Slow-release urea fertilizer and preparation method thereof

Abstract

The invention discloses a slow-release urea fertilizer and a preparation method thereof, and belongs to the technical field of agricultural fertilizers. The outer surface of the urea fertilizer core is sequentially coated with the polyvinyl alcohol-starch coating liquid layer and the biochar-based organic fertilizer powder layer from inside to outside, the prepared slow-release urea fertilizer has good biodegradability and fertilizer slow-release effect, wheat yield increase can be effectively promoted, and the physicochemical property of soil is improved; meanwhile, the raw materials used by the slow-release urea fertilizer are all natural degradable materials, the used charcoal-based organic fertilizer powder is derived from agricultural wastes, and preparation and application of the slow-release urea fertilizer have important practical significance for promoting agricultural green sustainable development and realizing waste resource utilization.

Description

Technical Field

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

[0002] World agriculture is facing the dual challenge of how to feed a growing global population while reducing the impact on the environment. In this process, the use of fertilizers, especially nitrogen fertilizers, plays a vital role. Nitrogen is a key nutrient required for plant growth and development, and nitrogen fertilizers (such as urea) are one of the most commonly used fertilizers in agriculture, designed to meet the nutritional needs of crops. However, the traditional use of urea has some problems, such as rapid volatilization of nitrogen, infiltration into groundwater, and low efficiency of nutrient absorption by plants. These problems not only lead to environmental pollution, but also reduce economic benefits.

[0003] Existing slow-release urea fertilizers usually rely on chemical coatings, such as polymer or resin-based materials, to regulate nutrient release. Although these coatings can effectively release nutrients to a certain extent, they have some limitations, including high production costs, limited biodegradability, and potential concerns about long-term environmental impacts. For example, Chinese Patent 202211732069.4 discloses a slow-release fertilizer with soil improvement function and a preparation method thereof, wherein the urea outer coating inner slow-release layer and the quick-release shell layer have an acidic soil improvement effect while also achieving a good slow-release effect. However, the coating formed by cross-linking the block epoxidized natural rubber used in the fertilizer degrades slowly in the soil, which may cause soil pollution. Another Chinese patent 201610755803.7 discloses a slow-release fertilizer and a preparation method thereof. Although its slow-release cycle can better meet the nutrient needs of medium- and short-term crops, it is not conducive to large-scale production and application due to its high production cost. At present, there is still a lack of a slow-release fertilizer that is inexpensive, environmentally friendly, and has good water retention performance, which limits the development and use of slow-release fertilizers. Summary of the invention

[0004] In view of the existing problems, the present invention provides a slow-release urea fertilizer and a preparation method thereof. The slow-release urea fertilizer is coated with a polyvinyl alcohol-starch coating liquid layer and a biochar-based organic fertilizer powder layer on the outer surface of the urea fertilizer core from the inside to the outside. The prepared slow-release urea fertilizer has good slow-release effect, good water retention, is environmentally friendly and low in price, can effectively promote wheat production and improve soil physical and chemical properties.

[0005] The technical solution of the present invention is as follows:

[0006] The invention provides a slow-release urea fertilizer, which consists of a urea fertilizer core, a polyvinyl alcohol-starch coating liquid layer and a biochar-based organic fertilizer powder layer which are sequentially coated on the outer surface of the urea fertilizer core from the inside to the outside.

[0007] Furthermore, the urea fertilizer core is a spherical urea fertilizer particle with a diameter of 2 to 4 mm.

[0008] The present invention also provides a method for preparing the above-mentioned slow-release urea fertilizer, comprising the following steps:

[0009] S1, mixing gelatinized starch with polyvinyl alcohol solution, maintaining the temperature of the mixed solution at 40-50° C., adding glycerol thereto and continuously stirring at 40-50° C. for at least 1 hour to obtain a polyvinyl alcohol-starch coating solution;

[0010] S2, mixing the biochar with the organic fertilizer, crushing, and sieving through an 80-100 mesh sieve to obtain a biochar-based organic fertilizer powder;

[0011] S3, preheating the coating machine to 40-50° C., placing urea fertilizer core particles, and rotating at a speed of 20-30 rpm; spraying the polyvinyl alcohol-starch coating liquid evenly on the surface of the urea fertilizer core particles to form a polyvinyl alcohol-starch coating liquid layer on the surface of the urea fertilizer core particles; and then evenly spraying the biochar-based organic fertilizer powder on the semi-dried polyvinyl alcohol-starch coating liquid layer to form a biochar-based organic fertilizer powder layer, thereby obtaining the slow-release urea fertilizer.

