A compound fertilizer for acid soil and a preparation method thereof

Abstract

The application belongs to the technical field of agricultural resources and environment, and particularly relates to a compound fertilizer for acid soil and a preparation method thereof, wherein the compound fertilizer is a compound particle with a double-layer structure; the compound particle comprises a core and a shell coated on the outer surface of the core; the core comprises chemical fertilizer, potassium-mineral fulvic acid and a first binder; and the shell comprises alkali lignin, potassium humate and a second binder; after the compound fertilizer is applied to acid soil, alkali lignin and potassium humate in the shell can rapidly react with hydrogen ions in the soil, are dissolved and form organic-inorganic composite colloid rich in functional groups such as carboxyl and phenolic hydroxyl groups, so that the soil pH value can be effectively improved and stabilized; relying on the physical barrier of the shell and the ion exchange and complex adsorption of the new colloid, the fixation of phosphorus and the leaching loss of nitrogen and potassium can be significantly reduced, the soil aggregate structure can be promoted, and long-acting fertilizer preservation and buffering capacity can be provided.

Description

Technical Field

[0001] This invention belongs to the field of agricultural resources and environmental technology, specifically a compound fertilizer for acidic soils and its preparation method. Background Technology

[0002] Soil acidification is a significant environmental problem facing agricultural production, particularly in red and yellow soil regions and some facility agriculture areas, where acidification is becoming increasingly severe. Acidic soils (pH typically below 5.5) cause a series of problems: first, increased activity of metal ions such as aluminum and manganese, which are toxic to crop roots; second, the fixation of nutrients such as phosphate, potassium, and calcium, leading to a sharp decline in their availability; and third, the deterioration of soil microbial community structure, hindering the decomposition of organic matter and nutrient cycling. These problems directly result in low fertilizer utilization, poor crop growth, and limited yields.

[0003] To combat soil acidification, the traditional approach is to apply alkaline amendments such as lime and dolomite powder. However, this method has significant drawbacks: 1) the improvement is not long-lasting, as the amendments are easily leached away, requiring frequent application; 2) it is labor-intensive and time-consuming, requiring separate application from fertilizers, increasing labor costs; 3) it can easily cause localized alkalization or drastic pH fluctuations, which can negatively impact root systems and microorganisms. Currently, most commercially available fertilizers do not organically combine acid-regulating functions with nutrient supply. Although conventional organic-inorganic compound fertilizers contain a certain amount of organic matter, their organic components (such as weathered coal and livestock manure) have low activity and slow reaction in acidic environments, resulting in weak acid-regulating effects and difficulty in controlling nutrient release rates, still facing serious leaching and fixation losses.

[0004] In the field of slow-release fertilizers, existing technologies mostly focus on physical coatings (such as polymer and sulfur coatings) or chemically synthesized slow-release substances (such as urea-formaldehyde). While these technologies can delay nutrient release, they are costly, and the non-degradable synthetic coating materials may pose environmental risks. More importantly, existing slow-release technologies are "passive" release technologies, failing to generate "active" interaction with the specific chemical environment of acidic soils. They cannot simultaneously resolve the contradiction between nutrient "release" and "preservation," meaning that under acidic conditions, even slowly released nutrients are easily fixed or leached away.

[0005] Therefore, the current technological field lacks an integrated fertilizer product that can intelligently respond to acidic soil environments and efficiently couple "acid-base neutralization," "controlled nutrient release," and "soil colloid construction." Developing a novel fertilizer that utilizes soil acidity as a trigger, constructs a fertilizer-retaining micro-pool using natural organic matter, and thus achieves low-cost, high-efficiency soil improvement and nutrient supply has become an urgent technical challenge to be solved in this field. Summary of the Invention

[0006] To solve the above-mentioned technical problems, the present invention provides a compound fertilizer for acidic soil, wherein the compound fertilizer is a compound granule with a double-layer structure; The composite particles include a core and a shell covering the outer surface of the core; Based on a total mass of 100 wt% for the compound fertilizer, the core has a mass fraction of 24 wt% to 65 wt%, the outer shell has a mass fraction of 35 wt% to 76 wt%, and the sum of the mass fractions of the core and the outer shell is 100 wt%. Based on a total mass of 100wt% of the compound fertilizer, the core comprises: 18wt%~50wt% of chemical fertilizer, 5wt%~10wt% of mineral-derived potassium humate, and 1wt%~5wt% of a first binder. Based on a total mass of 100wt% of the compound fertilizer, the outer shell comprises: 24wt%~46wt% alkali lignin, 10wt%~25wt% potassium humate, and 1wt%~5wt% a second binder.

