Preparation method of long-acting slow-release organic compound fertilizer

Abstract

The invention relates to the technical field of fertilizers, and discloses a preparation method of a long-acting slow-release organic compound fertilizer, which comprises the following steps: by taking methyl 3, 5-dihydroxybenzoate, hydrazine hydrate, biphenyl dicarboxaldehyde, D, L-lactide and the like as raw materials, carrying out a series of reactions to prepare hydroxyl-terminated polylactic acid; the preparation method comprises the following steps: taking polylactic acid as a raw material, sequentially reacting with D-cystine and isocyanate propyltriethoxysilane modified talcum powder to obtain modified polylactic acid and talcum powder modified polylactic acid, preparing the modified polylactic acid, the talcum powder modified polylactic acid and polylactic acid into a coating liquid, taking urea, fly ash and humic acid as a fertilizer core, spraying the modified polylactic acid coating liquid on the surface, and drying to obtain the fertilizer. The long-acting slow-release organic compound fertilizer is obtained. The organic compound fertilizer prepared by the invention has the characteristics of excellent slow release effect, damage resistance and environmental protection.

Description

Technical Field

[0001] The invention relates to the technical field of fertilizers, and in particular to a method for preparing a long-acting slow-release organic composite fertilizer. Background Art

[0002] my country is a large agricultural country with the largest area of ​​arable land. Agriculture has become a pillar industry in my country. In agricultural development, fertilizer plays a very positive role in the growth of crops. However, due to the differences in the properties of fertilizers themselves and the land environment, fertilizers have low utilization rates in actual application. This low utilization rate not only causes waste of resources, but also causes a series of environmental problems such as eutrophication of water bodies. Therefore, improving the utilization rate of fertilizers is one of the research hotspots in the fertilizer field.

[0003] Polylactic acid is an environmentally friendly polymer material. It can be used as a coating material to "embed" fertilizers. After fertilization, it can be gradually degraded and released in the soil. It is an environmentally friendly fertilizer coating material. However, polylactic acid is brittle and has poor mechanical properties. It is easy to break during fertilization and transportation, resulting in the release of nutrients. Therefore, the performance of polylactic acid materials needs to be improved to improve the utilization rate of fertilizers. Talc is a hydrated magnesium silicate whose main component is talc. It has good lubricity, fire resistance and other properties. It is widely used in plastics, rubber, coatings and other fields. Organic modification of talc can improve its dispersibility and compatibility in organic polymers. Summary of the invention

[0004] 1. Technical issues to be resolved

[0005] In view of the deficiencies in the prior art, the present invention provides a method for preparing a long-acting slow-release organic compound fertilizer. The prepared organic compound fertilizer has excellent slow-release effect, damage resistance and is green and environmentally friendly.

[0006] (II) Technical solution

[0007] A method for preparing a long-acting slow-release organic compound fertilizer, the preparation method is as follows:

[0008] (1) Adding hydroxy-terminated polylactic acid to a dichloromethane solvent, and then adding D-cystine, 4-dimethylaminopyridine (DMAP), and N,N-dicyclohexylcarbocyanine (DCC) thereto and stirring and dispersing, stirring and reacting for 20-30 hours at room temperature under nitrogen protection, and after the reaction is completed, precipitating with acetone, filtering, dissolving with chloroform, washing with deionized water, distilling off the solvent, and drying to obtain modified polylactic acid;

[0009] (2) adding the hydroxy-terminated polylactic acid to a toluene solvent, stirring and dispersing, and then adding isocyanate propyl triethoxysilane modified talc and 0.05-0.1 wt% stannous octoate thereto, controlling the temperature to 100-120° C., stirring and reacting for 2-3 hours, vacuum extracting the solvent, washing with deionized water, and drying to obtain talc-modified polylactic acid;

[0010] (3) adding polylactic acid, modified polylactic acid, and talc-modified polylactic acid into a dichloromethane solvent, stirring and dispersing, and preparing a modified polylactic acid coating liquid;

[0011] (4) adding urea, fly ash and humic acid into an electric grinder, grinding and mixing them evenly, and then using a disc granulator to obtain a granular fertilizer core with a particle size of 2-4 mm, and then placing it in a coating machine, and spraying the modified polylactic acid coating liquid evenly on the surface of the fertilizer core by a high-pressure spray gun, controlling the coating rate to 10%, and drying to obtain a long-acting slow-release organic compound fertilizer.

