A citric acid soluble slow-release fertilizer, a preparation method and application thereof

Abstract

This invention discloses a citric acid-soluble slow-release fertilizer, its preparation method, and its application. The stoichiometric formula of the active ingredient in the citric acid-soluble slow-release fertilizer is (NH4). a Ca b Zn c Mg d Fe e P n O 3n+1 a + 2(b + c + d + e) ​​= n + 2, n ≥ 2, and at least two of b, c, d, and e are greater than zero. This citric acid-soluble slow-release fertilizer is extremely stable in neutral, alkaline, and weakly acidic soil environments, releasing only a small amount of NH4+ through hydrolysis very slowly. + When plant roots secrete citric acid or the soil lacks corresponding metal ions, citrate ions chelate with metal ions, causing a decrease in the local microenvironment pH. This leads to the depolymerization of the polyphosphate skeleton under acidic conditions, rapidly releasing H2PO4. ‑ Ca 2+ Zn 2+ Mg 2+ Fe 2+ It provides nutrients in sync with crop needs, improving fertilizer efficiency and reducing pollution.

Description

Technical Field

[0001] This invention relates to the field of agricultural chemistry and fertilizer industry, and in particular to a citric acid-soluble slow-release fertilizer, its preparation method, and its application. Background Technology

[0002] Chemical fertilizers are the cornerstone of modern agriculture, but traditional fertilizers, especially water-soluble phosphate and nitrogen fertilizers, release nutrients too quickly after being applied to the soil and are easily lost through volatilization, leaching, and fixation, resulting in generally low utilization rates (approximately 30-35% for nitrogen fertilizers and 10-25% for phosphate fertilizers). This not only causes enormous economic waste but also leads to a series of environmental problems, such as eutrophication of water bodies and soil acidification and compaction.

[0003] Slow-release fertilizers are an effective way to solve the above problems. However, existing technologies still have significant limitations: (1) Coated slow-release fertilizer: mainly controls nutrient diffusion through polymer coating. Its disadvantages are high cost and the coating material is difficult to degrade, which may cause secondary pollution.

[0004] (2) Inhibitor-type slow-release fertilizer: Nitrogen conversion is delayed by adding urease inhibitors and nitrification inhibitors. Its disadvantage is that it is highly targeted and only applies to nitrogen fertilizers, and is ineffective for phosphorus, potassium and micronutrients.

[0005] (3) Chemically synthesized slow-release fertilizers: such as urea-formaldehyde (UF). Its disadvantages are that the release of nutrients is greatly affected by the activity of soil microorganisms, the release pattern is uncontrollable, and there may be a risk of formaldehyde pollution during the production process.

[0006] In particular, phosphorus and micronutrients (such as Ca, Zn, Mg, and Fe) are easily fixed in soil in insoluble forms, making it difficult to achieve effective slow release using traditional methods. Research has found that plants have evolved unique coping strategies: when phosphorus or micronutrients are deficient, the roots actively secrete organic acids such as citric acid to chelate and release the fixed nutrients by acidifying the rhizosphere soil. Summary of the Invention

[0007] To address the aforementioned shortcomings, the present invention aims to provide a citric acid-soluble slow-release fertilizer, its preparation method, and its application. Based on the biological mechanisms of plants, the present invention creatively designs a slow-release, high-efficiency fertilizer that responds to plant needs and intelligently releases nutrients.

[0008] The objective of this invention is achieved through the following technical solution: A citric acid-soluble slow-release fertilizer has an active ingredient with the stoichiometric formula (NH4). a Ca b Zn c Mg d Fe e Pn O 3n+1 a+2(b+c+d+e)=n+2, n≥2, and at least two of b, c, d and e are greater than zero.

[0009] Preferably, the citric acid-soluble slow-release fertilizer further includes 1-8% citrate by mass.

[0010] Preferably, the citrate is at least one of sodium citrate and potassium citrate.

[0011] The preparation method of the above-mentioned citric acid-soluble slow-release fertilizer includes the following steps: Prepare an aqueous solution of monoammonium phosphate, heat it to 80-90℃ and keep it at that temperature for later use; Weigh the corresponding dihydrogen phosphate salt according to the stoichiometric ratio of the active ingredient, add it to the monoammonium phosphate aqueous solution, adjust the pH to 5.0-6.5 and stir under 80-90℃ and a protective atmosphere to carry out the reaction, and obtain a viscous polymer slurry. The polymer slurry was cooled to room temperature, dried to constant weight, pulverized and sieved to obtain the citric acid-soluble slow-release fertilizer.

[0012] Preferably, the concentration of the monoammonium phosphate aqueous solution is 35-60 wt%.

[0013] Preferably, the molar ratio of P provided by the monoammonium phosphate aqueous solution to P provided by the dihydrogen phosphate is 1:0.8~1.5.

[0014] Preferably, the dihydrogen phosphate is at least one of calcium dihydrogen phosphate, zinc dihydrogen phosphate, magnesium dihydrogen phosphate, and ferrous dihydrogen phosphate.

