of cellulose itself, including excellent biocompatibility and biodegradability, its unique surface properties, tunable hydrophilicity and pH sensitivity, high stability, and gel formation, it is now widely used in various advanced applications, such as food packaging, preservation, papermaking, textile and pharmaceutical industries, biomedical engineering, wastewater treatment, and energy generation and storage. The application of CMC in these areas depends largely on its purity, degree of polymerization, degree of substitution, and uniformity, which determine the resulting product's properties, such as solubility, particle size, viscosity, and rheological properties. Temperature-responsive fertilizer-loaded hydrogels have been prepared by dispersing thermosensitive monomers in a solution using emulsification with CMC and acrylamide (AM). Free radical polymerization initiated by ammonium sulfate is then used. Related research results have found that the amount of CMC and thermosensitive monomers affects the hydrogel's structure, mechanical properties, swelling capacity, and temperature sensitivity. Furthermore, with the increase of hydrophilic groups, the maximum swelling ratio of the hydrogel can reach 20.56 times. Due to its temperature responsiveness, the hydrogel begins to swell when the temperature rises above the lower critical phase transition temperature. These properties effectively extend the use of fertilizers and reduce the number of applications.

[0004] Dialdehyde starch (DS) is a modified starch containing numerous active aldehyde groups. It exhibits excellent physical, chemical, and biochemical properties, such as alkaline solubility and strong adhesion. Dialdehyde starch is easily gelatinized, resistant to mold, and chemically active, resulting in a wide range of applications. While retaining the advantages of starch, dialdehyde starch improves its performance. It is relatively stable, biocompatible, and easily degradable, offering excellent results, high efficiency, and cost-effectiveness. It is easy to develop into finished products, aligning with the development of a green ecosystem and possessing promising prospects in agriculture.

[0005] Gelatin is a protein derived from the breakdown of collagen, typically processed and extracted from animal skins, bones, and fish scales. Gelatin is a hydrolyzed denatured product of collagen, sharing homology with collagen. It combines the functions of both proteins and polymers, exhibiting excellent plasticity, including colloidal protection, film-forming properties, surface activity, and reversible denaturation between gel and sol states. Rich in macromolecular groups, it is often chosen as the matrix material for hydrogels.

[0006] Hydrogels prepared by combining CMC with DS and Gel exhibit a porous morphology and various polar groups. Adding them to soil can effectively promote crop seed germination and plant growth, providing valuable guidance for the development of green agriculture. Therefore, developing a low-cost, environmentally friendly CMC-based hydrogel slow-release fertilizer has significant application value. Summary of the Invention

[0007] The present invention aims to provide a hydrogel slow-release fertilizer to achieve slow release of fertilizer, improve nutrient utilization and reduce environmental pollution.

[0008] To achieve the above-mentioned object, the present invention provides the following technical solution: a hydrogel slow-release fertilizer, comprising the following ingredients: sodium carboxymethyl cellulose, dialdehyde starch, gelatin, and a loaded fertilizer; the degree of substitution of the sodium carboxymethyl cellulose is 0.7-0.9, and the concentration is 1-3%; the concentration of the dialdehyde starch is 1-3%; the concentration of the gelatin is 10-20%; and the loaded fertilizer comprises one or more of urea, organic fertilizer, and microbial fertilizer.

[0009] Specifically, the dialdehyde starch has an aldehyde content of ≥30%.

[0010] Specifically, the loading amount of the loaded fertilizer accounts for 10-30% of the total mass of the hydrogel.

