Proline-modified urea-formaldehyde liquid suspension slow-release fertilizer and preparation method thereof

Abstract

The application relates to the technical field of liquid suspension fertilizers, and particularly discloses a proline-modified urea-formaldehyde liquid suspension slow-release fertilizer and a preparation method thereof. The proline-modified urea-formaldehyde liquid suspension slow-release fertilizer comprises the following raw materials in parts by weight: 18-25 parts of proline-modified urea-formaldehyde suspension, 5-15 parts of dolomite powder, 20-50 parts of conventional base fertilizer raw materials, 0.5-5 parts of a suspension stabilizer, and water to make up to 100 parts; the proline-modified urea-formaldehyde is formed by chemical bonding of proline and urea-formaldehyde. The proline-modified urea-formaldehyde liquid suspension slow-release fertilizer of the application realizes the improvement of the release performance of urea-formaldehyde through the chemical bonding of the secondary amine group of proline and the hydroxymethyl urea, meanwhile, the proline has a hydrophobic pyrrolidine skeleton and a hydrophilic carboxyl group, and in combination with the dolomite powder and the suspension stabilizer, a stable urea-formaldehyde suspension system is formed, thereby realizing the long-term physical stability of the high-concentration urea-formaldehyde liquid suspension fertilizer and the stable release of nutrients.

Description

Technical Field

[0001] This invention relates to the field of liquid suspension fertilizer technology, and in particular to a proline-modified urea-formaldehyde liquid suspension slow-release fertilizer and its preparation method. Background Technology

[0002] Urea-formaldehyde (UF) is the earliest biodegradable polymeric slow-release fertilizer successfully developed and commercially produced. Urea-formaldehyde can slowly degrade and gradually release nitrogen nutrients under the action of microorganisms, exhibiting higher nitrogen utilization compared to traditional fast-acting nitrogen fertilizers. However, after addition cross-linking reactions, conventional urea-formaldehyde has a high proportion of low-molecular-weight components, leading to excessively rapid release and nitrogen surplus in the early stages; while the high-molecular-weight components degrade too slowly, resulting in insufficient nitrogen supply in the later stages, making it difficult to match the nitrogen requirements of long-cycle crops throughout the season. Therefore, slowing down the early release rate of urea-formaldehyde through modification has become a research hotspot in this field. Chinese patent CN115849959A discloses an organic acid-modified urea-formaldehyde slow-release fertilizer, using citric acid and succinic acid to modify the urea-formaldehyde molecule. This method uses organic acids with low crop activity and high cost. The preparation process involves liquid-phase reaction, mixing, solid-phase extrusion, drying, and granulation, making the process complex and energy-intensive.

[0003] Compared to solid granules, the production process of liquid suspension fertilizers is simpler. It eliminates the need to dry the water used in the modified urea-formaldehyde preparation process and avoids the extrusion, drying, and granulation processes of granular fertilizers. The process simply involves thoroughly mixing the materials and homogenizing them using a shear emulsification pump. Liquid suspension fertilizers have lower processing costs, saving energy compared to granulation and drying. Furthermore, their fluidity and pumpability allow for easy transport using pumps, and application can be done through various methods such as mechanized direct application around the roots, drip irrigation, sprinkler irrigation, and fertigation. Liquid suspension fertilizers significantly reduce labor costs and intensity in fertilizer transport and application, laying a solid foundation for mechanized agricultural fertilization. Urea-formaldehyde liquid fertilizers are easily absorbed and utilized by crops, exhibiting high fertilizer utilization rates, and existing technologies already report on urea-formaldehyde liquid suspension fertilizers.

[0004] Liquid suspension slow-release fertilizers and solid slow-release fertilizers differ fundamentally in their slow-release mechanisms. Solid slow-release fertilizers include coated and chemical types. Coated slow-release fertilizers primarily rely on physical barrier mechanisms, forming a physical barrier by coating the fertilizer particles with one or more layers of hydrophobic material. Water penetrates into the coating to dissolve nutrients, which then diffuse outward through the membrane pores. The release rate is mainly controlled by factors such as the thickness and porosity of the coating material, as well as ambient temperature. Chemical solid slow-release fertilizers, on the other hand, rely on the gradual degradation of their macromolecular chains by soil microorganisms to release nutrients. However, their release rate is significantly affected by external factors such as soil type, pH, microbial activity, and moisture, resulting in poorer release uniformity and controllability. In contrast, the slow-release mechanism of liquid suspension slow-release fertilizers has different characteristics: In liquid suspension fertilizers, the slow-release components are uniformly dispersed in the suspension system in the form of tiny particles. The slow-release effect depends on the synergy of two levels: First, the chemical structure of the dispersed particles themselves determines the intrinsic rate of nutrient release—the higher the degree of polymerization and the longer the molecular chain, the lower the water solubility and the slower the nutrient release. Second, the physicochemical stability of the suspension system directly affects the sustainability and uniformity of the slow-release effect. The particles need to remain stably dispersed in the suspension without settling in order to be evenly distributed in the soil or irrigation water after application. However, many current liquid suspension fertilizers exhibit varying degrees of sedimentation and stratification problems, and sedimentation is more likely to occur during long-term storage. This difference means that the slow-release performance regulation of liquid suspension slow-release fertilizers cannot simply follow the technical route of solid slow-release fertilizers—relying solely on adjusting the degree of polymerization or coating thickness is insufficient to achieve the desired effect; it is also necessary to solve the synergistic optimization problem between suspension stability and slow-release performance. Therefore, developing a novel urea-formaldehyde liquid slow-release fertilizer with both excellent slow-release performance and good suspension stability is of great significance. Summary of the Invention