[0015] Preferably, the dihydrogen phosphate needs to be crushed and passed through a 200-mesh sieve before use.

[0016] Preferably, the protective atmosphere is a nitrogen atmosphere.

[0017] Preferably, the reaction time for stirring is 2.5 to 4.5 hours.

[0018] Preferably, drying to constant weight refers to drying to constant weight at 105~115℃.

[0019] Preferably, the crushing and sieving refers to sieving out particles with a diameter of 1~4mm.

[0020] The above-mentioned citric acid-soluble slow-release fertilizers are used in the preparation of soil base fertilizers or top dressings.

[0021] Compared with the prior art, the beneficial effects of the present invention include: (1) This invention achieves a high degree of fit between nutrient release kinetics and crop fertilizer requirements, significantly improving fertilizer utilization and thus effectively reducing the amount of fertilizer applied.

[0022] (2) This invention provides an efficient chemical regulation solution to the problem of phosphorus and trace elements fixation in soil.

[0023] (3) The entire preparation process of this invention produces no toxic or harmful substances, and the product has excellent environmentally friendly characteristics.

[0024] (4) The process route of this invention is simple and efficient, and has a significant cost advantage compared with coated fertilizers, and has good potential for industrial promotion. Attached Figure Description

[0025] Figure 1 NH4+ release characteristics of fertilizers A1-A3 and B1-B8 in soil culture experiments + -N cumulative release rate curvature plot.

[0026] Figure 2 PO4 in soil culture release characteristics experiment of fertilizers A1~A3 and B1~B8 3- -P cumulative release rate curvature plot. Detailed Implementation

[0027] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0028] As the cornerstone of modern agriculture, the efficiency of fertilizer utilization directly impacts agricultural production benefits and ecological environmental safety. However, traditional water-soluble fertilizers often face the dilemma of "rapid release and severe loss" after being applied to the soil. Data shows that the utilization rates of nitrogen and phosphorus fertilizers in the current season are only 30%–35% and 10%–25%, respectively, with the majority of nutrients lost through volatilization, leaching, or chemical fixation. This inefficient utilization not only causes enormous resource waste but also triggers a series of environmental crises such as eutrophication of water bodies and soil acidification and compaction.

[0029] To overcome this bottleneck, controlled-release fertilizer technology has emerged, but existing mainstream technologies still have significant limitations. Firstly, coated fertilizers, while effectively delaying nutrient release through physical barriers, are limited by high production costs and the risk of microplastic pollution from the difficult-to-degrade coating materials, restricting their large-scale application. Secondly, inhibitor-type fertilizers, while effectively delaying nitrogen conversion, have a single mechanism of action, effective only for nitrogen fertilizers and unable to address the loss of phosphorus, potassium, and micronutrients. Finally, chemically synthesized fertilizers (such as urea-formaldehyde) have nutrient release highly dependent on soil microbial activity, making precise control of release patterns difficult, and the production process carries the risk of formaldehyde residue.

[0030] Of particular concern is that phosphorus and micronutrients such as calcium, zinc, magnesium, and iron are readily chemically fixed in the soil, transforming into insoluble forms that are difficult for crops to absorb. Traditional slow-release technologies are often ineffective in addressing this issue. To tackle this problem, plant physiology research has discovered that plants have developed a sophisticated "self-rescue mechanism" during long-term evolution: when faced with phosphorus or micronutrient stress, the roots actively secrete organic acids such as citric acid. These organic acids acidify the rhizosphere microenvironment, releasing the fixed nutrients from the soil for plant absorption through chelation and dissolution. This natural wisdom provides a novel biomimetic approach for the design of new fertilizers.

[0031] In view of this, based on this biological process, the present invention creatively designs a slow-release, high-efficiency fertilizer that releases nutrients in response to plant needs. The specific technical solution of the present invention is as follows: A citric acid-soluble slow-release fertilizer has an active ingredient with the stoichiometric formula (NH4). a Ca b Zn c Mg d Fe e P n O 3n+1 a+2(b+c+d+e)=n+2, n≥2, and at least two of b, c, d and e are greater than zero.

[0032] Preferably, the citric acid-soluble slow-release fertilizer further includes 1-8% citrate by mass.

[0033] In this invention, the citric acid-soluble slow-release fertilizer also includes citrate at a mass of 1-8%. This ensures that when roots secrete citric acid or citrate is added to the slow-release fertilizer, the citrate ion (Citrate) is released. 3- It possesses extremely strong chelating ability, preferentially binding with Ca in polymer salts. 2+ Zn 2+ Fe 2+ The binding of metal ions disrupts the structural stability of the polymeric salt. Simultaneously, H... +The supply of H2PO4 caused a rapid drop in the local pH, which drastically accelerated the acid-catalyzed hydrolysis process of POP, leading to the rapid disintegration of the polymer framework and the release of a sufficient amount of H2PO4 in one step. - It contains chelated metal ions, which can be absorbed by plants when needed.

[0034] Preferably, the citrate is at least one of sodium citrate and potassium citrate.

