Liquid slow-release nitrogen fertilizer and its preparation method and device

Abstract

This invention discloses a liquid slow-release nitrogen fertilizer and its preparation method and apparatus, belonging to the field of fertilizers. The liquid slow-release nitrogen fertilizer is characterized by being generated by the reaction of urea, formaldehyde, and polymer additives. The polymer additives are generated by the reaction of modified sodium carboxymethyl cellulose and acrylate-acrylamide copolymer. The preparation method of the liquid slow-release nitrogen fertilizer is as follows: first, a urea-formaldehyde polymer and polymer additives are generated; then, the two are mixed to obtain a composite system; finally, the pH is adjusted, and the final product is obtained by low-temperature evaporation. The apparatus for preparing the liquid slow-release nitrogen fertilizer includes a reaction vessel with a low-temperature evaporation function. This liquid slow-release nitrogen fertilizer can solve the problems of fertilizer crystallization and freezing under low-temperature environments, which can lead to blockages in fertilizer pipelines and storage containers. It also maintains the slow-release performance of the fertilizer, making it suitable for storage, transportation, and use in cold regions, expanding the application range and applicable scenarios of fertilizers. Furthermore, it simplifies the device structure, reduces the floor space, and saves costs.

Description

Technical Field

[0001] This invention relates to the field of fertilizers, specifically to a liquid slow-release nitrogen fertilizer, a method for preparing the liquid slow-release nitrogen fertilizer, and an apparatus for preparing the liquid slow-release nitrogen fertilizer. Background Technology

[0002] Liquid nitrogen fertilizer, widely used in modern agricultural production, plays a crucial role in the cultivation of various crops, including grains, cash crops, and fruits and vegetables, due to its advantages such as complete nutrient dissolution, high absorption efficiency, convenient application, and flexible mixing with other fertilizers or pesticides. It can quickly replenish nitrogen nutrients to crops, effectively promoting leaf and branch growth, improving photosynthetic efficiency, and thus contributing to increased crop yield and quality. It is a vital support for ensuring large-scale, efficient agricultural production and has become an indispensable component of the current agricultural fertilization system, with wide application scenarios and huge market demand both nationally and globally.

[0003] However, conventional liquid nitrogen fertilizers currently on the market still have significant limitations in practical applications, especially in the storage, transportation, and use stages in cold regions. Due to their poor low-temperature stability, conventional liquid nitrogen fertilizers are prone to crystallization and freezing when the ambient temperature drops to a certain level (especially close to or below 0°C), leading to blockages in fertilizer pipelines and storage containers. This not only affects the normal storage and transportation efficiency of liquid nitrogen fertilizers and increases storage and transportation costs, but also prevents fertilization operations from proceeding smoothly due to blockages, and may even cause fertilizer waste and equipment damage. This greatly limits the promotion and application of liquid nitrogen fertilizers in cold northern regions and high-altitude, low-temperature areas, making it difficult to meet the needs of agricultural production in cold regions for liquid nitrogen fertilizers.

[0004] Furthermore, existing liquid nitrogen fertilizer production systems typically employ a multi-tank series structure, where each reactor is connected to a separate concentration tank, and each concentration tank is equipped with its own condenser. This one-to-one configuration of components results in significant redundancy in the equipment structure. This not only drastically increases the floor space required for the entire production system and makes the equipment layout extremely complex, but also significantly increases the overall procurement cost due to the large number of devices. Subsequent daily operation and maintenance costs also rise considerably. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a liquid slow-release nitrogen fertilizer and its preparation method and apparatus, which can solve the problem of fertilizer crystallization and freezing in low-temperature environments, which leads to blockage of fertilizer pipelines and storage containers, while taking into account the slow-release performance of the fertilizer, adapting to storage, transportation and use in cold regions, expanding the application range and applicable scenarios of fertilizer; and can also simplify the structure of the device, reduce the floor space, and save costs.

[0006] To solve this technical problem, the present invention adopts the following technical solution: A liquid slow-release nitrogen fertilizer is produced by the reaction of urea, formaldehyde and polymer additives, wherein the polymer additives are produced by the reaction of modified sodium carboxymethyl cellulose and acrylate-acrylamide copolymer. The modified sodium carboxymethyl cellulose is incorporating hydroxypropyl groups with a degree of substitution of 0.8 to 1.2; in the acrylate-acrylamide copolymer, the molar percentage of acrylamide monomer is 30% to 50%.

