Slow-release phosphorus fertilizer, its preparation method and application

Abstract

The application discloses a slow-release phosphorus fertilizer and a preparation method and application thereof, and an iron-phosphorus composite fertilizer is prepared by depositing iron minerals on the surface of the phosphorus fertilizer to form an iron oxide coating. The iron oxide coating can prevent the phosphorus fertilizer from absorbing moisture, reduce the volatilization loss of phosphorus, and enable the phosphorus fertilizer to achieve controlled release, thereby solving the problems of phosphorus fertilizer invalidation and excessive phosphorus loss. In addition, the iron minerals used for synthesizing the composite fertilizer can supplement trace element iron in the soil after being dissolved, and improve soil nutrition. The application also provides application of the slow-release phosphorus fertilizer. With dynamic changes of the soil environment humidity and oxidation-reduction conditions, the oxidation of ferrous iron on the surface of the controlled-release fertilizer into minerals and the dissolution and release of phosphorus have high controllability. The anaerobic-aerobic environment on the surface of the composite fertilizer can be changed by flooding and draining, and the controlled-release intelligent phosphorus fertilizer is formed. In addition, the controlled-release fertilizer is driven by water, and the form change of iron in different environments is used as a switch. The application method is simple, has no influence on the environment, and is suitable for large-scale agricultural operation.

Description

Technical Field

[0001] This invention belongs to the field of soil improvement technology, specifically relating to a slow-release phosphate fertilizer, its preparation method, and its application. Background Technology

[0002] Phosphorus is a constituent element of biological cells such as DNA, RNA, and phospholipids, and is also an essential nutrient for normal crop growth. However, the available phosphorus content in soil is very low, thus requiring the application of phosphate fertilizers in agricultural production to meet plant growth needs. The utilization rate of phosphate fertilizers applied to the soil is typically only 10%–25% in the current season, with the remaining large amount stored in an ineffective state. When the phosphorus level in the soil reaches a certain point, phosphorus adsorption sites become occupied, leading to near-saturation of the soil's phosphorus holding capacity. This results in a sharp increase in phosphorus loss, causing agricultural non-point source pollution and other problems. Therefore, there is an urgent need to develop a technology to improve phosphate fertilizer utilization and avoid environmental pollution caused by phosphate fertilizer inactivation and excessive phosphorus loss.

[0003] To address the problems of excessive application and insufficient utilization of phosphate fertilizers, microbial phosphate solubilization technology has been developed. However, this technology is costly, and the amount of phosphorus obtained from dissolution is limited. Additional phosphate fertilizer application is still required to meet the normal growth needs of crops, and it cannot fundamentally solve the environmental problems caused by insufficient phosphate fertilizer utilization and excessive application.

[0004] In conclusion, the need for developing low-cost, high-efficiency phosphate fertilizer utilization technologies remains, which is of great significance for improving soil quality, reducing fertilizer application, and minimizing environmental pollution in the development of green agriculture. Summary of the Invention

[0005] To overcome the problems existing in the prior art, one objective of this invention is to provide a slow-release phosphate fertilizer. A second objective is to provide a method for preparing the aforementioned slow-release phosphate fertilizer. A third objective is to provide applications of the aforementioned slow-release phosphate fertilizer. A fourth objective is to provide a method for improving soil.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] The first aspect of the present invention provides a slow-release phosphate fertilizer, comprising a phosphate fertilizer, wherein the surface of the phosphate fertilizer is covered with an iron oxide coating.

[0008] Preferably, the raw materials for preparing the slow-release phosphate fertilizer, by weight, include: 50 parts of iron compound, 250-500 parts of phosphate fertilizer, and 0.1-5 parts of organic acid.

[0009] More preferably, the raw materials for preparing the slow-release phosphate fertilizer, by weight, include: 50 parts of iron compound, 250-400 parts of phosphate fertilizer, and 0.1-3 parts of organic acid.

[0010] Preferably, the iron compound is derived from red mud or non-ferrous metals.

[0011] More preferably, the method for preparing the iron compound includes the following steps: crushing red mud and separating it by magnetic separation to obtain magnetic iron minerals and non-magnetic slag; adding the magnetic iron minerals to an inorganic alkaline solution for alkaline leaching to obtain the iron compound.

[0012] More preferably, the red mud is pulverized into powder with a particle size of less than 200 μm.