[0005] To address the above shortcomings, this invention provides a proline-modified urea-formaldehyde liquid suspension slow-release fertilizer and its preparation method, achieving high-concentration stable suspension and synergistic slow release of nutrients. The specific technical solution is as follows: A proline-modified urea-formaldehyde liquid suspension slow-release fertilizer comprises the following raw materials in parts by weight: 18-25 parts of proline-modified urea-formaldehyde suspension, 5-15 parts of dolomite powder, 20-50 parts of conventional basic fertilizer raw materials, 0.5-5 parts of suspension stabilizer, and water to make up to 100 parts; wherein the proline-modified urea-formaldehyde is formed by the reaction of proline and urea-formaldehyde to form a chemical bond.

[0006] In the technical solution of this invention, proline, as a functional protein amino acid, possesses multiple beneficial properties: on the one hand, it can enhance the plant's tolerance to osmotic stress, improve the crop's ability to resist adverse conditions such as low temperature, high temperature, and drought, while promoting the growth of aboveground parts, thus achieving stable and guaranteed yield; on the other hand, proline has a unique molecular structure, containing a secondary amine group, a carboxyl group, and a pyrrolidine ring. The secondary amine group can undergo a dehydration condensation reaction with hydroxymethylurea prepolymer under acidic catalytic conditions to form a stable chemical bond, thereby achieving the modification and grafting of proline onto the urea-formaldehyde molecular chain. More importantly, proline possesses both a hydrophobic pyrrolidine skeleton and a hydrophilic carboxyl group. This amphiphilic characteristic allows it to regulate the hydrophilic-hydrophobic balance on the surface of urea-formaldehyde particles after grafting, thus enabling synergistic formulation with dolomite powder and suspension stabilizers to construct a stable liquid-phase suspension system. This significantly improves suspension stability while optimizing the nutrient release rate, ultimately achieving synergistic optimization of slow-release performance and suspension stability.

[0007] Preferably, in the above-mentioned proline-modified urea-formaldehyde liquid suspension slow-release fertilizer, the conventional basic fertilizer raw materials are selected from agricultural-grade or ordinary industrial-grade raw materials, specifically one or more of urea, ammonium sulfate, ammonium chloride, monoammonium phosphate, potassium chloride, potassium sulfate, and molasses fermentation broth. The nitrogen, phosphorus, and potassium ratio can be adjusted according to the crop's nutrient requirements, and since all selected raw materials are agricultural-grade or ordinary industrial-grade, production costs are reduced while ensuring fertilizer efficiency, and a fast-and-slow-release nutrient supply system is formed with the proline-modified urea-formaldehyde.

[0008] Preferably, in the above-mentioned proline-modified urea-formaldehyde liquid suspension slow-release fertilizer, the suspension stabilizer includes xanthan gum, magnesium aluminum silicate, and sodium polyacrylate. Xanthan gum constructs a three-dimensional network structure, magnesium aluminum silicate forms a thixotropic gel network to prevent hard sedimentation, and sodium polyacrylate disperses particles through electrostatic repulsion and steric hindrance. The synergistic effect of these three components enhances the anti-settling ability and redispersibility of the high-solids-content suspension system.

[0009] Preferably, the above-mentioned proline-modified urea-formaldehyde liquid suspension slow-release fertilizer further includes 0.1-0.4 parts of tannic acid. This invention utilizes the multiple hydrogen bonds and coordination effects between the polyphenolic hydroxyl groups of tannic acid and proline-modified urea-formaldehyde to enhance slow-release stability; it also forms a hydration film on the particle surface, creating a steric hindrance effect, further strengthening the network structure of the suspension system, slowing sedimentation, and improving the suspension stability and slow-release properties of the liquid suspension slow-release fertilizer.