[0012] Furthermore, in step S1, starch and water are stirred and mixed at 60-70° C. for 0.5-1 h to obtain the gelatinized starch; and the polyvinyl alcohol solution is prepared by stirring polyvinyl alcohol and water at 80-90° C. for 1-2 h.

[0013] Furthermore, the starch may be corn starch.

[0014] Furthermore, the ratio of the starch mass to the water volume in the gelatinized starch in step S1 is

[0015] Furthermore, the ratio of the mass of polyvinyl alcohol to the volume of water in the polyvinyl alcohol solution in step S1 is

[0016] Furthermore, the biochar in step S2 is prepared by pyrolysis of crop straw; the crop straw includes one or more of rice straw, wheat straw, and nut shells.

[0017] Furthermore, the organic fertilizer in step S2 is an organic fertilizer with a total nutrient content of ≥4%; the organic fertilizer is obtained by composting animal manure and / or plant residues.

[0018] Furthermore, in step S2, the mass ratio of biochar to organic fertilizer is 1:4.

[0019] Different from the prior art, the present invention has the following beneficial effects:

[0020] 1. The outer surface of the urea fertilizer core of the present invention is coated with a polyvinyl alcohol-starch coating liquid layer and a biochar-based organic fertilizer powder layer in sequence from the inside to the outside. Compared with the existing polyvinyl alcohol coated slow-release fertilizer, the outer coating of the biochar-based organic fertilizer powder of the present invention enables the prepared slow-release urea fertilizer to achieve a better balance between the slow-release speed of the fertilizer effect and the biodegradability of the coated fertilizer; in the early stage of fertilization, the outer coating of the biochar-based organic fertilizer powder enables the coating film layer of the slow-release fertilizer to have a slower degradation rate, which can effectively exert the slow-release effect of the coating film layer, and then as time goes by, the biochar-based organic fertilizer powder layer and the polyvinyl alcohol-starch coating liquid layer are gradually separated, which can promote the degradation of the coating film layer of the slow-release fertilizer in the soil; the above characteristics enable the slow-release urea fertilizer provided by the present invention to have both good biodegradability and fertilizer slow-release effect.

[0021] 2. The preparation process of the slow-release urea fertilizer provided by the present invention is simple, and the prepared slow-release urea fertilizer has good slow-release performance and water-retention performance, and has a good improvement effect on the quality, yield and physical and chemical properties of crops, which can greatly improve the fertilizer utilization rate and reduce the fertilizer use cost; at the same time, the raw materials used for the coating film layer of the slow-release urea fertilizer are all natural degradable materials, and the biochar-based organic fertilizer powder used in the preparation process of the slow-release urea fertilizer comes from agricultural waste. The implementation of the preparation process has important practical significance for promoting the green and sustainable development of agriculture and realizing the utilization of waste resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The growth conditions of wheat at harvest treated with the slow-release urea fertilizer prepared in Example 1 and the fertilizers described in Comparative Examples 1 to 3 are shown.

[0023] Figure 2 The grain setting rate of wheat fertilized with the slow-release urea fertilizer prepared in Example 1 and the fertilizers described in Comparative Examples 1 to 3.

[0024] Figure 3 The number of grains per wheat plant treated with the slow-release urea fertilizer prepared in Example 1 and the fertilizers described in Comparative Examples 1 to 3.

[0025] Figure 4 The yield of wheat fertilized with the slow-release urea fertilizer prepared in Example 1 and the fertilizers described in Comparative Examples 1 to 3.

[0026] Figure 5 The thousand-grain weight of wheat fertilized with the slow-release urea fertilizer prepared in Example 1 and the fertilizers described in Comparative Examples 1 to 3.