[0007] Furthermore, the mass fraction of the kernel is any one of 24wt%, 28wt%, 32wt%, 35wt%, 40wt%, 45wt%, 50wt%, 55wt%, 60wt%, 65wt%, or a range between two of them. Furthermore, the mass fraction of the outer shell is any one of 35wt%, 40wt%, 45wt%, 50wt%, 55wt%, 60wt%, 65wt%, 70wt%, 76wt%, or a range between two of them. Furthermore, the mass fraction of fertilizer in the core is any one of 18wt%, 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, or a range between two of them. Furthermore, the mass fraction of potassium humate from mineral sources in the core is any one of 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, or a range between two of them. Furthermore, the mass fraction of alkali lignin in the shell is any one of 24wt%, 30wt%, 35wt%, 40wt%, 46wt%, or a range between two of them. Furthermore, the mass fraction of potassium humate in the outer shell is any one of 10wt%, 15wt%, 20wt%, 25wt%, or a range between two of them.

[0008] The fertilizer includes at least one of urea, ammonium sulfate, monoammonium phosphate, diammonium phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, potassium nitrate, and potassium sulfate. The fulvic acid content in the mineral-derived potassium fulvicate is greater than or equal to 50 wt%. The first binder is a clay-based substance, including one of bentonite, kaolin, or attapulgite.

[0009] Wherein, the purity of the alkali lignin is greater than or equal to 99%, and the pH value of the alkali lignin is 8 to 10; The humic acid content in the potassium humate is greater than or equal to 65 wt%.

[0010] Furthermore, the pH value of the alkali lignin is any one of 8, 9, 10, or a range between two of them.

[0011] The second binder is modified water-soluble sodium carboxymethyl cellulose, and the degree of substitution of the modified water-soluble sodium carboxymethyl cellulose is greater than or equal to 0.6.

[0012] The core has a particle size of 1-3 mm, the composite particles have a particle size of 2-4 mm, and the composite particles have a larger particle size than the core.

[0013] Furthermore, the particle size of the kernel is any one of 1 mm, 2 mm, 3 mm, or a range between two of them; Furthermore, the particle size of the compound particles is any one of 2 mm, 3 mm, and 4 mm, or a range between two of them.

[0014] To address the aforementioned technical problems, the present invention also provides a method for preparing the above-mentioned compound fertilizer for acidic soils, comprising the following steps: S1. The fertilizer, the mineral-derived potassium humate, and the first binder are mixed, granulated, and dried to obtain core particles; S2. Mix the alkali lignin, the potassium humate and the second binder with water to prepare a coating slurry; S3. The coating slurry is coated onto the outer surface of the core particles and dried to form the outer shell on the outer surface of the core particles, thus obtaining coated particles. S4. The coated particles are dried and sieved to obtain compound particles.

[0015] Specifically, step S1 involves crushing the fertilizer, the mineral-derived potassium humate, and the first binder into powders, passing them through an 80-mesh sieve, and then mixing them to obtain a mixture. The granulation process includes spraying granulation water into a rotary drum granulator, wherein the mass of the granulation water is 7wt% to 10wt% of the total mass of the mixture. Furthermore, the mass of the granulation water is any one of 7wt%, 8wt%, 9wt%, or 10wt% of the total mass of the mixture, or a range between two of these.

[0016] The drying process includes drying the granulated mixture at a temperature of 70~80°C until the moisture content of the mixture is less than or equal to 4wt%.

[0017] Specifically, step S2 involves first adding the second adhesive to water and stirring until completely dissolved, then sequentially adding the alkali lignin and the potassium humate, and preparing the coating slurry by high-speed shearing and stirring. The solid content of the coating slurry is 60% to 70%.

[0018] Furthermore, the solid content of the coating slurry is any one of 60%, 65%, 70%, or a range between two of them.