[0010] On the other hand, this application also provides a method for preparing the above-mentioned proline-modified urea-formaldehyde liquid suspension slow-release fertilizer, comprising the following steps: (1) Mix urea with formaldehyde solution and then react at pH 8.0-9.0 and 60-85℃ to generate hydroxymethylurea prepolymer; (2) Then, proline was added to the hydroxymethyl urea prepolymer, the pH was adjusted to 4.0-5.0, and the reaction was continued at 60-70℃ to obtain a proline-modified urea-formaldehyde suspension. (3) Water, suspension stabilizer, conventional basic fertilizer raw materials and dolomite powder are added sequentially to the proline-modified urea-formaldehyde suspension obtained in step (2), and the mixture is dispersed by high-speed shearing to obtain proline-modified urea-formaldehyde liquid suspension slow-release fertilizer.

[0011] In the preparation method of this invention, hydroxymethylurea prepolymer is first synthesized under alkaline conditions, and then proline is introduced under acidic conditions for modification, ensuring the high efficiency and controllability of chemical bonding; subsequently, through shear emulsification and homogenization dispersion processes, the components are uniformly mixed at the microscale to form a relatively stable suspension system.

[0012] Preferably, in the above preparation method, in step (1), the molar ratio of urea to formaldehyde is 1.2-1.9:1, and the reaction time is 30-60 min.

[0013] Preferably, in the above preparation method, the mass ratio of proline to urea is 1:5-30; and the reaction time in step (2) is 180-360 min.

[0014] Preferably, in the above preparation method, in step (3), tannic acid is added to the proline-modified urea-formaldehyde suspension, and the mixture is stirred and reacted at room temperature for 0.5-1 h. Then, conventional basic fertilizer raw materials, suspension stabilizers, and dolomite powder are added, followed by shear emulsification and homogenization dispersion to obtain proline-modified urea-formaldehyde liquid suspension slow-release fertilizer. First, tannic acid and proline-modified urea-formaldehyde are fully reacted to form a pre-crosslinked network before introducing other components, which is beneficial for constructing a more uniform and dense suspension system and further improving the long-term storage stability of the system.

[0015] Preferably, in the above preparation method, in step (3), the rotation speed of shear dispersion is 3000-7000 rpm and the dispersion time is 15-60 min.

[0016] Preferably, in the above preparation method, in step (3), after high-speed shear dispersion, the particle size D95 of the proline-modified urea-formaldehyde liquid suspension slow-release fertilizer is ≤50μm.

[0017] Compared with the prior art, the beneficial effects of the present invention are: 1. The proline-modified urea-formaldehyde liquid suspension slow-release fertilizer of the present invention improves the urea-formaldehyde release performance through the chemical bonding of the proline secondary amine group and hydroxymethyl urea. At the same time, proline has both a hydrophobic pyrrolidine skeleton and a hydrophilic carboxyl group. After the reaction, the surface of the urea-formaldehyde particles is in hydrophilic-hydrophobic balance. Combined with the compatibility of dolomite powder and suspension stabilizer, a stable liquid-phase suspension system is constructed, which improves the suspension stability and achieves synergistic optimization of nutrient release rate and suspension stability.

[0018] 2. The proline-modified urea-formaldehyde liquid suspension slow-release fertilizer of the present invention undergoes controlled dissociation in the soil in accordance with environmental changes, so that the nitrogen nutrient release curve is dynamically matched with the crop absorption pattern and the soil buffering capacity, overcoming the defects of traditional liquid suspension fertilizers that release too quickly in the early stage and have poor environmental adaptability.

[0019] 3. The proline-modified urea-formaldehyde liquid suspension slow-release fertilizer of the present invention, with the addition of a small amount of tannic acid, further enhances the slow-release performance, improves suspension stability, and increases fertilizer effectiveness and utilization. The polyphenolic component also has certain effects in improving crop stress resistance and inhibiting bacteria. Furthermore, the preparation process of the slow-release fertilizer of the present invention is compatible with agricultural-grade low-purity raw materials. Through stepwise reaction and optimized feeding sequence, production costs are effectively controlled while ensuring high product performance, demonstrating good prospects for industrial application. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 The infrared spectra of the proline-modified urea-formaldehyde prepared in Example 1 of this invention are compared with those of the unmodified urea-formaldehyde and pure proline in Comparative Example 1. Figure 2 The cumulative nitrogen release rate curves of proline-modified urea-formaldehyde in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 are shown. Figure 3 The curves show the effect of different amounts of tannic acid added on the growth rate of suspended particle size. Detailed Implementation

[0022] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. Unless otherwise defined, all technical terms used below have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing specific embodiments only and are not intended to limit the scope of protection of the present invention. Unless otherwise specifically stated, all raw materials, reagents, instruments, and equipment used in the present invention are commercially available or can be prepared by existing methods.