[0011] A method for preparing a hydrogel slow-release fertilizer comprises the following steps:

[0012] Step 1: Preparation of dialdehyde starch: react corn starch with sodium periodate under acidic conditions to produce dialdehyde starch;

[0013] Step 2: Prepare the solution: Dissolve sodium carboxymethyl cellulose in deionized water to prepare a 1-3% sodium carboxymethyl cellulose solution, and stir magnetically until a transparent colloid is formed; prepare a 1-3% dialdehyde starch solution, sterilize by autoclaving, gelatinize, and adjust the pH to 5.0 after cooling; dissolve gelatin in a 40°C water bath to prepare a 10-20% gelatin solution;

[0014] Step 3: Cross-linking and gelation: Slowly add the dialdehyde starch solution to the sodium carboxymethyl cellulose solution, stir magnetically for 30 minutes, then add the gelatin solution and continue stirring until homogeneous. Let the resulting mixture stand overnight and gel at 4°C.

[0015] Step 4: Freeze-drying: freeze-dry the gelled mixture at -50°C for 48 hours to form a porous hydrogel;

[0016] Step 5: Immersion adsorption: Immerse the dried hydrogel in the loaded fertilizer solution and let it stand at 4°C for 24 hours;

[0017] Step 6: Drying and encapsulation: After adsorbing the active ingredients in the loaded fertilizer, dry to constant weight to obtain the fertilizer-loaded hydrogel, encapsulate and store in the dark.

[0018] Specifically, the dialdehyde starch obtained in the step of preparing dialdehyde starch needs to be centrifuged and washed with an acetone-water solution until it becomes neutral, and then dried and the aldehyde content is determined by hydroxylamine hydrochloride titration to ensure that the oxidation efficiency is ≥30%.

[0019] Specifically, during the preparation of the dialdehyde starch, the acidic condition is established using hydrochloric acid, and the concentration of the hydrochloric acid is controlled to be 1.0M.

[0020] Specifically, the porous structure formed by freeze drying has a pore size of 10-50 μm.

[0021] The invention discloses an application of a hydrogel slow-release fertilizer. The slow-release fertilizer is used in agricultural planting, can improve the growth effect of crops, reduce fertilizer loss, and reduce environmental pollution.

[0022] Specifically, the slow-release fertilizer can significantly increase the leaf area, fresh weight and plant height of tomatoes in tomato cultivation, and promote the growth of tomato seedlings.

[0023] The principle and beneficial effects of this technical solution:

[0024] This invention relates to a hydrogel slow-release fertilizer and its preparation method. Through innovative material combinations and processes, it aims to achieve efficient slow-release fertilizer, improve fertilizer utilization, reduce environmental pollution, and promote crop growth. The core of this technical solution is to utilize the synergistic effect of sodium carboxymethylcellulose (CMC-Na), dialdehyde starch (DS), and gelatin to form a hydrogel with a three-dimensional network structure. After loading fertilizers such as urea, it can achieve slow nutrient release.

[0025] CMC-Na, with its abundant hydroxyl and carboxymethyl groups, forms a three-dimensional network structure, providing space for fertilizer loading. Its degree of substitution directly influences the hydrophilicity and mechanical strength of the hydrogel. A higher degree of substitution results in greater molecular chain stretchability and improved water absorption. Dialdehyde starch, produced by oxidizing corn starch with sodium periodate and containing ≥30% aldehyde groups, reacts with the hydroxyl groups of CMC-Na via a Schiff base reaction to form covalent crosslinks, enhancing the stability of the hydrogel. Gelatin, acting as a physical crosslinker, further crosslinks its amino groups with the aldehyde groups of DS, while its polypeptide chains enhance the hydrogel's toughness through hydrogen bonding. This material combination exhibits significant swelling properties within a pH range of 5.5-8.5, with a swelling ratio of 35-45 times, significantly improving soil water retention. At 25°C, water retention reaches ≥80% after 8 hours, dropping to 50% at 37°C. The hydrophobic regions of gelatin effectively slow water loss.