[0027] Figure 6The starch content of wheat fertilized with the slow-release urea fertilizer prepared in Example 1 and the fertilizers described in Comparative Examples 1 to 3.

[0028] Figure 7 The water retention of the slow-release urea fertilizer prepared in Example 1 and the fertilizers described in Comparative Examples 1 to 3.

[0029] Figure 8 The degradability of the slow-release urea fertilizer prepared in Example 1, the fertilizer described in Comparative Example 2, and the polyvinyl alcohol-starch film.

[0030] Fig. 9 NH4 in soil column leaching test of different fertilizers + Release curve.

[0031] Fig.10 This is the urea nitrogen release curve of different fertilizers in the soil column leaching test. DETAILED DESCRIPTION

[0032] The present invention will be further described below in conjunction with the accompanying drawings and preferred embodiments. The given embodiments are only for illustrating the present invention, rather than for limiting the scope of the present invention.

[0033] Unless otherwise specified, the materials, reagents, etc. used in the following examples can be obtained from commercial sources; the methods in the following examples are conventional methods unless otherwise specified.

[0034] The crop straw involved in the following embodiment can be prepared by the following method: washing the crop straw, and sequentially performing a killing treatment and a first homogenization treatment on the washed crop straw to obtain the crop straw powder.

[0035] The crop straw may be one or more of rice straw, wheat straw, and nut shells; there is no special requirement for the method of washing the crop straw, and the technology well known in the art can be used; the temperature for killing the crop straw used in the following embodiments is preferably 105° C.; the killing time may be 20 to 40 minutes, and preferably 30 minutes in the following embodiments;

[0036] The first homogenization treatment includes the steps of drying and cutting the crop straw after the killing; the drying temperature can be 60-90°C, and the drying temperature of the crop straw used in the following embodiment is 80°C; the drying time can be 24-48h, and the drying time of the crop straw used in the following embodiment is 24h; there is no special requirement for the cutting method, and the technology well known in the art can be used, and the length can be 5-10cm, and the length of the crop straw used in the following embodiment is 5cm, so as to facilitate a more uniform pyrolysis reaction. The above technical solution is conducive to obtaining crop straw suitable for subsequent operations.

[0037] The biochar involved in the following embodiments is obtained by pyrolysis treatment of the crop straw mentioned above, and the pyrolysis treatment includes the following steps: pyrolyzing and washing the crop straw in an oxygen-limited atmosphere, and subjecting the crop straw to a second homogenization treatment to obtain the biochar used in the following embodiments.

[0038] The pyrolysis temperature of the crop straw can be 300-500° C., preferably 500° C.; the pyrolysis time can be 0.5-2 hours, preferably 2 hours in the following embodiment; there is no special requirement for the pyrolysis equipment, and the technology well known in the art can be used; after the pyrolysis of the crop straw, the obtained biochar is rinsed, preferably with tap water; there is no special requirement for the method and number of rinsing, and the pH value of the solution after rinsing meets the requirements; the pH value of the solution after rinsing is preferably 6.0-8.0, more preferably 7.0;

[0039] The second homogenization treatment comprises the following steps: drying the washed biochar.

[0040] Example 1

[0041] This embodiment provides a method for preparing a slow-release urea fertilizer, comprising the following steps:

[0042] S1. 8 g corn starch and 150 mL water were stirred and mixed at 60° C. (stirring speed was 700 rpm) for 1 h to obtain gelatinized starch, and 12 g polyvinyl alcohol and 150 mL water were stirred and mixed at 90° C. (stirring speed was 700 rpm) for 2 h to obtain a polyvinyl alcohol solution; then the gelatinized starch was mixed with the polyvinyl alcohol solution, and after the temperature of the mixed solution dropped to 40° C., 3 mL glycerol was added thereto and stirred continuously at 40° C. (stirring speed was 700 rpm) for 1 h to obtain a polyvinyl alcohol-starch coating solution;

[0043] S2, mixing biochar and organic fertilizer in a mass ratio of 1:4, crushing, and sieving through a 100-mesh sieve to obtain biochar-based organic fertilizer powder; in this embodiment, the biochar is prepared by pyrolysis of rice straw, and the organic fertilizer is an organic fertilizer with a total nutrient content of ≥4%, and the organic fertilizer is obtained by composting chicken manure, cow manure and crop straw;