[0035] The preparation method of the above-mentioned citric acid-soluble slow-release fertilizer includes the following steps: Prepare an aqueous solution of monoammonium phosphate, heat it to 80-90℃ and keep it at that temperature for later use; Weigh the corresponding dihydrogen phosphate salt according to the stoichiometric ratio of the active ingredient, add it to the monoammonium phosphate aqueous solution, adjust the pH to 5.0-6.5 and stir under 80-90℃ and a protective atmosphere to carry out the reaction, and obtain a viscous polymer slurry. The polymer slurry was cooled to room temperature, dried to constant weight, pulverized and sieved to obtain the citric acid-soluble slow-release fertilizer.

[0036] In this invention, different dihydrogen phosphates are used as raw materials, and a polymerization reaction is carried out in a monoammonium phosphate solution under specific high-temperature conditions of 80-90°C to generate a water-insoluble multi-element polymeric phosphate complex with a low degree of polymerization (average degree of polymerization n=5-15). This reaction is not a simple mixing process, but a condensation reaction occurring under specific temperature, pH, and concentration conditions to form a -POP- covalent backbone, thereby incorporating NH4+. + and metal ions (M n+ The NH4+ in the polyphosphate complex is encapsulated within the structure. + Release is primarily achieved through the slow hydrolysis of terminal POP bonds, a process minimally affected by soil moisture, temperature, and pH, thus providing a highly stable and long-lasting supply of ammonium nitrogen. The degree of polymerization (n=5~15) is crucial; too low a degree results in increased water solubility and poor slow-release properties, while too high a degree leads to excessive stability and slow release even under acidic conditions. This complex is extremely stable in neutral, alkaline, and weakly acidic (pH≥5.5) soil environments, releasing only a very slow amount of NH4 through hydrolysis. + When plant roots secrete citric acid or the soil lacks corresponding metal ions, citrate ions chelate with metal ions, causing a drop in the local microenvironment pH (which can fall below 4.0). This causes the polyphosphate skeleton to depolymerize under acidic conditions, rapidly releasing H2PO4. - Ca 2+ Zn 2+ Mg 2+ Fe 2+ It provides nutrients that plants can directly absorb, thus synchronizing nutrient release with crop needs, significantly improving fertilizer utilization and reducing environmental pollution.

[0037] Preferably, the concentration of the monoammonium phosphate aqueous solution is 35-60 wt%.

[0038] Preferably, the molar ratio of P provided by the monoammonium phosphate aqueous solution to P provided by the dihydrogen phosphate is 1:0.8~1.5.

[0039] Preferably, the dihydrogen phosphate is at least one of calcium dihydrogen phosphate, zinc dihydrogen phosphate, magnesium dihydrogen phosphate, and ferrous dihydrogen phosphate.

[0040] In some embodiments of the present invention, the dihydrogen phosphate is at least one of calcium dihydrogen phosphate, zinc dihydrogen phosphate, magnesium dihydrogen phosphate, and ferrous dihydrogen phosphate, thereby providing trace elements such as Ca, Zn, Mg, and Fe to supplement the plant with rich nutrition.

[0041] Preferably, the dihydrogen phosphate needs to be crushed and passed through a 200-mesh sieve before use.

[0042] Preferably, the protective atmosphere is a nitrogen atmosphere.

[0043] Preferably, the reaction time for stirring is 2.5 to 4.5 hours.

[0044] Preferably, drying to constant weight refers to drying to constant weight at 105~115℃.

[0045] Preferably, the crushing and sieving refers to sieving out particles with a diameter of 1~4mm.

[0046] The above-mentioned citric acid-soluble slow-release fertilizers are used in the preparation of soil base fertilizers or top dressings.

[0047] Example 1 The raw materials and preparation method for a citric acid-soluble slow-release fertilizer (polyphosphate calcium zinc magnesium ferric ammonium slow-release fertilizer) are as follows: Raw materials: calcium dihydrogen phosphate (Ca(H2PO4)2, industrial grade), zinc dihydrogen phosphate (Zn(H2PO4)2, industrial grade), magnesium dihydrogen phosphate (Mg(H2PO4)2, industrial grade), ferrous dihydrogen phosphate (Fe(H2PO4)2, self-made, prepared by reaction of FeSO4 and NaH2PO4), ammonium phosphate (NH4H2PO4, industrial grade), ammonia (25%), potassium citrate (C6H5K3O7·H2O).

[0048] The preparation steps for ferrous dihydrogen phosphate are as follows: (1) Preparation of ferrous sulfate solution Weigh out a measured amount of FeSO4·7H2O, dissolve it in pre-deoxygenated deionized water, and prepare a solution with a concentration of 0.5~1.0 mol·L⁻¹. -1Ferrous sulfate solution.

[0049] (2) Preparation of potassium dihydrogen phosphate solution Weigh out KH₂PO₄ in a molar ratio of 2:1 to 2.2:1 with ferrous sulfate, dissolve it in deoxygenated deionized water, and prepare a solution of approximately 1 mol·L⁻¹. -1 The solution.