[0013] More preferably, the inorganic alkali in the inorganic alkali solution is selected from at least one of sodium carbonate, sodium hydroxide, and potassium carbonate.

[0014] More preferably, the concentration of the inorganic alkali in the inorganic alkali solution is 1 mol / L to 5 mol / L.

[0015] More preferably, the specific steps of the alkali leaching include: adding magnetic iron minerals to an inorganic alkali solution and stirring the mixture at 70-90°C.

[0016] More preferably, the method further includes the following steps: collecting the iron compound suspended on the surface of the solution after alkali leaching, drying it, and then pulverizing it to obtain the iron compound.

[0017] The iron ore raw material used for coating in this invention is extracted from waste materials such as red mud, which is of great significance for low cost and environmental protection.

[0018] Preferably, the phosphate fertilizer is selected from at least one of diammonium phosphate, calcium phosphate, and superphosphate.

[0019] Preferably, the organic acid is at least one of citric acid and malic acid.

[0020] Preferably, the raw materials for preparation further include 25 to 50 parts of lignin or its derivatives.

[0021] More preferably, the lignin derivative is selected from at least one of alkaline lignin, dealkalized lignin, and sodium lignin sulfonate.

[0022] The alkaline lignin used in this invention is used to prepare iron mineral coated alkaline lignin compound phosphate fertilizer. Alkaline lignin can not only accelerate iron reduction, but also serve as a carbon source to increase the organic carbon content of the soil, thus playing a role in soil improvement.

[0023] The second aspect of this invention provides a method for preparing the slow-release phosphate fertilizer described in the first aspect, comprising the following steps:

[0024] Phosphate fertilizer and organic acid are added to water and mixed to obtain a phosphate fertilizer mixed solution; the iron compound is added to the phosphate fertilizer mixed solution, and then an inorganic base is added to adjust the pH of the reaction system to 6.8-7.2. The reaction is carried out in a static environment to obtain the slow-release phosphate fertilizer.

[0025] Preferably, the mixing step specifically includes stirring at 40–70°C for 1–3 hours.

[0026] More preferably, the stirring speed is 300-800 rpm.

[0027] Preferably, the inorganic base is sodium hydroxide.

[0028] Preferably, when the raw materials for preparation also include lignin or its derivatives, phosphate fertilizer, lignin or its derivatives, and organic acid are added to water and mixed to obtain a phosphate fertilizer mixed solution.

[0029] Preferably, the ratio of phosphate fertilizer to water is (1-5) g: 10 mL.

[0030] Preferably, the iron compound is added to the phosphate fertilizer mixture at 40–70°C under stirring conditions.

[0031] Preferably, the iron compound is added in 1 to 5 portions.

[0032] Preferably, the step of carrying out the reaction in a static environment specifically includes: letting it stand at room temperature for 1-2 hours.

[0033] The third aspect of this invention provides the application of the slow-release phosphate fertilizer described in the first aspect in soil improvement or the preparation of soil conditioners.

[0034] The fourth aspect of the present invention provides a method for improving soil, comprising the following steps: applying the slow-release phosphate fertilizer described in the first aspect into the soil, controlling the anaerobic-aerobic environment in the soil, thereby controlling the release rate of phosphorus from the slow-release phosphate fertilizer.

[0035] Preferably, the anaerobic-aerobic environment in the soil is controlled by changing the water content in the soil.

[0036] Preferably, the slow-release phosphate fertilizer is applied to the soil of the potted plant system.

[0037] The beneficial effects of this invention are:

[0038] (1) This invention provides a slow-release phosphate fertilizer, which is prepared by precipitating iron minerals on the surface of the phosphate fertilizer to form an iron oxide coating. The iron oxide coating can prevent the phosphate fertilizer from absorbing moisture and reduce the volatilization loss of phosphorus, thus enabling the controlled release of the phosphate fertilizer and solving the problems of phosphate fertilizer failure and excessive phosphorus loss. In addition, the iron minerals used to synthesize the compound fertilizer can replenish the trace element iron in the soil after dissolution, thereby improving the soil nutrient content.