[0007] Preferably, in the modified sodium carboxymethyl cellulose, the basic structural unit of sodium carboxymethyl cellulose has the molecular formula C8H. 14 O8Na, after modification with hydroxypropyl-CH2CH(OH)CH3, has a repeating structural unit with the molecular formula C0. 11 H 20 O9Na.

[0008] Preferably, the acrylate-acrylamide copolymer is a random copolymer, and the repeating unit is composed of acrylate units and acrylamide units; the repeating unit of the acrylamide has the molecular formula C3H5NO.

[0009] More preferably, the acrylate is specifically methyl acrylate, with the repeating unit molecular formula C4H6O2; the structural formula of the acrylate-acrylamide copolymer is [CH2CH(COOCH3)]. m -[CH2CH(CONH2)] n , where n / (m+n) = 30~50%.

[0010] The preparation method of the liquid slow-release nitrogen fertilizer includes the following steps: Step 1: Mix formaldehyde and urea in a molar ratio of 1:1.5 to 1:2 and stir at 70 to 80°C for 60 to 120 minutes to generate urea-formaldehyde polymer; add modified sodium carboxymethyl cellulose and acrylate-acrylamide copolymer to water in a mass ratio of 2:1 to 3:1 to prepare a solution with a mass fraction of 8% to 12%, which is the polymer additive; Step 2: After the urea-formaldehyde polymer cools down to 40-50℃, slowly add the polymer additives. The amount of polymer additives added is 0.5%-1.5% of the mass of the urea-formaldehyde polymer. Stir and keep the reaction at the temperature for 20-30 minutes to obtain the composite system. Step 3: Add ammonia water to the composite system, adjust the pH to 6.5~7.5, and evaporate at low temperature to obtain a concentrated solution, which is the liquid slow-release nitrogen fertilizer.

[0011] Preferably, in step one, the stirring rate for preparing the polymer additive is 200-600 r / min, the temperature is maintained at 20-40℃, and the reaction time is 20-60 min.

[0012] The apparatus for preparing the liquid slow-release nitrogen fertilizer includes a reaction vessel. The upper end of the reaction vessel is provided with a formaldehyde inlet, a urea inlet, a polymer additive delivery port, and an ammonia delivery port. The lower end of the reaction vessel is provided with a discharge port. The reaction vessel is equipped with a stirring paddle and a temperature control device. The reaction vessel is also equipped with a condenser for low-temperature evaporation, and the condenser is connected to a vacuum pump.

[0013] Preferably, the urea inlet is connected to a urea dissolving tank for dissolving solid urea into a urea solution.

[0014] Preferably, a gas adsorption device is provided near the discharge outlet to adsorb unreacted waste gas.

[0015] Preferably, the condenser is also connected to a storage tank for recovering the condensate obtained from low-temperature evaporation.

[0016] The positive effects of this invention are as follows: First, this invention improves the low-temperature stability and slow-release performance of liquid slow-release nitrogen fertilizer by adding a polymer additive generated from the reaction of modified sodium carboxymethyl cellulose and acrylate-acrylamide copolymer to the urea-formaldehyde polymer obtained by the polymerization reaction of formaldehyde and urea. In the polymer additive, the modified sodium carboxymethyl cellulose, as the main additive, has hydroxypropyl groups that lower the freezing point of the system, preventing crystallization and freezing at -15℃. Simultaneously, the long-chain structure of the modified sodium carboxymethyl cellulose forms a network structure in the liquid, physically blocking the diffusion of urea molecules and achieving a slow-release effect. The acrylate-acrylamide copolymer, as a synergistic additive, forms a "hydrogen bond-network structure" composite system with urea molecules and formaldehyde condensate (methylene diurea), further slowing down the urea release rate.

[0017] Secondly, the reaction apparatus used in this invention combines the material reaction and low-temperature evaporation into a single tank, avoiding the structural redundancy caused by traditional multi-tank series systems. This simplifies the structure, reduces the floor space, and saves on procurement and maintenance costs. By dissolving solid urea into a urea solution before adding it to the reaction vessel, the apparatus prevents urea agglomeration and pipeline blockage, thus improving reaction efficiency. The apparatus incorporates an adsorption device to adsorb unreacted or released volatile components (such as formaldehyde and ammonia), enhancing system safety and aligning with green environmental protection principles. Furthermore, the inclusion of a storage tank allows for the recycling and reuse of the condensate obtained from low-temperature evaporation, further implementing the green environmental protection concept.