[0050] (3) Mixing and addition Under vigorous stirring and continuous nitrogen protection, potassium dihydrogen phosphate solution is slowly added dropwise to ferrous sulfate solution through a constant pressure dropping funnel at a rate of 1-2 drops per second. After the addition is complete, stirring is continued for 15-30 minutes to ensure thorough mixing. The solution is still light green at this point and generally does not produce any obvious precipitate.

[0051] (4) Adjust the pH to precipitate the product. The newly prepared concentration is 1 mol·L -1 NaOH is slowly added dropwise to the mixture described in step (3) under thorough stirring. When the pH of the solution reaches 3.0 to 3.8, a light green fine precipitate begins to appear in the system. Continue to slowly add alkali, strictly control the endpoint pH to 3.5 to 4.5. After reaching the target pH, stop adding alkali and continue stirring and aging for 30 to 60 minutes to obtain a suspension.

[0052] (5) Filtration and washing Under nitrogen protection, the suspension was filtered using a Buchner funnel. The filter cake was washed 2-3 times alternately with small amounts of deoxygenated deionized water and anhydrous ethanol to remove entrained potassium. + Na + SO4 2- After removing impurities and drying, a moist ferrous dihydrogen phosphate filter cake is obtained.

[0053] (6) Drying The filter cake was quickly transferred to a vacuum drying oven and vacuum dried at room temperature (40°C) until constant weight. The final product was a light green powder, which is hydrated ferrous dihydrogen phosphate.

[0054] The ratio (molar ratio in terms of P) is: Ca(H2PO4)2:Zn(H2PO4)2:Mg(H2PO4)2:Fe(H2PO4)2:NH4H2PO4 = 0.4:0.2:0.2:0.2:0.9.

[0055] Preparation steps: (1) In a 2L four-necked flask equipped with a stirrer, thermometer, reflux condenser and nitrogen inlet tube, add 600g of deionized water to dissolve 324g of NH4H2PO4 to prepare a 35% solution, heat to 85℃ and pass N2 through for protection. (2) Mix 147g Ca(H2PO4)2, 81g Zn(H2PO4)2, 68g Mg(H2PO4)2 and 78g Fe(H2PO4)2 (total 374g) and grind them, then pass them through a 200-mesh sieve to obtain a mixture. Add the mixture to the reaction flask of step (1) within 2 hours under stirring conditions. During this time, the pH of the reaction system is maintained at 5.8~6.0 by adding ammonia water dropwise. After the addition is completed, continue the reaction for 3 hours under 85℃ and N2 protection. The reactants gradually turn into a white viscous slurry. (3) After the reaction is complete, the viscous slurry described in step (2) is cooled to room temperature and poured out, and then dried in an oven at 110°C for 24 hours until constant weight. (4) The dried hard block described in step (3) is first crushed by a jaw crusher, then crushed by a roller crusher, sieved, and 1~3mm particles are taken as the finished fertilizer matrix. The average degree of polymerization is measured to be 9 by end-group titration. (5) Place the matrix particles in a fluidized bed and spray them with a pre-prepared 20wt% potassium citrate aqueous solution (the potassium citrate aqueous solution accounts for 3% of the matrix particles). Dry the particles at 80℃ until the moisture content is <2% to obtain the finished fertilizer, which is denoted as A1.

[0056] Example 2 The raw materials and preparation method for a citric acid-soluble slow-release fertilizer (polyphosphate calcium zinc magnesium ferric ammonium slow-release fertilizer) are as follows: Raw materials: calcium dihydrogen phosphate (Ca(H2PO4)2, industrial grade), zinc dihydrogen phosphate (Zn(H2PO4)2, industrial grade), magnesium dihydrogen phosphate (Mg(H2PO4)2, industrial grade), ferrous dihydrogen phosphate (Fe(H2PO4)2, self-made, prepared by reaction of FeSO4 and NaH2PO4), ammonium phosphate (NH4H2PO4, industrial grade), ammonia (25%), potassium citrate (C6H5K3O7·H2O).

[0057] The ratio (molar ratio in terms of P) is: Ca(H2PO4)2:Zn(H2PO4)2:Mg(H2PO4)2:Fe(H2PO4)2:NH4H2PO4 = 0.3:0.3:0.2:0.2:0.8.

[0058] Preparation steps: (1) In a 2L four-necked flask equipped with a stirrer, thermometer, reflux condenser and nitrogen inlet tube, add 400g of deionized water to dissolve 272g of NH4H2PO4 to prepare a 40% solution, heat to 90℃ and pass N2 through for protection. (2) Mix 103g Ca(H2PO4)2, 115g Zn(H2PO4)2, 65g Mg(H2PO4)2 and 74g Fe(H2PO4)2 (total 357g) and grind them, then pass them through a 200-mesh sieve to obtain a mixture. Add the mixture to the reaction flask of step (1) within 2 hours under stirring conditions. During this time, the pH of the reaction system is maintained at 5.8~6.0 by adding ammonia water dropwise. After the addition is completed, continue the reaction for 3 hours under 90℃ and N2 protection. The reactants gradually turn into a white viscous slurry. (3) After the reaction is complete, the viscous slurry described in step (2) is cooled to room temperature and poured out, and then dried in an oven at 110°C for 24 hours until constant weight. (4) The dried hard block described in step (3) is initially crushed by a jaw crusher, then crushed by a roller crusher, sieved, and 1-3 mm particles are taken as the finished fertilizer matrix. The average degree of polymerization is measured to be 11 by end-group titration. (5) Place the matrix particles in a fluidized bed, spray with a pre-prepared 20wt% potassium citrate aqueous solution (citric acid accounts for 3% of the matrix particles), and dry the particles at 80℃ until the moisture content is <2% to obtain the finished fertilizer, which is denoted as A2.