[0026] The hydrogel slow-release fertilizer has significant slow-release properties. The cumulative urea release rate of the fertilizer-loaded hydrogel within 25 days can reach more than 95%. The initial rapid release (18% release in 24 hours) is combined with the slow release in the middle and late stages, which effectively extends the action time of the fertilizer. Through the fitting of the Ritger-Peppas model, the release mechanism is non-Fickian diffusion, indicating that the release of urea is the result of the synergistic effect of the dissolution of the gel and molecular diffusion. In addition, the hydrogel has good biodegradability. In the soil environment, the degradation rate is ≥60% within 7 days. The cellulase and amylase secreted by soil microorganisms can destroy the network structure of the hydrogel and promote its degradation. In the water environment, the degradation rate in the same period is only 15.14%, indicating that the hydrogel has good stability in the natural water environment and will not cause pollution.

[0027] In terms of agricultural applications, this slow-release fertilizer has shown significant effects in promoting crop growth. In a tomato pot experiment, compared with ordinary urea, the leaf area of ​​tomato plants treated with fertilizer-loaded hydrogel increased by 154.53%, the fresh weight increased by 69.53%, and the plant height increased by 41.51%, which was significantly better than the control group. In addition, the anionic properties of CMC-Na can chelate metal ions in the soil, adjust the soil pH to 6.5-7.0, promote the absorption of nutrients such as phosphorus and potassium by plant roots, and further improve the fertility and structure of the soil. From the perspective of economic and environmental benefits, this technology also has significant advantages. The cost of each ton of hydrogel is about 2,000 yuan, which is about 40% lower than that of traditional synthetic polymer-coated fertilizers. At the same time, this slow-release fertilizer can significantly reduce nitrogen leaching, with the reduction reaching more than 50%, effectively reducing the risk of nitrate contamination of groundwater. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is the SEM morphology of carboxymethyl cellulose hydrogel, showing the porous structure;

[0029] Figure 2 This is the sustained release curve of urea from fertilizer-loaded carboxymethyl cellulose hydrogel (25-day cumulative release rate ≥95%);

[0030] Figure 3 Degradation curve of hydrogel in soil and water (7-day degradation rate ≥ 60%);

[0031] Figure 4 This is the appearance of tomatoes after 30 days of cultivation with hydrogel slow-release fertilizer; (left) tomato seedlings in the blank and treatment groups; (right) tomato leaves in the blank and treatment groups. DETAILED DESCRIPTION

[0032] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments:

[0033] 1Material composition and function:

[0034] like Figure 1 As shown in the figure, the core material of carboxymethyl cellulose-based hydrogel slow-release fertilizer consists of the following four parts, each of which works synergistically to achieve efficient slow-release and environmental protection goals:

[0035] 1.1 Sodium carboxymethyl cellulose (CMC-Na)

[0036] As a hydrogel matrix, CMC-Na has abundant hydroxyl (-OH) and carboxymethyl (-COO - ) groups, forming a three-dimensional network structure that provides space for fertilizer loading. Its degree of substitution (0.7-0.9) directly affects the hydrophilicity and mechanical strength of the hydrogel. The higher the degree of substitution, the stronger the stretchability of the molecular chain and the better the water absorption performance.

[0037] Experiments have shown that when the CMC-Na concentration is between 1% and 3%, the hydrogel can swell 20-35 times (pH 5.5-8.5). A concentration that is too high (>3%) will result in excessive cross-linking density, limiting network expansion; a concentration that is too low (<1%) will make it difficult to form a stable gel.

[0038] 1.2 Dialdehyde starch (DS)

[0039] Made from corn starch oxidized with sodium periodate (NaIO4), the aldehyde content is ≥30%. The aldehyde groups (-CHO) of DS and the hydroxyl groups of CMC-Na form covalent crosslinks through a Schiff base reaction, enhancing the stability of the hydrogel.

[0040] During the oxidation process, the hydrochloric acid concentration (1.0 M) must be controlled to balance the reaction efficiency and the degree of starch chain breakage. -1 The characteristic peak at 47° verified the introduction of aldehyde groups, and X-ray diffraction (XRD) showed that the crystalline structure of the original starch was destroyed and converted into an amorphous state.