[0012] Preferably, in (1), the usage ratio of hydroxy-terminated polylactic acid, D-cystine, DMAP and DCC is 1 g: (0.4-1) mmol: (0.1-0.15) mmol: (0.5-0.8) mmol.

[0013] Preferably, in (2), the usage ratio of the terminal hydroxyl polylactic acid, the isocyanate propyl triethoxysilane modified talc and stannous octoate is 1g:(0.3-0.6)g.

[0014] Preferably, in (3), the usage ratio of polylactic acid, modified polylactic acid and talc-modified polylactic acid is 1g:(0.5-1)g:(0.5-1)g; and the concentration of the modified polylactic acid coating solution is 0.1g / mg.

[0015] Preferably, in (4), the usage ratio of urea, fly ash and humic acid is 1g:(0.1-0.5)g:(0.1-0.5)g.

[0016] Preferably, in (1), the preparation method of the hydroxy-terminated polylactic acid is:

[0017] S1, adding methyl 3,5-dihydroxybenzoate to an ethanol solvent, stirring and dispersing, heating to 75-80°C, adding hydrazine hydrate thereto, continuing stirring and reacting for 2-4 hours, after the reaction is completed, filtering under reduced pressure, washing with deionized water, and drying to obtain 3,5-dihydroxybenzoic acid hydrazide;

[0018] S2, adding 3,5-dihydroxybenzoic acid hydrazide to dimethyl sulfoxide solvent, stirring and dispersing, then adding dimethyl sulfoxide solution of biphenyl dicarboxaldehyde thereto, ultrasonically dispersing, controlling the temperature to 80-90° C., reacting for 3-5 hours, filtering after the reaction, washing with deionized water, and drying to obtain tetrahydroxybiphenyl dihydrazone;

[0019] S3. Add D,L-lactide, tetrahydroxybiphenyl dihydrazone and stannous octoate into a polymerization tube, seal the tube under vacuum, and react at 130-140°C for 12-24 hours under magnetic stirring. After the reaction, use dichloromethane as solvent, precipitate with methanol, filter and dry to obtain hydroxy-terminated polylactic acid.

[0020] Further preferably, in S1, the usage ratio of methyl 3,5-dihydroxybenzoate and hydrazine hydrate is 1 mol:(2-2.4) mol.

[0021] Further preferably, in S2, the usage ratio of 3,5-dihydroxybenzoic acid hydrazide and biphenyl dicarboxaldehyde is (2-2.5) mol:1 mol.

[0022] Further preferably, in S3, the usage ratio of D,L-lactide, tetrahydroxybiphenyl dihydrazone and stannous octoate is (50-200) mol:1 mol:(0.1-0.2) mol.

[0023] (III) Beneficial technical effects

[0024] The invention uses 3,5-dihydroxybenzoic acid methyl ester and hydrazine hydrate as raw materials to obtain 3,5-dihydroxybenzoic acid hydrazide, then performs Schiff base reaction on the 3,5-dihydroxybenzoic acid hydrazone with biphenyl dicarboxaldehyde to obtain tetrahydroxybiphenyl dicarboxylic acid hydrazone, utilizes the hydroxyl contained in the tetrahydroxybiphenyl dicarboxylic acid hydrazone to perform ring-opening polymerization reaction with D,L-lactide to obtain terminal hydroxyl polylactic acid, then sequentially reacts the tetrahydroxybiphenyl dicarboxylic acid with D-cystine and isocyanate propyl triethoxysilane modified talc to obtain modified polylactic acid and talc modified polylactic acid, prepares the two and polylactic acid into coating liquid, uses urea, fly ash and humic acid as fertilizer core, sprays the modified polylactic acid coating liquid on the surface and dries, and obtains long-acting slow-release organic composite fertilizer.

