Multi-stage slow-release fertilizer pill production method suitable for whole period of crops

Abstract

The invention discloses a production method of multi-stage slow-release fertilizer pills suitable for the whole cycle of crops. The production method comprises the steps of core fertilizer pill preparation, middle layer wrapping, outer layer preparation and post-treatment. The core layer takes nitrogen, phosphorus, potassium, organic matters and the like as raw materials, and super absorbent resin and nano montmorillonite are added to form an expandable porous structure; the middle layer adopts a chitosan-polylactic acid mixed film, and swelling regulation and control are realized through a modified sodium alginate binder; and the outer layer is a coupling layer formed by cordierite ceramic particles and a photo-thermal response material. After the produced multi-stage slow-release fertilizer pill is dried at the temperature of 60-70 DEG C, the nutrient release error of the product within the period of 90-180 days is smaller than or equal to 5%, the nitrogen phosphorus and potassium utilization rate is increased by 25%-35%, the soil leaching loss is reduced by 40%-50%, the release stability under the extreme climate is improved by 60%-70%, the nutrient requirements of crops in the seedling stage, the growth stage and the mature stage can be accurately met, and the yield of the crops is increased. The problems that traditional chemical fertilizers are not matched in release and slow-release fertilizers are poor in stability are solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of agricultural fertilizer, in particular to a multi-stage slow-release fertilizer pellet production method suitable for the whole cycle of crops. BACKGROUND

[0002] At present, the traditional fertilizer widely used in agricultural production has the problem of mismatch between nutrient release rate and crop demand. The release of nutrients is too fast in the seedling stage, which easily leads to seedling burn, the release is insufficient in the growth stage, which affects the development of crops, and the release is excessive in the mature stage, which causes resource waste and soil pollution.

[0003] Although the existing slow-release fertilizer can delay the release through coating technology, it still has defects. For example, the single coating structure cannot cope with the complex environmental changes such as soil pH, temperature and humidity, the release stability is poor, and under extreme weather conditions such as continuous rain or high temperature and drought, the nutrient loss rate is as high as 50% or more. In addition, the interlayer bonding force of the fertilizer pellet is weak, which is easy to peel off in the soil, leading to burst release or stagnation, and the nutrient utilization rate is only 30%-40%. In addition, the fertilizer pellet lacks gradient release design for the whole cycle of crops (seedling stage, growth stage and mature stage), and cannot accurately match the nutrient demand at different stages, such as high nitrogen demand in the seedling stage, balanced phosphorus and potassium demand in the growth stage, and low nutrient demand in the mature stage.

[0004] Therefore, it is necessary to provide a multi-stage slow-release fertilizer pellet production method suitable for the whole cycle of crops to solve the above technical problems. SUMMARY

[0005] The purpose of the present application is to provide a multi-stage slow-release fertilizer pellet production method suitable for the whole cycle of crops to solve the problems of mismatch between nutrient release of traditional fertilizer and crop demand, poor ability of existing slow-release fertilizer to cope with complex environment, weak interlayer bonding force and inability to accurately match the nutrient demand of crops in the whole cycle.

[0006] To achieve the above purpose, the present application provides the following technical scheme: a multi-stage slow-release fertilizer pellet production method suitable for the whole cycle of crops, comprising the following steps: (1) Core fertilizer pellet preparation Mix 60-75 parts of total nitrogen, phosphorus and potassium, 10-20 parts of organic matter and 5-8 parts of trace elements, add 5-10 parts of starch grafted acrylic acid superabsorbent resin and 1-2 parts of nano montmorillonite, stir uniformly after adding water, and then mix and granulate to form core fertilizer pellets with a porous structure that can expand on the surface; (2) Middle layer wrapping Mix chitosan and polylactic acid at a mass ratio of 1:1-2:1, add modified sodium alginate adhesive containing 10-20 parts of sodium carboxymethyl cellulose, dissolve in a solvent to form a solution with a concentration of 10%-20%, coat the core fertilizer pellets of step (1), and form a swelling film on the surface to obtain fertilizer pellets wrapped with a middle layer; (3) Preparation of outer layer Mix 60-70 parts cordierite ceramic particles, 2-5 parts photoresponsive titanium dioxide, and 1-3 parts thermosensitive poly(N-isopropylacrylamide), add 10%-15% polyvinyl alcohol binder to make a slurry, coat the pellets from step (2) to form a photothermal coupling layer, and obtain pellets with an outer coating. (4) Post-processing The fertilizer pellets from step (3) are dried at 60-70℃ until the moisture content is less than 5% to obtain multi-stage slow-release fertilizer pellets.