[0044] S3, preheating the coating machine to 50°C, placing spherical urea fertilizer core particles with a diameter of 2 to 4 mm, and rotating at a speed of 30 rpm; spraying the polyvinyl alcohol-starch coating liquid evenly on the surface of the urea fertilizer core particles to form a polyvinyl alcohol-starch coating liquid layer on the surface of the urea fertilizer core particles; and then evenly spraying the biochar-based organic fertilizer powder on the semi-dried polyvinyl alcohol-starch coating liquid layer to form a biochar-based organic fertilizer powder layer, thereby obtaining the slow-release urea fertilizer.

[0045] The urea fertilizer core in the slow-release urea fertilizer prepared by the preparation method is a spherical urea fertilizer particle with a diameter of 2 to 4 mm, and the outer surface of the urea fertilizer core is coated with a polyvinyl alcohol-starch coating liquid layer and a biochar-based organic fertilizer powder layer in sequence from the inside to the outside.

[0046] Example 2

[0047] This embodiment provides a method for preparing a slow-release urea fertilizer, comprising the following steps:

[0048] S1. 8 g corn starch and 150 mL water were stirred and mixed at 70° C. (stirring speed was 900 rpm) for 0.5 h to obtain gelatinized starch, and 12 g polyvinyl alcohol and 150 mL water were stirred and mixed at 100° C. (stirring speed was 900 rpm) for 1 h to obtain a polyvinyl alcohol solution; then the gelatinized starch was mixed with the polyvinyl alcohol solution, and after the temperature of the mixed solution dropped to 50° C., 3 mL glycerol was added thereto and stirred continuously at 50° C. (stirring speed was 900 rpm) for 1 h to obtain a polyvinyl alcohol-starch coating solution;

[0049] S2. Mixing biochar and organic fertilizer in a mass ratio of 1:4, crushing, and sieving through an 80-mesh sieve to obtain biochar-based organic fertilizer powder; wherein the biochar is prepared by pyrolysis of crop straw; the crop straw used in this embodiment is wheat straw; the organic fertilizer used in this embodiment is an organic fertilizer with a total nutrient content of ≥4%, which is obtained by composting chicken manure, cow manure and plant residues, and the plant residues are crop straw;

[0050] S3, preheating the coating machine to 40°C, placing spherical urea fertilizer core particles with a diameter of 2 to 4 mm, and rotating at a speed of 20 rpm; spraying the polyvinyl alcohol-starch coating liquid evenly on the surface of the urea fertilizer core particles to form a polyvinyl alcohol-starch coating liquid layer on the surface of the urea fertilizer core particles; and then evenly spraying the biochar-based organic fertilizer powder on the semi-dried polyvinyl alcohol-starch coating liquid layer to form a biochar-based organic fertilizer powder layer, thereby obtaining the slow-release urea fertilizer.

[0051] The slow-release urea fertilizer prepared by the preparation method provided in this embodiment has a composition and structure similar to those in Example 1.

[0052] Example 3

[0053] This embodiment provides a method for preparing a slow-release urea fertilizer, comprising the following steps:

[0054] S1. 8 g corn starch and 150 mL water were stirred and mixed at 65° C. (stirring speed was 800 rpm) for 0.8 h to obtain gelatinized starch, and 12 g polyvinyl alcohol and 150 mL water were stirred and mixed at 95° C. (stirring speed was 800 rpm) for 1.5 h to obtain a polyvinyl alcohol solution; then the gelatinized starch was mixed with the polyvinyl alcohol solution, and after the temperature of the mixed solution dropped to 45° C., 3 mL glycerol was added thereto and stirred continuously at 45° C. (stirring speed was 800 rpm) for 1 h to obtain a polyvinyl alcohol-starch coating solution;

[0055] S2. Mixing biochar and organic fertilizer in a mass ratio of 1:4, crushing, and sieving through a 90-mesh sieve to obtain biochar-based organic fertilizer powder; wherein the biochar is prepared by pyrolysis of crop straw; the crop straw used in this embodiment is nut shells; the organic fertilizer used in this embodiment is an organic fertilizer with a total nutrient content of ≥4%, and the organic fertilizer is obtained by composting chicken manure, cow manure and plant residues, and the plant residues are crop straws;

