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

Abstract

The present invention relates to the technical field of fertilizers, and discloses a preparation method of a long-acting slow-release organic compound fertilizer. The present invention uses methyl 3,5-dihydroxybenzoate, hydrazine hydrate, biphenyl dialdehyde, D,L-lactide, etc. as raw materials, and through a series of reactions, prepares hydroxy-terminated polylactic acid. Then, it reacts with D-cystine and isocyanate propyltriethoxysilane-modified talc powder in sequence to obtain modified polylactic acid and talc powder-modified polylactic acid. The two are prepared into a coating solution with polylactic acid, and urea, fly ash, and humic acid are used as the fertilizer core. The modified polylactic acid coating solution is sprayed on the surface and dried to obtain a long-acting slow-release organic compound fertilizer. The organic compound fertilizer prepared by the present invention has relatively excellent slow-release effects, damage resistance performance, and the characteristics of being green and environmentally friendly.

Description

Technical Field

[0001] The present invention relates to the technical field of fertilizers, and particularly to a preparation method of a long-acting slow-release organic compound fertilizer. Background Art

[0002] China is a large agricultural country with the largest cultivated land area, and agriculture has become a pillar industry in China. In the development of agriculture, fertilizers play a very positive role in the growth of crops. However, due to the nature of fertilizers themselves and the differences in land environments, there are problems of low utilization rate in the actual application of fertilizers. This low utilization rate not only causes waste of resources but also leads to a series of environmental problems such as water eutrophication. 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, and after fertilization, it can gradually degrade and release in the soil. It is an environmentally friendly fertilizer coating material. However, polylactic acid is brittle and has poor mechanical properties, and it is easy to break during fertilization and transportation, resulting in the release of nutrients. Therefore, it is necessary to improve the performance of polylactic acid materials to improve the utilization rate of fertilizers. Talc powder is a magnesium silicate hydrate mainly composed of talc, with good lubricity, fire resistance and other properties, and is widely used in fields such as plastics, rubber, and coatings. Organic modification of talc powder can improve its dispersion performance and compatibility in organic polymers. Summary of the Invention

[0004] (1) Technical Problems to be Solved

[0005] Aiming at the deficiencies of the prior art, the present invention provides a preparation method of a long-acting slow-release organic compound fertilizer. The prepared organic compound fertilizer has relatively excellent slow-release effects, damage resistance performance and the characteristics of environmental friendliness.

[0006] (2) Technical Solutions

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

[0008] (1) Add hydroxyl-terminated polylactic acid to dichloromethane solvent, then add D-cystine, 4-dimethylaminopyridine (DMAP), and N,N'-dicyclohexylcarbodiimide (DCC) thereto and stir for dispersion. Under nitrogen protection, stir and react at room temperature for 20 - 30 h. After the reaction is completed, precipitate with acetone, filter, dissolve with chloroform, wash with deionized water, evaporate the solvent, and dry to obtain modified polylactic acid;

[0009] (2) Add the hydroxyl-terminated polylactic acid to toluene solvent, stir and disperse it, then add isocyanate propyltriethoxysilane modified talc powder and 0.05 - 0.1 wt% stannous octoate thereto, control the temperature at 100 - 120 °C, stir and react for 2 - 3 h, vacuum extract the solvent, wash with deionized water, and dry to obtain talc powder modified polylactic acid;

[0010] (3) Add the polylactic acid, modified polylactic acid, and talc powder modified polylactic acid to dichloromethane solvent, stir and disperse them to prepare a modified polylactic acid coating solution;

[0011] (4) Add urea, fly ash, and humic acid to an electric grinder, grind and mix them evenly, then use a disc granulator to obtain granular fertilizer cores with a particle size of 2 - 4 mm. Then place them in a coating machine, and evenly spray the modified polylactic acid coating solution on the surface of the fertilizer cores through a high-pressure spray gun, control the coating rate at 10%, and dry to obtain a long-acting slow-release organic compound fertilizer.