[0025] The hydroxy-terminated polylactic acid prepared by the present invention is a four-arm star polylactic acid with a relatively novel structure. Compared with single-chain polylactic acid, it contains more long-chain structures that can be cross-linked to form a cross-linked network structure. This structure can not only disperse the impact force when the material is subjected to external impact, but also improve the mechanical properties of the coating material. Moreover, this cross-linked network structure can delay the time for nutrients to dissolve out of the film, achieving a slow-release effect. In addition, the coating material prepared by the present invention also has talcum powder. After talcum powder is organically modified, it can be evenly dispersed in the coating material. When subjected to external stress, it can be used as a stress concentration point to absorb more impact energy, further improve the mechanical properties of the coating material, reduce the loss of fertilizer in the transportation process and fertilization process, and improve the utilization rate of fertilizer.

[0026] In addition, the modified polylactic acid prepared by the present invention contains dynamic covalent disulfide bonds and acylhydrazone bonds. When the polylactic acid material is damaged under external conditions, the dynamic covalent bonds and acylhydrazone bonds therein can achieve self-repair of the material, improve the density and uniformity of the coating material, and at the same time, the dynamic covalent network cross-linking improves the flexibility of the coating material, promotes the dissipation of the collision energy of the fertilizer particles, reduces the damage of the coating material, and further improves the fertilizer nutrient release cycle. The organic compound fertilizer prepared by the present invention has been tested and confirmed to have good slow-release effect and damage resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is the reaction route of hydroxy-terminated polylactic acid. DETAILED DESCRIPTION

[0028] In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively below, and preferred embodiments of the present invention are provided. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0029] The preparation method of isocyanate propyl triethoxysilane modified talc powder is as follows: talc powder is added to 75% ethanol aqueous solution, ultrasonically dispersed, and then isocyanate propyl triethoxysilane is added thereto, the temperature is raised to 70°C, and the reaction is stirred for 2 hours. After the reaction is completed, the mixture is cooled to room temperature, filtered, washed with ethanol, and dried to obtain isocyanate propyl triethoxysilane modified talc powder.

[0030] Example 1

[0031] (1) 0.1 mol of methyl 3,5-dihydroxybenzoate is added to an ethanol solvent, stirred and dispersed, and the temperature is raised to 80° C., 0.2 mol of hydrazine hydrate is added thereto, and the stirring reaction is continued for 4 hours. After the reaction is completed, the mixture is filtered under reduced pressure, washed with deionized water, and dried to obtain 3,5-dihydroxybenzoic acid hydrazide.

[0032] (2) Add 0.2 mol of 3,5-dihydroxybenzoic acid hydrazide to dimethyl sulfoxide solvent, stir and disperse, then add dimethyl sulfoxide solution containing 0.08 mol of biphenyl dicarboxaldehyde, ultrasonically disperse, control the temperature to 85°C, react for 4 hours, filter after the reaction, wash with deionized water, and dry to obtain tetrahydroxybiphenyl dicarboxylic acid hydrazone.

[0033] (3) Add 0.5 mol of D,L-lactide, 10 mmol of tetrahydroxybiphenyl dihydrazone, and 2 mmol of stannous octoate into a polymerization tube, seal the tube in vacuum, and react at 135°C for 24 hours under magnetic stirring. After the reaction, add dichloromethane solvent, methanol precipitation, filter, and dry to obtain terminal hydroxyl polylactic acid.

[0034] (4) 20 g of hydroxy-terminated polylactic acid was added to a dichloromethane solvent, and then 8 mmol of D-cystine, 3 mmol of DMAP, and 16 mmol of DCC were added thereto and stirred and dispersed. Under nitrogen protection, the reaction was stirred at room temperature for 24 h. After the reaction was completed, acetone was precipitated, filtered, dissolved in chloroform, washed with deionized water, the solvent was evaporated, and dried to obtain modified polylactic acid.

[0035] (5) Add 20 g of terminal hydroxyl polylactic acid to toluene solvent, stir and disperse, then add 6 g of isocyanate propyl triethoxysilane modified talc and 0.06 wt% of stannous octoate, control the temperature to 115 ° C, stir and react for 2.5 hours, vacuum extract the solvent, wash with deionized water, and dry to obtain talc-modified polylactic acid.