[0056] S3, preheating the coating machine to 45°C, placing urea fertilizer core particles, and rotating at a speed of 25rpm; spraying the polyvinyl alcohol-starch coating liquid evenly on the surface of the urea fertilizer core particles to form a polyvinyl alcohol-starch coating liquid layer on the surface of the urea fertilizer core particles; and then evenly spraying the biochar-based organic fertilizer powder on the semi-dried polyvinyl alcohol-starch coating liquid layer to form a biochar-based organic fertilizer powder layer, thereby obtaining the slow-release urea fertilizer.

[0057] The slow-release urea fertilizer prepared by the preparation method provided in this embodiment has a composition and structure similar to those in Example 1.

[0058] Comparative Example 1

[0059] This comparative example provides a biochar-based organic fertilizer urea granule, which is prepared by mixing and granulating biochar-based organic fertilizer powder and urea granules.

[0060] The preparation method of the biochar-based organic fertilizer urea granules comprises the following steps:

[0061] S1. Mixing biochar and organic fertilizer in a mass ratio of 1:4, crushing, and sieving through a 100-mesh sieve to obtain biochar-based organic fertilizer powder; wherein the biochar is prepared by pyrolysis of crop straw; the crop straw used in this comparative example is rice straw; the organic fertilizer used in this comparative example is an organic fertilizer with a total nutrient content of ≥4%, which is obtained by composting chicken manure, cow manure and plant residues, and the plant residues are crop straws;

[0062] S2, crushing the urea granules through a 100-mesh sieve to obtain urea powder;

[0063] S3, mixing the biochar-based organic fertilizer powder and the urea powder in a mass ratio of 1:9 to obtain a biochar-based organic fertilizer urea mixed powder;

[0064] S4. Transfer the biochar-based organic fertilizer urea mixed powder to a disc granulator, set the tilt angle to 45° and the rotation speed to 20 rpm, until fertilizer particles are formed to obtain moist biochar-based organic fertilizer urea particles; then dry the moist biochar-based organic fertilizer urea particles in an oven at 40°C for 1 hour to obtain biochar-based organic fertilizer urea particles.

[0065] Comparative Example 2

[0066] This comparative example provides a polyvinyl alcohol-starch urea coated fertilizer, which consists of a urea fertilizer core and a polyvinyl alcohol-starch coating liquid layer coated on the outer surface of the urea fertilizer core;

[0067] Wherein, the urea fertilizer core is a spherical urea fertilizer particle with a diameter of 2 to 4 mm;

[0068] The preparation method of the polyvinyl alcohol-starch urea coated fertilizer comprises the following steps:

[0069] S1. 8 g corn starch and 150 mL water were stirred and mixed at 60° C. (stirring speed was 700 rpm) for 1 h to obtain gelatinized starch, and 12 g polyvinyl alcohol and 150 mL water were stirred and mixed at 90° C. (stirring speed was 700 rpm) for 2 h to obtain a polyvinyl alcohol solution; then the gelatinized starch was mixed with the polyvinyl alcohol solution, and after the temperature of the mixed solution dropped to 40° C., glycerol was added thereto and stirred continuously at 40° C. (stirring speed was 700 rpm) for 1 h to obtain a polyvinyl alcohol-starch coating solution;

[0070] S2, preheating the coating machine to 50°C, placing spherical urea fertilizer core particles with a diameter of 2 to 4 mm, and rotating at a speed of 30 rpm; spraying the polyvinyl alcohol-starch coating liquid evenly on the surface of the spherical urea fertilizer core particles and forming a polyvinyl alcohol-starch coating liquid layer on the surface of the urea fertilizer core particles, thereby obtaining the polyvinyl alcohol-starch urea coated fertilizer.

[0071] Comparative Example 3

[0072] This comparative example provides a urea fertilizer, which is a spherical urea fertilizer particle with a diameter of 2 to 4 mm and a nitrogen content of 46%.