[0012] Preferably, in the step (1), the dosage ratio of the hydroxyl-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 the step (2), the dosage ratio of the hydroxyl-terminated polylactic acid, isocyanate propyltriethoxysilane modified talc powder, and stannous octoate is 1 g : (0.3 - 0.6) g.

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

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

[0016] Preferably, in the step (1), the preparation method of the hydroxyl-terminated polylactic acid is as follows:

[0017] S1. Add methyl 3,5-dihydroxybenzoate to ethanol solvent, stir and disperse it, heat up to 75 - 80 °C, add hydrazine hydrate thereto, continue to stir and react for 2 - 4 h. After the reaction ends, perform vacuum filtration, wash with deionized water, and dry to obtain methyl 3,5-dihydroxybenzohydrazide;

[0018] S2. Add 3,5-dihydroxybenzhydrazide into dimethyl sulfoxide solvent, stir and disperse it, then add the dimethyl sulfoxide solution of biphenyl dialdehyde into it, ultrasonically disperse it, control the temperature at 80 - 90 °C, react for 3 - 5 h. After the reaction, filter, wash with deionized water, and dry to obtain tetrahydroxybiphenyl diacylhydrazone;

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

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

[0021] Further preferably, in S2, the dosage ratio of 3,5-dihydroxybenzhydrazide to biphenyl dialdehyde is (2 - 2.5) mol:1 mol.

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

[0023] (III) Beneficial technical effects

[0024] In the present invention, using methyl 3,5-dihydroxybenzoate and hydrazine hydrate as raw materials, 3,5-dihydroxybenzhydrazide is obtained. Then, it undergoes a Schiff base reaction with biphenyl dialdehyde to obtain tetrahydroxybiphenyl diacylhydrazone. The hydroxyl groups contained in tetrahydroxybiphenyl diacylhydrazone are used to carry out a ring-opening polymerization reaction with D,L-lactide to obtain hydroxyl-terminated polylactic acid. Then, it reacts with D-cystine and isocyanate propyltriethoxysilane-modified talc powder in sequence to obtain modified polylactic acid and talc powder-modified polylactic acid. The two are prepared into a coating solution with polylactic acid. Using urea, fly ash, and humic acid as the fertilizer core, the modified polylactic acid coating solution is sprayed on the surface and dried to obtain a long-acting slow-release organic compound fertilizer.

[0025] The hydroxyl-terminated polylactic acid prepared by the present invention is a four-armed star-shaped polylactic acid with a novel structure. Compared with single-chain polylactic acid, it contains more long-chain structures that can crosslink with each other to form a crosslinked network structure. This structure can not only disperse the impact force and improve the mechanical properties of the coating material when the material is subjected to external impact, but also 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 contains talc powder. After organic modification of the talc powder, it can be evenly dispersed in the coating material. When subjected to external stress, it can act as a stress concentration point, absorb more impact energy, further improve the mechanical properties of the coating material, reduce the loss of fertilizer during transportation and fertilization, and improve the utilization rate of fertilizer.

[0026] In addition, the modified polylactic acid prepared by the present invention contains dynamic covalent bonds, disulfide bonds and hydrazone bonds. When the polylactic acid material is damaged under external conditions, the dynamic covalent bonds and hydrazone bonds can achieve self-repair of the material, improve the compactness and uniformity of the coating material. At the same time, the dynamic covalent network crosslinking improves the flexibility of the coating material, promotes the dissipation of the collision energy of fertilizer particles, reduces the damage of the coating material, and further extends the fertilizer nutrient release period. The organic compound fertilizer prepared by the present invention has been experimentally proven to have good slow-release effect and damage resistance performance. Brief Description of the Drawings

[0027] Figure 1 is the reaction route of hydroxyl-terminated polylactic acid. Detailed Embodiments

[0028] 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 given. 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] Preparation method of isocyanate propyltriethoxysilane modified talc powder: Add talc powder to a 75% ethanol aqueous solution, ultrasonically disperse it, then add isocyanate propyltriethoxysilane to it, heat up to 70 °C, stir and react for 2 h. After the reaction is completed, cool to room temperature, filter by suction, wash with ethanol, and dry to obtain isocyanate propyltriethoxysilane modified talc powder.