[0007] The multi-stage slow-release fertilizer pellets produced by the method provided in this invention can precisely match the nutrient requirements of crops throughout their entire life cycle. During the seedling stage, the outer light-temperature coupling layer rapidly releases nitrogen through pore regulation under specific light and temperature conditions, meeting the high nitrogen demand of seedlings and preventing seedling burn. During the growth period, the cross-layer mass transfer channels formed by the middle and core layers cause the nutrient release rate to decrease exponentially, releasing phosphorus and potassium evenly to ensure crop growth. During the maturity stage, the core fertilizer pellet slowly releases the remaining nutrients, working in conjunction with the outer layer regulation mechanism to reduce excessive release and soil pollution. In addition, under extreme environments, the core layer pore expansion rate remains stable during continuous rain, and the middle layer regulation channels prevent nutrient burst release, reducing leaching losses by 40%-50%. Under high temperature and drought conditions, the outer layer pores shrink, reducing loss and increasing release stability by 60%-70% under extreme climate conditions. Simultaneously, the cumulative utilization rate of nitrogen, phosphorus, and potassium increases by 25%-35% over a 90-180 day period, with a release cycle error of ≤5%. The chute-type continuous production system used in this invention improves interlayer peel strength by 40%-50%, particle size deviation by ≤±0.2mm, and production efficiency by 30%.

[0008] Preferably, in step (1), the water content is controlled at 15%-20% and the granulation pressure is 0.3-0.5MPa during the granulation of the core fertilizer layer. Granulation is carried out at 40-50℃ for 10-15 minutes to form initial pores with a pore size of 5-10μm on its surface. After the starch-grafted acrylic superabsorbent resin absorbs water, the pores expand to 50-100μm. Furthermore, the nano-montmorillonite inhibits its excessive expansion through the interlayer structure, so that the volume expansion rate of the pores is 300%-400% when the humidity is ≥60%.

[0009] This invention expands the initial pores by using starch-grafted acrylic superabsorbent resin in the core layer of the slow-release fertilizer to absorb water, providing a channel for nutrient release. The nano-montmorillonite inhibits excessive expansion through its interlayer structure, controlling the volume expansion rate under high humidity to 300%-400%, thus avoiding structural cracking.

[0010] Preferably, in step (2), a porous chitosan-polylactic acid membrane is formed after coating the core pellets. The membrane has a pore size of 1-5 μm and a distribution density of 50-100 pores. After the membrane absorbs water and swells, it forms microporous channels of 10-30 μm. These microporous channels, together with the pores of the core layer that expand to 50-100 μm, constitute a translayer mass transfer channel, causing the nutrient release rate to decrease exponentially, with an attenuation constant k of 0.015-0.025 / day. The hydroxyl groups of the outer polyvinyl alcohol binder used in step (3) permeate through the microporous channels of the intermediate layer and form a chelating effect with the carboxyl groups of the starch-grafted acrylic superabsorbent resin in the core layer. This makes the swelling rate of the intermediate layer synchronized with the pore expansion rate of the core layer, with a determining coefficient. ≥0.95.

[0011] In this invention, step (2) involves forming a porous chitosan-polylactic acid membrane on the outside of the core pellet, with an initial pore size of 1-5 μm and a distribution density of 50-100 pores. After absorbing water and swelling, it forms microporous channels of 10-30 μm, which, together with the pores of the core layer expanding to 50-100 μm, constitute a cross-layer mass transfer channel. This causes the nutrient release rate to decay exponentially (decay constant k is 0.015-0.025 / day), precisely matching the dynamic nutrient requirements of crops throughout their entire life cycle. The decay constant k reflects the rate of nutrient release decay over time; the smaller the k, the slower the release and the longer the fertilizer effect. This exponential release, achieved through cross-layer mass transfer, matches the needs of crops throughout their entire life cycle. Simultaneously, the hydroxyl groups of the outer polyvinyl alcohol binder permeate through the microporous channels of the intermediate layer, forming a chelating effect with the carboxyl groups of the starch-grafted acrylic superabsorbent resin in the core layer. This ensures that the swelling rate of the intermediate layer is highly synchronized with the pore expansion rate of the core layer (determinant coefficient). ≥0.95), significantly enhancing interlayer bonding, avoiding unstable nutrient release caused by interlayer delamination, ultimately achieving efficient, stable nutrient release adapted to all stages of crop growth, and determining the coefficient of determination. It reflects the synchronicity between the swelling of the intermediate layer and the pore expansion of the core layer. The closer the value is to 1, the tighter the interlayer bonding and the less likely it is to be peeled off, ensuring stable release.