[0073] Fertilizer performance testing

[0074] (1) Effects of different fertilization treatments on wheat plant growth

[0075] Wheat (Yangmai No. 39) was planted on a non-arable land. After ten years of forest cultivation, this land was used to grow food crops for the first time. The planting period is from November 2023 to May 2024. Each plot is 4m×5m in size and is separated by a 30cm wide and 40cm deep trench. 35kg of N:P:K (28:0:5) binary fertilizer was applied to each plot as basal fertilizer. The slow-release urea fertilizer prepared by the preparation method of the slow-release urea fertilizer described in Example 1 and the fertilizer described in Comparative Examples 1 to 2 (containing 5.56kg of nitrogen) were applied to the plots respectively, and a blank control group (no fertilizer) and the urea fertilizer described in Comparative Example 3 were set as control treatments.

[0076] Result analysis:

[0077] After planting, the growth of wheat was observed at harvest time. Figure 1 ( Figure 1 is a graph showing the growth of wheat at harvest, U-PVA / ST / BOF represents the treatment result of the slow-release urea fertilizer of Example 1, U-PVA / ST represents the treatment result of the polyvinyl alcohol-starch urea coated fertilizer of Comparative Example 2, U / BOF represents the treatment result of the biochar-based organic fertilizer urea granules of Comparative Example 1, Urea represents the treatment result of the urea fertilizer of Comparative Example 3, and CK represents the treatment result of the blank control); Figure 1 It can be seen that the wheat plant growth height and ear length treated with biochar-based organic fertilizer coating fertilizer were significantly higher than those of other treatments. The blank control group showed slow growth, characterized by shorter plants and fewer grains per ear. It can be seen that biochar-based organic fertilizer coating fertilizer can effectively promote plant growth and enhance its absorption and utilization of nutrients. It can not only meet the nutritional needs of plants, but also help save fertilization costs and reduce negative impacts on the environment, reflecting its environmentally friendly characteristics. Its excellent slow-release performance not only improves the utilization efficiency of fertilizers, but also promotes the healthy growth of plants.

[0078] (2) Effects of different fertilization treatments on wheat seed setting rate, number of grains per plant, wheat yield, thousand-grain weight, and starch content

[0079] The results are as follows Figures 2 to 6 As shown, U-PVA / ST / BOF represents the treatment results of the slow-release urea fertilizer of Example 1, U-PVA / ST represents the treatment results of the polyvinyl alcohol-starch urea coated fertilizer of Comparative Example 2, U / BOF represents the treatment results of the biochar-based organic fertilizer urea granules of Comparative Example 1, Urea represents the treatment results of the urea fertilizer of Comparative Example 3, and CK represents the treatment results of the blank control.

[0080] The granulation rate of the slow-release urea fertilizer of the present invention is 84.06%, which is significantly higher than about 77.65% of the polyvinyl alcohol-starch urea coated fertilizer and the biochar-based organic fertilizer urea granules, while the granulation rate of the blank control group is only 49.62% ( Figure 2 In addition, the number of grains per plant also increased significantly, by 65%-94%. On average, the wheat plant treated with the slow-release urea fertilizer of the present invention produced 45 grains, while the yield of the other groups was about 39 grains, an increase of 15.38% ( Figure 3 ). Grain setting rate refers to the proportion of successfully developed grains after pollination, which indirectly reflects the yield potential of wheat. A high grain setting rate is associated with a large number of well-developed grains, while a low grain setting rate may result in fewer grains or poor seed development. Various factors affect the grain setting rate of wheat, including soil nutrient content; the lack of essential nutrients will have an adverse effect on the grain setting rate. The nutrients of the slow-release urea fertilizer of the present invention are gradually released, ensuring that wheat obtains a continuous supply of nutrients, further improving the grain setting rate and the number of grains per plant.

