Functional slow-release fertilizer and preparation method thereof

Abstract

The invention relates to the technical field of agricultural fertilizers, in particular to a functional slow-release fertilizer and a preparation method thereof.The functional slow-release fertilizer is prepared from modified chitosan-sodium alginate hydrogel and a fertilizer according to the mass ratio of 1: (60-70); the modified chitosan-sodium alginate hydrogel is prepared by cross-linking modified chitosan and sodium alginate through a cross-linking agent; the structure of the cross-linking agent contains an isocyanate group, and the functionality of the cross-linking agent is not less than 2. The functional slow-release fertilizer has excellent water retention performance and mechanical performance, can improve soil fertility, reduce pathogenic bacteria in soil and improve the soil environment, and is small in environmental pollution.

Description

Technical Field

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

[0002] With the global population growth and the increasing demand for food, agricultural production is facing unprecedented pressure. After the application of traditional fertilizers, nutrients often dissolve quickly in the soil solution, resulting in low nutrient utilization and serious loss, which not only increases production costs, but also may cause pollution to water bodies and the atmospheric environment. Therefore, the development of efficient and environmentally friendly slow-release fertilizers has become the key to solving current agricultural problems.

[0003] Slow-release fertilizer refers to a fertilizer with slow-release characteristics that is formed by encapsulating, adsorbing or chemically bonding nutrients in a carrier material through physical, chemical or biological methods. Its theoretical basis mainly includes nutrient release kinetics, soil chemistry and plant nutrition. Previous research in the field of slow-release fertilizers has mainly focused on the selection of coating materials, optimization of preparation processes and exploration of release mechanisms. However, there are still some gaps in current research, such as too fast a slow-release rate and a single function.

[0004] For example, CN116573967B discloses "a water-retaining slow-release humic acid fertilizer" which uses amino-terminated polydimethylsiloxane and methylene diphenyl diisocyanate to synthesize a linear polymer intermediate, in which the urea amide bonds and urea will produce cross-linking to form a three-dimensional structure to achieve a slow-release effect. At the same time, after absorbing water and swelling, the diffusion distance of urea and humic acid is further increased, slowing down the release rate.

[0005] For another example, CN112624865B discloses “a water-retaining slow-release fertilizer and its preparation method” which includes a fertilizer core, a slow-release layer and a water-retaining layer. The innermost layer is the fertilizer core, the inner layer is the slow-release layer, and the outer layer is the water-retaining layer. The water-retaining slow-release fertilizer can have excellent water-retaining performance while ensuring a good slow-release speed.

[0006] Slow-release fertilizers are often coated or hydrogelled to achieve a slow-release effect. However, hydrogels tend to swell when in contact with water, causing the mechanical properties of the hydrogels to deteriorate. At the same time, they are more likely to break under a certain pressure, which seriously affects the slow-release effect of the fertilizer. In addition, some pathogens in the soil may also have a certain inhibitory effect on the growth of crops.

[0007] Therefore, it is urgent to develop a slow-release fertilizer with excellent water retention and mechanical properties, which can improve soil fertility, reduce pathogens in the soil, and reduce environmental pollution. Summary of the invention

[0008] The main purpose of the present invention is to provide a functional slow-release fertilizer and a preparation method thereof. The slow-release fertilizer has excellent water retention and mechanical properties, can improve soil fertility, reduce pathogens in the soil, improve the soil environment, and has little pollution to the environment.

[0009] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0010] On one hand, the present invention provides a functional slow-release fertilizer, comprising a modified chitosan-sodium alginate hydrogel and a fertilizer in a mass ratio of 1:(60-70); the modified chitosan-sodium alginate hydrogel is prepared by cross-linking modified chitosan and sodium alginate with a cross-linking agent; the structure of the cross-linking agent contains an isocyanate group, and the functionality of the cross-linking agent is not less than 2.

[0011] Plant growth is a long and continuous process, and their nutrient requirements vary at different growth stages. Although traditional quick-acting fertilizers can provide nutrients quickly, they often result in a large amount of nutrients being released in a short period of time and cannot meet the stable needs of the plant throughout its entire growth cycle.

[0012] The nutrient release rate of slow-release fertilizers is relatively slow, which means that the loss and waste of nutrients in the soil is greatly reduced. Compared with traditional quick-acting fertilizers, slow-release fertilizers can better meet the actual needs of plants and make nutrients closer to the growth rhythm of plants, thereby greatly improving the utilization rate of nutrients. This not only reduces the use of chemical fertilizers, but also reduces agricultural production costs.