[0019] Specifically, S3 involves feeding the core particles into a rotary drum coating machine while simultaneously introducing hot air at a temperature of 50-70°C to spray the coating slurry onto the surface of the core particles, thereby forming the outer shell by coating the core particles and obtaining the coated particles.

[0020] Specifically, step S4 involves drying the coated particles in a fluidized bed at 50-60°C until the moisture content of the coated particles is less than or equal to 3 wt%, thereby obtaining the composite particles.

[0021] When the compound fertilizer prepared by this invention is applied to acidic soil, the alkali lignin and potassium humate in its outer shell can quickly react with soil hydrogen ions, dissolve and form an organic-inorganic complex colloid rich in functional groups such as carboxyl and phenolic hydroxyl groups. This can effectively increase and stabilize the soil pH value. Relying on the physical barrier of the outer shell and the ion exchange and complexation adsorption of the newly formed colloid, it can significantly reduce the fixation of phosphorus and the leaching loss of nitrogen and potassium, promote the formation of soil aggregate structure, and provide long-term fertilizer retention and buffering capacity. Detailed Implementation

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

[0023] This invention provides a method for preparing a compound fertilizer for acidic soils, comprising the following steps: S1. The fertilizer, the mineral-derived potassium humate, and the first binder are mixed, granulated, and dried to obtain core particles; In this step, the fertilizer, the mineral-derived potassium humate, and the first binder are crushed and mixed evenly. Then, the mixed materials are fed into a granulation device, and an appropriate amount of water is sprayed in during the granulation process of the rotating drum to promote the agglomeration of the materials into granules. Finally, the wet granules after granulation are dried, cooled, and sieved to obtain granules that meet the expected particle size as the core particles for later use.

[0024] S2. Mix the alkali lignin, the potassium humate and the second binder with water to prepare a coating slurry; In this step, to ensure the uniformity of the slurry and the final film-forming and curing effect of the outer shell, the second adhesive is first added to an appropriate amount of clean water and completely dissolved under continuous stirring to form an adhesive solution; then, the formulated amounts of the alkali lignin and the potassium humate powder are added to the adhesive solution in sequence, and the mixture is stirred thoroughly using a stirring device to prepare a uniform, viscous, and pumpable slurry with a suitable solid content, which is the coating slurry.

[0025] S3. The coating slurry is coated onto the outer surface of the core particles and dried to form the outer shell on the outer surface of the core particles, thus obtaining coated particles. In this step, the core particles obtained in the previous process are fed into the coating equipment as the mother core; while keeping the mother core (core particles) in a uniform rolling state, the coating slurry is uniformly sprayed onto the outer surface of the mother core; the key is that hot air needs to be introduced simultaneously during the slurry spraying process so that the sprayed coating slurry coats the surface of the mother core layer by layer and dries and solidifies into a film in time, ultimately forming a dense outer shell.

[0026] S4. The coated particles are dried and sieved to obtain compound particles.

[0027] In this step, in order to further reduce the product moisture and improve the physical strength of the coated particles, the coated particles are subjected to a final drying process; after drying and cooling, they are sieved by a sieving device to remove unqualified fine powder, and finally obtain compound particles with uniform particle size.

[0028] Example 1 The components and their contents in this embodiment are as follows: Core components: 78 kg urea, 114 kg diammonium phosphate, 116 kg potassium sulfate, 80 kg mineral-derived potassium humate (humic acid content 55%), 20 kg bentonite; Shell: 392 kg of alkali lignin (purity 99%, pH 9.2), 170 kg of potassium humate (humic acid content 70%), and 30 kg of modified water-soluble sodium carboxymethyl cellulose (degree of substitution 0.8).

[0029] A method for preparing a compound fertilizer for acidic soils includes the following steps: S1. Urea, diammonium phosphate, potassium sulfate, mineral-derived potassium humate and bentonite are crushed to pass through an 80-mesh sieve and then mixed evenly. The mixture is fed into a rotary drum granulator and water equivalent to 10wt% of the total mass of the mixture is sprayed in for granulation. The mixture is dried at 75℃ until the moisture content is 4wt%. After cooling and sieving, 2mm core particles are obtained.