[0059] Example 3 The raw materials and preparation method for a citric acid-soluble slow-release fertilizer (polyphosphate calcium zinc magnesium ferric ammonium slow-release fertilizer) are as follows: Raw materials: calcium dihydrogen phosphate (Ca(H2PO4)2, industrial grade), zinc dihydrogen phosphate (Zn(H2PO4)2, industrial grade), magnesium dihydrogen phosphate (Mg(H2PO4)2, industrial grade), ferrous dihydrogen phosphate (Fe(H2PO4)2, self-made, prepared by reaction of FeSO4 and NaH2PO4), ammonium phosphate (NH4H2PO4, industrial grade), ammonia (25%), potassium citrate (C6H5K3O7·H2O).

[0060] The ratio (molar ratio in terms of P) is: Ca(H2PO4)2:Zn(H2PO4)2:Mg(H2PO4)2:Fe(H2PO4)2:NH4H2PO4 = 0.2:0.3:0.3:0.2:1.1.

[0061] Preparation steps: (1) In a 2L four-necked flask equipped with a stirrer, thermometer, reflux condenser and nitrogen inlet tube, add 400g of deionized water to dissolve 396g of NH4H2PO4 to prepare a 50% solution, heat to 90℃ and pass N2 through for protection. (2) 73g Ca(H2PO4)2, 121g Zn(H2PO4)2, 102g Mg(H2PO4)2 and 78g Fe(H2PO4)2 (total 374g) were mixed and ground and passed through a 200-mesh sieve to obtain a mixture. The mixture was added to the reaction flask of step (1) within 2 hours under stirring conditions. During this time, the pH of the reaction system was maintained at 5.8~6.0 by adding ammonia water dropwise. After the addition was completed, the reaction was continued for 3 hours under N2 protection at 90℃. The reactants gradually turned into a white viscous slurry. (3) After the reaction is complete, the viscous slurry described in step (2) is cooled to room temperature and poured out, and then dried in an oven at 110°C for 24 hours until constant weight. (4) The dried hard block described in step (3) is initially crushed by a jaw crusher, then crushed by a roller crusher, sieved, and 1-3 mm particles are taken as the finished fertilizer matrix. The average degree of polymerization is measured to be 13 by end-group titration. (5) Place the matrix particles in a fluidized bed, spray with a pre-prepared 20wt% potassium citrate aqueous solution (citric acid accounts for 3% of the matrix particles), and dry the particles at 80℃ until the moisture content is <2% to obtain the finished fertilizer, which is denoted as A3.

[0062] Comparative Example 1 A raw material and preparation method for a polyammonium phosphate blended fertilizer are disclosed below: Raw materials: calcium dihydrogen phosphate (Ca(H2PO4)2, industrial grade), zinc dihydrogen phosphate (Zn(H2PO4)2, industrial grade), magnesium dihydrogen phosphate (Mg(H2PO4)2, industrial grade), ferrous dihydrogen phosphate (Fe(H2PO4)2, self-made, prepared by reaction of FeSO4 and NaH2PO4), ammonium phosphate (NH4H2PO4, industrial grade), ammonia (25%), potassium citrate (C6H5K3O7·H2O).

[0063] The ratio (molar ratio in terms of P) is: Ca(H2PO4)2:Zn(H2PO4)2:Mg(H2PO4)2:Fe(H2PO4)2:NH4H2PO4 = 0.4:0.2:0.2:0.2:0.9.

[0064] Preparation steps: (1) In a 2L four-necked flask equipped with a stirrer, thermometer, reflux condenser and nitrogen inlet tube, add 300g of deionized water and dissolve 324g of NH4H2PO4 to prepare a 52% solution. Heat to 85℃ and introduce N2 for protection. During this period, maintain the pH of the reaction system at 5.8~6.0 by adding ammonia dropwise. Continue the reaction for 3h at 85℃ under N2 protection. The reactants gradually turn into a white viscous slurry. (2) After the reaction is complete, the slurry is poured out and dried in an oven at 110°C for 24 hours. The dried hard block is crushed by a jaw crusher and its average degree of polymerization is measured to be 11 by end-group titration, thus obtaining ammonium polyphosphate.

[0065] (3) Mix 147g Ca(H2PO4)2, 81g Zn(H2PO4)2, 68g Mg(H2PO4)2 and 78g Fe(H2PO4)2 (total 374g) and grind them, then pass them through a 200-mesh sieve to obtain a mixture. Grind the mixture together with ammonium polyphosphate using a roller mill, sieve it, and take 1~3mm particles as the finished fertilizer matrix.

