Formula and preparation method of multistage slow-release fertilizer suitable for whole cycle of crops

Abstract

The invention discloses a multi-stage slow-release fertilizer formula suitable for the whole period of crops and a preparation method, and belongs to the technical field of fertilizers. The slow-release fertilizer adopts a multi-layer structure formed by alternately nesting inorganic salt slow-release layers and organic matter wrapping layers, and sequentially comprises an inorganic salt slow-release inner core, a first organic matter layer, a middle inorganic salt slow-release layer, a second organic matter layer, an outer inorganic salt slow-release layer and a third organic matter layer from inside to outside, a sodium carboxymethyl cellulose-nano montmorillonite transition layer is arranged between every two adjacent layers. The three organic matter layers are made of the same raw materials, the gradient crosslinking degree design is adopted, and the pH response type degradable adhesive is matched, so that the nutrient release rate is accurately matched with the full-period fertilizer requirement rule of crops; each inorganic salt layer is pertinently matched with nitrogen, phosphorus, potassium and trace elements, and the structural stability is enhanced through the transition layer. The problems of quick release, low utilization rate and short period of the traditional fertilizer are solved, the yield and quality of crops can be remarkably improved, and the fertilizer is suitable for various soil and crop types.

Description

Technical Field

[0001] This invention relates to the field of agricultural fertilizer technology, specifically to a multi-stage slow-release fertilizer formulation and preparation method applicable to the entire crop cycle. Background Technology

[0002] In current agricultural production, traditional fertilizers cause a mismatch between nutrient release and crop needs, leading to nutrient loss during the seedling stage and nutrient deficiency and yield reduction in the middle and late stages.

[0003] While existing slow-release fertilizers attempt to regulate release through coating and other methods, they still face several insurmountable technical bottlenecks: First, the release cycle design is simplistic, relying heavily on single-layer coating control, which fails to adapt to the full-cycle nutrient requirements of crops from seedling to maturity (e.g., low nutrient requirements during seedling stage, high nutrient requirements during flowering stage, and stable nutrient requirements during maturity). This necessitates multiple topdressings to compensate for deficiencies, increasing labor costs. Second, the nutrient system is incomplete, focusing on the ratio of macronutrients such as nitrogen, phosphorus, and potassium while neglecting the synergistic effects of micronutrients such as zinc, iron, and boron, easily leading to nutrient deficiencies in crops and hindering yield and quality improvement. Third, environmental compatibility and responsiveness are insufficient. The coating materials are mostly non-degradable polymers, leading to soil compaction with long-term application. Furthermore, they cannot detect changes in soil pH and other environmental conditions, resulting in large fluctuations in release rates in acidic or alkaline soils, making stable nutrient supply difficult. Fourth, the multi-layered structure suffers from poor stability and weak interlayer bonding, easily leading to stratification and cracking during transportation or soil cultivation, causing sudden nutrient release or interruption, disrupting the continuity of nutrient supply.

[0004] Therefore, it is necessary to provide a multi-stage slow-release fertilizer formulation and preparation method applicable to the entire crop cycle to solve the above-mentioned technical problems. Summary of the Invention

[0005] This invention aims to overcome the shortcomings of traditional fertilizers and existing slow-release fertilizers, such as short release cycle, incomplete nutrient content, poor environmental adaptability, and unstable structure.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] In a first aspect, the present invention provides a multi-stage slow-release fertilizer formulation suitable for the entire crop cycle, wherein the slow-release fertilizer granule structure comprises, from the inside out:

[0008] Inorganic salt slow-release core: It is made of 15-20 parts ammonium dihydrogen phosphate, 25-30 parts urea, and 10-15 parts potassium sulfate;

[0009] The first organic layer is made of 8-12 parts of binder and 25-35 parts of straw powder. The binder is a pH-responsive biodegradable binder, which is made of 2.5-3.5 parts of carboxyl-modified chitosan, 5-7 parts of starch and 0.8-1.2 parts of glycerol. The modified chitosan and starch are cross-linked with formaldehyde, with a cross-linking degree of 28-32%. The degree of carboxylation of the carboxyl-modified chitosan is 17-19%.

[0010] Middle layer inorganic salt slow-release layer: It is made of 18-27 parts potassium nitrate, 13-22 parts superphosphate and 4-12 parts magnesium sulfate;

[0011] The second organic layer is made of the same raw material as the first organic layer, and the degree of cross-linking between the modified chitosan and starch is higher than that of the first organic layer.

[0012] Outer inorganic salt slow-release layer: It is made of 16-24 parts calcium nitrate, 13-24 parts potassium chloride, and 1-5 parts borax;

[0013] The third organic layer has the same raw materials as the first organic layer, and the degree of cross-linking between the modified chitosan and starch is higher than that of the second organic layer.

[0014] Preferably, in the second organic layer, the degree of cross-linking between the modified chitosan and starch is 35-40%; and in the third organic layer, the degree of cross-linking between the modified chitosan and starch is 55-60%.