[0023] This invention provides a proline-modified urea-formaldehyde liquid suspension slow-release fertilizer, comprising the following raw materials in parts by weight: 18-25 parts of proline-modified urea-formaldehyde suspension, 5-15 parts of dolomite powder, 20-50 parts of conventional basic fertilizer raw materials, 0.5-5 parts of suspension stabilizer, and water to make up to 100 parts; wherein the proline-modified urea-formaldehyde is formed by the reaction of proline and urea-formaldehyde to form a chemical bond.

[0024] The preparation of the proline-modified urea-formaldehyde suspension includes: (1) Mix urea with formaldehyde solution and then react at pH 8.0-9.0 and 60-85℃ to generate hydroxymethylurea prepolymer; (2) Then, proline is added to the hydroxymethylurea prepolymer, the pH is adjusted to 4.0-5.0, and the reaction is continued at 60-70℃ to obtain proline-modified urea-formaldehyde suspension.

[0025] Proline-modified urea-formaldehyde involves the reaction of proline and urea-formaldehyde to form a chemical bond. This chemical bond is not a simple physical mixture, but rather a chemical modification achieved through the condensation reaction between the secondary amine group in the proline molecule and the hydroxymethyl group in the urea-formaldehyde prepolymer. Proline is grafted onto the polymer chain of urea-formaldehyde, significantly altering the molecular properties of urea-formaldehyde. Compared to other amino acids, the proline introduced in this invention increases steric hindrance, which helps the particles wet and disperse in the aqueous phase and prevents rapid sedimentation caused by hydrophobic aggregation.

[0026] Example 1 (Proline modification, NPK mass ratio 160-60-90) A proline-modified urea-formaldehyde liquid suspension slow-release fertilizer, with a nitrogen, phosphorus, and potassium content of 310 g / L (NPK mass ratio 160-60-90), is composed of the following raw materials by weight: 18 parts proline-modified urea-formaldehyde suspension, 13 parts urea, 10 parts ammonium sulfate, 8 parts monoammonium phosphate, 11 parts potassium chloride, 8 parts molasses fermentation liquid (potassium content approximately 6%), 12 parts dolomite powder, 0.3 parts xanthan gum, 0.1 parts magnesium aluminum silicate, 0.4 parts sodium polyacrylate, and water to make up to 100 parts.

[0027] Preparation of proline-modified urea-formaldehyde: Take 120g of urea, 100g of 37% formaldehyde solution (molar ratio of approximately 1.62:1), and 40g of water, adjust the pH to 8.5, and react at 70℃ for 45 min; add 12g of proline (proline to urea weight ratio of 1:10, molar ratio of 1:19.2), adjust the pH to 4.5, and react at 60℃ for 240 min to obtain a proline-modified urea-formaldehyde suspension.

[0028] A method for preparing a proline-modified urea-formaldehyde liquid suspension slow-release fertilizer includes the following steps: (1) Add water and suspension stabilizer to proline-modified urea-formaldehyde suspension and shear at 5000 rpm for 10 min; (2) Add urea, ammonium sulfate, monoammonium phosphate, and potassium chloride, and continue stirring; (3) Add dolomite powder, adjust pH to 8, and shear at 5000 rpm for 15 min to obtain proline modified urea-formaldehyde liquid suspension slow-release fertilizer. The particle size D95 in the liquid suspension slow-release fertilizer is ≤50μm.

[0029] Example 2 (proline-modified NPK mass ratio 170-70-80) A proline-modified urea-formaldehyde liquid suspension slow-release fertilizer, with a nitrogen, phosphorus, and potassium content of 320 g / L (NPK mass ratio 170-70-80), is composed of the following raw materials by weight: 25 parts proline-modified urea-formaldehyde suspension, 6 parts urea, 19 parts ammonium sulfate, 10 parts agricultural-grade monoammonium phosphate, 11 parts potassium chloride, 15 parts dolomite powder, 0.4 parts xanthan gum, 0.2 parts magnesium aluminum silicate, 0.3 parts sodium polyacrylate, and water to make up to 100 parts.

[0030] Preparation of proline-modified urea-formaldehyde: Take 120g of urea, 100g of 37% formaldehyde solution (molar ratio of approximately 1.62:1), and 40g of water, adjust the pH to 8.5, and react at 70℃ for 45 min; add 10g of proline (proline to urea weight ratio of 1:12), adjust the pH to 4.5, and react at 60℃ for 240 min to obtain a proline-modified urea-formaldehyde suspension.

[0031] A method for preparing a proline-modified urea-formaldehyde liquid suspension slow-release fertilizer includes the following steps: (1) Add water and suspension stabilizer to proline-modified urea-formaldehyde suspension and shear at high speed for 10 min at 5000 rpm; (2) Add urea, ammonium sulfate, monoammonium phosphate, and potassium chloride, and continue stirring; (3) Add dolomite powder, adjust pH to 8, and shear at 5000 rpm for 15 min to obtain proline modified urea-formaldehyde liquid suspension slow-release fertilizer. The particle size D95 in the liquid suspension slow-release fertilizer is ≤50μm.