[0012] Preferably, in step (2), the modified sodium alginate binder has a carboxyl methyl substitution degree of 0.3-0.5, and its molecular chain carboxyl groups form ionic bonds with a bond energy of 30-50 kJ / mol with chitosan amino groups; when the modified sodium alginate absorbs water and swells, it will form an elastic gel layer, which can adjust the pore opening degree of the chitosan-polylactic acid membrane by changing its own swelling degree, so that the sieve pore size of the pores can vary with the swelling degree in the range of 5-50 μm.

[0013] This invention utilizes a porous membrane formed by chitosan and polylactic acid in the intermediate layer. After swelling by absorbing water, the membrane forms a translayer mass transfer channel with the pores of the core layer. The carboxyl groups of the modified sodium alginate binder form ionic bonds with the amino groups of chitosan. The gel layer adjusts the pore opening and closing degree through changes in swelling, achieving an exponential decrease in the nutrient release rate. Furthermore, the hydroxyl groups of the outer polyvinyl alcohol binder form a chelating effect with the carboxyl groups of the superabsorbent resin in the core layer, synchronizing the swelling rate of the intermediate layer with the pore expansion rate of the core layer.

[0014] Preferably, in step (3), the fertilizer pellets with the intermediate layer are placed into a granulator and granulated at 50-60°C for 15-20 minutes to form a photothermal coupling layer on the surface of the slurry; wherein, titanium dioxide expands the pore size of the ceramic channel by 30%-50% when exposed to light ≥2000 lux, and poly(N-isopropylacrylamide) shrinks the channel by 20%-30% when exposed to temperature ≥25°C.

[0015] Preferably, in step (3), the surface of the outer cordierite ceramic particles is loaded with 20-50nm nano-sized titanium dioxide. The photogenerated electron-hole pairs generated under 400-700nm wavelength light trigger the low critical dissolution temperature of poly(N-isopropylacrylamide) to drift by ±2℃ through interfacial charge transfer. This photoinduced temperature-sensitive characteristic shift forms a cascade amplification mechanism of light-temperature response, which enables the autonomous adjustment of the outer layer nutrient release rate to 40%-60% in an environment with a day-night temperature difference ≥10℃.

[0016] Preferably, in step (3), the outer cordierite ceramic particles... Content is 50-70 parts The content is 30-45 parts, and the mass loss rate is ≤5% in soil with pH 3-11.

[0017] Preferably, the cordierite ceramic particles have an average pore size of 2-5 μm, and unqualified particles are removed by screening.

[0018] This invention utilizes nano-titanium dioxide on the surface of the outer cordierite ceramic particles to generate photogenerated electron-hole pairs under light irradiation, triggering a low critical dissolution temperature drift of poly(N-isopropylacrylamide) and forming a cascade amplification mechanism of light-temperature response. When the light irradiation is ≥2000 lux, the ceramic channels expand, and when the temperature is ≥25℃, the channels shrink. In an environment with a day-night temperature difference ≥10℃, the nutrient release rate of the outer layer can be autonomously regulated by 40%-60%.

[0019] Preferably, the nitrogen, phosphorus, and potassium ratio in the core fertilizer layer raw material is: 20-25 parts nitrogen, and the rest phosphorus. Counted as 15-18 portions, potassium element The total is 15-22 portions.

[0020] Preferably, the multi-stage slow-release fertilizer pellets have a nutrient release cycle error of ≤5% within a period of 90-180 days, increase the cumulative utilization rate of nitrogen, phosphorus and potassium by 25%-35%, reduce soil leaching loss by 40%-50%, and improve release stability under extreme climate conditions by 60%-70%.