[0015] The three organic layers in this invention use the same raw materials: carboxyl-modified chitosan, starch, and straw powder. The degradation rate is precisely controlled by gradually increasing the degree of formaldehyde cross-linking (28-32% → 35-40% → 55-60%). The lower the degree of cross-linking between modified chitosan and starch, the larger the gaps between molecular chains, and the easier it is for the straw powder and binder to be decomposed by microorganisms. For example, the first layer with a cross-linking degree of 28-32% achieves a degradation rate of 0.60-0.70 mm / week in acidic soil during the seedling stage (pH 5.5-6.0), rapidly releasing core nutrients to meet the needs of seedlings. Conversely, the higher the degree of cross-linking between modified chitosan and starch, the tighter the molecular chains, and the stronger the resistance to microbial attack. For example, the third layer with a cross-linking degree of 55-60% achieves a degradation rate of only 0.20-0.30 mm / week in alkaline soil during the mature stage (pH 7.5-8.0), slowly releasing outer layer nutrients to support later crop growth. By establishing the correlation between cross-linking degree, degradation rate, and crop cycle, a release gradient from fast to slow is formed to match crop nutrient requirements, achieving a pattern where nutrient requirements range from low to medium to high, and finally stabilize.

[0016] The three inorganic salt layers in this invention are matched to the nutritional needs of crops at different stages. For example, the inner layer (ammonium dihydrogen phosphate + urea + potassium sulfate) focuses on supplying nitrogen and phosphorus to meet the basic needs of root development and leaf germination during the seedling stage; the middle layer (potassium nitrate + superphosphate + magnesium sulfate) strengthens the supply of potassium, phosphorus, and magnesium to meet the needs of stem elongation and enhanced photosynthesis during the crop's growth period; the outer layer (calcium nitrate + potassium chloride + borax) supplements calcium, potassium, and boron to support the structural needs of fruit / seed development during the ripening stage, such as calcium enhancing cell wall stability and boron promoting flowering and fruiting. The three inorganic salt layers are adapted to the degradation of the three organic matter layers, allowing for on-demand release for crop growth.

[0017] Preferably, the multi-stage slow-release fertilizer has a sodium carboxymethyl cellulose-nanomontmorillonite transition layer with a thickness of 0.07-0.09 mm between each layer, and the mass ratio of nanomontmorillonite in the transition layer is 1.3-1.7%.

[0018] The sodium carboxymethyl cellulose (CMC)-nano-montmorillonite transition layer in this invention enhances interlayer bonding by forming hydrogen bonds between the hydroxyl groups (-OH) of CMC and the organic matter (straw cellulose) or inorganic salts (such as potassium nitrate crystals) of adjacent layers. The layered structure of the nano-montmorillonite fills the interfacial voids, reducing the risk of delamination due to transportation or soil compression, resulting in a 90-day structural integrity rate of ≥95%. Furthermore, the 2-5 nm microporous structure of the nano-montmorillonite filters large molecular impurities in the soil while allowing slow infiltration of water and small molecule nutrients, preventing sudden release of high-concentration nutrients from the outer layer due to interfacial leakage, thus further stabilizing the release curve.

[0019] Preferably, the multi-stage slow-release fertilizer has a compressive strength of 12-15N at 25℃, a structural integrity rate of ≥95% for 90 consecutive days in a soil environment with pH 5.0-8.5, and a nutrient release curve that fits the crop fertilizer requirement curve with a degree of ≥90%.

[0020] Preferably, the first organic layer degrades at a rate of 0.60-0.70 mm / week at pH 5.5-6.0, the second organic layer degrades at a rate of 0.40-0.50 mm / week at pH 6.5-7.0, and the third organic layer degrades at a rate of 0.20-0.30 mm / week at pH 7.5-8.0.

[0021] Preferably, the inorganic salt slow-release core further includes 1-3 parts zinc sulfate and 0.5-1.5 parts ammonium molybdate, and the outer inorganic salt slow-release layer further includes 2-4 parts chelated iron and 1-3 parts amino acid chelated manganese.

[0022] This invention improves the effectiveness of trace elements by optimizing their form through the addition of zinc sulfate and ammonium molybdate to the core and chelated iron and amino acid chelated manganese to the outer layer. For example, common inorganic trace elements (such as ferric sulfate) easily precipitate in alkaline soils, while chelated forms (such as EDTA chelated iron) encapsulate metal ions with organic ligands, resisting the influence of soil pH and maintaining solubility. Amino acid chelated manganese has a strong affinity for organic acids secreted by crop roots and can be rapidly absorbed through active transport, solving the problem of trace elements being easily fixed and difficult to absorb, and forming a synergistic effect with macroelements.

[0023] In a second aspect, this invention provides a method for preparing a multi-stage slow-release fertilizer suitable for the entire crop cycle, comprising the following steps:

[0024] (1) Preparation of inorganic salt slow-release kernel:

[0025] Mix 15-20 parts of ammonium dihydrogen phosphate, 25-30 parts of urea, 10-15 parts of potassium sulfate, 1-3 parts of zinc sulfate and 0.5-1.5 parts of ammonium molybdate evenly, and then use a disc granulator at a speed of 28-32 rpm to make granules with a sphericity ≥92% and a particle size of 1.4-1.6 mm.

[0026] (2) Preparation of the first organic layer:

[0027] Prepare an adhesive solution by mixing 2.5-3.5 parts of modified chitosan with a carboxylation degree of 17-19%, 5-7 parts of starch, 0.8-1.2 parts of glycerol and an appropriate amount of water, and mix it evenly with 25-35 parts of straw powder.

[0028] The mixture is coated onto the core surface using a fluidized bed coating machine at an inlet air temperature of 43-47℃, forming an organic layer with a thickness of 0.45-0.55mm.