[0030] Example 1

[0031] (1) Add 0.1 mol of methyl 3,5-dihydroxybenzoate to an ethanol solvent, stir to disperse, heat up to 80 °C, add 0.2 mol of hydrazine hydrate to it, continue to stir and react for 4 h. After the reaction is completed, filter under reduced pressure, wash with deionized water, and dry to obtain 3,5-dihydroxybenzohydrazide.

[0032] (2) Add 0.2 mol of 3,5-dihydroxybenzohydrazide to a dimethyl sulfoxide solvent, stir to disperse, and then add a dimethyl sulfoxide solution containing 0.08 mol of biphenyl dialdehyde to it. Ultrasonically disperse, control the temperature at 85 °C, react for 4 h. After the reaction is completed, filter, wash with deionized water, and dry to obtain tetrahydroxybiphenyl diacylhydrazone.

[0033] (3) Add 0.5 mol of D,L-lactide, 10 mmol of tetrahydroxybiphenyl diacylhydrazone, and 2 mmol of stannous octoate to a polymerization tube, seal the tube under vacuum, and react at 135 °C with magnetic stirring for 24 h. After the reaction is completed, precipitate with dichloromethane solvent and methanol, filter, and dry to obtain hydroxyl-terminated polylactic acid.

[0034] (4) Add 20 g of hydroxyl-terminated polylactic acid to a dichloromethane solvent, and then add 8 mmol of D-cystine, 3 mmol of DMAP, and 16 mmol of DCC to it. Stir to disperse, under nitrogen protection, stir and react at room temperature for 24 h. After the reaction is completed, precipitate with acetone, filter, dissolve in chloroform, wash with deionized water, evaporate the solvent, and dry to obtain modified polylactic acid.

[0035] (5) Add 20 g of hydroxyl-terminated polylactic acid to a toluene solvent, stir to disperse, and then add 6 g of isocyanate propyltriethoxysilane-modified talc powder and 0.06 wt% of stannous octoate to it. Control the temperature at 115 °C, stir and react for 2.5 h, evacuate the solvent under vacuum, wash with deionized water, and dry to obtain talc powder-modified polylactic acid.

[0036] (6) Add 20 g of polylactic acid, 10 g of modified polylactic acid, and 10 g of talc powder-modified polylactic acid to a dichloromethane solvent, stir to disperse, and prepare a modified polylactic acid coating solution with a concentration of 0.1 g / mg.

[0037] (7) Add 100 g of urea, 40 g of fly ash, and 30 g of humic acid to an electric grinder, grind and mix evenly, and then use a disk granulator to obtain granular fertilizer cores with a particle size of 3 mm. Then place them in a coating machine, the rotation speed of the coating machine is 35 rpm, the temperature is 70 °C, and the hot air flow rate is 400 m 3 / min. Spray the modified polylactic acid coating solution evenly on the surface of the fertilizer cores through a high-pressure spray gun, control the coating rate at 10%, and dry to obtain a long-acting slow-release organic compound fertilizer.

[0038] Example 2

[0039] (1) 0.1 mol of methyl 3,5-dihydroxybenzoate was added to an ethanol solvent, stirred and dispersed, heated to 75 °C, 0.24 mol of hydrazine hydrate was added thereto, and the mixture was continuously stirred and reacted for 3 h. After the reaction was completed, filtration was carried out under reduced pressure, washed with deionized water, and dried to obtain methyl 3,5-dihydroxybenzoyl hydrazide.