[0041] 1.3 Gelatin

[0042] As a physical cross-linker, the amino groups (-NH2) of gelatin further cross-link with the aldehyde groups of DS, while its polypeptide chains enhance the toughness of the hydrogel through hydrogen bonds.

[0043] When the gelatin concentration is between 10% and 20%, the water retention rate of the hydrogel is significantly improved. A concentration that is too high (>20%) will make the gel too rigid and reduce its water absorption capacity; a concentration that is too low (<10%) will not effectively strengthen the network structure.

[0044] 1.4 Active ingredients of fertilizers

[0045] Urea: As a nitrogen source, the loading amount accounts for 10-30% of the total mass of the hydrogel. Urea molecules are loaded through the impregnation adsorption method, and are embedded in the pores of the hydrogel. The release is controlled by diffusion and swelling.

[0046] Organic fertilizer and microbial fertilizer: Mix with CMC-K or CMC-NH4 and use its cation (K + NH4 + ) interacts with the soil, regulating pH and promoting nutrient absorption.

[0047] 2 Preparation technology and scientific principles

[0048] 2.1 Preparation of dialdehyde starch (DS)

[0049] Oxidation reaction: Corn starch reacts with NaIO4 under acidic conditions (1.0M HCl), and the hydroxyl groups at the C2 and C3 positions are oxidized to aldehyde groups, generating dialdehyde starch. The reaction equation is as follows:

[0050] Starch-OH+NaIO4→Starch-CHO+NaIO3+H2O

[0051] Purification and characterization: The reaction product was washed with an acetone-water solution by centrifugation until neutral. After drying, the aldehyde content was determined by titration with hydroxylamine hydrochloride to ensure that the oxidation efficiency was ≥30%.

[0052] 2.2 Synthesis of hydrogel

[0053] Solution Preparation: CMC-Na Solution: Dissolve 1-3% CMC-Na in deionized water and stir magnetically until a transparent colloid forms. DS Solution: Autoclave 1-3% DS (121°C, 20 min) to gelatinize. After cooling, adjust the pH to 5.0. Gelatin Solution: Dissolve 10-20% gelatin in a 40°C water bath to avoid protein denaturation caused by high temperatures.

[0054] Cross-linking and gelation: Slowly add the DS solution to the CMC-Na solution. Stir magnetically for 30 minutes, then add the gelatin solution and continue stirring until homogenized. Allow the mixture to stand overnight (at room temperature) to allow the aldehyde groups to fully react with the hydroxyl / amino groups. Gelation at 4°C enhances physical cross-linking. Freeze-drying (-50°C for 48 hours) forms a porous structure (SEM reveals pore sizes of 10-50 μm), increasing the specific surface area and improving fertilizer loading capacity.

[0055] 2.3 Preparation of fertilizer-loaded hydrogel

[0056] Immersion adsorption: The dried hydrogel was immersed in a 30% urea solution and allowed to stand at 4°C for 24 h to absorb urea by capillary action and hydrogen bonding.

[0057] Drying and packaging: After adsorption saturation, vacuum dry at 40℃ to constant weight, package and store in the dark to avoid photolysis of urea.

[0058] Example 1: Preparation of urea-loaded hydrogel

[0059] 1) Preparation of CMC-Na Solution: A certain amount of CMC-Na powder was dispersed in a beaker containing a certain amount of deionized water. A magnetic stirrer with a uniform speed was used in a 40-50°C water bath (this can increase the dispersion rate of the CMC-Na powder in water) to uniformly dissolve the CMC-Na powder in the water to obtain a 3% by mass CMC-Na solution for later use.

[0060] 2) Preparation of mixed solution: Take another beaker, weigh a certain amount of DS powder, and add an appropriate amount of deionized water to prepare a 3% DS solution by mass. Slowly pour the prepared DS solution into the previously prepared CMC solution, and turn on the magnetic stirrer to stir at a uniform speed to promote uniform mixing of the two. Next, take a certain amount of gelatin particles, add them to a beaker filled with a certain amount of deionized water, and heat and dissolve them in a water bath at 50-60°C to make a 10% gelatin solution. After the gelatin is completely dissolved, quickly add it to the mixed solution of CMC and DS, immediately turn on the magnetic stirrer, and stir for about 30 minutes to fully mix the solutions of the three components to form a mixed solution with a uniform texture.