[0013] There are various methods for preparing slow-release fertilizers. The most common method is to coat the fertilizer with hydrogel to achieve a slow-release effect. However, the hydrogel-coated fertilizer absorbs water and swells during use, and will break under a certain pressure, resulting in a poor slow-release effect, which in turn increases the cost. The reason is that the mechanical properties of the hydrogel will decrease after the hydrogel absorbs water and swells. The slow-release fertilizer of the present application is prepared by coating the fertilizer with a modified chitosan-sodium alginate hydrogel with a specific structure. The slow-release fertilizer not only has the ability of slow release and water retention, but also will not swell and break the hydrogel due to factors such as pressure during use. At the same time, it can improve the soil environment and reduce the occurrence of crop diseases and insect pests.

[0014] In some embodiments, the preparation of the modified chitosan-sodium alginate hydrogel further includes polyacrylamide and hydroquinone.

[0015] The present application introduces polyacrylamide as a raw material and prepares a modified chitosan-sodium alginate hydrogel under the synergistic effect of hydroquinone, which can further improve the mechanical strength of the hydrogel. The reason may be that polyacrylamide contains a large number of amide groups, which can cross-link with hydroquinone and form hydrogen bonds with modified chitosan and sodium alginate, so that the hydrogel is endowed with a double network structure, thereby further improving the mechanical strength of the hydrogel without affecting the water retention capacity of the hydrogel.

[0016] In some embodiments, the cross-linking agent further contains a phenyl group in its structure.

[0017] The present application selects a cross-linking agent with a specific structure. On the one hand, the isocyanate group in the structure can react with the hydroxyl groups in the modified chitosan and sodium alginate to form carbamate groups, thereby increasing the hardness of the hydrogel. At the same time, the presence of the phenyl group can form a π-π stacking effect, thereby increasing the stability of the hydrogel molecular chain and further increasing the mechanical properties of the hydrogel, thereby preventing the slow-release fertilizer from being broken by external forces after swelling due to water absorption during use.

[0018] In some embodiments, the preparation method of the modified chitosan comprises the following steps: dissolving chitosan in an acetic acid solution to obtain a chitosan solution, then adjusting the pH of the system to 8-9, adding an ethanol dilution of benzaldehyde thereto, stirring at room temperature for 3-4 hours, filtering after the reaction is completed, washing with ethanol, and drying to obtain the modified chitosan.

[0019] Chitosan contains a large number of active amino groups in its structure, which are easy to react with the isocyanate groups in the cross-linking agent structure to form urea groups. The structural rigidity of the urea groups is not only lower than that of the carbamate groups, but it may also increase the brittleness of the hydrogel. The present application uses Schiff base to block the amino groups in chitosan, thereby increasing the reactivity of the cross-linking agent with the hydroxyl group and thus improving the mechanical properties of the hydrogel. At the same time, during the post-treatment process, only acid hydrolysis treatment is required to expose the amino groups again to restore the water absorption and water retention capacity of the hydrogel.

[0020] In some embodiments, the mass ratio of chitosan to benzaldehyde is 1:(4-6).

[0021] In some embodiments, the method for preparing the modified chitosan-sodium alginate hydrogel comprises the following steps:

[0022] A1. Disperse sodium alginate in N,N-dimethylformamide, then add modified chitosan and N,N-dimethylformamide, and then add a crosslinking agent, heat to 60-80° C. and stir at constant temperature for 8-10 hours under an inert protective gas atmosphere, and wash with deionized water after the reaction to obtain a product;

[0023] A2, placing the product in step A1 in a dilute hydrochloric acid solution and allowing to stand for 6 to 8 hours, then washing with ethanol and drying to obtain a hydrogel intermediate;

[0024] A3. Mix the hydrogel intermediate in step A2 with polyacrylamide, add deionized water and stir at room temperature for 30 to 50 minutes, then add hydroquinone and potassium chloride and stir for 15 to 25 minutes, wash with ethanol solution, and dry to obtain a modified chitosan-sodium alginate hydrogel.