[0021] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention achieves gradient nutrient release through a multi-level structural design consisting of a core layer, a middle layer, and an outer layer, combined with the responsive characteristics of each layer's materials. The core layer, with its starch-grafted acrylic superabsorbent resin and nano-montmorillonite, works synergistically to control the pore expansion rate at 300%-400% when humidity is ≥60%, providing sufficient nutrients for seedlings. The middle layer, a chitosan-polylactic acid membrane, forms a cross-layer mass transfer channel with the core layer's pores, causing the nutrient release rate to decrease exponentially (k=0.015-0.025 / day), meeting the balanced needs of the growth period. The outer light-temperature coupling layer, through the cascade response of titanium dioxide and poly(N-isopropylacrylamide), adjusts the release range to 40%-60% when the diurnal temperature difference is ≥10℃, adapting to the low nutrient requirements of the maturity stage, fundamentally avoiding the problems of seedling burn, insufficient growth period, and waste during maturity associated with traditional chemical fertilizers.

[0022] 2. Compared with existing single-coating slow-release fertilizers, the slow-release fertilizer prepared by this invention has a multi-level structure with multiple regulatory mechanisms. For example, the middle layer of modified sodium alginate gel can adjust the pore size (5-50μm) through swelling to cope with soil pH fluctuations; the outer layer of cordierite ceramic particles (… 50-70 servings (30-45 parts) showed a mass loss rate of ≤5% in soil with pH 3-11. Combined with light- and temperature-responsive materials, this improved release stability under extreme climates by 60%-70%. Simultaneously, the chelation effect between the outer polyvinyl alcohol hydroxyl groups and the core carboxyl groups (…) ≥0.95) enhances interlayer bonding, increases interlayer peel strength by 40%-50%, reduces the risk of explosive release or stagnation, increases the cumulative utilization rate of nitrogen, phosphorus and potassium from 30%-40% of the existing technology to 25%-35%, and reduces soil leaching loss by 40%-50%.

[0023] 3. This invention employs a chute-type continuous production system to achieve precise preparation of multi-layered nested organic-inorganic-organic structures, with particle size deviation ≤ ±0.2 mm and production efficiency increased by 30%. The expandable porous structure of the core layer, the swelling and mass transfer synergistic mechanism of the middle layer, and the light-temperature response cascade amplification effect of the outer layer form an organic whole. This ensures stability with a nutrient release error of ≤ 5% within a 90-180 day cycle, and also reduces negative environmental impacts through interfacial interactions between materials, such as interlayer inhibition of nano-montmorillonite and photogenerated charge transfer of titanium dioxide. Attached Figure Description

[0024] Figure 1 Line graphs showing the increase in cumulative utilization rate of nitrogen, phosphorus, and potassium for the slow-release fertilizers prepared in Examples 1-3 and Comparative Examples 1-3; Figure 2 Line graphs showing the reduction in soil leaching loss of the slow-release fertilizers prepared in Examples 1-3 and Comparative Examples 1-3. Detailed Implementation

[0025] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. 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.

[0026] Example 1: Core fertilizer pellet preparation: Mix 60 parts of total nitrogen, phosphorus, and potassium (20 parts nitrogen, 10 parts phosphorus, and 10 parts potassium). A total of 15 portions, potassium Mix 15 parts of organic matter, 10 parts of trace elements, and 5 parts of starch-grafted acrylic superabsorbent resin and 1 part of nano-montmorillonite. Add water and stir until the water content is 15%. Granulate for 10 minutes at a granulation pressure of 0.3 MPa and a temperature of 40°C to form core fertilizer pellets with an initial surface pore size of 5 μm.

[0027] Intermediate layer coating: Chitosan and polylactic acid are mixed in a mass ratio of 1:1, and modified sodium alginate binder containing 10% sodium carboxymethyl cellulose (carboxymethyl substitution degree 0.3) is added. This mixture is dissolved in a solvent to prepare a 10% concentration solution, which is then used to coat the core pellets, forming pores with a diameter of 1 μm and a distribution density of 50. Chitosan-polylactic acid membrane.

[0028] Outer layer preparation: 60 parts of cordierite ceramic particles ( 50 copies 30 parts (average pore size 2μm), 2 parts 20nm nano titanium dioxide, and 1 part thermosensitive poly(N-isopropylacrylamide) were mixed and 10% polyvinyl alcohol binder was added to make a slurry. The intermediate layer pellets were granulated at 50℃ for 15 minutes to form a photothermal coupling layer.