[0081] The wheat yield of the slow-release urea fertilizer of the present invention was 4426 kg / ha, while the yield of other fertilization treatments was about 3100 kg / ha, an increase of 40%-44% ( Figure 4 The thousand-grain weight of wheat treated with the slow-release urea fertilizer of the present invention is 23% higher than that of the other fertilization groups ( Figure 5 In addition, the starch content of the grains was also measured. The starch content of wheat treated with the slow-release urea fertilizer and the polyvinyl alcohol-starch urea coated fertilizer of the present invention reached 33.88% and 30.55%, respectively, while the starch content of the biochar-based organic fertilizer urea granules and the pure urea treatment was about 21% ( Figure 6 ). Nutrients play a vital role in grain development, especially in the grain filling period, and effective nutrient absorption is crucial to grain growth. The slow-release urea fertilizer of the present invention has excellent slow-release properties, ensuring that wheat obtains sufficient nutrient supply during the critical stages of grain filling and maturity. In addition, the externally applied organic fertilizer is rich in nitrogen, phosphorus, potassium, trace elements and other nutrients, which promotes wheat growth and development and improves photosynthetic efficiency.

[0082] (3) Water retention and biodegradability tests of different fertilizers

[0083] 1) Water retention

[0084] The water retention of the fertilizers in Example 1 and Comparative Examples 1 to 3 was verified respectively. Each fertilizer corresponded to one treatment, and each treatment was tested three times in parallel. The results are shown in Table 1. Figure 7 (exist Figure 7, U-PVA / ST / BOF represents the treatment result of Example 1, U-PVA / ST represents the treatment result of Comparative Example 2, U / BOF represents the treatment result of Comparative Example 1, Urea represents the treatment result of Comparative Example 3, and CK represents the treatment result of the blank control).

[0085] Depend on Figure 7 It can be seen that after 43 days of soil water retention test, the water retention rates of the blank control group, urea, biochar-based organic fertilizer urea granules, polyvinyl alcohol-starch urea coated fertilizer and slow-release urea fertilizer were 0.01%, 1.99%, 3.63%, 2.90% and 6.68%, respectively, indicating that slow-release urea fertilizer has good soil water retention capacity.

[0086] 2) Biodegradability

[0087] The degradation rates of the slow-release urea fertilizer prepared in Example 1, the fertilizer described in Comparative Example 2, and the polyvinyl alcohol-starch film were tested and statistically analyzed. Each fertilizer corresponds to one treatment, and each treatment is repeated 3 times. The results are shown in Figure 8 (exist Figure 8 In the figure, U-PVA / ST / BOF represents the treatment result of Example 1, U-PVA / ST represents the treatment result of Comparative Example 2, and PVA / Starch Film represents polyvinyl alcohol-starch film).

[0088] Depend on Figure 8 The results showed that the degradation rate of the polyvinyl alcohol-starch film was 53.54% after 90 days. Although the surface of the film was still intact, it showed signs of mold and thinning. Given enough time, the sample will eventually degrade completely. Both the polyvinyl alcohol-starch urea coated fertilizer and the slow-release urea fertilizer continued to degrade, with degradation rates of 94.22% and 77.44% after 30 days, respectively. The degradation rate of the polyvinyl alcohol-starch urea coated fertilizer was faster than that of the slow-release urea fertilizer. The slower degradation rate of the coating material provides a better guarantee for its slow-release performance. By the 90th day, the degradation rates of the polyvinyl alcohol-starch urea coated fertilizer and the slow-release urea fertilizer reached 95.13% and 89.50%, respectively. Over time, the degradation rate of the slow-release urea fertilizer gradually increased, which was due to the separation of the biochar layer, which allowed the internal polyvinyl alcohol-starch coating liquid layer to further degrade.

[0089] (4) Soil column leaching test

[0090] The fertilizers obtained in Example 1 and Comparative Examples 1-2 and the common urea in Comparative Example 3 were subjected to soil column leaching tests, and the NH4 + The release of nitrogen and urea nitrogen, among which the release curves of nitrogen, phosphorus and potassium in different treatments are shown in Fig. 9and Fig.10 ( Fig. 9 and Fig.10 , U-PVA / ST / BOF represents the treatment result of Example 1, U-PVA / ST represents the treatment result of Comparative Example 2, U / BOF represents the treatment result of Comparative Example 1, Urea represents the treatment result of Comparative Example 3, and CK represents the treatment result of the blank control).