[0040] (2) 0.2 mol of methyl 3,5-dihydroxybenzoyl hydrazide was added to a dimethyl sulfoxide solvent, stirred and dispersed, and then a dimethyl sulfoxide solution containing 0.08 mol of biphenyl dialdehyde was added thereto, ultrasonically dispersed, the temperature was controlled at 85 °C, and the reaction was carried out for 3 h. After the reaction was completed, filtration was carried out, washed with deionized water, and dried to obtain tetrahydroxybiphenyl diacyl hydrazone.

[0041] (3) 1 mol of D,L-lactide, 10 mmol of tetrahydroxybiphenyl diacyl hydrazone, and 2 mmol of stannous octanoate were added to a polymerization tube, sealed under vacuum, and reacted at 135 °C under magnetic stirring for 20 h. After the reaction was completed, dichloromethane solvent was used, precipitated with methanol, filtered, and dried to obtain hydroxyl-terminated polylactic acid.

[0042] (4) 20 g of hydroxyl-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, stirred and dispersed. Under nitrogen protection, the mixture was stirred and reacted at room temperature for 20 h. After the reaction was completed, precipitation was carried out with acetone, filtered, dissolved in chloroform, washed with deionized water, the solvent was evaporated, and dried to obtain modified polylactic acid.

[0043] (5) 20 g of hydroxyl-terminated polylactic acid was added to a toluene solvent, stirred and dispersed, and then 8 g of isocyanate propyltriethoxysilane-modified talc powder and 0.08 wt% of stannous octanoate were added thereto. The temperature was controlled at 120 °C, and the mixture was stirred and reacted for 2 h. The solvent was extracted under vacuum, washed with deionized water, and dried to obtain talc powder-modified polylactic acid.

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

[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 granular fertilizer cores with a particle size of 2 mm were obtained by using a disk granulator. Then, they were placed in a coating machine. The rotation speed of the coating machine was 35 rpm, the temperature was 70 °C, and the hot air flow rate was 400 m 3 / min, the modified polylactic acid coating solution is evenly sprayed on the surface of the fertilizer core through a high-pressure spray gun, the coating rate is controlled to be 10%, and then dried to obtain the 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, heated to 80 °C, 0.22 mol of hydrazine hydrate is added thereto, and stirring reaction is continued for 2 h. After the reaction is completed, vacuum filtration is carried out, washed with deionized water, and dried to obtain methyl 3,5-dihydroxybenzohydrazide.

[0048] (2) 0.2 mol of methyl 3,5-dihydroxybenzohydrazide is added to a dimethyl sulfoxide solvent, stirred and dispersed, and then a dimethyl sulfoxide solution containing 0.08 mol of biphenyl dialdehyde is added thereto, ultrasonically dispersed, the temperature is controlled at 90 °C, and the reaction is carried out for 4 h. After the reaction is completed, filtration is carried out, washed with deionized water, and dried to obtain tetrahydroxybiphenyl diacylhydrazone.

[0049] (3) 2 mol of D,L-lactide, 10 mmol of tetrahydroxybiphenyl diacylhydrazone, and 1 mmol of stannous octanoate are added to a polymerization tube, vacuum sealed, and reacted at 140 °C under magnetic stirring for 12 h. After the reaction is completed, precipitated with methanol in a dichloromethane solvent, filtered, and dried to obtain hydroxyl-terminated polylactic acid.

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

[0051] (5) 20 g of hydroxyl-terminated polylactic acid is added to a toluene solvent, stirred and dispersed, and then 10 g of isocyanate propyltriethoxysilane-modified talc powder and 0.1 wt% of stannous octanoate are added thereto, the temperature is controlled at 110 °C, and the reaction is carried out for 2 h. The solvent is extracted under vacuum, washed with deionized water, and dried to obtain talc powder-modified polylactic acid.