[0032] Example 3 A proline-modified urea-formaldehyde liquid suspension slow-release fertilizer, with a nitrogen, phosphorus, and potassium content of 310 g / L (NPK mass ratio 160-60-90), is composed of the following raw materials by weight: 18 parts proline-modified urea-formaldehyde suspension, 13 parts urea, 10 parts ammonium sulfate, 8 parts monoammonium phosphate, 11 parts potassium chloride, 8 parts molasses fermentation liquid (potassium content approximately 6%), 12 parts dolomite powder, 0.25 parts tannic acid, 0.3 parts xanthan gum, 0.1 parts magnesium aluminum silicate, 0.4 parts sodium polyacrylate, and water to make up to 100 parts.

[0033] Preparation of a proline-modified urea-formaldehyde suspension: Take 120g of urea and 100g of 37% formaldehyde solution (molar ratio approximately 1.62:1), add 40g of water, adjust the pH to 8.5, and react at 70℃ for 45min; add 12g of proline (proline to urea weight ratio of 1:10), adjust the pH to 4.5, and react at 60℃ for 240min to obtain the proline-modified urea-formaldehyde suspension.

[0034] A method for preparing a proline-modified urea-formaldehyde liquid suspension slow-release fertilizer includes the following steps: (1) Add tannic acid to proline-modified urea-formaldehyde suspension, stir at 500 rpm for 30 min at room temperature, then add water and suspension stabilizer, and shear at 5000 rpm for 10 min. (2) Add urea, ammonium sulfate, monoammonium phosphate, and potassium chloride, and continue stirring; (3) Add dolomite powder, adjust pH to 8, and shear at 5000 rpm for 15 min to obtain proline modified urea-formaldehyde liquid suspension slow-release fertilizer. The particle size D95 in the liquid suspension slow-release fertilizer is ≤50μm.

[0035] Comparative Example 1 (Unmodified) This comparative example differs from Example 1 in that it does not modify urea-formaldehyde. Specifically, it is a urea-formaldehyde liquid suspension slow-release fertilizer, which, by weight, consists of the following raw materials: 18 parts urea-formaldehyde, 13 parts urea, 10 parts ammonium sulfate, 8 parts monoammonium phosphate, 11 parts potassium chloride, 8 parts molasses fermentation liquid (potassium content approximately 6%), 12 parts dolomite powder, 0.3 parts xanthan gum, 0.1 parts magnesium aluminum silicate, 0.4 parts sodium polyacrylate, and water to make up to 100 parts.

[0036] Preparation of a urea-formaldehyde suspension: Take 120g of urea, 100g of 37% formaldehyde solution (molar ratio of approximately 1.62:1), and 40g of water, adjust the pH to 8.5, and react at 70℃ for 45 min; adjust the pH to 4.5, and react at 60℃ for 240 min to obtain the urea-formaldehyde suspension.

[0037] A method for preparing a urea-formaldehyde liquid suspension slow-release fertilizer includes the following steps: (1) Add water and suspension stabilizer to urea-formaldehyde suspension and shear at high speed for 10 min at 5000 rpm; (2) Add urea, ammonium sulfate, monoammonium phosphate, and potassium chloride, and continue stirring; (3) Add dolomite powder, adjust pH to 8, and shear at 5000 rpm for 15 min to obtain urea-formaldehyde liquid suspension slow-release fertilizer. The particle size D95 in the liquid suspension slow-release fertilizer is ≤50μm.

[0038] Comparative Example 2 (Glycine Modification) The difference between this comparative example and Example 1 is that glycine was used to replace proline in the same molar amount to modify urea-formaldehyde, while the rest is the same as in Example 1.

[0039] Comparative Example 3 (Histidine Modification) The difference between this comparative example and Example 1 is that the same molar amount of histidine is used to replace proline in modifying urea-formaldehyde, while the rest is the same as in Example 1.

[0040] Comparative Example 4 (Physical blending of proline) The difference between this comparative example and Example 1 is that Example 1 used proline-modified urea-formaldehyde, while this comparative example uses a mixture of proline and unmodified urea-formaldehyde suspension fertilizer. The mixing ratio is 0.79 parts proline replacing an equal proportion of water in the suspension fertilizer.

[0041] Comparative Example 5 (Physically Mixed Proline Slow-Release Granular Fertilizer 18-7-10) The preparation method of proline physical mixed slow-release granular fertilizer, with a nitrogen, phosphorus and potassium content of 350g / kg (NPK mass ratio 180-70-100), is composed of the following raw materials by weight: 0.79 parts proline, 12 parts coated urea, 8 parts urea, 20 parts ammonium sulfate, 15 parts ammonium chloride, 16 parts monoammonium phosphate, 17 parts potassium chloride, and the remainder is made up to 100 parts with kaolin.