[0036] (6) 20 g of polylactic acid, 10 g of modified polylactic acid, and 10 g of talc-modified polylactic acid were added to a dichloromethane solvent, stirred and dispersed, and a modified polylactic acid coating solution with a concentration of 0.1 g / mg was prepared.

[0037] (7) 100 g of urea, 40 g of fly ash, and 30 g of humic acid were added to an electric grinder, ground and mixed evenly, and then a disc granulator was used to obtain a granular fertilizer core with a particle size of 3 mm, which was then placed in a coating machine with a speed of 35 rpm, a temperature of 70°C, and a hot air flow rate of 400 m 3 / min, and evenly spray the modified polylactic acid coating liquid on the surface of the fertilizer core through a high-pressure spray gun, controlling the coating rate to 10%, and drying to obtain a long-acting slow-release organic compound fertilizer.

[0038] Example 2

[0039] (1) 0.1 mol of methyl 3,5-dihydroxybenzoate is added to an ethanol solvent, stirred and dispersed, and the temperature is raised to 75° C., 0.24 mol of hydrazine hydrate is added thereto, and the stirring reaction is continued for 3 hours. After the reaction is completed, the mixture is filtered under reduced pressure, washed with deionized water, and dried to obtain 3,5-dihydroxybenzoic acid hydrazide.

[0040] (2) Add 0.2 mol of 3,5-dihydroxybenzoic acid hydrazide to dimethyl sulfoxide solvent, stir and disperse, then add dimethyl sulfoxide solution containing 0.08 mol of biphenyl dicarboxaldehyde, ultrasonically disperse, control the temperature to 85°C, react for 3 hours, filter after the reaction, wash with deionized water, and dry to obtain tetrahydroxybiphenyl dicarboxylic acid hydrazone.

[0041] (3) Add 1 mol of D,L-lactide, 10 mmol of tetrahydroxybiphenyl dihydrazone, and 2 mmol of stannous octoate into a polymerization tube, seal the tube in vacuum, and react at 135°C for 20 h under magnetic stirring. After the reaction, add dichloromethane solvent, methanol precipitation, filter, and dry to obtain terminal hydroxyl polylactic acid.

[0042] (4) 20 g of hydroxy-terminated polylactic acid was added to a dichloromethane solvent, and then 10 mmol of D-cystine, 2 mmol of DMAP, and 15 mmol of DCC were added thereto and stirred and dispersed. Under nitrogen protection, the reaction was stirred at room temperature for 20 h. After the reaction was completed, acetone was precipitated, filtered, dissolved in chloroform, washed with deionized water, the solvent was evaporated, and dried to obtain modified polylactic acid.

[0043] (5) Add 20 g of terminal hydroxyl polylactic acid to toluene solvent, stir and disperse, then add 8 g of isocyanate propyl triethoxysilane modified talc and 0.08 wt% of stannous octoate, control the temperature to 120°C, stir and react for 2 h, vacuum extract the solvent, wash with deionized water, and dry to obtain talc-modified polylactic acid.

[0044] (6) 20 g of polylactic acid, 14 g of modified polylactic acid, and 12 g of talc-modified polylactic acid were added to a dichloromethane solvent, stirred and dispersed, and a modified polylactic acid coating solution with a concentration of 0.1 g / mg was prepared.

[0045] (7) 100 g of urea, 30 g of fly ash, and 20 g of humic acid were added to an electric grinder, ground and mixed evenly, and then a disc granulator was used to obtain a granular fertilizer core with a particle size of 2 mm, which was then placed in a coating machine with a speed of 35 rpm, a temperature of 70°C, and a hot air flow rate of 400 m 3 / min, and evenly spray the modified polylactic acid coating liquid on the surface of the fertilizer core through a high-pressure spray gun, controlling the coating rate to 10%, and drying to obtain a long-acting slow-release organic compound fertilizer.

[0046] Example 3

[0047] (1) 0.1 mol of methyl 3,5-dihydroxybenzoate is added to an ethanol solvent, stirred and dispersed, and the temperature is raised to 80° C., 0.22 mol of hydrazine hydrate is added thereto, and the stirring reaction is continued for 2 h. After the reaction is completed, the mixture is filtered under reduced pressure, washed with deionized water, and dried to obtain 3,5-dihydroxybenzoic acid hydrazide.