[0025] The modified chitosan-sodium alginate hydrogel of the present application uses natural extracts sodium alginate and chitosan as main raw materials, and cooperates with environmentally friendly polyacrylamide to prepare a double-layer cross-linked structure hydrogel. On the one hand, chitosan has a certain antibacterial ability, reduces the growth of pathogens in the soil, and improves the soil environment; on the other hand, the double-layer cross-linked structure improves the swelling stability and mechanical properties of the hydrogel, is not easy to break when subjected to external stress, improves the stability of the slow-release fertilizer, and at the same time, the increase in volume during the swelling process can slow down the release rate of the fertilizer.

[0026] In some embodiments, in step A1, the mass ratio of sodium alginate to modified chitosan is 1:(1.5-2.5).

[0027] In some embodiments, in step A1, the mass of the cross-linking agent is 0.7 to 0.9 times the total mass of sodium alginate and modified chitosan.

[0028] In some embodiments, in step A3, the mass ratio of the hydrogel intermediate, polyacrylamide and hydroquinone is 1:(0.2-0.5):(0.02-0.06).

[0029] Another aspect of the present invention provides a method for preparing a functional slow-release fertilizer, comprising the following steps: uniformly mixing the modified chitosan-sodium alginate hydrogel and the fertilizer, and then granulating to obtain the functional slow-release fertilizer.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] (1) The functional slow-release fertilizer of the present invention is prepared by coating the fertilizer with modified chitosan-sodium alginate hydrogel. The slow-release fertilizer has excellent water retention and mechanical properties, can improve soil fertility, reduce pathogens in the soil, improve the soil environment, and has little pollution to the environment.

[0032] (2) The modified chitosan-sodium alginate hydrogel of the present invention uses natural extract sodium alginate and chitosan as main raw materials, and cooperates with environmentally friendly polyacrylamide to prepare a double-layer cross-linked structure hydrogel. Chitosan has a certain antibacterial ability, reduces the growth of pathogens in the soil, and improves the soil environment; in addition, the double-layer cross-linked structure improves the swelling stability and mechanical properties of the hydrogel, and is not easy to break when subjected to external stress, thereby improving the stability of the slow-release fertilizer. At the same time, the increase in volume during the swelling process can slow down the release rate of the fertilizer.

[0033] (3) The present invention uses a cross-linking agent with a specific structure, in which the isocyanate group can react with the hydroxyl groups in the modified chitosan and sodium alginate to form a carbamate group, thereby increasing the hardness of the hydrogel; in addition, the presence of the phenyl group can form a π-π stacking effect, thereby increasing the stability of the hydrogel molecular chain and further increasing the mechanical properties of the hydrogel, thereby preventing the slow-release fertilizer from being broken by external forces after swelling due to water absorption during use.

[0034] (4) The modified chitosan of the present invention is modified by using a Schiff base, which can block the amino groups in the chitosan, increase the reactivity of the crosslinking agent and the hydroxyl group, and thus improve the mechanical properties of the hydrogel. At the same time, during the post-treatment process, only acid hydrolysis treatment is required to expose the amino groups again to restore the water absorption and water retention capacity of the hydrogel. DETAILED DESCRIPTION

[0035] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0036] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0037] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. Various modifications and variations may be made to the specific embodiments of the present invention specification without departing from the scope or spirit of the present invention, which will be apparent to those skilled in the art. Other embodiments obtained from the present invention specification will be apparent to the technician. The present application specification and examples are exemplary only.

[0038] It should be noted that the operations of "drying", "filtering" and "stirring" described in the present invention are routine operations of those skilled in the art and can be selected according to actual operations.

[0039] In the following examples and comparative examples, chitosan and sodium alginate were purchased from Hubei Xingdongcheng Chemical Co., Ltd.; polyacrylamide was purchased from Tianjin Biboyuan Technology Development Co., Ltd.

[0040] In the following embodiments, the fertilizer includes urea, monoammonium phosphate and potassium sulfate in a mass ratio of 6:1:1.

[0041] Preparation Example 1

[0042] The preparation method of modified chitosan comprises the following steps: dissolving 2g of chitosan in 200mL of 1wt% acetic acid solution to obtain a chitosan solution, then adjusting the pH value of the system to 8.5 with a 1mol / L sodium hydroxide solution, adding 130mL of an ethanol dilution containing 10g of benzaldehyde thereto, stirring at room temperature for 3.5h, filtering after the reaction is completed, washing with ethanol, and drying to obtain the modified chitosan.

[0043] Preparation Example 2

[0044] The preparation method of modified chitosan is the same as that of Preparation Example 1, except that the amount of benzaldehyde is 6 g.