[0018] Third, this invention uses urea-formaldehyde polymer obtained by the polymerization reaction of formaldehyde and urea as the core material for achieving slow-release of liquid nitrogen fertilizer. This is because urea and formaldehyde first undergo an addition reaction under neutral or weakly alkaline conditions, where the amino group (-NH2) of the urea molecule undergoes nucleophilic addition with the carbonyl group (C=O) of the formaldehyde molecule, generating hydroxymethylurea intermediates such as monohydroxymethylurea and dihydroxymethylurea. Subsequently, under alkaline conditions and heating, a condensation reaction occurs, where the hydroxymethylurea intermediates undergo dehydration condensation between themselves or with urea molecules, forming chain-like or slightly branched urea-formaldehyde prepolymers (such as methylene diurea and dimethylene triurea) connected by methylene bridges (-CH2-). The slow-release mechanism involves the generated urea-formaldehyde prepolymer being unable to be directly absorbed by crops in the soil. It needs to be gradually decomposed by microorganisms to slowly release urea and ammonium nitrogen, thereby extending the fertilizer's effective period. Simultaneously, the polymer adjuvants added to the system further delay nitrogen release through physical barriers and hydrogen bonding, enhancing the slow-release effect. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the overall structure of an embodiment of the device used in this invention.

[0020] The following are the labels in the diagram: 1. Reactor; 2. Urea inlet; 3. Formaldehyde inlet; 4. Polymer additive inlet; 5. Ammonia inlet; 6. Urea dissolving tank; 7. Vacuum pump; 8. Storage tank; 9. Condenser; 10. Temperature display device; 11. Gas adsorption device; 12. Discharge port; 13. Heating device; 14. Stirring paddle.

[0021] Figure 2 It is a curve showing the cumulative nitrogen release rate over time. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1 A liquid slow-release nitrogen fertilizer is produced by the reaction of urea, formaldehyde, and polymer additives. Specifically, urea and formaldehyde first undergo an addition-condensation reaction to generate urea-formaldehyde polymers of different chain lengths. Then, polymer additives generated by the reaction of modified sodium carboxymethyl cellulose and acrylate-acrylamide copolymers are added. After low-temperature evaporation, the liquid slow-release nitrogen fertilizer is obtained.

[0024] The modified sodium carboxymethyl cellulose incorporates a hydroxypropyl group with a degree of substitution of 0.8, wherein the basic structural unit of sodium carboxymethyl cellulose has the molecular formula C8H. 14 O8Na, after modification with hydroxypropyl-CH2CH(OH)CH3, has a repeating structural unit with the molecular formula C0. 11 H 20 O9Na.

[0025] The acrylate-acrylamide copolymer is a random copolymer, with repeating units composed of acrylate units and acrylamide units. The acrylamide monomer accounts for 30% of the total molar composition, and the repeating unit of acrylamide has the molecular formula C3H5NO. Specifically, the acrylate is methyl acrylate, with the repeating unit having the molecular formula C4H6O2. The structural formula of the acrylate-acrylamide copolymer is [CH2CH(COOCH3)]. m -[CH2CH(CONH2)] n , where n / (m+n) = 30%.

[0026] like Figure 1 As shown, the reaction apparatus used to prepare the liquid slow-release nitrogen fertilizer includes a reaction vessel 1, made of SUS316L stainless steel. The inner liner of the vessel is shot-blasted and mirror-polished to improve corrosion resistance and cleanliness. The effective volume of the vessel is 5 m³ (diameter 2 m × height 1.6 m), and the wall thickness is 8 mm.