[0048] (2) Add 0.2 mol of 3,5-dihydroxybenzoic acid hydrazide to dimethyl sulfoxide solvent, stir and disperse, then add dimethyl sulfoxide solution containing 0.08 mol of biphenyl dicarboxaldehyde, ultrasonically disperse, control the temperature to 90°C, react for 4 hours, filter after the reaction, wash with deionized water, and dry to obtain tetrahydroxybiphenyl dicarboxylic acid hydrazone.

[0049] (3) Add 2 mol of D,L-lactide, 10 mmol of tetrahydroxybiphenyl dihydrazone, and 1 mmol of stannous octoate into a polymerization tube, seal the tube in vacuum, and react at 140°C for 12 h under magnetic stirring. After the reaction, add dichloromethane solvent, methanol precipitation, filter, and dry to obtain terminal hydroxyl polylactic acid.

[0050] (4) 20 g of hydroxy-terminated polylactic acid was added to a dichloromethane solvent, and then 15 mmol of D-cystine, 2.5 mmol of DMAP, and 10 mmol of DCC were added thereto and stirred and dispersed. Under nitrogen protection, the reaction was stirred at room temperature for 30 h. After the reaction was completed, acetone was precipitated, filtered, dissolved in chloroform, washed with deionized water, the solvent was evaporated, and dried to obtain modified polylactic acid.

[0051] (5) Add 20 g of terminal hydroxyl polylactic acid to toluene solvent, stir and disperse, then add 10 g of isocyanate propyl triethoxysilane modified talc and 0.1 wt% of stannous octoate, control the temperature to 110°C, stir and react for 2 h, vacuum extract the solvent, wash with deionized water, and dry to obtain talc-modified polylactic acid.

[0052] (6) 20 g of polylactic acid, 18 g of modified polylactic acid, and 16 g of talc-modified polylactic acid were added to a dichloromethane solvent, stirred and dispersed, and a modified polylactic acid coating solution with a concentration of 0.1 g / mg was prepared.

[0053] (7) 100 g of urea, 20 g of fly ash, and 50 g of humic acid were added to an electric grinder, ground and mixed evenly, and then a disc granulator was used to obtain a granular fertilizer core with a particle size of 2 mm, which was then placed in a coating machine with a speed of 35 rpm, a temperature of 70°C, and a hot air flow rate of 400 m 3 / min, and evenly spray the modified polylactic acid coating liquid on the surface of the fertilizer core through a high-pressure spray gun, controlling the coating rate to 10%, and drying to obtain a long-acting slow-release organic compound fertilizer.

[0054] Example 4

[0055] (1) 0.1 mol of methyl 3,5-dihydroxybenzoate is added to an ethanol solvent, stirred and dispersed, and the temperature is raised to 80° C., 0.2 mol of hydrazine hydrate is added thereto, and the stirring reaction is continued for 3 h. After the reaction is completed, the mixture is filtered under reduced pressure, washed with deionized water, and dried to obtain 3,5-dihydroxybenzoic acid hydrazide.

[0056] (2) Add 0.16 mol of 3,5-dihydroxybenzoic acid hydrazide to dimethyl sulfoxide solvent, stir and disperse, then add dimethyl sulfoxide solution containing 0.08 mol of biphenyl dicarboxaldehyde, ultrasonically disperse, control the temperature to 80°C, react for 5 hours, filter after the reaction, wash with deionized water, and dry to obtain tetrahydroxybiphenyl dicarboxylic acid hydrazone.

[0057] (3) Add 2 mol of D,L-lactide, 10 mmol of tetrahydroxybiphenyl dihydrazone, and 1 mmol of stannous octoate into a polymerization tube, seal the tube in vacuum, and react at 130°C for 24 hours under magnetic stirring. After the reaction, add dichloromethane solvent, methanol precipitation, filter, and dry to obtain terminal hydroxyl polylactic acid.

[0058] (4) 20 g of hydroxy-terminated polylactic acid was added to a dichloromethane solvent, and then 20 mmol of D-cystine, 3 mmol of DMAP, and 15 mmol of DCC were added thereto and stirred and dispersed. Under nitrogen protection, the reaction was stirred at room temperature for 25 h. After the reaction was completed, acetone was precipitated, filtered, dissolved in chloroform, washed with deionized water, the solvent was evaporated, and dried to obtain modified polylactic acid.