[0039] (2) This invention also provides the application of the above-mentioned slow-release phosphate fertilizer. With the dynamic changes in soil humidity and redox conditions, the oxidation of ferrous iron on the surface of the controlled-release fertilizer and the release of phosphorus are highly controllable. When the soil moisture content increases, anaerobic conditions are formed, the reduction of iron by soil microorganisms is enhanced, the coated iron ore slowly dissolves, and phosphorus is released for crop absorption. When the soil moisture is drained and oxygen enters the soil pores, ferrous iron is oxidized and re-forms a film on the surface of the compound fertilizer. It then dissolves and releases phosphorus again in the next flooding cycle, effectively avoiding the problems of phosphate fertilizer loss and insufficient utilization. Therefore, the anaerobic-aerobic environment on the surface of the compound fertilizer can be changed by flooding and drainage to form a controlled-release intelligent phosphate fertilizer. Furthermore, this controlled-release fertilizer is water-driven and uses the changes in the form of iron in different environments as a switch. The application method is simple, has no impact on the environment, and is suitable for large-scale agricultural operations. Attached Figure Description

[0040] Figure 1 The image shows a scanning electron microscope (SEM) image of the compound fertilizer in Example 1.

[0041] Figure 2 Here is a scanning electron microscope (SEM) image of the compound fertilizer in Example 2;

[0042] Figure 3 This is a trend graph showing the amount of Fe(II) generated by hydrochloric acid extraction in experimental group 1 of Experimental Example 1;

[0043] Figure 4 The trend of soluble phosphorus formation in experimental group 1 and the control group in Experiment Example 1;

[0044] Figure 5 This is a trend graph showing the amount of Fe(II) generated by hydrochloric acid extraction in experimental group 2 of Experimental Example 1;

[0045] Figure 6 The trend of soluble phosphorus formation in experimental group 2 and the control group in Experiment Example 1;

[0046] Figure 7 This is a trend graph showing the amount of Fe(II) produced by hydrochloric acid extraction in the experimental and control groups in Experiment Example 2.

[0047] Figure 8 The trend of soluble phosphorus formation in the experimental and control groups in Experiment Example 2 is shown. Detailed Implementation

[0048] The present invention will be further described in detail below through specific embodiments. Unless otherwise specified, the raw materials used in the following embodiments can be obtained from conventional commercial channels or prepared and isolated through simple synthesis; unless otherwise specified, the processes employed are conventional processes in the art.

[0049] Example 1

[0050] This embodiment provides a slow-release phosphate fertilizer, the preparation process of which is as follows:

[0051] S1. The red mud was pulverized using a ball mill and then sieved to obtain powder with a particle size of less than 200 μm. Most of the iron ore powder was then collected using a magnetic separator. A sodium carbonate solution (3 mol / L) was prepared, and the red mud residue was mixed in the sodium carbonate solution and heated and stirred at 80°C until no more bubbles were generated. The iron compounds suspended on the surface of the solution were collected, dried, and pulverized for use in phosphate fertilizer coating.

[0052] S2. Take a 2L beaker and add 1000mL of deionized water and a magnetic stir bar. Turn on the heating magnetic stirrer and set the speed to 500rpm and the temperature to 50℃. Gradually add a mixture of diammonium phosphate (300g) and citric acid (0.5g) until the solution is saturated. A small amount of precipitate will be formed at the bottom, and the system will become a suspension. Continue stirring for 2 hours to obtain the pretreated phosphate fertilizer solution.

[0053] S3. Maintaining heating and stirring conditions, add 50g of the iron compound obtained in step (1) five times (10g / time) to the above suspension. After mixing evenly, add 4.0mM sodium hydroxide solution dropwise using a syringe, while monitoring the pH of the system in real time using a pH meter. After the pH stabilizes at 6.8-7.2, stop stirring and heating, cool to room temperature, and let stand for 1 hour to generate a large amount of yellowish-brown precipitate. Filter using a Buchner funnel and filter paper, and air-dry the collected solid to obtain a controlled-release phosphate fertilizer sample, which is an iron mineral coated composite phosphate fertilizer. Observing the structure of the composite fertilizer by scanning electron microscopy, a large number of iron oxide particles were observed on the surface, proving that an iron oxide coating was formed on the surface of the composite fertilizer. Figure 1 ).

[0054] Example 2

[0055] This embodiment provides a slow-release phosphate fertilizer, the preparation process of which is as follows:

[0056] S1. The red mud was pulverized using a ball mill and then sieved to obtain powder with a particle size of less than 200 μm. Most of the iron ore powder was then collected using a magnetic separator. A sodium carbonate solution (3 mol / L) was prepared, and the red mud residue was mixed in the sodium carbonate solution and heated and stirred at 80°C until no more bubbles were generated. The iron compounds suspended on the surface of the solution were collected, dried, and pulverized for use in phosphate fertilizer coating.