[0042] The preparation method is as follows: First, mix crystalline or powdered proline, urea, ammonium sulfate, ammonium chloride, monoammonium phosphate, potassium chloride, and kaolin, then extrude and granulate them, and then mix them with coated urea to obtain the final product.

[0043] The infrared spectra of the proline-modified urea-formaldehyde prepared in Example 1 and the unmodified urea-formaldehyde prepared in Comparative Example 1 are shown in the figure. Figure 1 As shown in the figure, in the infrared spectrum of pure urea-formaldehyde, 3327 cm⁻¹ -1 For secondary amide NH stretching, 3208 cm -1 For primary amide NH stretching, 1550 cm -1 The coupling between NH bending and CN stretching is observed. After proline grafting, the intensity of the 1550 absorption peak decreases because the number of NH bonds in the system decreases after proline grafting; the NH peak at 3327 broadens with a blue shift, indicating that the number of carboxyl and hydroxyl groups in the system increases after the introduction of proline, forming a more complex and stronger hydrogen bond network; the primary amide peak at 3208 weakens, attributed to reaction consumption; a new shoulder peak appears at 1710, attributed to the C=O stretching vibration of the residual free carboxyl group of proline. These changes collectively indicate that proline has been successfully grafted onto urea-formaldehyde.

[0044] Experimental Example 1: Nitrogen Release Rate Determination The nitrogen release rate of the following fertilizers prepared using the soil culture-leaching method was determined: proline-modified urea-formaldehyde liquid suspension slow-release fertilizer prepared in Example 1; tannic acid-proline-modified urea-formaldehyde liquid suspension slow-release fertilizer prepared in Example 3; unmodified urea-formaldehyde liquid suspension slow-release fertilizer prepared in Comparative Example 1; glycine-modified urea-formaldehyde liquid suspension slow-release fertilizer prepared in Comparative Example 2; and histidine-modified urea-formaldehyde liquid suspension slow-release fertilizer prepared in Comparative Example 3. The tested soil was alluvial soil with a pH of 6.7, air-dried, and passed through a 2mm sieve. A sample equivalent to 200 mg of N was weighed and mixed with 100 g of soil, then packed into a leaching column (3 cm inner diameter, 30 cm height), with quartz sand and glass wool at the bottom. The column was kept at a constant temperature of 25℃ and leached with 50 mL of deionized water weekly. The nitrogen content of the leached liquid was measured, and monitoring was conducted continuously for 60 days. Each group had three replicates.

[0045] Depend on Figure 2 It can be seen that, in terms of the cumulative nitrogen nutrient release rate, unmodified urea-formaldehyde releases too quickly in the early stage (reaching 13% on day 1 and 39% on day 7), easily causing initial nutrient loss. After proline modification, the initial release rate is significantly slowed down (only 4.2% on day 1 and 11.5% on day 7), effectively avoiding early nutrient loss. Compared with other amino acid-modified samples, proline modification is more effective than glycine (10.7% on day 1 and 27% on day 7) and histidine (7% on day 1 and 23% on day 7). The above results indicate that proline-modified urea-formaldehyde effectively solves the defect of excessively rapid initial release of unmodified urea-formaldehyde, and the modification effect is better than glycine and histidine.

[0046] Experimental Example 2: Suspension Stability Test Five samples—proline-modified urea-formaldehyde suspension fertilizer from Example 1, tannic acid-proline-modified urea-formaldehyde suspension fertilizer from Example 3, glycine-modified urea-formaldehyde suspension fertilizer from Comparative Example 2, histidine-modified urea-formaldehyde suspension fertilizer from Comparative Example 3, and unmodified urea-formaldehyde suspension fertilizer from Comparative Example 1—were placed in uniform storage containers, sealed, and stored at room temperature for 90 days. After the standing period, the degree of separation and color difference changes in the supernatant of the samples were visually compared to determine the stratification level. The viscosity before and after storage was measured using a rotational viscometer at 30 rpm and a constant temperature of 25°C, and the viscosity fluctuation rate was calculated. The median particle size (D50) of the suspended particles before and after storage was measured using a laser particle size analyzer, and the particle size growth rate was calculated. The pH value before and after storage was measured using a pH meter, and the pH fluctuation amplitude (ΔpH) was recorded. The results are shown in Table 1.