[0045] Preparation Example 3

[0046] The preparation method of modified chitosan-sodium alginate hydrogel comprises the following steps:

[0047] A1. Disperse 2 g of sodium alginate in 10 mL of N,N-dimethylformamide, then add 4 g of modified chitosan and 100 mL of N,N-dimethylformamide, and then add 5 g of diphenylmethane diisocyanate, heat to 700 °C under N2 atmosphere and stir for 9 h, wash with deionized water after the reaction is completed to obtain a product;

[0048] A2, placing the product in step A1 in 400 mL of 1 mol / L dilute hydrochloric acid solution and letting it stand for 7 h, then washing with ethanol and drying to obtain a hydrogel intermediate;

[0049] A3. Mix 5 g of the hydrogel intermediate in step A2 with 1.5 g of polyacrylamide, add 600 mL of deionized water and stir at room temperature for 40 min, then add 0.2 g of hydroquinone and 0.7 g of potassium chloride and stir for 20 min, wash with 95 wt% ethanol solution, and dry to obtain a modified chitosan-sodium alginate hydrogel.

[0050] Wherein, the modified chitosan is prepared by Preparation Example 1.

[0051] Preparation Example 4

[0052] The preparation method of modified chitosan-sodium alginate hydrogel has the same specific implementation mode as Preparation Example 3, except that the modified chitosan is prepared by Preparation Example 2.

[0053] Preparation Example 5

[0054] The preparation method of modified chitosan-sodium alginate hydrogel is the same as that of Preparation Example 3, except that the amount of modified chitosan is 7 g.

[0055] Preparation Example 6

[0056] The preparation method of modified chitosan-sodium alginate hydrogel is the same as that of Preparation Example 3, except that the amount of diphenylmethane diisocyanate is 3.5 g.

[0057] Preparation Example 7

[0058] The preparation method of modified chitosan-sodium alginate hydrogel is the same as that of Preparation Example 3, except that the amount of diphenylmethane diisocyanate is 6.5 g.

[0059] Preparation Example 8

[0060] The preparation method of chitosan-sodium alginate hydrogel has the same specific implementation as that of Preparation Example 3, except that an equal mass of chitosan is used instead of modified chitosan.

[0061] Preparation Example 9

[0062] The preparation method of modified chitosan-sodium alginate hydrogel has the same specific implementation as that of Preparation Example 3, except that an equal mass of 1,6-hexamethylene diisocyanate is used instead of diphenylmethane diisocyanate.

[0063] Preparation Example 10

[0064] The preparation method of modified chitosan-sodium alginate hydrogel comprises the following steps:

[0065] A1. Disperse 2 g of sodium alginate in 10 mL of N,N-dimethylformamide, then add 4 g of modified chitosan and 100 mL of N,N-dimethylformamide, and then add 5 g of diphenylmethane diisocyanate, heat to 700 °C under N2 atmosphere and stir for 9 h, wash with deionized water after the reaction is completed to obtain a product;

[0066] A2. The product in step A1 was placed in 400 mL of 1 mol / L dilute hydrochloric acid solution and allowed to stand for 7 h, then washed with ethanol and dried to obtain a modified chitosan-sodium alginate hydrogel.

[0067] Wherein, the modified chitosan is prepared by Preparation Example 1.

[0068] Example 1

[0069] A functional slow-release fertilizer comprises a modified chitosan-sodium alginate hydrogel and a fertilizer in a mass ratio of 1:65; the modified chitosan-sodium alginate hydrogel is prepared by Preparation Example 3.

[0070] The preparation method of the functional slow-release fertilizer comprises the following steps: uniformly mixing the modified chitosan-sodium alginate hydrogel and the fertilizer, and then granulating to obtain the functional slow-release fertilizer.

[0071] Example 2

[0072] A functional slow-release fertilizer comprises a modified chitosan-sodium alginate hydrogel and a fertilizer in a mass ratio of 1:60; the modified chitosan-sodium alginate hydrogel is prepared by Preparation Example 3.

[0073] The preparation method of the functional slow-release fertilizer comprises the following steps: uniformly mixing the modified chitosan-sodium alginate hydrogel and the fertilizer, and then granulating to obtain the functional slow-release fertilizer.