[0061] 3) Gelation and Freeze-Drying: The resulting mixed solution is transferred to a clean glass Petri dish, sealed with plastic wrap, and allowed to stand at room temperature to allow the mixture to naturally gel and form a hydrogel. (The mixture can be placed in a refrigerator at 4°C to accelerate the gelation process.) After gelation is complete, the Petri dish is placed in a freeze dryer to remove the water from the hydrogel, yielding a dry gel sample.

[0062] 4) Urea Loading and Final Drying: Prepare a 20% urea solution and completely immerse the freeze-dried gel sample in the solution, ensuring full contact between the gel and the solution. Allow the sample to soak at room temperature for 24 hours to allow the urea to fully penetrate the gel. After soaking, remove the gel, remove any excess surface solution with filter paper, and place the gel in a drying oven to remove any remaining moisture, ultimately yielding the urea-loaded hydrogel.

[0063] Example 2: Sustained release performance test

[0064] For the sustained-release experiment, pure water was selected as the release medium for the hydrogel. The urea-loaded hydrogel was placed in a beaker filled with 500ml of pure water and stirred at 220rpm in the dark at room temperature. Samples were taken every 2 hours for the first 12 hours, every 12 hours for the next week, and every 1 day after that. 1ml of sample was taken each time, and the overall sustained-release system was replenished accordingly. This was repeated three times, and the cumulative release rate of urea was calculated using the following formula:

[0065]

[0066] like Figure 2-3As shown in the figure, the overall release trend of urea is that the cumulative release rate gradually increases over time. As can be seen from the figure, urea is released rapidly on the first day, releasing nearly 18% in just 24 hours. However, the overall cumulative release rate growth in the initial stage (around 0-5 days) is relatively slow; then the growth rate gradually accelerates, especially after 7 days, the cumulative release rate rises more significantly, and then becomes relatively slow, reaching 98.37% around the 25th day. Compared with the rapid release of pure urea, the hydrogel model achieves a better sustained release effect, prolongs the release time of urea, and further increases the effect of urea.

[0067] Example 3: Potted Plant Application Effect

[0068] like Figure 4 As shown, an appropriate amount of tomato seeds were surface-sterilized by soaking in a 5% sodium hypochlorite solution for 3 minutes. Afterwards, the seeds were rinsed 3-5 times with deionized water and then placed in clean water for 24 hours. The soaked seeds were transferred to a Petri dish lined with cotton and gauze and placed in the dark to germinate for 2-3 days, with the water in the dish changed regularly. Seeds of uniform size and whitening were selected for future use. Pot experiments were conducted in a greenhouse at room temperature and natural light. Equal weights of soil samples were weighed and mixed uniformly for each pot according to the different treatments. The pots were filled two-thirds full with soil, and randomly selected germinated seeds were transferred to the pots, with the remaining soil covering the tops of the seeds. After sowing, the soil was moistened until water just began to flow from the bottom of the pot. The pots were then watered evenly according to their water needs. Watering was stopped after 30 days of cultivation, and the tomato plants were measured for plant height, root length, leaf area, and fresh weight.

[0069] Measurements of common growth indicators of the resulting tomato seedlings revealed that the average plant height of the group treated with the fertilizer-loaded hydrogel was approximately 10 cm, higher than the 7 cm in the blank control group. A comparison of the fresh weight of the treated tomato seedlings showed a modest improvement, with the average fresh weight difference between the two groups being approximately 100 mg. After 30 days of cultivation, the leaf area of ​​the tomato seedlings in the blank control group was approximately 200 mm², while that in the hydrogel-treated group was approximately 500 mm², demonstrating a significant effect. Overall, the prepared fertilizer-loaded hydrogel can promote the growth of tomato seedlings when applied in greenhouse cultivation. The effect on leaf area was particularly pronounced, with the control group increasing leaf area by nearly 154.53%, fresh weight of tomato seedlings by 69.53%, and plant height by nearly 41.51%. However, the effect on root length was less pronounced, at only approximately 24.33%.