[0027] The upper end of the reactor 1 is equipped with a formaldehyde inlet 3, a urea inlet 2, a polymer additive conveying inlet 4, and an ammonia water conveying inlet 5. All inlets and conveying inlets are equipped with valves to control the entry of each material. The urea inlet 2 is connected to a urea dissolving tank 6, which is made of 304 stainless steel and has a volume of 2 m³. The tank has a double-layer jacket for heating with hot oil or steam, with the temperature controlled between 25℃ and 40℃ to improve the urea dissolution rate and solubility. The urea solution enters the reactor 1 through the outlet pipe of the urea dissolving tank 6 and then through the urea inlet 2, preventing pipe blockage caused by urea agglomeration and improving reaction efficiency. The outlet pipe is wrapped with an insulation layer to prevent crystallization of the high-concentration urea solution due to temperature drop during transport.

[0028] The lower end of the reactor 1 is provided with an outlet 12, and a gas adsorption device 11 is provided near the outlet 12. The gas adsorption device 11 is filled with an adsorption medium such as activated carbon, which can adsorb unreacted or released volatile components (such as formaldehyde, ammonia, etc.), improve the safety of the system, and conform to the concept of green environmental protection.

[0029] The reactor 1 is equipped with a stirring paddle 14 and a temperature control device. The top of the stirring paddle 14 is connected to the top of the reactor 1 (which is equipped with a variable frequency motor), and the stirring paddle 14 is driven by the variable frequency motor. The temperature control device includes a temperature display device 10 and a heating device 13. The sensing end of the temperature display device 10 is located on the inner top of the reactor 1, and the display end of the temperature display device 10 is located on the outer side of the reactor 1. The heating device 13 is a jacketed hot oil (or steam) circulation system fixed to the outer wall of the reactor 1. By forming a closed cavity in the jacket and circulating hot oil (or steam) into the cavity, heat is transferred to the interior of the reactor 1.

[0030] The reactor 1 is also equipped with a vertical shell-and-tube condenser 9 for low-temperature evaporation, with a condensation area of ​​15 m² and circulating water as the cooling medium. The condenser 9 is connected to a two-stage rotary vane vacuum pump 7, which, upon startup, can stably maintain a negative pressure of 0.02 MPa within the reactor 1, with the vacuum degree continuously adjustable within the range of 0.01-0.05 MPa. By combining the material reaction and low-temperature evaporation in the same tank, the structural redundancy caused by traditional multi-tank series systems is avoided, simplifying the structure, reducing the floor space, and saving on procurement and maintenance costs. The condenser 9 is also connected to a storage tank 8 for recycling and reusing the condensate obtained from low-temperature evaporation, further implementing the concept of green environmental protection.

[0031] The method for preparing the liquid slow-release nitrogen fertilizer using the above-described reaction apparatus is as follows: Step 1: Dissolve 1200 kg of solid urea in urea dissolving tank 6 to form a urea solution. If formaldehyde and urea are mixed at a molar ratio of 1:2, then the amount of solid formaldehyde should be 300 kg. Prepare a 37% formaldehyde solution, then the mass of the formaldehyde solution will be approximately 811 kg.

[0032] Open formaldehyde inlet 3 and urea inlet 2 to mix formaldehyde and urea in reactor 1 at a stable molar ratio of 1:2. Turn on the stirrer 14 and heating device 13 to heat and stir reactor 1 at a stirring rate of 150 r / min, setting the heating rate to no more than 3℃ / min to avoid local overheating and ensure uniform molecular weight distribution of the product. When the temperature reaches 70~80℃, maintain the temperature stable using the temperature control device, allowing the formaldehyde and urea in reactor 1 to react for 60 min, producing a total of 1320 kg of methylene diurea prepolymer (belonging to the category of urea-formaldehyde polymers). After the reaction, turn off the heating device 13 and simultaneously close the valves of formaldehyde inlet 3 and urea inlet 2.

[0033] Modified sodium carboxymethyl cellulose and acrylate-acrylamide copolymer were added to water at a mass ratio of 3:1. The mixture was stirred at a speed of 200 r / min, kept at a temperature of 20℃, and reacted for 20 min to prepare a solution with a mass fraction of 8%, which is the polymer additive.

[0034] Step 2: After the temperature in reactor 1 drops to 50℃, open the polymer additive delivery port and slowly add the polymer additive. The amount of polymer additive added is 0.5% of the mass of the methylene diurea prepolymer. In Step 1, the mass of the methylene diurea prepolymer was 1320 kg, therefore the amount of polymer additive added is 6.6 kg.

[0035] The stirring rate was adjusted to 500 r / min, and the reaction was kept at a constant temperature for 30 min to obtain the composite system. After the reaction, the valve of the polymer additive delivery port 4 was closed.