[0066] (4) Place the matrix particles in a fluidized bed, spray with a pre-prepared 20wt% potassium citrate aqueous solution (citric acid accounts for 3% of the matrix particles), and dry the particles at 80℃ until the moisture content is <2% to obtain the finished fertilizer, denoted as B1.

[0067] Comparative Example 2 A raw material and preparation method for a polyammonium phosphate blended fertilizer are disclosed below: Raw materials: calcium dihydrogen phosphate (Ca(H2PO4)2, industrial grade), zinc dihydrogen phosphate (Zn(H2PO4)2, industrial grade), magnesium dihydrogen phosphate (Mg(H2PO4)2, industrial grade), ferrous dihydrogen phosphate (Fe(H2PO4)2, self-made, prepared by reaction of FeSO4 and NaH2PO4), ammonium phosphate (NH4H2PO4, industrial grade), and ammonia (25%).

[0068] The ratio (molar ratio in terms of P) is: Ca(H2PO4)2:Zn(H2PO4)2:Mg(H2PO4)2:Fe(H2PO4)2:NH4H2PO4 = 0.4:0.2:0.2:0.2:0.9.

[0069] Preparation steps: (1) In a 2L four-necked flask equipped with a stirrer, thermometer, reflux condenser and nitrogen inlet tube, add 300g of deionized water and dissolve 324g of NH4H2PO4 to prepare a 52% solution. Heat to 85℃ and introduce N2 for protection. During this period, maintain the pH of the reaction system at 5.8~6.0 by adding ammonia dropwise. Continue the reaction for 3h at 85℃ under N2 protection. The reactants gradually turn into a white viscous slurry. (2) After the reaction is complete, the slurry is poured out and dried in an oven at 110°C for 24 hours. The dried hard block is crushed by a jaw crusher and its average degree of polymerization is measured to be 11 by end-group titration, thus obtaining ammonium polyphosphate.

[0070] (3) Mix 147g Ca(H2PO4)2, 81g Zn(H2PO4)2, 68g Mg(H2PO4)2 and 78g Fe(H2PO4)2 (total 374g) and grind them, then pass them through a 200-mesh sieve to obtain a mixture. Grind the mixture together with ammonium polyphosphate using a roller mill, sieve it, and take 1~3mm particles as the finished fertilizer, which is denoted as B2.

[0071] Comparative Example 3 The raw materials and preparation method for a high-calcium polyphosphate ammonium slow-release fertilizer are as follows: Raw materials: calcium dihydrogen phosphate (Ca(H2PO4)2, industrial grade), ammonium phosphate (NH4H2PO4, industrial grade), ammonia (25%), potassium citrate (C6H5K3O7·H2O).

[0072] The ratio (molar ratio in terms of P) is: Ca(H2PO4)2 : NH4H2PO4 = 1 : 0.9.

[0073] Preparation steps: (1) In a 2L four-necked flask equipped with a stirrer, thermometer, reflux condenser and nitrogen inlet tube, add 600g of deionized water to dissolve 324g of NH4H2PO4 to prepare a 35% solution. Heat to 85℃ and purge with N2 for protection.

[0074] (2) Grind 367g of Ca(H2PO4)2 through a 200-mesh sieve. Add Ca(H2PO4)2 powder to the reaction flask within 2 hours while stirring. During this time, maintain the pH of the reaction system at 5.8~6.0 by adding ammonia water dropwise. After the addition is complete, continue the reaction for 3 hours under 85℃ and N2 protection. The reactant gradually turns into a white viscous slurry.

[0075] (3) After the reaction is completed, the slurry is poured out and dried in an oven at 110°C for 24 hours. The dried hard block is then crushed by a jaw crusher and then crushed by a roller crusher. After sieving, 1-3 mm particles are taken as the finished fertilizer matrix. The average degree of polymerization is 5 when the end-group titration method is used.

[0076] (4) Place the matrix particles in a fluidized bed, spray with a pre-prepared 20wt% potassium citrate aqueous solution (citric acid accounts for 3% of the matrix particles), and dry the particles at 80℃ until the moisture content is <2% to obtain the finished fertilizer, which is designated as B3.

[0077] Comparative Example 4 The raw materials and preparation method for a high-magnesium polyphosphate calcium ammonium slow-release fertilizer are as follows: Raw materials: calcium dihydrogen phosphate (Mg(H2PO4)2, industrial grade), ammonium phosphate (NH4H2PO4, industrial grade), ammonia (25%), potassium citrate (C6H5K3O7·H2O).

[0078] The ratio (molar ratio based on P) is: Mg(H2PO4)2 : NH4H2PO4 = 1 : 0.9.

[0079] Preparation steps: (1) In a 2L four-necked flask equipped with a stirrer, thermometer, reflux condenser and nitrogen inlet tube, add 600g of deionized water to dissolve 324g of NH4H2PO4 to prepare a 35% solution. Heat to 85℃ and purge with N2 for protection.