[0030] S2. Add modified water-soluble sodium carboxymethyl cellulose to an appropriate amount of water and stir to dissolve. Then add alkali lignin and potassium humate in sequence. Stir at high speed for 20 minutes to prepare a coating slurry with a solid content of 64%.

[0031] S3. The core particles are fed into the rotary drum coating machine, and 65°C hot air is simultaneously introduced to atomize and spray the coating slurry onto the surface of the core particles to form a shell, thus obtaining coated particles.

[0032] S4. The coated particles are dried in a fluidized bed at 55°C until the moisture content is 3wt%, and then sieved to obtain compound particles with a particle size of 3mm.

[0033] Example 2 Unlike Example 1, the components and their contents in this example are as follows: Core components: 117 kg urea, 171 kg diammonium phosphate, 174 kg potassium sulfate, 80 kg mineral-derived potassium humate (humic acid content 60%), 20 kg attapulgite. Shell: 280 kg of alkali lignin (purity 99%, pH 9.5), 138 kg of potassium humate (humic acid content 65%), and 20 kg of modified water-soluble sodium carboxymethyl cellulose (degree of substitution 0.7).

[0034] In step S1, the mass of water sprayed during granulation is equivalent to 7 wt% of the total mass of the mixture. In step S3, the temperature of the hot air introduced is 60°C.

[0035] Comparative Example 1 Unlike Example 1, the preparation method in Example 1 was not used. Instead, the components of the core and the components of the shell were mixed and prepared into homogeneous mixed particles by conventional extrusion granulation.

[0036] Comparative Example 2 Unlike Example 1, potassium humate was not added to the outer shell; instead, it was replaced by an equal mass of alkali lignin.

[0037] Comparative Example 3 Unlike Example 1, no alkali lignin was added to the outer shell; instead, it was replaced by potassium humate at an equal mass.

[0038] Comparative Example 4 Unlike Example 1, this is a common granular organic-inorganic compound fertilizer (total nutrients 30%, organic matter 20%, pH 7, without core-shell structure).

[0039] Comparative Example 5 Unlike Example 1, with a total mass of 100wt% of compound fertilizer, the core accounts for 80wt% and the outer shell accounts for 20wt%, and the proportions of the internal components are the same as in Example 1.

[0040] Comparative Example 6 Unlike Example 1, with a total mass of 100wt% of compound fertilizer, the core accounts for 20wt% and the outer shell accounts for 80wt%, and the proportions of the internal components are the same as in Example 1.

[0041] The compound fertilizers prepared in the examples and comparative examples were applied: Typical southern red soil (initial pH=5.0) was used for pot cultivation of maize, with each pot containing the same amount of soil. The maize was cultivated until maturity. The results of each example or comparative example were obtained by averaging the results of five repeated pot cultivations. The treatments were the example group (applied with the compound fertilizers prepared in Example 1 and Example 2, respectively) and the comparative group (applied with the compound fertilizers from Comparative Examples 1, 2, 3, 4, 5, and 6, respectively) which were given equal amounts of nitrogen, phosphorus, and potassium nutrients. A blank control group without fertilizer was also set up.

[0042] Examples 1 and 2 raised the rhizosphere pH to 6.2-6.3, significantly higher than the control groups, indicating that the core-shell structure combined with alkali lignin and potassium humate has a significant acid-regulating ability.

[0043] The example group was superior to the comparative group in both plant height and yield per plant. Among them, the comparative group 6 (high outer shell ratio) was better than the comparative group 5 (high kernel ratio), but both were lower than the example group, indicating that the appropriate inner and outer shell ratio and the synergistic effect of the components are the key.

[0044] The leachate was collected using the bottom seepage collection method. The group of examples showed the lowest loss of nitrogen, phosphorus, and potassium, indicating that the core-shell structure effectively reduced nutrient leaching and improved fertilizer utilization efficiency.

[0045] When the compound fertilizer prepared by this invention is applied to acidic soil, the alkali lignin and potassium humate in its outer shell can quickly react with soil hydrogen ions, dissolve and form an organic-inorganic complex colloid rich in functional groups such as carboxyl and phenolic hydroxyl groups. This can effectively increase and stabilize the soil pH value. Relying on the physical barrier of the outer shell and the ion exchange and complexation adsorption of the newly formed colloid, it can significantly reduce the fixation of phosphorus and the leaching loss of nitrogen and potassium, promote the formation of soil aggregate structure, and provide long-term fertilizer retention and buffering capacity.