[0029] Post-processing: Dry at 60℃ to a moisture content of 4% to obtain multi-stage slow-release fertilizer pellets.

[0030] Example 2: Core fertilizer pellet preparation: A total nitrogen, phosphorus, and potassium content of 68 parts (nitrogen 23 parts, phosphorus 23 parts, and potassium 40 parts) is prepared. A total of 16 portions, potassium Mix 29 parts of organic matter, 15 parts of trace elements, and 6 parts of micronutrients. Add 8 parts of starch-grafted acrylic superabsorbent resin and 1.5 parts of nano-montmorillonite. Add water and stir until the water content is 18%. Granulate for 13 minutes at a granulation pressure of 0.4 MPa and a temperature of 45°C to form core fertilizer pellets with an initial surface pore size of 8 μm.

[0031] Intermediate layer coating: Chitosan and polylactic acid are mixed at a mass ratio of 1.5:1, and a modified sodium alginate binder containing 15% sodium carboxymethyl cellulose (carboxymethyl substitution degree 0.4) is added. This mixture is dissolved in a solvent to prepare a 15% concentration solution, which is then used to coat the core pellets, forming pores with a diameter of 3 μm and a distribution density of 75. Chitosan-polylactic acid membrane.

[0032] Outer layer preparation: 65 parts of cordierite ceramic particles ( 60 copies 38 parts (average pore size 3.5 μm), 3.5 parts (35 nm nano titanium dioxide), and 2 parts (thermosensitive poly(N-isopropylacrylamide)) were mixed and 13% polyvinyl alcohol binder was added to make a slurry. The intermediate layer pellets were granulated at 55°C for 18 min to form a photothermal coupling layer.

[0033] Post-processing: Dry at 65℃ to a moisture content of 3% to obtain multi-stage slow-release fertilizer pellets.

[0034] Example 3: Core fertilizer pellet preparation: Mix 75 parts of total nitrogen, phosphorus, and potassium (25 parts nitrogen, 10 parts phosphorus, and 10 parts potassium). A total of 18 portions, potassium Mix 22 parts of organic matter, 20 parts of trace elements, and 8 parts of micronutrients. Add 10 parts of starch-grafted acrylic superabsorbent resin and 2 parts of nano-montmorillonite. Add water and stir until the water content is 20%. Granulate for 15 minutes at a granulation pressure of 0.5 MPa and a temperature of 50°C to form core fertilizer pellets with an initial surface pore size of 10 μm.

[0035] Intermediate layer coating: Chitosan and polylactic acid are mixed at a mass ratio of 2:1, and a modified sodium alginate binder containing 20% ​​sodium carboxymethyl cellulose (carboxymethyl substitution degree 0.5) is added. This mixture is dissolved in a solvent to prepare a 20% concentration solution, which is then used to coat the core pellets, forming pores with a diameter of 5 μm and a distribution density of 100. Chitosan-polylactic acid membrane.

[0036] Outer layer preparation: 70 parts of cordierite ceramic particles ( 70 copies 45 parts (average pore size 5μm), 5 parts 50nm nano titanium dioxide, and 3 parts thermosensitive poly(N-isopropylacrylamide) were mixed and 15% polyvinyl alcohol binder was added to make a slurry. The intermediate layer pellets were granulated at 60℃ for 20 minutes to form a photothermal coupling layer.

[0037] Post-processing: Dry at 70℃ to a moisture content of 2% to obtain multi-stage slow-release fertilizer pellets.

[0038] The multi-stage slow-release fertilizer pellets of this invention are prepared through the following production system: At least three stages of sluices (e.g., a first-stage organic matter sluice, a first-stage inorganic salt sluice, and a second-stage organic matter sluice) are arranged sequentially along the material rolling direction, with a material addition area between each two stages of sluices, containing a built-in spray device and a powder distributor. Specific powders are spread on the tracks of each stage of the sluices: the inorganic salt sluices are spread with inorganic salt powder containing nitrogen, phosphorus, and potassium (thickness 0.5-1 mm), and the organic matter sluices are spread with organic powders such as humic acid and chitosan-polylactic acid (thickness 0.3-0.8 mm). The track slope is 5°-10° to ensure uniform particle rolling. Adjacent sluices are connected by an arc-shaped transition plate to achieve continuous production.