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

[0053] (7) Add 100 g of urea, 20 g of fly ash, and 50 g of humic acid to an electric grinder, grind and mix them evenly, then use a disc granulator to obtain granular fertilizer cores with a particle size of 2 mm. Then place them in a coating machine. The rotation speed of the coating machine is 35 rpm, the temperature is 70 °C, and the hot air flow rate is 400 m 3 / min. Spray the modified polylactic acid coating solution evenly on the surface of the fertilizer cores through a high-pressure spray gun, control the coating rate to be 10%, and dry to obtain a long-acting slow-release organic compound fertilizer.

[0054] Example 4

[0055] (1) Add 0.1 mol of methyl 3,5-dihydroxybenzoate to an ethanol solvent, stir and disperse, heat up to 80 °C, add 0.2 mol of hydrazine hydrate to it, continue stirring and reacting for 3 h. After the reaction is completed, carry out vacuum filtration, wash with deionized water, and dry to obtain methyl 3,5-dihydroxybenzohydrazide.

[0056] (2) Add 0.16 mol of methyl 3,5-dihydroxybenzohydrazide to a dimethyl sulfoxide solvent, stir and disperse, then add a dimethyl sulfoxide solution containing 0.08 mol of biphenyl dialdehyde to it, carry out ultrasonic dispersion, control the temperature to be 80 °C, react for 5 h. After the reaction is completed, filter, wash with deionized water, and dry to obtain tetrahydroxybiphenyl diacylhydrazone.

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

[0058] (4) Add 20 g of hydroxyl-terminated polylactic acid to a dichloromethane solvent, then add 20 mmol of D-cystine, 3 mmol of DMAP, and 15 mmol of DCC to it and stir and disperse. Under nitrogen protection, stir and react at room temperature for 25 h. After the reaction is completed, carry out acetone precipitation, filter, dissolve in chloroform, wash with deionized water, evaporate the solvent, and dry to obtain modified polylactic acid.

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

[0060] (6) Add 20 g of polylactic acid, 20 g of modified polylactic acid, and 20 g of talc-modified polylactic acid to dichloromethane solvent, stir and disperse to prepare a modified polylactic acid coating solution with a concentration of 0.1 g / mg.

[0061] (7) Add 100 g of urea, 50 g of fly ash, and 10 g of humic acid to an electric grinder, grind and mix evenly, then use a disc granulator to obtain granular fertilizer cores with a particle size of 4 mm. Then place them in a coating machine. The rotation speed of the coating machine is 35 rpm, the temperature is 70 °C, and the hot air flow rate is 400 m 3 / min. Spray the modified polylactic acid coating solution evenly on the surface of the fertilizer cores through a high-pressure spray gun, control the coating rate to be 10%, and dry 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 generally the same as that in Example 1. The main difference is that: in step (6), it does not contain modified polylactic acid.

[0064] Comparative Example 2

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

[0066] Referring to GB / T23348-2009, test the nutrient release rate of the fertilizer. The specific steps are as follows: Add 10 g of the fertilizer sample to a glass bottle containing 200 mL of deionized water, and determine its 24-hour nutrient release rate and the number of days when the total nutrient rate release amount exceeds 80% by the Kjeldahl method. Take this number of days as the fertilizer nutrient release period.

[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] As can be seen from the table, the organic compound fertilizer prepared by the present invention has a good slow-release effect, and the slow-release effect of the fertilizer with both modified polylactic acid and talc-modified polylactic acid added is better than that of the single addition.

[0070] Add 10 g of the fertilizer to a conical flask, then add 6 steel balls to it. Place the conical flask on a shaker and shake it at a speed of 250 r / min. Take out the fertilizer sample at the 60th minute and test the initial nutrient release rate with reference 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] As can be seen from the table, the slow-release effect of the fertilizer after the steel balls are mixed in is still good. This is because it contains reversible covalent bonds, which can spontaneously repair the damage on the surface of the fertilizer, improve the compactness and uniformity of the coating material, thereby prolonging the nutrient controlled-release life of the fertilizer and achieving the purpose of good long-term slow release.

[0074] The above-described embodiments merely represent several implementation manners of the present invention. The description thereof is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended 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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