[0047] Table 1 Comparison of suspension stability test results for each group of liquid suspension fertilizers Comparison of the stability indicators of the five samples showed that the proline-modified urea-formaldehyde suspension fertilizer and the tannic acid-proline-modified urea-formaldehyde suspension fertilizer were significantly better than the other control groups in all parameters. The histidine-modified sample was second best, followed by the glycine-modified sample, while the unmodified urea-formaldehyde suspension fertilizer performed the worst in all parameters. Regarding stratification, the proline-modified and tannic acid-proline-modified samples remained homogeneous and did not stratify after 90 days of storage. The histidine and glycine-modified samples showed only slight stratification, while the unmodified sample showed obvious stratification, indicating that amino acid modification can effectively inhibit solid-liquid phase separation.

[0048] Quantitatively, the proline-modified and tannic acid-proline-modified samples showed significantly better viscosity fluctuation (+7.2%, +5.3%), particle size growth rate (+6.2%, +3.5%), and pH decrease (-0.06, -0.08) than the other three samples. Histidine-modified samples outperformed glycine-modified samples in all indicators, and both were superior to the unmodified samples (viscosity fluctuation +22.5%, particle size growth rate +19.6%, pH decrease -0.41).

[0049] Based on the above results, the order of room temperature storage stability of the five fluid fertilizers is as follows: tannic acid-proline modified urea-formaldehyde suspension fertilizer > proline modified urea-formaldehyde suspension fertilizer > histidine modified urea-formaldehyde suspension fertilizer > glycine modified urea-formaldehyde suspension fertilizer > unmodified urea-formaldehyde suspension fertilizer. This indicates that amino acid modification, especially proline modification, can effectively inhibit the particle growth and aggregation of urea-formaldehyde in the fluid fertilizer system, decrease the pH of the buffer system, and significantly improve the overall stability of the suspension system.

[0050] To further illustrate the effect of tannic acid on the suspension stability of proline-modified urea-formaldehyde liquid suspension fertilizer, suspension stability tests were conducted with different tannic acid addition amounts (except for changes in tannic acid addition amount, everything else was the same as in Example 3) (stored for 90 days). The test results are shown below. Figure 3 As shown in the figure, if the amount of tannic acid added is too small, the effect on improving suspension stability is not obvious; if the amount added is too large, the particle size growth rate increases significantly, which actually reduces suspension stability.

[0051] Experiment Example 3: Field Fertilizer Trial A fertilizer efficacy trial was conducted in Guliu Village, Dawan Town, Xingbin District, Laibin City, Guangxi Zhuang Autonomous Region. Sugarcane, variety Liucheng 05136, was planted on March 12th. Basal fertilizer consisted of 8 kg of urea and 12 kg of monoammonium phosphate per mu (approximately 0.067 hectares). The planting row spacing was 1.2 meters. Topdressing was applied during the hilling-up period. Each treatment covered 10 mu (approximately 6.7 hectares) on May 26th. Liquid fertilizer was applied directly without water using a manual back-mounted applicator. The suspended fertilizer was applied approximately 15 cm beside the sugarcane rows using a strip application method, at a rate of 100 kg per mu, followed by hilling. Granular fertilizer was manually spread directly about 15 cm beside the sugarcane rows, then covered with soil and hilled up, with the same application rate as the liquid fertilizer treatment. After the trial, effective stems, average plant height, and average stem diameter were measured on July 16th, September 5th, and December 13th, and the theoretical yield was calculated. The formula for calculating the theoretical yield is: Yield per mu (tons) = Weight of a single stem (kg) × Number of effective stems per mu / 1000 The formula for calculating the weight of a single stem (in kilograms) is as follows: Single stem weight = 0.7854 × stem diameter 2 × (plant height - 30) × specific gravity / 1000 0.7854: The constant of pi (π) divided by 4.

[0052] Stem diameter (cm): The diameter of the middle part of the sugarcane stem.

[0053] Plant height (cm): The height from the base of the sugarcane stalk to the highest visible leaf ring.

[0054] 30: A constant (in centimeters) deducted to remove the ineffective portion at the base of the sugarcane stalk.

[0055] Specific gravity: The density of sugarcane stalks, usually taken as 1.0.