[0059] (5) Add 20 g of terminal hydroxyl polylactic acid to toluene solvent, stir and disperse, then add 12 g of isocyanate propyl triethoxysilane modified talc and 0.05 wt% of stannous octoate, control the temperature to 100 ° C, stir and react for 3 hours, vacuum extract the solvent, wash with deionized water, and dry to obtain talc modified polylactic acid.

[0060] (6) 20 g of polylactic acid, 20 g of modified polylactic acid, and 20 g of talc-modified polylactic acid were added to a dichloromethane solvent, stirred and dispersed, and a modified polylactic acid coating solution with a concentration of 0.1 g / mg was prepared.

[0061] (7) 100 g of urea, 50 g of fly ash, and 10 g of humic acid were added to an electric grinder, ground and mixed evenly, and then a disc granulator was used to obtain a granular fertilizer core with a particle size of 4 mm, which was then placed in a coating machine with a speed of 35 rpm, a temperature of 70°C, and a hot air flow rate of 400 m 3 / min, and evenly spray the modified polylactic acid coating liquid on the surface of the fertilizer core through a high-pressure spray gun, controlling the coating rate to 10%, and drying to obtain a long-acting slow-release organic compound fertilizer.

[0062] Comparative Example 1

[0063] The preparation method of the long-acting slow-release organic compound fertilizer provided in this comparative example is substantially the same as that in Example 1, the main difference being that modified polylactic acid is not contained in step (6).

[0064] Comparative Example 2

[0065] The preparation method of the long-acting slow-release organic compound fertilizer provided in this comparative example is substantially the same as that in Example 1, the main difference being that the step (6) does not contain talc-modified polylactic acid.

[0066] Refer to GB / T23348-2009 to test the nutrient release rate of fertilizer. The specific steps are as follows: add 10g of fertilizer sample into a glass bottle filled with 200mL of deionized water, and use the Kjeldahl method to determine its 24h nutrient release rate and the number of days when the total nutrient release rate exceeds 80%. The number of days is used as the nutrient release period of the fertilizer.

[0067] Table 1:

[0068] 24h nitrogen release rate (%) Nutrient release period (days) Example 1 12.3 24 Example 2 9.8 28 Example 3 6.2 34 Example 4 4.5 36 Comparative Example 1 15.8 21 Comparative Example 2 16.2 21

[0069] It can be seen from the table that the organic compound fertilizer prepared by the present invention has a good sustained release effect, and the sustained release effect of the fertilizer with the modified polylactic acid and the talc-modified polylactic acid added at the same time is better than that of the fertilizer with only one added.

[0070] Add 10g of fertilizer into a conical flask, then add 6 steel balls into it, place the conical flask on a shaker, shake it at a speed of 250r / min, take out the fertilizer sample at the 60th minute, and test the initial nutrient release rate according to GB / T23348-2009.

[0071] Table 2:

[0072] Initial nutrient release rate (%) Example 1 5.0 Example 2 4.6 Example 3 4.1 Example 4 3.8 Comparative Example 1 6.9 Comparative Example 2 7.1

[0073] It can be seen from the table that the slow-release effect of the fertilizer after being mixed with steel balls is still good. This is because the reversible covalent bonds contained in the fertilizer can spontaneously repair the damage on the surface of the fertilizer and improve the density and uniformity of the coating material, thereby increasing the nutrient controlled-release life of the fertilizer and achieving better long-term slow-release purposes.