[0057] S2. Take a 2L beaker and add 1000mL of deionized water and a magnetic stir bar. Turn on the heated magnetic stirrer and set the speed to 500rpm and the temperature to 50℃. Gradually add a mixture of diammonium phosphate (300g), alkaline lignin (30g) and citric acid (0.5g) until the solution is saturated. A small amount of precipitate will form at the bottom, and the system will become a suspension. Continue stirring for 2 hours to obtain the pretreated phosphate fertilizer solution.

[0058] S3. Maintaining heating and stirring conditions, add 50g of the iron compound obtained in step (1) to the suspension in five portions (10g / time). After mixing evenly, add 4.0mM sodium hydroxide solution dropwise using a syringe, while simultaneously monitoring the pH of the system in real time using a pH meter. Once the pH stabilizes at 6.8-7.2, stop stirring and heating, cool to room temperature, and let stand for 1 hour to generate a large amount of yellowish-brown precipitate. Filter the precipitate using a Buchner funnel and filter paper, and air-dry the collected solid to obtain a controlled-release phosphate fertilizer sample, which is an iron mineral-coated alkaline lignin compound phosphate fertilizer. Observing the structure of the compound fertilizer using a scanning electron microscope, a large number of iron oxide particles were observed on the surface, proving that an iron oxide coating was formed on the surface of the compound fertilizer. Figure 2 ).

[0059] Experimental Example 1

[0060] In Example 1 of this experiment, iron-reducing bacteria and iron mineral-coated compound phosphate fertilizer were added to a pure culture system. The periodic changes in redox conditions during waterlogging and drainage in soil were simulated by changing the gas atmosphere in the vials. The dissolution of the coating and the release of phosphate fertilizer were examined by measuring the changes in ferrous and phosphorus concentrations in the system, and the effect of changes in redox conditions on the controlled release of compound fertilizer was evaluated.

[0061] 1. Iron-reducing bacteria culture

[0062] First, prepare the LB medium (the medium required for the activation and culture of Shewanella oneidensis MR-1 strain). The nutrient composition of LB medium (1.0L) is as follows: 10g tryptone, 5g yeast extract, and 10g sodium chloride. After preparation, adjust the pH of the medium to approximately 7.0 using hydrochloric acid and sodium hydroxide. For the preparation of LB solid medium, add 15g of agar (1.5% agar content) to the LB liquid medium preparation steps. Dispense the solid LB medium into 250mL Erlenmeyer flasks, adding 100mL of culture medium to each flask, and sterilize the culture medium using an autoclave.

[0063] After sterilization of LB solid medium, the culture medium is cooled to approximately 50°C in a laminar flow hood. It is then poured into sterile petri dishes in a sterile operating table and allowed to solidify. The MR-1 bacterial culture, preserved in 50% glycerol (v / v), is removed from a -80°C freezer and quickly placed on ice to thaw, ensuring the ice tube is thawed on ice to avoid prolonged exposure to room temperature. Once the bacterial culture begins to thaw, a small amount of the culture is streaked onto an LB agar plate using a disposable sterile inoculation loop in a sterile operating table. The plates are then inverted and incubated in a 30°C biochemical incubator in the dark for 18-24 hours. After a single colony forms on the plate, one independent colony is inoculated into a conical flask containing 100 mL of culture medium. The conical flask is then placed in a 30°C shaking incubator at 180 rpm for approximately 12-16 hours to promote the bacteria into the logarithmic growth phase. Once the bacteria in the liquid culture medium showed significant growth, they were inoculated again onto solid culture medium (streaking) under aseptic conditions. Then, under aseptic conditions, a single colony of MR-1 was picked using a disposable sterile inoculating loop and inoculated into an Erlenmeyer flask containing 100 mL of culture medium. The flask was incubated under constant temperature and shaking conditions (30°C, 180 rpm) until the logarithmic growth phase, approximately 12-16 hours. The incubated culture was centrifuged to obtain a bacterial precipitate. The precipitate was washed 3-4 times with PIPES buffer (pH 7.0). After the final wash, the resulting bacterial culture was dispersed in PIPES buffer, shaken well, and the OD value of the culture was adjusted. 600 To the appropriate range.