[0091] from Figures 9-10 It can be seen that urea has the fastest nitrogen release rate and the largest release amount. In 28 days, the released nitrogen accounts for 49.9% of the total nitrogen. This rapid release is a typical feature of traditional fertilizers. Although it can quickly provide nutrients in the early stage, it may also cause a sharp increase in soil nitrogen content, followed by a sharp drop in the release rate. This rapid release method is not conducive to a sustained and effective nutrient supply, and may cause nitrogen leaching and reduce the long-term nitrogen absorption efficiency of plants.

[0092] In contrast, at the end of the 28-day treatment, the total nitrogen release of the polyvinyl alcohol-starch urea coated fertilizer and the slow-release urea fertilizer was 42.7% and 36.4%, respectively. Among them, the slow-release urea fertilizer had the slowest nitrogen release rate. During the 28-day leaching period, its release rate gradually stabilized, and the cumulative release was only 18.3%. This lower release rate and extended release period help reduce nitrogen leaching, improve nitrogen effectiveness, and enable plants to continuously absorb nutrients during a longer growth cycle. Its excellent slow-release performance can not only provide plants with the required nutrients in a balanced manner, but also reduce nutrient loss, improve fertilizer utilization, and thus reduce fertilization costs.

[0093] Although the above embodiment describes the present invention in detail, it is only a part of the embodiments of the present invention, not all of the embodiments. People can also obtain other embodiments based on this embodiment without creativity, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A slow-release urea fertilizer, characterized in that: The slow-release urea fertilizer consists of a urea fertilizer core, a polyvinyl alcohol-starch coating liquid layer and a biochar-based organic fertilizer powder layer which are sequentially coated on the outer surface of the urea fertilizer core from the inside to the outside.

2. The slow-release urea fertilizer according to claim 1, characterized in that The urea fertilizer core is a spherical urea fertilizer particle with a diameter of 2 to 4 mm.

3. A method for preparing a slow-release urea fertilizer according to claim 1 or 2, characterized in that: The steps include: S1, mixing gelatinized starch with polyvinyl alcohol solution, maintaining the temperature of the mixed solution at 40-50° C., adding glycerol thereto and continuously stirring at 40-50° C. for at least 1 hour to obtain a polyvinyl alcohol-starch coating solution; S2, mixing the biochar with the organic fertilizer, crushing, and sieving through an 80-100 mesh sieve to obtain a biochar-based organic fertilizer powder; S3, preheating the coating machine to 40-50° C., placing urea fertilizer core particles, and rotating at a speed of 20-30 rpm; spraying the polyvinyl alcohol-starch coating liquid evenly on the surface of the urea fertilizer core particles to form a polyvinyl alcohol-starch coating liquid layer on the surface of the urea fertilizer core particles; and then evenly spraying the biochar-based organic fertilizer powder on the semi-dried polyvinyl alcohol-starch coating liquid layer to form a biochar-based organic fertilizer powder layer, thereby obtaining the slow-release urea fertilizer.

4. The method for preparing the slow-release urea fertilizer according to claim 3, characterized in that: In step S1, starch and water are stirred and mixed at 60-70° C. for 0.5-1 h to obtain the gelatinized starch; the starch is corn starch; and the polyvinyl alcohol solution is prepared by stirring polyvinyl alcohol and water at 80-90° C. for 1-2 h.

5. The method for preparing the slow-release urea fertilizer according to claim 4, characterized in that: The ratio of starch mass to water volume in the gelatinized starch in step S1 is 6. The method for preparing the slow-release urea fertilizer according to claim 4, characterized in that: The ratio of the mass of polyvinyl alcohol to the volume of water in the polyvinyl alcohol solution in step S1 is 7. The slow-release urea fertilizer according to claim 3, characterized in that: The biochar in step S2 is prepared by pyrolysis of crop straw; the crop straw includes one or more of rice straw, wheat straw, and nut shells.

8. The slow-release urea fertilizer according to claim 3, characterized in that: The organic fertilizer in step S2 is an organic fertilizer with a total nutrient content of ≥4%; the organic fertilizer is obtained by composting animal manure and / or plant residues.

9. The method for preparing the slow-release urea fertilizer according to claim 3, characterized in that: The mass ratio of biochar to organic fertilizer in step S2 is 1:4.

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