[0056] Table 2 Results of field fertilizer trials The experimental results show that, in the first survey on July 16 (51 days after the experiment), the number of effective stems, plant height, and theoretical yield of Examples 1 and 3 were all lower than those of Comparative Examples 1, 4, and 5, with the theoretical yield being approximately 0.2-0.4 tons lower. The main reason is that proline, after modifying urea-formaldehyde, reduced the nitrogen release rate in the first 50 days. In the early stages, the number of effective stems per acre in Examples 1 and 3 was lower than in other groups. Reasonable nitrogen fertilizer supply eliminated many sugarcane seedlings that would not have eventually formed stems, preventing these plants from absorbing fertilizer and wasting nutrients, ultimately failing to achieve effective yield. In the second survey on September 5 (102 days after the experiment), the data for Examples 1 and 3 were not significantly different from those of other groups, proving that a more even release of nitrogen fertilizer allowed the sugarcane growth to gradually catch up, and the even release of proline played a significant role in promoting mid-stage growth of the sugarcane. In December… On the 13th (201 days after the experiment), the third survey showed that the theoretical yield of Example 1 reached 7.13 tons, and the theoretical yield of Example 3 reached 7.43 tons. Compared with Comparative Examples 1, 4, and 5, Example 1 increased yield by 24.22%, 12.47%, and 19.83%, respectively, while Example 3 increased yield by 29.44%, 17.19%, and 24.87%, respectively. This indicates that the nitrogen fertilizer released by proline after modification with urea-formaldehyde and combined with tannic acid modification is more in line with the nutrient requirements of sugarcane, enabling the later stage of topdressing 100 days after the sugarcane to better meet its nitrogen requirements, thus resulting in higher yield. At the same time, the sugar content of the proline-modified urea-formaldehyde suspension fertilizer group reached more than 15%, which was significantly higher than that of other control groups. This indicates that after the released proline is absorbed by the sugarcane, it enhances the sugarcane's resistance to stress (especially drought and cold), providing physiological support for the stable accumulation of sucrose.

[0057] The foregoing description of specific exemplary embodiments of the invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed, and it will be apparent that many changes and variations can be made in accordance with the foregoing teachings. The exemplary embodiments were chosen and described in order to explain the specific principles of the invention and its practical application, thereby enabling those skilled in the art to implement and utilize various different exemplary embodiments of the invention, as well as various different choices and variations. The scope of the invention is intended to be defined by the claims and their equivalents.

Claims

1. A proline-modified urea-formaldehyde liquid suspension slow-release fertilizer, characterized in that, The raw materials include the following parts by weight: 18-25 parts of proline-modified urea-formaldehyde suspension, 5-15 parts of dolomite powder, 20-50 parts of conventional basic fertilizer raw materials, 0.5-5 parts of suspension stabilizer, and water to make up to 100 parts; the proline-modified urea-formaldehyde is formed by the reaction of proline and urea-formaldehyde to form a chemical bond.

2. The proline-modified urea-formaldehyde liquid suspension slow-release fertilizer according to claim 1, characterized in that, The conventional basic fertilizer raw materials are one or more of the following: urea, ammonium sulfate, ammonium chloride, monoammonium phosphate, potassium chloride, potassium sulfate, and molasses fermentation liquid.

3. The proline-modified urea-formaldehyde liquid suspension slow-release fertilizer according to claim 1, characterized in that, The suspension stabilizers include xanthan gum, magnesium aluminum silicate, and sodium polyacrylate.

4. The proline-modified urea-formaldehyde liquid suspension slow-release fertilizer according to claim 1, characterized in that, It also includes 0.1-0.4 parts of tannic acid.

5. A method for preparing a proline-modified urea-formaldehyde liquid suspension slow-release fertilizer as described in any one of claims 1-4, characterized in that, Includes the following steps: (1) Mix urea with formaldehyde solution and then react at pH 8.0-9.0 and 60-85℃ to generate hydroxymethylurea prepolymer; (2) Then, proline was added to the hydroxymethyl urea prepolymer, the pH was adjusted to 4.0-5.0, and the reaction was continued at 60-70℃ to obtain a proline-modified urea-formaldehyde suspension. (3) Water, suspension stabilizer, conventional basic fertilizer raw materials and dolomite powder are added sequentially to the proline-modified urea-formaldehyde suspension obtained in step (2), and dispersed by high-speed shearing to obtain proline-modified urea-formaldehyde liquid suspension slow-release fertilizer.

6. The preparation method according to claim 5, characterized in that, In step (1), the molar ratio of urea to formaldehyde is 1.2-1.9:1, and the reaction time is 30-60 min.

7. The preparation method according to claim 5, characterized in that, The mass ratio of proline to urea is 1:5-30; the reaction time in step (2) is 180-360 min.

8. The preparation method according to claim 5, characterized in that, In step (3), tannic acid is added to the proline-modified urea-formaldehyde suspension, and the mixture is stirred at room temperature for 0.5-1 h. Then, water, suspension stabilizer, conventional basic fertilizer raw materials, and dolomite powder are added for high-speed shear dispersion to obtain proline-modified urea-formaldehyde liquid suspension slow-release fertilizer.

9. The preparation method according to claim 5, characterized in that, In step (3), the rotation speed of shear dispersion is 3000-7000 rpm, and the dispersion time is 15-60 min.

10. The preparation method according to claim 5, characterized in that, In step (3), after high-speed shear dispersion, the particle size D95 of the proline-modified urea-formaldehyde liquid suspension slow-release fertilizer is ≤50μm.

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