[0064] 2. Experimental Methods

[0065] The effect of iron-reducing bacteria on the controlled release of compound fertilizer under alternating anaerobic and aerobic conditions was investigated. 30 mL of OD200 was added to a 50 mL vial. 600 A 0.1g MR-1 bacterial culture was used to construct a microcosm culture system. The controlled-release phosphate fertilizer anaerobic system was added in a clean bench, with nitrogen aeration for 40 minutes. Experimental group 1 consisted of PIPES buffer, the compound phosphate fertilizer from Example 1, and sodium acetate, with the compound phosphate fertilizer concentration at 10g / L and the sodium acetate concentration at 5mmol / L. Experimental group 2 was set up by replacing the compound phosphate fertilizer with that from Example 2, with the rest of the setup the same as experimental group 1. The control group received the same amount of dipotassium hydrogen phosphate as the experimental groups. Both experimental and control groups were then incubated in a constant temperature shaking incubator at 30℃ and 180rpm for 20 days.

[0066] After the anaerobic stage of culture is completed, remove the rubber stopper and aluminum cap from the vial and place it on a shaker for further culture (at a rate of 180 rpm). Air is introduced into the vial, and moisture gradually evaporates, entering the aerobic stage, which lasts for 10 days.

[0067] 3. Analytical Methods

[0068] (1) Extraction and determination of ferrous iron

[0069] Hydrochloric acid-extractable Fe(II): Shake the sample in the vial well, and transfer 0.2 mL of the sample suspension to 0.2 mL of hydrochloric acid. Extract for 60 minutes at 30°C and 180 rpm in a constant temperature shaker. After extraction, centrifuge, and then transfer 0.2 mL of the supernatant to 0.4 mL of sodium acetate buffer and 0.1 mL of o-phenanthroline chromogenic reagent. After developing in the dark for 10 minutes, measure the absorbance of the sample at 510 nm and calculate the actual concentration of hydrochloric acid-extractable ferrous iron in the sample using the ferrous standard as a reference.

[0070] (2) Dissolved phosphorus: The vial was placed in an anaerobic glove box for 2 hours to allow the particulate matter in the system to settle. The aluminum cap and rubber stopper were opened, and 2.5 mL of the supernatant was drawn with a syringe. 1.8 mL of the supernatant was filtered through a 0.22 μm needle filter. After removing iron ions from the sample by hydrogen column dilution to an appropriate factor, the soluble phosphorus content of the sample was determined by ion chromatography. The actual concentration of soluble phosphorus in the sample was calculated based on the phosphate mark.

[0071] 4. Experimental Results

[0072] (1) Experimental results of experimental group 1

[0073] Changes in ferrous and phosphorus concentrations at different stages of anaerobic culture, as follows: Figure 3 and Figure 4 As shown, during the anaerobic stage, the ferrous iron concentration gradually increased over time, indicating that the iron oxides on the surface of the compound fertilizer gradually dissolved. Simultaneously, the soluble phosphorus concentration in the system also slowly increased over time. The increase in soluble phosphorus concentration was slow, but began to rise rapidly from day 10, indicating that as the iron oxides on the surface of the compound fertilizer dissolved, pores were formed, and the mineral-encapsulated phosphorus was gradually released into the system. In contrast, the phosphate fertilizer in the control group dissolved more rapidly, reaching its peak on day 15 and remaining thereafter. This result demonstrates that iron-coated controlled-release fertilizer can control the slow release of phosphorus under anaerobic conditions through iron reduction.

[0074] When oxygen was introduced into the reaction system, the concentrations of ferrous iron and soluble phosphorus decreased rapidly, reaching 3 mg / L and 120 mg / L respectively on day 25 (day 5 after the reaction conditions were changed). During the subsequent reaction period, ferrous iron decreased to approximately 0 mg / L, and soluble phosphorus decreased to 60 mg / L. The phosphate concentration in the control group remained unchanged during the aerobic phase. This result indicates that ferrous iron in the experimental system was re-oxidized into ore, and most of the phosphorus was re-encapsulated and stored within the coating, demonstrating that iron ore coatings can be regenerated and phosphorus can be re-encapsulated under aerobic conditions.