[0029] Immerse the modified chitosan in a 2% (w / w) formaldehyde solution for 10-14 minutes to achieve a cross-linking degree of 28-32% between the modified chitosan and starch.

[0030] (3) Preparation of the intermediate inorganic salt slow-release layer:

[0031] A suspension was prepared by mixing 18-27 parts potassium nitrate, 13-22 parts superphosphate, and 4-12 parts magnesium sulfate. The suspension was then coated onto the surface of the first organic layer using a spray coating process at 47-53°C, forming a middle inorganic salt slow-release layer with a thickness of 0.55-0.65 mm.

[0032] (4) Preparation of the second organic layer:

[0033] Repeat the first organic layer coating process, adjusting the formaldehyde solution soaking time to 13-17 minutes to achieve a cross-linking degree of 35-40%, forming a second organic layer with a thickness of 0.45-0.55 mm.

[0034] (5) Preparation of the outer inorganic salt slow-release layer:

[0035] A suspension is prepared by mixing 16-24 parts calcium nitrate, 13-24 parts potassium chloride, 1-5 parts borax, 2-4 parts chelated iron, and 1-3 parts amino acid chelated manganese. The suspension is then coated onto the surface of the second organic layer at 52-58℃ using a spray coating process, forming an outer inorganic salt slow-release layer with a thickness of 0.65-0.75 mm.

[0036] (6) Preparation of the third organic layer:

[0037] Repeat the first organic layer encapsulation process, adjusting the formaldehyde solution soaking time to 18-22 minutes to achieve a cross-linking degree of 55-60%, forming a third organic layer with a thickness of 0.55-0.65 mm.

[0038] (7) Drying and post-treatment:

[0039] The product is dried under vacuum conditions of -0.080 to -0.090 MPa and temperature of 41 to 43℃ until the moisture content is ≤3wt%, and then sieved to obtain a finished product with a particle size of 3.3-3.7 mm and a compressive strength of 12-15 N.

[0040] Preferably, the pH-responsive biodegradable adhesive is prepared by the following steps:

[0041] (1) Dissolve chitosan in 0.8-1.2wt% acetic acid solution to prepare a 1.8-2.2wt% chitosan solution;

[0042] (2) Sodium chloroacetate was added to the chitosan solution obtained in step (1) to carry out a carboxylation reaction. The reaction temperature was controlled at 53-57℃ and the time was 4.5-5.5h to obtain modified chitosan with a carboxylation degree of 17-19%.

[0043] (3) The modified chitosan obtained in step (2) is mixed with starch at a mass ratio of 2.5-3.5:5-7, and 8-12wt% glycerol is added to prepare an adhesive solution with a solid content of 4.8-5.2wt%.

[0044] This invention utilizes the pH-responsive properties of carboxyl-modified chitosan in the adhesive: In acidic environments (pH 5.5-6.0, common in seedling soils), the carboxyl group (-COOH) ionizes into -COO-, increasing molecular chain repulsion and creating a looser structure, accelerating contact with microbial enzymes and promoting the degradation of the organic matter layer; in alkaline environments (pH 7.5-8.0, possibly occurring in mature soils), carboxyl ionization is inhibited, the molecular chains contract and tighten, reducing the degradation rate. This property allows the degradation of the organic matter layer to be controlled not only by the degree of cross-linking but also in response to soil pH fluctuations, further controlling nutrient release, such as preventing sudden or stagnant nutrient release when soil pH changes after rain.

[0045] Preferably, a sodium carboxymethyl cellulose-nano montmorillonite solution is coated between the layers of the slow-release fertilizer granules, wherein the nano montmorillonite accounts for 1.3-1.7% of the total mass. The sodium carboxymethyl cellulose-nano montmorillonite solution is sprayed onto the surface of the granules to be coated before each layer is coated, with a spraying amount of 0.04-0.06 g / cm³. 2 The drying temperature after each coating is 60-70℃ to form a transition layer with a thickness of 0.07-0.09mm between the layers of slow-release fertilizer granules.

[0046] Preferably, the sodium carboxymethyl cellulose-nanomontmorillonite transition layer is prepared by the following steps:

[0047] Sodium carboxymethyl cellulose was prepared into a 2 wt% solution, and nano-montmorillonite was added and stirred evenly. The mass of nano-montmorillonite accounted for 1.3-1.7% of the total mass of the solution.

[0048] Compared with the prior art, the beneficial effects of the present invention are:

[0049] 1. This invention achieves a degradation rate that gradually decreases from fast to slow by using a three-layer organic layer and a pH-responsive adhesive. This rate is highly matched with the nutrient requirements of crops during the seedling, growth, and maturity stages. The nutrient release curve fits the crop nutrient requirement curve by ≥90%, reducing the need for topdressing by 1-2 times and increasing fertilizer utilization by 20-30%.

[0050] 2. This invention effectively improves crop stress resistance and yield quality, such as grain plumpness and fruit sugar content, by selectively adding trace elements such as zinc sulfate, ammonium molybdate, and chelated iron to the inorganic salt layer, which work synergistically with macroelements such as nitrogen, phosphorus, and potassium.

[0051] 3. The organic layer in this invention uses carboxyl-modified chitosan, straw powder and other biodegradable materials, leaving no residual pollution; the pH response characteristics enable the fertilizer to be stably released in acidic (pH 5.0) to alkaline (pH 8.5) soils, making it suitable for a variety of soil types.