[0039] During production, the initial inorganic salt particles enter from the beginning of the first-stage organic matter sluice. During rolling, they adhere to organic powder, forming a base. After being sprayed with water (controlling the water content to 15%-20%) in the material addition zone, they enter the first-stage inorganic salt sluice, where they adhere to inorganic salt powder containing highly absorbent resin and nano-montmorillonite, forming a core fertilizer layer with a diameter of 1.0-1.5 mm. Subsequently, the fertilizer pellets enter the next-stage material addition zone, are sprayed with modified sodium alginate binder, and then enter the second-stage organic matter sluice, where they adhere to organic powder, forming a 50-100 μm thick intermediate layer (diameter increased to 1.2-1.8 mm). Next, after being sprayed with polyvinyl alcohol binder in the addition zone, they enter the second-stage inorganic salt sluice, where they adhere to a mixture of cordierite ceramic particles and light / temperature responsive material powder. Under the assistance of hot air at 50-60℃, an outer layer of 80-120 μm thickness (final diameter 1.5-2.5 mm) is formed. Finally, the product is dried at 60-70℃ until the moisture content is <5%, and then sieved to obtain a finished product with an organic-inorganic-organic multilayer nested structure.

[0040] This system achieves precise nesting of multi-layered structures through continuous chute production, increasing interlayer peel strength by 40%-50% and particle size deviation ≤±0.2mm. The core layer of nano-montmorillonite can control the expansion rate to 300%-400% under high humidity, while the outer layer of light-temperature responsive material can adjust the expansion rate by 40%-60% under extreme climates. This not only solves the problems of poor interlayer bonding and unstable release in traditional processes, but also increases production efficiency by 30%, meeting the precise nutrient release needs of crops throughout their entire life cycle.

[0041] Comparative Example 1: The difference from Example 2 is that the intermediate layer of step (2) is missing, and the core pellet is directly coated with the outer layer.

[0042] Comparative Example 2: The difference from Example 2 is that the outer layer uses only cordierite ceramic particles and polyvinyl alcohol binder, and does not contain titanium dioxide and poly(N-isopropylacrylamide).

[0043] Comparative Example 3: Commercially available single-chitosan-coated slow-release fertilizer is used, with the core fertilizer formula the same as in Example 2, but without intermediate or outer layer structures.

[0044] To analyze the performance differences between the fertilizer pellets prepared in Examples 1-3 and Comparative Examples 1-3, the present invention provides the following test methods: I. Sample Preparation: Fertilizer pellets were prepared according to Examples 1-3 and Comparative Examples 1-3. 30 parallel samples were prepared for each group. The particle diameter was uniformly controlled at 3.0±0.2 mm. The pellets were placed in a desiccator (humidity 30%) and stored for 24 hours for later use.

[0045] II. Performance Testing Methods 1. Nutrient release cycle and error determination The constant temperature water bath method (temperature 25℃±1℃, 500mL deionized water) was used. The sample (5g) was placed in a dialysis bag (molecular weight cutoff 8000-14000Da). 20mL of sample was taken daily (with an equal amount of deionized water added simultaneously). The nitrogen content was determined by ultraviolet spectrophotometer (detection wavelength 660nm), phosphorus by molybdenum antimony colorimetric method, and potassium by flame photometry. The time when the cumulative nutrient release reached 80% was recorded as the release cycle. The periodic error was calculated as the ratio of the periodic standard deviation of 30 samples to the average value.

[0046] 2. Cumulative utilization rate test of nitrogen, phosphorus and potassium Wheat seedlings (three-leaf stage) were selected for pot experiments. Each pot contained 5 kg of soil (1.5% organic matter content) and 10 g of sample fertilizer was applied. The control group was treated with the same amount of commercially available compound fertilizer. The plants were harvested after 30 days, and the total nitrogen, phosphorus and potassium content of the above-ground parts was measured. The increase was calculated by (absorption amount of experimental group - absorption amount of blank group) / total fertilizer application × 100% and compared with commercially available compound fertilizer.

[0047] 3. Determination of soil leaching loss The soil column leaching method was used (soil column height 30cm, diameter 10cm). 5g of sample was applied to each column, simulating natural rainfall (leaching twice a week, 500mL of deionized water each time). The total nitrogen, phosphorus and potassium content of the leachate was measured, and the leaching loss rate was calculated as: total nutrients in the leachate / total fertilizer applied × 100%. The reduction rate was compared with that of commercially available compound fertilizer.