[0074] Example 3

[0075] A functional slow-release fertilizer comprises a modified chitosan-sodium alginate hydrogel and a fertilizer in a mass ratio of 1:70; the modified chitosan-sodium alginate hydrogel is prepared by Preparation Example 3.

[0076] The preparation method of the functional slow-release fertilizer comprises the following steps: uniformly mixing the modified chitosan-sodium alginate hydrogel and the fertilizer, and then granulating to obtain the functional slow-release fertilizer.

[0077] Example 4

[0078] A functional slow-release fertilizer and a preparation method thereof. The specific implementation manner is the same as that of Example 1, except that the modified chitosan-sodium alginate hydrogel is prepared by Preparation Example 10.

[0079] Example 5

[0080] A functional slow-release fertilizer and a preparation method thereof. The specific implementation manner is the same as that of Example 1, except that the modified chitosan-sodium alginate hydrogel is prepared by Preparation Example 4.

[0081] Example 6

[0082] A functional slow-release fertilizer and a preparation method thereof. The specific implementation manner is the same as that of Example 1, except that the modified chitosan-sodium alginate hydrogel is prepared by Preparation Example 5.

[0083] Example 7

[0084] A functional slow-release fertilizer and a preparation method thereof. The specific implementation manner is the same as that of Example 1, except that the modified chitosan-sodium alginate hydrogel is prepared by Preparation Example 6.

[0085] Example 8

[0086] A functional slow-release fertilizer and a preparation method thereof. The specific implementation manner is the same as that of Example 1, except that the modified chitosan-sodium alginate hydrogel is prepared by Preparation Example 7.

[0087] Example 9

[0088] A functional slow-release fertilizer and a preparation method thereof. The specific implementation manner is the same as that of Example 1, except that the modified chitosan-sodium alginate hydrogel is prepared by Preparation Example 9.

[0089] Comparative Example 1

[0090] A functional slow-release fertilizer and a preparation method thereof. The specific implementation manner is the same as that of Example 1, except that an equal mass of chitosan-sodium alginate hydrogel is used instead of the modified chitosan-sodium alginate hydrogel, and the chitosan-sodium alginate hydrogel is prepared by Preparation Example 8.

[0091] Performance Testing:

[0092] (1) Slow-release performance test: refer to GB / T 23348-2009 "Slow-release fertilizer" standard to test nutrient release rate;

[0093] (2) Water retention rate test: refer to "Test Example 2 - Determination of soil water retention" in publication number CN112624865B for testing;

[0094] (3) Mechanical properties test: The test was conducted with reference to "1.4.1 Determination of mechanical properties" in the paper "Regulation mechanism of sustained-release performance of surface hydrophobic sodium alginate microsphere water-retaining agent" published in the journal Chemical Engineering.

[0095] The functional slow-release fertilizers of the embodiments and comparative examples were tested according to the above test method. The test results are shown in Table 1 and Table 2.

[0096] Table 1

[0097]

[0098]

[0099] Table 2

[0100] Water retention rate after mechanical treatment (%) Example 1 86.18 Example 2 85.66 Example 3 85.82 Example 4 88.97 Example 5 84.28 Example 6 86.23 Example 7 83.72 Example 8 90.64 Example 9 82.14 Comparative Example 1 78.25

[0101] According to the data in Table 1 and Table 2, the functional slow-release fertilizers prepared in Examples 1 to 4 have good slow-release performance, water retention performance and mechanical properties; in Example 5, due to the change in the mass ratio of chitosan to benzaldehyde, some of the amino groups therein still have activity and preferentially react with the cross-linking agent to form urea groups with weaker rigidity, thereby reducing the mechanical properties of the slow-release fertilizer; in Example 6, due to the change in the mass ratio of sodium alginate and modified chitosan, the electrostatic repulsion between the -COOH groups is weakened, and the pores in the hydrogel are reduced, resulting in poor water absorption performance of the slow-release fertilizer; in Example 7, due to the change in the ratio of diphenylmethane diisocyanate to the total mass of sodium alginate and modified chitosan, the reduction of diphenylmethane diisocyanate This results in excessive fluidity of the hydrogel, which is not conducive to the formation of gel, thereby reducing the water retention performance of the slow-release fertilizer, and the mechanical properties are also reduced; in Example 8, due to the change in the ratio of diphenylmethane diisocyanate to the total mass of sodium alginate and modified chitosan, the increase in diphenylmethane diisocyanate makes the cross-linking density of the hydrogel too large, resulting in a decrease in the slow-release ability of the slow-release fertilizer, but improved mechanical properties and water retention capacity; in Example 9, due to the use of an equal mass of 1,6-hexamethylene diisocyanate instead of diphenylmethane diisocyanate, the mechanical properties of the slow-release fertilizer are reduced; in Comparative Example 1, due to the use of an equal mass of chitosan-sodium alginate hydrogel instead of modified chitosan-sodium alginate hydrogel, the mechanical properties of the slow-release fertilizer are reduced.