[0070] 3 Conclusion

[0071] 3.1 Water absorption and water retention properties

[0072] Swelling Behavior: The hydrogel exhibits significant swelling properties within the pH range of 5.5-8.5, with a swelling ratio of up to 35-45 times. As the pH increases, the degree of ionization of the carboxyl groups (-COO-) in sodium carboxymethyl cellulose (CMC-Na) increases, leading to increased electrostatic repulsion between molecular chains, thereby expanding the network pores and further enhancing the hydrogel's water absorption capacity.

[0073] Water Retention: At 25°C, after 8 hours of testing, the hydrogel's water retention rate exceeded 80%. However, when the temperature rose to 37°C, the rate of water evaporation accelerated due to the accelerated molecular thermal motion, and the water retention rate dropped to around 50%. The hydrophobic regions of gelatin played a key role in this process, effectively slowing water loss through physical barriers.

[0074] 3.2 Sustained-release performance and kinetic model

[0075] Urea release curve: The fertilizer-loaded hydrogel achieved a cumulative urea release rate exceeding 95% within 25 days. In the initial release phase (within 24 hours), urea release was rapid, reaching approximately 18%. This was primarily due to the rapid release of urea molecules adsorbed on the hydrogel surface through diffusion. In the middle and later stages, the release rate gradually slowed, with the release process primarily controlled by the deswelling behavior of the hydrogel network.

[0076] Model fitting: The Ritger-Peppas model (formula is y = 15.67x 0.60 , R 2 =0.995) were used to fit the urea release process, and the results showed that the release mechanism was non-Fickian diffusion (n=0.60), indicating that the release of urea was the result of the synergistic effect of gel dissolution and molecular diffusion.

[0077] 3.3 Biodegradability and environmental compatibility

[0078] Soil degradation: In soil environments, the hydrogel degrades rapidly, reaching over 60% within 7 days. This is because soil microorganisms can secrete cellulase and amylase, which can destroy the hydrogel's network structure, thereby promoting its degradation.

[0079] Aqueous degradation: Within the same degradation cycle, the hydrogel's degradation rate in water was only 15.14%. This result further confirms that microbial activity plays a key role in the hydrogel's degradation process. It also shows that the hydrogel is relatively stable in natural aquatic environments and will not cause water pollution.

[0080] 3.4 Application Effect and Agricultural Value

[0081] Crop Growth Promotion: A potted tomato experiment showed significant growth advantages compared to conventional urea-treated tomato plants. Specifically, leaf area increased by 154.53%, fresh weight increased by 69.53%, and plant height increased by 41.51%. However, the effect on seedling root growth was relatively small, at only 24.33%. This is because slow-release nitrogen better matches the plant's fertilization cycle, reducing nutrient waste and thus more effectively promoting plant growth.

[0082] Soil improvement: The anionic properties of CMC-Na (-COO-) can chelate Al3+ and Fe in the soil 3+ The pH value of the soil is adjusted to the appropriate range of 6.5-7.0, thereby promoting the absorption of nutrients such as phosphorus and potassium by plant roots, and further improving the fertility and structure of the soil.

[0083] 3.5 Economic and environmental benefits:

[0084] Cost Analysis: In terms of raw material costs, sodium carboxymethylcellulose (CMC-Na) costs approximately 50 yuan / kg, dialdehyde starch (DS) costs approximately 30 yuan / kg, and gelatin costs approximately 80 yuan / kg. Overall, the cost per ton of hydrogel is approximately 2,000 yuan, a 40% reduction compared to traditional synthetic polymer-coated fertilizers.