[0080] (2) Grind 340g of Mg(H2PO4)2 through a 200-mesh sieve. Add the Mg(H2PO4)2 powder to the reaction flask within 2 hours while stirring. During this time, maintain the pH of the reaction system at 5.8~6.0 by adding ammonia water dropwise. After the addition is complete, continue the reaction for 3 hours under 85℃ and N2 protection. The reactant gradually turns into a white viscous slurry.

[0081] (3) After the reaction is completed, the slurry is poured out and dried in an oven at 110°C for 24 hours. The dried hard blocks are then crushed by a jaw crusher and then crushed by a roller crusher. After sieving, 1-3 mm particles are taken as the finished fertilizer matrix. The average degree of polymerization is 6 when the end-group titration method is used.

[0082] (4) Place the matrix particles in a fluidized bed, spray with a pre-prepared 20wt% potassium citrate aqueous solution (citric acid accounts for 3% of the matrix particles), and dry the particles at 80℃ until the moisture content is <2% to obtain the finished fertilizer, which is designated as B4.

[0083] Comparative Example 5 The raw materials and preparation method for powdered polycalcium, zinc, magnesium, and ferric ammonium slow-release fertilizer used in drip irrigation are as follows: Raw materials: Same as in Example 1; Preparation steps: Compared with Example 1, the only difference is that after crushing in step (4), the product obtained is sieved through a 100-mesh sieve (<0.15mm) and is recorded as B5.

[0084] Comparative Example 6 The raw materials and preparation method for blended slow-release compound fertilizers containing polyphosphate, calcium, zinc, magnesium, iron, and ammonium are as follows: Raw materials: calcium dihydrogen phosphate (Ca(H2PO4)2, industrial grade), zinc dihydrogen phosphate (Zn(H2PO4)2, industrial grade), magnesium dihydrogen phosphate (Mg(H2PO4)2, industrial grade), ferrous dihydrogen phosphate (Fe(H2PO4)2, self-made, prepared by reaction of FeSO4 and NaH2PO4), monoammonium phosphate (NH4H2PO4, industrial grade), ammonia (25%), potassium citrate (C6H5K3O7·H2O, industrial grade), potassium chloride (KCl, industrial grade), urea (CO(NH2)2, industrial grade).

[0085] Preparation steps: Polymeric calcium zinc magnesium iron ammonium phosphate slow-release fertilizer A1 from Example 1 is physically mixed with potassium chloride (KCl) and urea in a ratio of N-P2O5-K2O=15-15-15 to obtain a compound fertilizer containing polymeric calcium zinc magnesium iron ammonium phosphate slow-release fertilizer, denoted as B6.

[0086] Comparative Example 7 The raw materials and preparation methods for ordinary mixed fertilizers are as follows: Raw materials: calcium dihydrogen phosphate (Ca(H2PO4)2, industrial grade), zinc dihydrogen phosphate (Zn(H2PO4)2, industrial grade), magnesium dihydrogen phosphate (Mg(H2PO4)2, industrial grade), ferrous dihydrogen phosphate (Fe(H2PO4)2, self-made, prepared by reaction of FeSO4 and NaH2PO4), monoammonium phosphate (NH4H2PO4, industrial grade), potassium citrate (C6H5K3O7·H2O, industrial grade).

[0087] Preparation steps: According to the component ratio in Example 1, monoammonium phosphate, calcium dihydrogen phosphate, zinc dihydrogen phosphate, magnesium dihydrogen phosphate, ferrous dihydrogen phosphate and potassium citrate were simply physically mixed and dried to obtain a mixture, denoted as B7.

[0088] Comparative Example 8 Commercially available coated urea Raw material: Sulfur-coated urea produced by Yunnan Yuntianhua Co., Ltd., purchased online.

[0089] Steps: Purchase commercially available sulfur-coated urea (SCU), labeled B8.

[0090] Effect Experiment Example Experiment 1: Determination of Citric Acid Solubility Accurately weigh 0.500 g (dry basis) of each fertilizer sample into an Erlenmeyer flask, add 100 mL of 2 wt% citric acid solution, and shake for 1 h at 25℃ and 150 rpm. Filter and determine the dissolution rate of metal ions such as P and Zn in the filtrate. The dissolution rate obtained by shaking in water for 1 h is used as the conventional release rate control. The test results are shown in Table 1.

[0091] Table 1 Results of Citric Acid Solubility Test

[0092] As shown in Table 1, the citric acid-soluble slow-release fertilizer prepared in this invention exhibits extremely low P and Zn solubility in water (below 2.5%), demonstrating excellent slow-release performance. In a 2 wt% citric acid solution, the dissolution rate of P and Zn significantly increases, confirming its superior citric acid-responsive release characteristics. Comparing Example 1 and Comparative Example 5, it can be seen that the smaller the particle size, the faster the release rate of nutrients. Comparative Example 1 and Comparative Example 6 show that Example 1 of this invention, when mixed with general fertilizers such as potassium and nitrogen fertilizers, still maintains stable slow-release performance. In contrast, the comparative example exhibits a high dissolution rate in water (above 40%), and the dissolution rate further increases in citric acid solution, indicating a lack of effective slow-release mechanism and acid-responsive characteristics.