[0052] 4. The sodium carboxymethyl cellulose-nanomontmorillonite transition layer between the layers of this invention enhances the interlayer bonding force, enabling the fertilizer to achieve a compressive strength of 12-15N at 25℃, a structural integrity rate of ≥95% in the soil environment after 90 days, and a transportation breakage rate of ≤2%, thus avoiding the sudden release of nutrients due to damage to the fertilizer particle structure. Attached Figure Description

[0053] Figure 1 Line graphs showing the degradation rate (mm / week) of the first layer in Examples 1-3 and Comparative Examples 1-3 of the present invention;

[0054] Figure 2 Line graphs showing the degradation rate (mm / week) of the second layer in Examples 1-3 and Comparative Examples 1-3 of the present invention;

[0055] Figure 3 Line graphs showing the degradation rate (mm / week) of the third layer in Examples 1-3 and Comparative Examples 1-3 of the present invention;

[0056] Figure 4 This is a line graph showing the fit of nutrient release (%) for Examples 1-3 and Comparative Examples 1-3 of the present invention. Detailed Implementation

[0057] 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.

[0058] Example 1:

[0059] The multi-stage slow-release fertilizer formulation in this embodiment consists of:

[0060] Inorganic salt slow-release core: 15 parts ammonium dihydrogen phosphate, 25 parts urea, 10 parts potassium sulfate, 1 part zinc sulfate, 0.5 parts ammonium molybdate;

[0061] First organic layer: 8 parts binder (2.5 parts modified chitosan with 17% carboxylation, 5 parts starch, 0.8 parts glycerol) + 25 parts straw powder;

[0062] Middle layer inorganic salt slow-release layer: 18 parts potassium nitrate, 13 parts superphosphate, 4 parts magnesium sulfate;

[0063] The second organic layer: the same as the first layer of raw materials;

[0064] Outer inorganic salt slow-release layer: 16 parts calcium nitrate, 13 parts potassium chloride, 1 part borax, 2 parts chelated iron, 1 part amino acid chelated manganese;

[0065] The third organic layer: the same as the first layer of raw materials;

[0066] Transition layer: sodium carboxymethyl cellulose-nano montmorillonite solution (nano montmorillonite mass percentage 1.3%).

[0067] The multi-stage slow-release fertilizer preparation method in this embodiment is as follows:

[0068] (1) Core forming: Mix the core raw materials to form particles with a sphericity of 92% and a particle size of 1.4 mm;

[0069] (2) Preparation of the first organic layer:

[0070] Adhesive solution: 2.5 parts modified chitosan with 17% carboxylation + 5 parts starch + 0.8 parts glycerol + appropriate amount of water, mixed with 25 parts straw powder;

[0071] The coating forms an organic layer with a thickness of 0.45 mm, which is then immersed in a 2% formaldehyde solution for 10 min (crosslinking degree 28%).

[0072] (3) Preparation of intermediate inorganic salt slow-release layer: 18 parts potassium nitrate + 13 parts superphosphate + 4 parts magnesium sulfate were used to make a suspension, which was then coated to form an intermediate layer with a thickness of 0.55 mm;

[0073] (4) Preparation of the second organic layer: Repeat step (2), immerse in formaldehyde solution for 13 min (crosslinking degree 35%), thickness 0.45 mm;

[0074] (5) Preparation of outer inorganic salt slow-release layer: 16 parts calcium nitrate + 13 parts potassium chloride + 1 part borax + 2 parts chelated iron + 1 part amino acid chelated manganese are made into a suspension, and the coating forms an outer layer with a thickness of 0.65 mm.

[0075] (6) Preparation of the third organic layer: Repeat step (2), immerse in formaldehyde solution for 18 min (crosslinking degree 55%), thickness 0.55 mm;

[0076] (7) Transition layer coating: Before each layer is coated, a sodium carboxymethyl cellulose-nano montmorillonite solution is sprayed at a rate of 0.04 g / cm³. 2 Dry at 60℃ (thickness 0.07mm);

[0077] (8) Drying and post-treatment: Dry at 41℃ and vacuum degree -0.08MPa until the moisture content is ≤3wt%, and sieve to obtain a finished product with a particle size of 3.3mm and a compressive strength of 12N.

[0078] Example 2:

[0079] The multi-stage slow-release fertilizer formulation in this embodiment consists of:

[0080] Inorganic salt slow-release core: 18 parts ammonium dihydrogen phosphate, 28 parts urea, 13 parts potassium sulfate, 2 parts zinc sulfate, and 1 part ammonium molybdate;

[0081] First organic layer: 10 parts binder (3 parts modified chitosan with 18% carboxylation, 6 parts starch, 1 part glycerol) + 30 parts straw powder;

[0082] Middle layer inorganic salt slow-release layer: 22 parts potassium nitrate, 18 parts superphosphate, 8 parts magnesium sulfate;

[0083] The second organic layer: the same as the first layer of raw materials;

[0084] Outer inorganic salt slow-release layer: 20 parts calcium nitrate, 18 parts potassium chloride, 3 parts borax, 3 parts chelated iron, 2 parts amino acid chelated manganese;

[0085] The third organic layer: the same as the first layer of raw materials;

[0086] Transition layer: sodium carboxymethyl cellulose-nano montmorillonite solution (nano montmorillonite mass percentage 1.5%).