[0036] Step 3: Open ammonia water inlet 5 and add ammonia water to the composite system in reactor 1. Adjust the pH to 6.5 and close the valve of ammonia water inlet 5. Turn on vacuum pump 7 and condenser 9 to perform low-temperature evaporation of the liquid in reactor 1, maintaining the temperature below 60℃. Evaporate approximately 30% of the water and recover it to storage tank 8. After low-temperature evaporation, turn off vacuum pump 7 and condenser 9, and open outlet 12 to obtain concentrated liquid (concentration reaching 25-30% solids), which is liquid slow-release nitrogen fertilizer. During the product discharge process, the activated carbon in the adsorption device can adsorb the unreacted waste gas.

[0037] Example 2 Compared to Example 1, the difference in the composition of the liquid slow-release nitrogen fertilizer in this example is as follows: The modified sodium carboxymethyl cellulose has a hydroxypropyl group introduced into it, with a degree of substitution of 1.2.

[0038] The acrylamide monomer has a molar percentage of 50%.

[0039] The structural formula of the acrylate-acrylamide copolymer is [CH2CH(COOCH3)]. m -[CH2CH(CONH2)] n , where n / (m+n)=50%.

[0040] Compared to Example 1, the difference in the preparation method of liquid slow-release nitrogen fertilizer in this example is as follows: Step 1: Dissolve 900 kg of solid urea in urea dissolving tank 6 to form a urea solution. If formaldehyde and urea are mixed at a molar ratio of 1:1.5, then the amount of solid formaldehyde should be 300 kg. Prepare a 37% formaldehyde solution, then the mass of the formaldehyde solution will be approximately 811 kg.

[0041] Formaldehyde and urea reacted in reactor 1 for 120 minutes, producing a total of 990 kg of methylene diurea prepolymer.

[0042] A 12% (w / w) solution was prepared by mixing modified sodium carboxymethyl cellulose and acrylate-acrylamide copolymer at a mass ratio of 2:1, a stirring rate of 600 r / min, a temperature of 40℃, and a reaction time of 60 min. This solution is the polymer additive.

[0043] Step 2: After the temperature in reactor 1 drops to 40°C, open the polymer additive delivery port. The amount of polymer additive added is 1.5% of the mass of the methylene diurea prepolymer. In Step 1, the mass of the methylene diurea prepolymer was 990 kg, therefore the amount of polymer additive added is 14.85 kg.

[0044] The stirring speed was adjusted to 800 r / min, and the reaction was kept at this temperature for 20 min.

[0045] Step 3: Adjust the pH to 7.5.

[0046] Low temperature stability test: Without adding polymer additives, a control sample was obtained based on Example 1.

[0047] Take 500 mL each of the liquid slow-release nitrogen fertilizer samples prepared in Examples 1 and 2, and the control sample without polymer additives, and place them in a constant temperature low-temperature chamber (-20±1℃). Five parallel samples are prepared for each group. After 48 hours, the samples are removed, allowed to return to room temperature (25℃), and then the following measurements are performed. The average value of each group is taken as the result: Ice precipitation amount: The sample is filtered through a 100μm filter, the filter residue is collected and weighed, and converted into the ice precipitation amount per 100mL of sample (g / 100 mL).

[0048] Transmittance: The transmittance (%) of the sample was measured at 600 nm using a spectrophotometer.

[0049] Dynamic viscosity: The viscosity of the sample (mPa·s) was measured at 25°C using a rotational viscometer.

[0050] Table 1. Results of Low Temperature Stability Test

[0051] Comparative analysis: The amount of ice precipitation decreased significantly: the amount of ice precipitation in the control sample was 18 g / 100 mL, in Example 1 it was 6 g / 100 mL, and in Example 2 it was 3 g / 100 mL, which were reduced by approximately 66.7% and 83.3% respectively, indicating that the polymer additive can effectively inhibit low-temperature nucleation and crystal growth.

[0052] Improved transmittance: The transmittance of the control sample was 42%, while that of Example 1 and Example 2 increased to 78% and 85% respectively, indicating that the clarity of the sample was significantly improved, the suspension of microcrystals / microices at low temperature was significantly reduced, and the reliability of delivery was higher.