[0048] 4. Stability testing for release in extreme climates Continuous rain simulation: The sample was placed in a constant temperature and humidity chamber (temperature 20℃, humidity 90%), and the nutrient release was measured daily. The percentage of the maximum fluctuation value (maximum value - minimum value) of the release over 10 days was calculated relative to the average value.

[0049] High temperature and drought simulation: The sample was placed in a light incubator (temperature 35℃, light intensity 8000 lux, humidity 30%), and the release fluctuation value was measured over 10 days using the same method.

[0050] 5. Determination of interlayer peel strength Using a universal testing machine (tensile rate 5 mm / min), the sample was fixed in a fixture, and the maximum tensile force during interlayer peeling was measured. The peel strength was calculated as: maximum tensile force / sample cross-sectional area (unit: MPa). High humidity expansion rate determination: The sample was soaked in deionized water for 24 hours, and the diameter before and after expansion was measured with Vernier calipers. The expansion rate was calculated as: (expanded volume - initial volume) / initial volume × 100%.

[0051] All experimental data were taken as the average of three replicates. SPSS 22.0 software was used for analysis of variance (P<0.05 was considered statistically significant), and the results were rounded to one decimal place.

[0052] The experimental data from the above experiments are as follows:

[0053] Experimental data show that the multi-stage slow-release fertilizer pellets of Examples 1-3 outperformed Comparative Examples 1-3 in all aspects of performance. Regarding nutrient release cycle and error, the release cycles of Examples 1-3 were 150 days, 165 days, and 180 days, respectively, with cycle errors all ≤3% and exhibiting exponential decay, meeting the gradient requirements of the entire crop cycle. In contrast, Comparative Example 1 (without the intermediate layer) had a release cycle of only 90 days with an error of 8%, Comparative Example 2 (without the outer light and temperature material) had a cycle of 120 days with an error of 7%, and Comparative Example 3 (commercially available single-coating) had a cycle of 100 days with an error of 10%, all exhibiting short cycles and poor stability. In terms of the cumulative utilization rate of nitrogen, phosphorus, and potassium, Examples 1-3 achieved a 25%-35% increase, and soil leaching losses decreased by 40%-50%, significantly higher than the 10%-18% and 15%-25% increases of the Comparative Examples. This is attributed to the cross-layer mass transfer channels between the core and intermediate layers and the regulation by the outer layer. Under extreme climate conditions, during continuous rainy weather, the release fluctuation of the embodiment was 10%-15%, compared to 35%-50% in the comparative embodiment; during high temperature and drought, the fluctuation was 12%-18% in the embodiment, compared to 38%-55% in the comparative embodiment, demonstrating the advantage of the light-temperature response cascade mechanism of the outer layer of the embodiment. Regarding interlayer bonding performance, the peel strength of the embodiment was 1.2-1.8 MPa, and the high humidity expansion rate was 300%-400%, while the strength of the comparative embodiment was only 0.5-0.8 MPa, with no controlled expansion. The interlayer synchronization advantage of the embodiment can prevent structural fracture or peeling caused by asynchronous changes in each layer.

[0054] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A method for producing multi-stage slow-release fertilizer pellets suitable for the entire crop cycle, characterized in that, Includes the following steps: (1) Preparation of core fertilizer pellets Mix 60-75 parts of total nitrogen, phosphorus and potassium, 10-20 parts of organic matter and 5-8 parts of trace elements, add 5-10 parts of starch-grafted acrylic superabsorbent resin and 1-2 parts of nano montmorillonite, add water and stir evenly, then mix and granulate to form a core fertilizer pellet with an expandable porous structure on the surface. (2) Intermediate layer wrapping Chitosan and polylactic acid are mixed in a mass ratio of 1:1-2:1, and modified sodium alginate binder containing 10-20 parts of sodium carboxymethyl cellulose is added. The mixture is dissolved in a solvent to prepare a solution with a concentration of 10%-20%, which is used to coat the core pellets of step (1) to form a swelling film on its surface, thereby obtaining pellets with an intermediate layer. (3) Preparation of outer layer Mix 60-70 parts cordierite ceramic particles, 2-5 parts photoresponsive titanium dioxide, and 1-3 parts thermosensitive poly(N-isopropylacrylamide), add 10%-15% polyvinyl alcohol binder to make a slurry, coat the pellets from step (2) to form a photothermal coupling layer, and obtain pellets with an outer coating. (4) Post-processing The fertilizer pellets from step (3) are dried at 60-70℃ until the moisture content is less than 5% to obtain multi-stage slow-release fertilizer pellets.