[0093] Experiment 2: Soil-based incubation release characteristics The soil leaching column method was used in the experiment. A fertilizer sample with an equal phosphorus content was uniformly mixed with 500g of air-dried soil and then loaded into the leaching column. The column was periodically leached with deionized water, and the leachate was collected and the NH4 content was determined. + -N and PO4 3- -P cumulative release rate. On day 30 of cultivation, 50 mL of 0.1 M potassium citrate solution was added to half of the parallel samples to simulate 2% citric acid secretion from the rhizosphere, and nutrient release was subsequently monitored. NH4 + -N and PO4 3- -P cumulative release rate curvature as Figure 1 and 2 As shown.

[0094] See Figures 1-2 We know that within the first 30 days of the citric acid-soluble slow-release fertilizer prepared by this invention, NH4 + -N and PO4 3- The release of phosphorus (P) was slow and steady, with cumulative release rates of approximately 25% and 8%, respectively. After the addition of potassium citrate, a significant peak in nutrient release occurred, followed by a substantial increase in P release over the next week. For Comparative Example 7, over 80% of the nutrients were released during the first leaching, and release was almost complete after one week. For Comparative Example 8, its NH4+... + -N release curves exhibit an S-shaped curve, but PO4 release curves exhibit an S-shaped curve. 3- -P has no slow-release effect because it is a pure nitrogen slow-release fertilizer.

[0095] Experiment 3: Verification of the effect of potted plants Using maize as the indicator crop, three treatments were established: a blank control (CK), a treatment treated with fertilizer B7 (Comparative Example 7), and a treatment treated with fertilizer A1 (Example 1) (with total N, P, and Zn nutrients equal to the B7 treatment). Each pot contained 5 kg of soil, 5 seeds were sown, and after emergence, 2 seedlings were transplanted. Biomass and nutrient uptake were measured after 60 days of growth. The test results are shown in Table 2.

[0096] Table 2. Results of the potted plant effect test

[0097] As shown in Table 2, compared with the ordinary mixed fertilizer B7, the slow-release fertilizer A1 treatment of the present invention significantly improved the biomass, phosphorus and zinc uptake, and apparent phosphorus utilization of corn, proving its superiority in practical applications.

[0098] The above embodiments merely illustrate implementation methods of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A citric acid-soluble slow-release fertilizer, characterized in that, The stoichiometric formula of its active ingredient is (NH4). a Ca b Zn c Mg d Fe e P n O 3n+1 a+2(b+c+d+e)=n+2, n≥2, and at least two of b, c, d and e are greater than zero.

2. The citric acid-soluble slow-release fertilizer according to claim 1, characterized in that, The citric acid-soluble slow-release fertilizer also includes citrate salts at a mass of 1-8%.

3. The citric acid-soluble slow-release fertilizer according to claim 2, characterized in that, The citrate is at least one of sodium citrate and potassium citrate.

4. The method for preparing the citric acid-soluble slow-release fertilizer according to any one of claims 1 to 3, characterized in that, Includes the following steps: Prepare an aqueous solution of monoammonium phosphate, heat it to 80-90℃ and keep it at that temperature for later use; Weigh the corresponding dihydrogen phosphate salt according to the stoichiometric ratio of the active ingredient, add it to the monoammonium phosphate aqueous solution, adjust the pH to 5.0-6.5 and stir under 80-90℃ and a protective atmosphere to carry out the reaction, and obtain a viscous polymer slurry. The polymer slurry was cooled to room temperature, dried to constant weight, pulverized and sieved to obtain the citric acid-soluble slow-release fertilizer.

5. The method for preparing the citric acid-soluble slow-release fertilizer according to claim 4, characterized in that, The concentration of the monoammonium phosphate aqueous solution is 35~60wt%.

6. The method for preparing the citric acid-soluble slow-release fertilizer according to claim 4, characterized in that, The molar ratio of P provided by the monoammonium phosphate aqueous solution to P provided by the dihydrogen phosphate is 1:0.8~1.

5.

7. The method for preparing the citric acid-soluble slow-release fertilizer according to claim 4, characterized in that, The dihydrogen phosphate is at least one of calcium dihydrogen phosphate, zinc dihydrogen phosphate, magnesium dihydrogen phosphate, and ferrous dihydrogen phosphate; and / or the dihydrogen phosphate needs to be pulverized and passed through a 200-mesh sieve before use; and / or the protective atmosphere is a nitrogen atmosphere.

8. The method for preparing the citric acid-soluble slow-release fertilizer according to claim 4, characterized in that, The reaction time for stirring is 2.5 to 4.5 hours.

9. The method for preparing the citric acid-soluble slow-release fertilizer according to claim 4, characterized in that, The drying to constant weight refers to drying to constant weight at 105~115℃; and / or the pulverizing and sieving refers to sieving out particles with a particle size of 1~4mm.

10. The use of the citric acid-soluble slow-release fertilizer according to any one of claims 1 to 3 in the preparation of soil base fertilizer or top dressing.

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