[0075] (2) Experimental results of experimental group 2

[0076] Changes in ferrous and phosphorus concentrations at different stages of anaerobic culture, as follows: Figure 5 and Figure 6 As shown in the figure, compared with experimental group 1, the concentrations of ferrous iron and phosphorus increased rapidly over time during the anaerobic stage, reaching their highest values ​​in the stable phase after 15-20 days of cultivation. This indicates that the addition of alkaline lignin effectively accelerated the dissolution of iron ore and the release of phosphorus. This result suggests that the dissolution of iron ore and the release of phosphorus in the anaerobic stage system are mainly due to electron transfer between iron-reducing bacteria and iron minerals.

[0077] Similar to Experimental Group 1, when oxygen was introduced into the reaction system, the concentrations of ferrous iron and soluble phosphorus in the experimental group decreased rapidly, reaching 15 mg / L and 400 mg / L respectively on day 25 (the 5th day after the reaction conditions were changed). During the subsequent reaction period, the ferrous iron concentration decreased to approximately 0 mg / L, regenerating the iron ore coating and sealing most of the phosphorus. The soluble phosphorus concentration in the control group also followed the same pattern as in Example 1, with the concentration rapidly dissolving and reaching its peak in the early anaerobic stage, remaining constant in the subsequent aerobic stage.

[0078] Experiment Example 2

[0079] In this experiment, an iron-coated alkaline lignin compound controlled-release phosphate fertilizer was added to a potted plant system. The soil moisture conditions were altered to establish a periodic anaerobic-aerobic cycle. The concentrations of ferrous iron and soluble phosphorus at different stages were measured to assess the slow-release properties of the compound phosphate fertilizer and evaluate its application effect in a real soil setting.

[0080] 1. Experimental conditions

[0081] The experimental soil samples (topsoil depth 0-20 cm) were collected from paddy fields in Jiangmen City, Guangdong Province. They were dried in the laboratory, air-dried naturally at room temperature in a cool, shaded place, impurities removed, ground, and passed through a 2 mm sieve before being stored in plastic containers for chemical analysis. The basic physicochemical properties of the soil are as follows: pH 4.25, total organic matter content 45.2 g / kg, total iron 15.0 g / kg. Experimental setup: In the rice pot experiment, each pot (φ=25cm; H=35cm) contained 5 kg of soil, with 10 g of the iron mineral-coated alkaline lignin compound phosphate fertilizer from Example 2 added and thoroughly mixed. The control group received the same amount of dipotassium hydrogen phosphate as the experimental group. 3.6 L of water was added, and the mixture was allowed to stabilize for 2-3 days. Rice seedlings were then transplanted into the pots, with one seedling per hole, and three holes per pot. During the rice flooding stage: the rice was irrigated with deionized water under natural conditions, without applying fertilizer, and the water depth in each pot was maintained at least 5 cm above the soil surface. Eight sampling points were set up according to the rice growth cycle (seedling stage, tillering stage, jointing stage, booting stage, heading stage, flowering stage, and grain-filling and ripening stage). Samples were taken at days 0, 15, 30, and 60 of the flooding stage to determine the content of dissolved Fe(II) and dissolved phosphorus in the system. After the flooding stage, the water in the flowerpots was allowed to evaporate naturally, at which point the soil environment entered a low-moisture, aerobic condition. The reaction time was 40 days.

[0082] 2. Experimental Results

[0083] The changes in ferrous and soluble phosphorus content at different culture stages are as follows: Figure 7 and Figure 8 As shown in the figure, during flooding, the concentrations of ferrous iron and phosphorus extracted by hydrochloric acid gradually increased, reaching 11.8 g / kg and 1.3 g / kg respectively at 30 days, indicating that phosphorus was released into the soil after the iron ore dissolved. Similar to the phosphorus release pattern in anaerobic microenvironment cultivation, the phosphorus release rate gradually increased over time, indicating that the iron ore coating of compound fertilizer can stably control the release of phosphorus in actual soil environments, avoiding rapid phosphorus loss. From 30 to 60 days of flooding, the concentrations of ferrous iron and soluble phosphorus remained relatively stable, and the released iron and phosphorus could be absorbed by rice. In the control group, under the action of alkaline lignin, the iron in the soil was reduced by microorganisms during the anaerobic stage, and the ferrous iron concentration also increased. The phosphate fertilizer dissolved rapidly, and the soluble phosphorus concentration reached its highest point at 15 days of flooding, then slowly decreased in the subsequent stages of flooding, indicating that excessive phosphorus dissolved in the soil could not be absorbed by crops in time, resulting in loss and adsorption.