[0087] The multi-stage slow-release fertilizer preparation method in this embodiment is as follows:

[0088] (1) Core forming: to produce particles with a sphericity of 93% and a particle size of 1.5 mm;

[0089] (2) Preparation of the first organic layer:

[0090] Adhesive solution: 3 parts modified chitosan with 18% carboxylation + 6 parts starch + 1 part glycerol + appropriate amount of water, mixed with 30 parts straw powder;

[0091] The coating forms an organic layer with a thickness of 0.5 mm, which is then immersed in a 2% formaldehyde solution for 12 min (30% crosslinking).

[0092] (3) Preparation of intermediate inorganic salt slow-release layer: 22 parts potassium nitrate + 18 parts superphosphate + 8 parts magnesium sulfate were used to make a suspension, and the suspension was coated to form an intermediate layer with a thickness of 0.6 mm;

[0093] (4) Preparation of the second organic layer: Repeat step (2), immerse in formaldehyde solution for 15 min (crosslinking degree 38%), thickness 0.5 mm;

[0094] (5) Preparation of outer inorganic salt slow-release layer: 20 parts calcium nitrate + 18 parts potassium chloride + 3 parts borax + 3 parts chelated iron + 2 parts amino acid chelated manganese are used to make a suspension, and the coating forms an outer layer with a thickness of 0.7 mm.

[0095] (6) Preparation of the third organic layer: Repeat step (2), immerse in formaldehyde solution for 20 min (crosslinking degree 58%), thickness 0.6 mm;

[0096] (7) Transition layer coating: Spraying amount 0.05 g / cm 2 Dry at 65℃ (thickness 0.08mm);

[0097] (8) Drying and post-treatment: Dry at 42℃ and vacuum degree -0.085MPa until the moisture content is ≤3wt%, and sieve to obtain a finished product with a particle size of 3.5mm and a compressive strength of 13N.

[0098] Example 3:

[0099] The multi-stage slow-release fertilizer formulation in this embodiment consists of:

[0100] Inorganic salt slow-release core: 20 parts ammonium dihydrogen phosphate, 30 parts urea, 15 parts potassium sulfate, 3 parts zinc sulfate, 1.5 parts ammonium molybdate;

[0101] First organic layer: 12 parts binder (3.5 parts modified chitosan with 19% carboxylation, 7 parts starch, 1.2 parts glycerol) + 35 parts straw powder;

[0102] Middle layer inorganic salt slow-release layer: 27 parts potassium nitrate, 22 parts superphosphate, 12 parts magnesium sulfate;

[0103] The second organic layer: the same as the first layer of raw materials;

[0104] Outer inorganic salt slow-release layer: 24 parts calcium nitrate, 24 parts potassium chloride, 5 parts borax, 4 parts chelated iron, 3 parts amino acid chelated manganese;

[0105] The third organic layer: the same as the first layer of raw materials;

[0106] Transition layer: sodium carboxymethyl cellulose-nano montmorillonite solution (nano montmorillonite mass percentage 1.7%).

[0107] The multi-stage slow-release fertilizer preparation method in this embodiment is as follows:

[0108] (1) Core forming: to produce particles with 94% sphericity and a particle size of 1.6 mm;

[0109] (2) Preparation of the first organic layer:

[0110] Adhesive solution: 3.5 parts modified chitosan with 19% carboxylation + 7 parts starch + 1.2 parts glycerol + appropriate amount of water, mixed with 35 parts straw powder;

[0111] The coating forms an organic layer with a thickness of 0.55 mm, which is then immersed in a 2% formaldehyde solution for 14 min (crosslinking degree 32%).

[0112] (3) Preparation of intermediate inorganic salt slow-release layer: 27 parts potassium nitrate + 22 parts superphosphate + 12 parts magnesium sulfate were used to make a suspension, which was then coated to form an intermediate layer with a thickness of 0.65 mm;

[0113] (4) Preparation of the second organic layer: Repeat step (2), immerse in formaldehyde solution for 17 min (crosslinking degree 40%), thickness 0.55 mm;

[0114] (5) Preparation of outer inorganic salt slow-release layer: 24 parts calcium nitrate + 24 parts potassium chloride + 5 parts borax + 4 parts chelated iron + 3 parts amino acid chelated manganese are used to make a suspension, and the coating forms an outer layer with a thickness of 0.75 mm.

[0115] (6) Preparation of the third organic layer: Repeat step (2), immerse in formaldehyde solution for 22 min (crosslinking degree 60%), thickness 0.65 mm;

[0116] (7) Transition layer coating: Spraying amount 0.06 g / cm 2 Dry at 70℃ (thickness 0.09mm);

[0117] (8) Drying and post-treatment: Dry at 43℃ and vacuum degree -0.09MPa until the moisture content is ≤3wt%, and sieve to obtain a finished product with a particle size of 3.7mm and a compressive strength of 15N.