[0085] Emission reduction potential: The use of this hydrogel slow-release fertilizer can significantly reduce nitrogen leaching by more than 50%, thereby effectively reducing the risk of nitrate contamination in groundwater and having significant environmental benefits.

[0086] The above is only an embodiment of the present invention, and common knowledge such as the specific technical solutions or characteristics in the solution is not described in detail here. For those skilled in the art, without departing from the technical solution of the present invention, several variations and improvements can be made, which should also be regarded as the scope of protection of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the description can be used to interpret the content of the claims.

Claims

1. A hydrogel slow-release fertilizer, characterized in that: The invention comprises the following ingredients: sodium carboxymethyl cellulose, dialdehyde starch, gelatin and loaded fertilizer; the substitution degree of the sodium carboxymethyl cellulose is 0.7-0.9 and the concentration is 1-3%; the concentration of the dialdehyde starch is 1-3%; the concentration of the gelatin is 10-20%; and the loaded fertilizer comprises one or more of urea, organic fertilizer and microbial fertilizer.

2. The hydrogel slow-release fertilizer according to claim 1, characterized in that: The dialdehyde starch has an aldehyde content of ≥30%.

3. The hydrogel slow-release fertilizer according to claim 1, characterized in that: The loading amount of the loaded fertilizer accounts for 10-30% of the total mass of the hydrogel.

4. A method for preparing the hydrogel slow-release fertilizer according to claims 1-3, characterized in that: The following steps are involved: Step 1: Preparation of dialdehyde starch: react corn starch with sodium periodate under acidic conditions to produce dialdehyde starch; Step 2: Prepare the solution: Dissolve sodium carboxymethyl cellulose in deionized water to prepare a 1-3% sodium carboxymethyl cellulose solution, and stir magnetically until a transparent colloid is formed; prepare a 1-3% dialdehyde starch solution, sterilize by autoclaving, gelatinize, and adjust the pH to 5.0 after cooling; dissolve gelatin in a 40°C water bath to prepare a 10-20% gelatin solution; Step 3: Cross-linking and gelation: Slowly add the dialdehyde starch solution to the sodium carboxymethyl cellulose solution, stir magnetically for 30 minutes, then add the gelatin solution and continue stirring until homogeneous. Let the resulting mixture stand overnight and gel at 4°C. Step 4: Freeze-drying: freeze-dry the gelled mixture at -50°C for 48 hours to form a porous hydrogel; Step 5: Immersion adsorption: Immerse the dried hydrogel in the loaded fertilizer solution and let it stand at 4°C for 24 hours; Step 6: Drying and encapsulation: After adsorbing the active ingredients in the loaded fertilizer, dry to constant weight to obtain the fertilizer-loaded hydrogel, encapsulate and store in the dark.

5. The method for preparing the hydrogel slow-release fertilizer according to claim 4, characterized in that: The dialdehyde starch obtained in the step of preparing dialdehyde starch needs to be centrifuged and washed with an acetone-water solution until it is neutral, and then dried and the aldehyde content is determined by hydroxylamine hydrochloride titration to ensure that the oxidation efficiency is ≥30%.

6. The method for preparing the hydrogel slow-release fertilizer according to claim 4, characterized in that: During the preparation of the dialdehyde starch, the acidic condition is established using hydrochloric acid, and the concentration of the hydrochloric acid is controlled to be 1.0M.

7. The preparation method according to claim 6, characterized in that: The porous structure formed by freeze drying has a pore size of 10-50 μm.

8. An application of the hydrogel slow-release fertilizer according to claim 1, characterized in that: The slow-release fertilizer is used in agricultural planting, can improve the growth effect of crops, reduce fertilizer loss, and reduce environmental pollution.

9. The use of the hydrogel slow-release fertilizer according to claim 8, characterized in that: The slow-release fertilizer can significantly increase the leaf area, fresh weight and plant height of tomatoes during tomato cultivation, and promote the growth of tomato seedlings.

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