[0046] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A compound fertilizer for acidic soils, characterized in that, The compound fertilizer is a compound granule with a double-layer structure. The composite particles include a core and a shell covering the outer surface of the core; Based on a total mass of 100 wt% for the compound fertilizer, the core has a mass fraction of 24 wt% to 65 wt%, the outer shell has a mass fraction of 35 wt% to 76 wt%, and the sum of the mass fractions of the core and the outer shell is 100 wt%. Based on a total mass of 100wt% of the compound fertilizer, the core comprises: 18wt%~50wt% of chemical fertilizer, 5wt%~10wt% of mineral-derived potassium humate, and 1wt%~5wt% of a first binder. Based on a total mass of 100wt% of the compound fertilizer, the outer shell comprises: 24wt%~46wt% alkali lignin, 10wt%~25wt% potassium humate, and 1wt%~5wt% a second binder.

2. The compound fertilizer for acidic soil according to claim 1, characterized in that, The fertilizer includes at least one of urea, ammonium sulfate, monoammonium phosphate, diammonium phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, potassium nitrate, and potassium sulfate. The fulvic acid content in the mineral-derived potassium fulvicate is greater than or equal to 50 wt%. The first binder is a clay-based substance, including one of bentonite, kaolin, or attapulgite.

3. The compound fertilizer for acidic soil according to claim 1, characterized in that, The purity of the alkali lignin is greater than or equal to 99%, and the pH value of the alkali lignin is 8 to 10. The humic acid content in the potassium humate is greater than or equal to 65 wt%.

4. A compound fertilizer for acidic soils according to claim 1, characterized in that, The second binder is modified water-soluble sodium carboxymethyl cellulose, wherein the degree of substitution of the modified water-soluble sodium carboxymethyl cellulose is greater than or equal to 0.

6.

5. A compound fertilizer for acidic soils according to claim 1, characterized in that, The core has a particle size of 1-3 mm, the compound particles have a particle size of 2-4 mm, and the particle size of the compound particles is larger than that of the core.

6. The method for preparing compound fertilizer for acidic soil as described in any one of claims 1 to 5, characterized in that, Includes the following steps: S1. The fertilizer, the mineral-derived potassium humate, and the first binder are mixed, granulated, and dried to obtain core particles; S2. Mix the alkali lignin, the potassium humate and the second binder with water to prepare a coating slurry; S3. The coating slurry is coated onto the outer surface of the core particles and dried to form the outer shell on the outer surface of the core particles, thus obtaining coated particles. S4. The coated particles are dried and sieved to obtain compound particles.

7. The method for preparing compound fertilizer for acidic soil according to claim 6, characterized in that, Specifically, step S1 involves crushing the fertilizer, the mineral-derived potassium humate, and the first binder into powders, passing them through an 80-mesh sieve, and then mixing them to obtain a mixture. The granulation process includes spraying granulation water into a rotary drum granulator, wherein the mass of the granulation water is 7wt% to 10wt% of the total mass of the mixture. The drying process includes drying the granulated mixture at a temperature of 70~80°C until the moisture content of the mixture is less than or equal to 4wt%.

8. The method for preparing compound fertilizer for acidic soil according to claim 6, characterized in that, Specifically, step S2 involves first adding the second adhesive to water and stirring until completely dissolved, then sequentially adding the alkali lignin and the potassium humate, and preparing the coating slurry by high-speed shearing and stirring. The solid content of the coating slurry is 60% to 70%.

9. The method for preparing compound fertilizer for acidic soil according to claim 6, characterized in that, S3 specifically involves feeding the core particles into a rotary drum coating machine while simultaneously introducing hot air at a temperature of 50~70℃, spraying the coating slurry onto the surface of the core particles, so that the coating slurry coats the surface of the core particles to form the outer shell, thereby obtaining the coated particles.

10. The method for preparing compound fertilizer for acidic soil according to claim 6, characterized in that, Specifically, step S4 involves drying the coated particles in a fluidized bed at 50-60°C until the moisture content of the coated particles is less than or equal to 3 wt%, thereby obtaining the composite particles.

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