2. The production method according to claim 1, characterized in that, In step (1), the water content is controlled at 15%-20% and the granulation pressure is 0.3-0.5MPa during the granulation of the core fertilizer layer. Granulation is carried out at 40-50℃ for 10-15min to form initial pores with a pore size of 5-10μm on its surface. After starch grafted with acrylic superabsorbent resin absorbs water, the pores expand to 50-100μm. Furthermore, nano-montmorillonite inhibits its excessive expansion through the interlayer structure, so that the volume expansion rate of the pores is 300%-400% when the humidity is ≥60%.

3. The production method according to claim 1, characterized in that, In step (2), a porous chitosan-polylactic acid membrane is formed after coating the core pellets. The membrane has a pore size of 1-5 μm and a distribution density of 50-100 pores. After the membrane absorbs water and swells, it forms microporous channels of 10-30 μm. These microporous channels, together with the pores of the core layer that expand to 50-100 μm, constitute a translayer mass transfer channel, causing the nutrient release rate to decrease exponentially, with an attenuation constant k of 0.015-0.025 / day. The hydroxyl groups of the outer polyvinyl alcohol binder used in step (3) permeate through the microporous channels of the intermediate layer and form a chelating effect with the carboxyl groups of the starch-grafted acrylic superabsorbent resin in the core layer. This makes the swelling rate of the intermediate layer synchronized with the pore expansion rate of the core layer, with a determining coefficient. ≥0.

95.

4. The production method according to claim 1, characterized in that, In step (2), the degree of carboxymethyl substitution of the modified sodium alginate binder is 0.3-0.5, and its molecular chain carboxyl groups form ionic bonds with a bond energy of 30-50 kJ / mol with chitosan amino groups. When the modified sodium alginate absorbs water and swells, it forms an elastic gel layer. The opening and closing degree of the pores of the chitosan-polylactic acid membrane can be adjusted by the change of its own swelling degree, so that the sieve pore size of the pores can vary in the range of 5-50 μm with the swelling degree.

5. The production method according to claim 3, characterized in that, In step (3), the fertilizer pellets with the intermediate layer are placed into a granulator and granulated at 50-60℃ for 15-20 minutes to form a photothermal coupling layer on the surface of the slurry; wherein, titanium dioxide expands the pore size of the ceramic channel by 30%-50% when exposed to light ≥2000 lux, and poly(N-isopropylacrylamide) shrinks the channel by 20%-30% when exposed to temperature ≥25℃.

6. The production method according to claim 1, characterized in that, In step (3), the surface of the outer cordierite ceramic particles is loaded with 20-50nm nano-sized titanium dioxide. The photogenerated electron-hole pairs generated under 400-700nm wavelength light trigger the low critical dissolution temperature of poly(N-isopropylacrylamide) to drift by ±2℃ through interfacial charge transfer. This photoinduced temperature-sensitive characteristic shift forms a cascade amplification mechanism of light-temperature response, which enables the autonomous adjustment of the nutrient release rate of the outer layer to 40%-60% in an environment with a day-night temperature difference ≥10℃.

7. The production method according to claim 1, characterized in that, In step (3), the outer layer of cordierite ceramic particles Content is 50-70 parts The content is 30-45 parts, and the mass loss rate is ≤5% in soil with pH 3-11.

8. The production method according to claim 1, characterized in that, The cordierite ceramic particles have an average pore size of 2-5 μm, and unqualified particles are removed by screening.

9. The production method according to claim 1, characterized in that, The nitrogen, phosphorus, and potassium ratio in the core fertilizer layer raw materials is: 20-25 parts nitrogen, and the rest phosphorus. Counted as 15-18 portions, potassium element The total is 15-22 portions.

10. The production method according to any one of claims 1-9, characterized in that, The multi-stage slow-release fertilizer pellets have a nutrient release cycle error of ≤5% within a period of 90-180 days, increase the cumulative utilization rate of nitrogen, phosphorus and potassium by 25%-35%, reduce soil leaching loss by 40%-50%, and improve release stability by 60%-70% under extreme climate conditions.

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