[0084] After the waterlogging cultivation ended, the water in the flowerpots was allowed to evaporate naturally, at which point the soil environment entered the aerobic stage. Once most of the water was drained and the soil entered the aerobic stage, the concentrations of hydrochloric acid-extractable ferrous iron and soluble phosphorus in the experimental group system decreased. On the 90th day of the total reaction (i.e., the 20th day after drainage), the concentrations of hydrochloric acid-extractable ferrous iron and soluble phosphorus decreased to 0.2 g / kg and 0.1 g / kg, respectively. The decrease in ferrous and phosphorus concentrations is because the iron mineral coating reformed and re-encapsulated the released phosphorus, with some phosphorus remaining free in the soil for crop absorption. In the aerobic stage, some phosphorus in the control group was adsorbed and fixed as iron oxidized and mineralized, while some continued to be lost, resulting in a continuous decrease in concentration to 0.5 g / kg, but not all of it was sealed.

[0085] In summary, the iron ore-coated composite phosphate fertilizer of this invention can control the release and sequestration of phosphorus by adjusting the system's moisture content. During the plant growth stage, increasing soil moisture promotes the dissolution of the iron ore coating, slowly releasing phosphorus for plant absorption. Subsequently, during plant maturity and harvest, reducing soil moisture content regenerates the iron ore coating, sequestering the remaining phosphorus for use in the next planting cycle. This technology can effectively improve phosphate fertilizer utilization and avoid economic losses and environmental pollution caused by phosphate fertilizer runoff.

[0086] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A slow-release phosphate fertilizer, characterized in that, Includes phosphate fertilizer, the surface of which is covered with an iron oxide coating; The raw materials for preparing the slow-release phosphate fertilizer, by weight, include: 50 parts of iron compound, 250-500 parts of phosphate fertilizer, and 0.1-5 parts of organic acid; The iron compound is derived from red mud or non-ferrous metal slag; The method for preparing the iron compound includes the following steps: crushing red mud and separating it by magnetic separation to obtain magnetic iron minerals and non-magnetic slag; adding the magnetic iron minerals to an inorganic alkaline solution for alkaline leaching to obtain the iron compound; The preparation method of the slow-release phosphate fertilizer includes the following steps: Phosphate fertilizer and organic acid are added to water and mixed to obtain a phosphate fertilizer mixed solution; the iron compound is added to the phosphate fertilizer mixed solution, and then an inorganic base is added to adjust the pH of the reaction system to 6.8-7.

2. The reaction is carried out in a static environment to obtain the slow-release phosphate fertilizer.

2. The slow-release phosphate fertilizer according to claim 1, characterized in that, The phosphate fertilizer is selected from at least one of diammonium phosphate, calcium phosphate, and superphosphate. And / or, the organic acid is at least one of citric acid and malic acid.

3. The slow-release phosphate fertilizer according to claim 1, characterized in that, The raw materials for preparation also include 25-50 parts of lignin or its derivatives; The lignin derivative is selected from at least one of alkaline lignin, dealkalized lignin, and sodium lignin sulfonate.

4. The method for preparing the slow-release phosphate fertilizer according to any one of claims 1 to 3, characterized in that, Includes the following steps: Phosphate fertilizer and organic acid are added to water and mixed to obtain a phosphate fertilizer mixed solution; the iron compound is added to the phosphate fertilizer mixed solution, and then an inorganic base is added to adjust the pH of the reaction system to 6.8-7.

2. The reaction is carried out in a static environment to obtain the slow-release phosphate fertilizer.

5. The method for preparing slow-release phosphate fertilizer according to claim 4, characterized in that, The mixing step specifically includes: stirring at 40~70℃ for 1-3 h; And / or, the step of carrying out the reaction in a static environment specifically includes: standing at room temperature for 1-2 hours.

6. The use of the slow-release phosphate fertilizer according to any one of claims 1 to 3 in soil improvement or preparation of soil conditioners.

7. A method for improving soil, characterized in that, The method includes the following steps: applying the slow-release phosphate fertilizer according to any one of claims 1 to 3 into the soil, controlling the anaerobic-aerobic environment in the soil, thereby controlling the release rate of phosphorus in the slow-release phosphate fertilizer.

8. The method for improving soil according to claim 7, characterized in that, The anaerobic-aerobic environment in the soil can be controlled by altering the soil water content.

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