[0053] Slight changes in viscosity: The viscosity of the control sample was 185 mPa·s, while that of Example 1 and Example 2 was 142 mPa·s and 136 mPa·s, respectively. This indicates that the polymer additive did not cause significant thickening while improving antifreeze stability, making it easier to pump and spray.

[0054] The above data shows that by adding polymer additives to the fertilizer formulation, the product can maintain high light transmittance, low ice precipitation, and suitable viscosity at -20℃, which greatly improves the low-temperature stability of liquid slow-release nitrogen fertilizer and makes it suitable for storage and application in cold regions.

[0055] Sustained-release performance test: Without adding polymer additives, a control sample was obtained based on Example 1.

[0056] Take 500 mL each of the liquid slow-release nitrogen fertilizer samples prepared in Examples 1 and 2, and the control sample without polymer adjuvants, and evaluate the release according to the soil culture / water extraction evaluation method. Add the samples to a simulated soil system (25℃, pH 7.0, 60% field capacity) with an equal nitrogen content, and take samples at 3, 7, 14, 21, and 28 days. Extract with 2 mol / L KCl to determine inorganic nitrogen (NH4+). + -N and NO3 - -N) and calculate the cumulative release rate; 5 parallel samples are set for each group, and the average value of the results is taken.

[0057] Table 2 Results of sustained-release performance test

[0058] Comparative analysis: Table 2 combined Figure 2 It can be seen that, compared with the control sample, the cumulative release rates of Examples 1 and 2 were significantly lower at 14 days, 21 days, and 28 days. The control sample reached 98% at 28 days, while Examples 1 and 2 were 78% and 72%, respectively, indicating that the addition of polymer additives can effectively prolong the nitrogen release process. Among them, Example 2, due to its higher degree of hydroxypropyl substitution and higher proportion of acrylamide, has a stronger hydrogen bond / network structure and a more obvious sustained release.

Claims

1. A liquid slow-release nitrogen fertilizer, characterized in that: It is produced by the reaction of urea, formaldehyde and polymer additives, wherein the polymer additives are produced by the reaction of modified sodium carboxymethyl cellulose and acrylate-acrylamide copolymer; The modified sodium carboxymethyl cellulose incorporates hydroxypropyl groups with a degree of substitution of 0.8 to 1.2; the acrylate-acrylamide copolymer contains 30% to 50% acrylamide monomer in molar proportion; and the basic structural unit of the modified sodium carboxymethyl cellulose has the molecular formula C8H. 14 O8Na, after modification with hydroxypropyl-CH2CH(OH)CH3, has a repeating structural unit with the molecular formula C0. 11 H 20 O9Na; The acrylate-acrylamide copolymer is a random copolymer, with repeating units composed of acrylate units and acrylamide units; the repeating unit of the acrylamide has the molecular formula C3H5NO; the acrylate is specifically methyl acrylate, whose repeating unit has the molecular formula C4H6O2; the structural formula of the acrylate-acrylamide copolymer is [CH2CH(COOCH3)]. m -[CH2CH(CONH2)] n Where n / (m+n) = 30~50%; The liquid slow-release nitrogen fertilizer is prepared through the following steps: Step 1: Mix formaldehyde and urea in a molar ratio of 1:1.5 to 1:2 and stir at 70 to 80°C for 60 to 120 minutes to generate urea-formaldehyde polymer; add modified sodium carboxymethyl cellulose and acrylate-acrylamide copolymer to water in a mass ratio of 2:1 to 3:1 to prepare a solution with a mass fraction of 8% to 12%, which is the polymer additive; Step 2: After the urea-formaldehyde polymer cools down to 40-50℃, slowly add the polymer additives. The amount of polymer additives added is 0.5%-1.5% of the mass of the urea-formaldehyde polymer. Stir and keep the reaction at the temperature for 20-30 minutes to obtain the composite system. Step 3: Add ammonia water to the composite system, adjust the pH to 6.5~7.5, and evaporate at low temperature to obtain a concentrated solution, which is the liquid slow-release nitrogen fertilizer.

2. The liquid slow-release nitrogen fertilizer according to claim 1, characterized in that: In step one, the stirring rate for preparing the polymer additive is 200-600 r / min, the temperature is maintained at 20-40℃, and the reaction time is 20-60 min.

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