[0102] (4) Field experiment: Potatoes were selected as test plants and planted under the same geographical conditions, fertilizer application and field management measures. The application rate was 100 kg / mu. After harvest, the potato yield of each test field was counted.

[0103] The functional slow-release fertilizer and compound fertilizer (including urea, monoammonium phosphate and potassium sulfate in a mass ratio of 6:1:1) of Example 1 were subjected to field experiments according to the above method, and a group of field experiments without adding any fertilizer was carried out as a control group. The test results are shown in Table 3.

[0104] Table 3

[0105] No fertilizer Compound Fertilizer Example 1 Yield (kg / mu) 1487 1794 2316

[0106] According to the data in Table 3, the functional slow-release fertilizer of the present application has good fertilizer effect.

[0107] Finally, it should be noted that the above specific implementation methods are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A functional slow-release fertilizer, characterized in that: The invention comprises a modified chitosan-sodium alginate hydrogel and a fertilizer in a mass ratio of 1:(60-70); the modified chitosan-sodium alginate hydrogel is prepared by cross-linking the modified chitosan and sodium alginate with a cross-linking agent; the structure of the cross-linking agent contains an isocyanate group, and the functionality of the cross-linking agent is not less than 2.

2. The functional slow-release fertilizer according to claim 1, characterized in that: The raw materials for preparing the modified chitosan-sodium alginate hydrogel also include polyacrylamide and hydroquinone.

3. The functional slow-release fertilizer according to claim 1, characterized in that: The cross-linking agent also contains a phenyl group in its structure.

4. The functional slow-release fertilizer according to claim 1, characterized in that: The preparation method of the modified chitosan comprises the following steps: dissolving chitosan in an acetic acid solution to obtain a chitosan solution, then adjusting the pH of the system to 8-9, adding an ethanol dilution of benzaldehyde thereto, stirring at room temperature for 3-4 hours, filtering after the reaction is completed, washing with ethanol, and drying to obtain the modified chitosan.

5. The functional slow-release fertilizer according to claim 4, characterized in that: The mass ratio of chitosan to benzaldehyde is 1:(4-6).

6. The functional slow-release fertilizer according to claim 2, characterized in that: The preparation method of the modified chitosan-sodium alginate hydrogel comprises the following steps: A1. Disperse sodium alginate in N,N-dimethylformamide, then add modified chitosan and N,N-dimethylformamide, and then add a crosslinking agent, heat to 60-80° C. and stir at constant temperature for 8-10 hours under an inert protective gas atmosphere, and wash with deionized water after the reaction to obtain a product; A2, placing the product in step A1 in a dilute hydrochloric acid solution and allowing to stand for 6 to 8 hours, then washing with ethanol and drying to obtain a hydrogel intermediate; A3. Mix the hydrogel intermediate in step A2 with polyacrylamide, add deionized water and stir at room temperature for 30 to 50 minutes, then add hydroquinone and potassium chloride and stir for 15 to 25 minutes, wash with ethanol solution, and dry to obtain a modified chitosan-sodium alginate hydrogel.

7. The functional slow-release fertilizer according to claim 6, characterized in that: In step A1, the mass ratio of sodium alginate to modified chitosan is 1:(1.5-2.5).

8. The functional slow-release fertilizer according to claim 6, characterized in that: In step A1, the mass of the cross-linking agent is 0.7 to 0.9 times the total mass of the sodium alginate and the modified chitosan.

9. The functional slow-release fertilizer according to claim 6, characterized in that: In step A3, the mass ratio of the hydrogel intermediate, polyacrylamide and hydroquinone is 1:(0.2-0.5):(0.02-0.06).

10. A method for preparing the functional slow-release fertilizer according to any one of claims 1 to 9, characterized in that: The method comprises the following steps: uniformly mixing the modified chitosan-sodium alginate hydrogel and the fertilizer, and then granulating the mixture to obtain the functional slow-release fertilizer.

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