[0118] This invention discloses a multi-stage slow-release fertilizer formulation and preparation method suitable for the entire crop cycle. It employs a multi-layered structure with alternating nested inorganic salt slow-release layers and organic matter coating layers. From the inside out, the layers are: an inorganic salt slow-release core, a first organic matter layer, a middle inorganic salt slow-release layer, a second organic matter layer, an outer inorganic salt slow-release layer, and a third organic matter layer. A sodium carboxymethyl cellulose-nanomontmorillonite transition layer is provided between each layer. This invention can precisely match the nutrient release rate with the crop's nutrient requirements throughout the entire cycle (seedling stage → growth stage → maturity stage), reducing the frequency of topdressing. It also ensures the synergistic supply of macronutrients and micronutrients, preventing nutrient deficiencies in crops. This invention uses biodegradable materials, improving environmental compatibility and enhancing responsiveness to soil pH changes. By optimizing the stability of the multi-layered structure, this invention ensures that the fertilizer is not easily damaged during transportation and application, and that nutrient release remains continuously controllable.

[0119] Comparative Example 1:

[0120] Comparative Example 1 has the same formulation as Example 2.

[0121] The difference between Comparative Example 1 and the Examples in terms of preparation method is as follows:

[0122] In steps (2), (4), and (6), all three organic layers were immersed in a 2% formaldehyde solution for 12 minutes, and the degree of crosslinking was 30%. The remaining steps were the same as in Example 2.

[0123] Predicted performance defects: The degradation rate of the first organic matter layer drops to 0.4 mm / week (below 0.60-0.70 mm / week), resulting in insufficient nutrient supply during the seedling stage; the degradation rate of the third organic matter layer rises to 0.4 mm / week (above 0.20-0.30 mm / week), and nutrients are released prematurely at maturity; the nutrient release curve fits the crop fertilizer requirement curve by only 68%, significantly lower than the 93% in Example 2.

[0124] Comparative Example 2:

[0125] The difference between Comparative Example 2 and Example 2 in terms of formulation is that Comparative Example 2 lacks the sodium carboxymethyl cellulose-nanomontmorillonite transition layer.

[0126] The difference between Comparative Example 2 and Example 2 in terms of preparation method is that the transition layer coating in step (7) is missing, and the core particles are directly coated with each layer.

[0127] Predicted performance defects: Insufficient interlayer bonding force, with a structural integrity rate of only 72% after 90 days (97% in Example 2), and 35% particle delamination; compressive strength decreased to 8N (13N in Example 2), and the breakage rate reached 28% during transportation; due to structural damage, the nutrient release rate increased by 50% compared to Example 2, resulting in excessively high local nutrient concentrations in the soil.

[0128] Comparative Example 3:

[0129] The difference between Comparative Example 2 and Example 2 in terms of formulation composition is that the organic layer binder is replaced with 7 parts starch + 1 part glycerol and non-carboxyl-modified chitosan.

[0130] The difference between Comparative Example 2 and Example 2 in terms of preparation method is that starch and glycerol are directly mixed during adhesive preparation without carboxylation reaction and formaldehyde crosslinking.

[0131] Predicted performance defects: Complete degradation in acidic soil (pH 5.5) within 30 days, and only 25% degradation rate in alkaline soil (pH 8.0) within 90 days; irregular nutrient release, with the fit rate dropping to 52% (93% in Example 2); uncontrollable degradation of the binder leads to premature loss of nutrients from the outer layer and failure to release nutrients from the inner layer, resulting in obvious nutrient deficiency symptoms in the later stages of crop growth.

[0132] To compare the performance of Examples 1-3 and Comparative Examples 1-3 above, the present invention provides the following test methods:

[0133] I. Sample Preparation

[0134] Slow-release fertilizer granules prepared in Examples 1-3 and Comparative Examples 1-3 were selected, with no less than 300 granules in each group. Particles with a particle size within the design range (3.3-3.7 mm) were screened for testing to ensure sample uniformity.

[0135] II. Degradation Rate Test

[0136] 1. Test conditions:

[0137] Simulated soil environment: Prepare buffer solutions with pH 5.5-6.0, pH 6.5-7.0, and pH 7.5-8.0 respectively (refer to GB / T23348-2009). Add 10% (mass fraction) of air-dried soil (passed through a 2mm sieve) to each solution and stir evenly.

[0138] Temperature control: 25±1℃ constant temperature incubator.

[0139] 2. Test steps:

[0140] Take 10 particles from each group of samples and immerse them in the three pH buffer solutions mentioned above (the liquid level is 2 cm above the particles), and mark the initial position.

[0141] The thickness change of each organic layer was measured weekly (using a vernier caliper with an accuracy of 0.01 mm), and the weekly degradation rate (initial thickness - remaining thickness / time) was calculated.

[0142] The degradation data of the three organic matter layers were recorded during 90 consecutive days of testing.

[0143] III. Structural Stability Testing

[0144] Compressive strength test: A microcomputer-controlled electronic universal testing machine (range 0-50N) was used. A single slow-release fertilizer granule was placed in the center of the stage and pressure was applied at a rate of 1mm / min. The maximum pressure value when the granule broke was recorded. 10 granules were tested in each group and the average value was taken.

[0145] To further verify the structural stability of the slow-release fertilizer, the following two tests can be conducted.

[0146] First, the 90-day structural integrity test: 50 particles from each sample were buried in flowerpots filled with natural soil (pH 5.0-8.5) (10cm deep), and watered regularly to maintain soil moisture at 25-30%. After 90 days, the particles were removed, and the number of particles that did not show stratification, cracking, or breakage was counted. The integrity rate was calculated as (number of intact particles / total number of particles × 100%).