[0074] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for preparing a long-acting slow-release organic compound fertilizer, characterized in that: The preparation method is as follows: (1) Adding hydroxy-terminated polylactic acid to a dichloromethane solvent, and then adding D-cystine, DMAP, and DCC thereto, stirring and dispersing, stirring and reacting for 20-30 hours at room temperature under nitrogen protection, and after the reaction is completed, precipitating with acetone, filtering, dissolving with chloroform, washing with deionized water, distilling off the solvent, and drying to obtain modified polylactic acid; (2) adding the hydroxy-terminated polylactic acid to a toluene solvent, stirring and dispersing, and then adding isocyanate propyl triethoxysilane modified talc and 0.05-0.1 wt% stannous octoate thereto, controlling the temperature to 100-120° C., stirring and reacting for 2-3 hours, vacuum extracting the solvent, washing with deionized water, and drying to obtain talc-modified polylactic acid; (3) adding polylactic acid, modified polylactic acid, and talc-modified polylactic acid into a dichloromethane solvent, stirring and dispersing, and preparing a modified polylactic acid coating liquid; (4) adding urea, fly ash and humic acid into an electric grinder, grinding and mixing them evenly, and then using a disc granulator to obtain a granular fertilizer core with a particle size of 2-4 mm, and then placing it in a coating machine, and spraying the modified polylactic acid coating liquid evenly on the surface of the fertilizer core by a high-pressure spray gun, controlling the coating rate to 10%, and drying to obtain a long-acting slow-release organic compound fertilizer.

2. The method for preparing the long-acting slow-release organic compound fertilizer according to claim 1, characterized in that: In the above (1), the usage ratio of terminal hydroxyl polylactic acid, D-cystine, DMAP and DCC is 1g: (0.4-1) mmol: (0.1-0.15) mmol: (0.5-0.8) mmol.

3. The method for preparing the long-acting slow-release organic compound fertilizer according to claim 1, characterized in that: In the above (2), the usage ratio of terminal hydroxyl polylactic acid, isocyanate propyl triethoxysilane modified talc and stannous octoate is 1g:(0.3-0.6)g.

4. The method for preparing the long-acting slow-release organic compound fertilizer according to claim 1, characterized in that: In the above (3), the usage ratio of polylactic acid, modified polylactic acid and talc-modified polylactic acid is 1g:(0.5-1)g:(0.5-1)g; the concentration of modified polylactic acid coating solution is 0.1g / mg.

5. The method for preparing the long-acting slow-release organic compound fertilizer according to claim 1, characterized in that: In the above (4), the usage ratio of urea, fly ash and humic acid is 1g:(0.1-0.5)g:(0.1-0.5)g.

6. The method for preparing the long-acting slow-release organic compound fertilizer according to claim 1, characterized in that: In the above (1), the preparation method of hydroxy-terminated polylactic acid is: S1, adding methyl 3,5-dihydroxybenzoate to an ethanol solvent, stirring and dispersing, heating to 75-80°C, adding hydrazine hydrate thereto, continuing stirring and reacting for 2-4 hours, after the reaction is completed, filtering under reduced pressure, washing with deionized water, and drying to obtain 3,5-dihydroxybenzoic acid hydrazide; S2, adding 3,5-dihydroxybenzoic acid hydrazide to dimethyl sulfoxide solvent, stirring and dispersing, then adding dimethyl sulfoxide solution of biphenyl dicarboxaldehyde thereto, ultrasonically dispersing, controlling the temperature to 80-90° C., reacting for 3-5 hours, filtering after the reaction, washing with deionized water, and drying to obtain tetrahydroxybiphenyl dihydrazone; S3. Add D,L-lactide, tetrahydroxybiphenyl dihydrazone and stannous octoate into a polymerization tube, seal the tube under vacuum, and react at 130-140°C for 12-24 hours under magnetic stirring. After the reaction, use dichloromethane as solvent, precipitate with methanol, filter and dry to obtain hydroxy-terminated polylactic acid.

7. The method for preparing the long-acting slow-release organic compound fertilizer according to claim 6, characterized in that: In the S1, the usage ratio of methyl 3,5-dihydroxybenzoate and hydrazine hydrate is 1 mol:(2-2.4) mol.

8. The method for preparing the long-acting slow-release organic compound fertilizer according to claim 6, characterized in that: In S2, the usage ratio of 3,5-dihydroxybenzoic acid hydrazide and biphenyl dicarboxaldehyde is (2-2.5) mol:1 mol.

9. The method for preparing the long-acting slow-release organic compound fertilizer according to claim 6, characterized in that: In S3, the usage ratio of D,L-lactide, tetrahydroxybiphenyl dihydrazone and stannous octoate is (50-200) mol:1 mol:(0.1-0.2) mol.

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