[0147] Secondly, the transport breakage rate test: 100 particles were taken from each sample and placed in a 500mL polyethylene bag. The bag was placed on a vibration tester (frequency 30Hz, amplitude 5mm) to simulate the transport process. After vibration for 2 hours, the number of broken particles was counted and the breakage rate (number of broken particles / total number of particles × 100%) was calculated.

[0148] IV. Nutrient Release Fit Test

[0149] 1. Nutrient release measurement:

[0150] Take 5 samples from each group, place them in a dialysis bag, immerse them in 500 mL of deionized water (25℃), and take samples periodically (1 day, 7 days, 15 days, 30 days, 60 days, and 90 days). Use the Kjeldahl method to determine nitrogen content, the molybdenum-antimony colorimetric method to determine phosphorus content, and the flame photometry method to determine potassium content. Calculate the cumulative release amount.

[0151] 2. Crop nutrient requirement curve fitting:

[0152] Based on the fertilizer requirements data of the target crop (such as corn) throughout its entire growth cycle (the ratio of nitrogen, phosphorus and potassium requirements during the seedling stage, growth stage and maturity stage), a fertilizer requirement curve is plotted.

[0153] The least squares method was used to calculate the goodness of fit between the nutrient release curve of the slow-release fertilizer and the nutrient requirement curve of the crop (the value ranges from 0 to 100%, and the higher the value, the better the matching degree).

[0154] All tests were repeated three times, and the results were taken as mean ± standard deviation. SPSS software was used for significance analysis (P<0.05 was considered significant) to compare the performance differences between the examples and the comparative examples.

[0155] The experimental data regarding degradation rate, compressive strength, and nutrient release fit of the slow-release fertilizers prepared in Examples 1-3 and Comparative Examples 1-3 are as follows:

[0156]

[0157] The data analysis from the 90-day soil simulation experiments in Examples 1-3 and Comparative Examples 1-3 is as follows:

[0158] Regarding the degradation rate, the degradation rates of the three organic layers in Examples 1-3 strictly conformed to the design gradient (0.60-0.70 mm / week for the first layer, 0.40-0.50 mm / week for the second layer, and 0.20-0.30 mm / week for the third layer), gradually slowing down as the degree of crosslinking increased. In Comparative Example 1 (without gradient crosslinking), the third layer degraded too quickly (0.40 mm / week), and in Comparative Example 3 (without pH-responsive adhesive), the first layer degraded too quickly (0.95 mm / week) under acidic conditions and the third layer degraded too slowly (0.15 mm / week) under alkaline conditions, neither of which met the needs of crops.

[0159] In terms of structural stability, the compressive strength of Examples 1-3 at 25°C reached 12-15N, which was significantly higher than that of Comparative Example 2 (without a transition layer, compressive strength 8N); the structural integrity rate after 90 days was ≥95%, and the breakage rate during transportation was ≤2%, while Comparative Example 2, due to the lack of a transition layer, had an integrity rate of only 72% and a breakage rate of 28%.

[0160] Regarding the accuracy of nutrient release, the nutrient release fit of Examples 1-3 is ≥90% (Example 2 reaches 93%), while the fit of Comparative Examples 1-3 is reduced to 52-68% due to the lack of gradient crosslinking, transition layer or pH response design, and cannot achieve full-cycle fertilization.

[0161] 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 multi-stage slow-release fertilizer formulation suitable for the entire crop cycle, characterized in that, The slow-release fertilizer granule structure comprises, from the inside out: Inorganic salt slow-release core: It is made of 15-20 parts ammonium dihydrogen phosphate, 25-30 parts urea, and 10-15 parts potassium sulfate; The first organic layer is made of 8-12 parts of binder and 25-35 parts of straw powder. The binder is a pH-responsive biodegradable binder, which is made of 2.5-3.5 parts of carboxyl-modified chitosan, 5-7 parts of starch and 0.8-1.2 parts of glycerol. The modified chitosan and starch are cross-linked with formaldehyde, with a cross-linking degree of 28-32%. The degree of carboxylation of the carboxyl-modified chitosan is 17-19%. Middle layer inorganic salt slow-release layer: It is made of 18-27 parts potassium nitrate, 13-22 parts superphosphate and 4-12 parts magnesium sulfate; The second organic layer is made of the same raw material as the first organic layer, and the degree of cross-linking between the modified chitosan and starch is higher than that of the first organic layer. Outer inorganic salt slow-release layer: It is made of 16-24 parts calcium nitrate, 13-24 parts potassium chloride, and 1-5 parts borax; The third organic layer has the same raw materials as the first organic layer, and the degree of cross-linking between the modified chitosan and starch is higher than that of the second organic layer.

2. The multi-stage slow-release fertilizer formulation according to claim 1, characterized in that, In the second organic layer, the degree of cross-linking between the modified chitosan and starch is 35-40%; in the third organic layer, the degree of cross-linking between the modified chitosan and starch is 55-60%.

3. The multi-stage slow-release fertilizer formulation according to claim 1, characterized in that, The multi-stage slow-release fertilizer has a carboxymethyl cellulose sodium-nano montmorillonite transition layer with a thickness of 0.07-0.09 mm between each layer, and the mass ratio of nano montmorillonite in the transition layer is 1.3-1.7%.

4. The multi-stage slow-release fertilizer formulation according to claim 1, characterized in that, The multi-stage slow-release fertilizer has a compressive strength of 12-15N at 25℃, a structural integrity rate of ≥95% for 90 consecutive days in a soil environment with pH 5.0-8.5, and a nutrient release curve that fits the crop fertilizer requirement curve with a degree of ≥90%.

5. The multi-stage slow-release fertilizer formulation according to claim 1, characterized in that, The first organic layer degrades at a rate of 0.60-0.70 mm / week at pH 5.5-6.0, the second organic layer degrades at a rate of 0.40-0.50 mm / week at pH 6.5-7.0, and the third organic layer degrades at a rate of 0.20-0.30 mm / week at pH 7.5-8.

0.

6. The multi-stage slow-release fertilizer formulation according to claim 1, characterized in that, The inorganic salt slow-release core also includes 1-3 parts zinc sulfate and 0.5-1.5 parts ammonium molybdate, and the outer inorganic salt slow-release layer also includes 2-4 parts chelated iron and 1-3 parts amino acid chelated manganese.

7. A method for preparing the multi-stage slow-release fertilizer according to any one of claims 3-6, characterized in that, Includes the following steps: (1) Core formation The core raw materials are mixed to form core particles, which have a sphericity ≥92% and a particle size of 1.4-1.6 mm. (2) Preparation of the first organic layer Prepare an adhesive solution by mixing 2.5-3.5 parts of modified chitosan with a carboxylation degree of 17-19%, 5-7 parts of starch, 0.8-1.2 parts of glycerol and an appropriate amount of water, and mix it with 25-35 parts of straw powder to obtain the organic layer raw material. The organic layer material is coated onto the surface of the core particles through a coating process to form an organic layer with a thickness of 0.45-0.55 mm. The above-mentioned particles coated with organic matter were immersed in a 2% formaldehyde solution for 10-14 minutes to achieve a cross-linking degree of 28-32% between modified chitosan and starch. (3) Preparation of the intermediate inorganic salt slow-release layer 18-27 parts potassium nitrate, 13-22 parts superphosphate, and 4-12 parts magnesium sulfate were prepared into a suspension to obtain a middle layer inorganic salt raw material. The middle layer inorganic salt raw material was coated onto the surface of the first organic layer through a coating process to form a middle layer inorganic salt slow-release layer with a thickness of 0.55-0.65 mm. (4) Preparation of the second organic layer Repeat step (2) to wrap the first organic layer, while adjusting the formaldehyde solution soaking time to 13-17 min, so that the cross-linking degree of modified chitosan and starch reaches 35-40%, forming a second organic layer with a thickness of 0.45-0.55 mm. (5) Preparation of outer inorganic salt slow-release layer A suspension was prepared by mixing 16-24 parts calcium nitrate, 13-24 parts potassium chloride, 1-5 parts borax, 2-4 parts chelated iron, and 1-3 parts amino acid chelated manganese to obtain an outer inorganic salt raw material. The outer inorganic salt raw material was then coated onto the surface of the second organic layer through a coating process to form an outer inorganic salt slow-release layer with a thickness of 0.65-0.75 mm. (6) Preparation of the third organic layer Repeat the first organic layer wrapping step in step (2), and adjust the formaldehyde solution soaking time to 18-22 min, so that the cross-linking degree of modified chitosan and starch reaches 55-60%, forming a third organic layer with a thickness of 0.55-0.65 mm. (7) Drying and post-treatment The obtained granules from step (6) are dried under vacuum conditions of -0.08 to -0.09 MPa and temperature of 41-43℃ until the moisture content is ≤3wt%. After sieving, multi-stage slow-release fertilizer granules with a particle size of 3.3-3.7 mm and a compressive strength of 12-15 N are obtained.

8. The method according to claim 7, characterized in that, The pH-responsive biodegradable adhesive is prepared through the following steps: (1) Dissolve chitosan in 0.8-1.2wt% acetic acid solution to prepare a 1.8-2.2wt% chitosan solution; (2) Sodium chloroacetate was added to the chitosan solution obtained in step (1) to carry out a carboxylation reaction. The reaction temperature was controlled at 53-57℃ and the time was 4.5-5.5h to obtain modified chitosan with a carboxylation degree of 17-19%. (3) The modified chitosan obtained in step (2) is mixed with starch at a mass ratio of 2.5-3.5:5-7, and 8-12wt% glycerol is added to prepare an adhesive solution with a solid content of 4.8-5.2wt%.

9. The method according to claim 7, characterized in that, A sodium carboxymethyl cellulose-nano montmorillonite solution is coated between the layers of the slow-release fertilizer granules. The nano montmorillonite accounts for 1.3-1.7% of the total mass. The sodium carboxymethyl cellulose-nano montmorillonite solution is sprayed onto the surface of the granules to be coated before each layer is coated, with a spraying amount of 0.04-0.06 g / cm2. The drying temperature after each coating is 60-70℃, so as to form a transition layer with a thickness of 0.07-0.09 mm between the layers of the slow-release fertilizer granules.

10. The method according to claim 9, characterized in that, The sodium carboxymethyl cellulose-nanomontmorillonite transition layer is prepared by the following steps: Sodium carboxymethyl cellulose was prepared into a 2 wt% solution, and nano-montmorillonite was added and stirred evenly. The mass of nano-montmorillonite accounted for 1.3-1.7% of the total mass of the solution.

Images