Method for preparing intelligent controlled-release fertilizer by using water to drive iron switch

Abstract

The invention relates to the technical field of soil improvement, and discloses a preparation method of an intelligent controlled-release fertilizer using water to drive an iron switch. The slow / controlled-release fertilizer is of a core-shell structure, the core of the slow / controlled-release fertilizer is a porous carrier loaded with nutrient elements, and the core shell of the slow / controlled-release fertilizer is an iron mineral film layer. According to the slow / controlled-release fertilizer provided by the invention, the nutrient elements are loaded on the porous carrier, then the carrier is coated with the iron mineral film layer, and the slow / controlled-release fertilizer can be combined with soil moisture conditions to induce soil microorganisms to change the pore size of the iron mineral film layer on the surface of a material, so that the release rate of the nutrient elements is regulated and controlled, and the efficient utilization of the nutrient elements is promoted; meanwhile, the porous carrier has the effects of keeping soil moisture and increasing soil pores, so that the sustained and controlled release fertilizer can also optimize the soil structure and enhance the air permeability and water retention of the soil; the slow / controlled-release fertilizer has the advantages of easily available raw materials, low cost, environmental friendliness, simple preparation method, low energy consumption and no generation of solid waste, and is suitable for popularization and application.

Description

Technical Field

[0001] The present invention relates to the technical field of soil improvement, and particularly relates to a method for preparing an intelligent controlled-release fertilizer using a water-driven iron switch. Background Art

[0002] Fertilizer is an important source of the nutrient elements required for the growth of crops. Its input accounts for about 50% of the total input of agricultural production and is an important part of agricultural production. Fertilizer is easily lost in farmland. To ensure that crops fully obtain nutrients, a large amount of fertilizer needs to be applied, resulting in frequent problems of over-fertilization, which not only causes waste of resources but also triggers a series of environmental problems such as non-point source pollution and soil compaction. Therefore, developing smart agriculture and developing an intelligent fertilizer with high efficiency and low cost are of great significance for promoting the development of high-quality agriculture.

[0003] Slow / controlled-release fertilizer is a kind of fertilizer that changes the release rate of the effective components in the fertilizer through physical, chemical or biological methods to meet the long-term nutritional needs of agricultural crops. Most of the existing slow / controlled-release fertilizers cannot balance safety, harmlessness and low cost. For example, CN 109928823 A discloses a method for preparing a coated slow-release fertilizer. The organic slow-release coating prepared by using materials such as polyvinyl alcohol and methyl cellulose in this technical solution has good effects, but the continuous input of polyvinyl alcohol and other difficult-to-degrade organic substances into farmland soil may cause a series of negative environmental effects; CN 116425588 A discloses a trace element amino acid chelate / urea formaldehyde slow-release fertilizer and its reactive extrusion preparation. This technical solution uses vegetable oil and starch as the basic materials for the coating, which has the advantages of environmental friendliness, but the raw materials are relatively expensive compared with general materials. In addition, crops have different nutrient requirements under different environmental conditions. Most of the existing slow / controlled-release fertilizers can only disorderly slow down the release rate of nutrients and cannot directionally regulate the nutrient release according to crop growth and nutrient requirements. Summary of the Invention

[0004] The present invention aims to solve at least one of the above technical problems existing in the prior art. For this reason, one of the purposes of the present invention is to provide a slow / controlled-release fertilizer; the second purpose of the present invention is to provide a preparation method of this slow / controlled-release fertilizer; the third purpose of the present invention is to provide an application of this slow / controlled-release fertilizer.

[0005] In order to achieve the above purposes, the technical solutions adopted by the present invention are as follows:

[0006] The first aspect of the present invention provides a slow / controlled-release fertilizer. The slow / controlled-release fertilizer has a core-shell structure, wherein the core is a porous carrier loaded with nutrient elements, and the core-shell is an iron mineral film layer.

[0007] In some embodiments of the present invention, the porous carrier is selected from at least one of diatomite, bentonite, zeolite, montmorillonite, expanded vermiculite, sepiolite or attapulgite.

[0008] In some specific embodiments of the present invention, the porous carrier is diatomite.

[0009] In some embodiments of the present invention, the particle size of the porous carrier is less than 50 μm.

[0010] In some specific embodiments of the present invention, the particle size of the porous carrier is less than 48 μm.

[0011] In some embodiments of the present invention, the moisture content of the porous carrier is 60%-80%.

[0012] In some specific embodiments of the present invention, the moisture content of the porous carrier is 60%-75%.

[0013] In some embodiments of the present invention, the nutrient elements include at least one of the essential nutrient elements and beneficial nutrient elements required for plant growth.

[0014] In some embodiments of the present invention, the essential nutrient elements include at least one of zinc, phosphorus, and potassium.

[0015] In some specific embodiments of the present invention, the essential nutrient elements are phosphorus and potassium.

[0016] In some embodiments of the present invention, the beneficial nutrient element includes selenium.

[0017] In some embodiments of the present invention, the sustained-release fertilizer comprises the following preparation raw materials: a porous carrier, a ferrous salt, a binder, a microbial activation induction adjuvant, a nutrient element salt, water, and an alkali solution.

[0018] In some embodiments of the present invention, the ferrous salt is selected from at least one of ferrous sulfate, ferrous chloride, ferrous perchlorate, ammonium ferrous sulfate, and ferrous oxalate.

[0019] In some embodiments of the present invention, the concentration of the binder is 0.5 wt%-1.5 wt%.

[0020] In some embodiments of the present invention, the binder is selected from at least one of chromatographic silica gel and silica sol.

[0021] In some embodiments of the present invention, the concentration of the microbial activation induction adjuvant is 1.5 wt%-2.5 wt%.

[0022] In some embodiments of the present invention, the microbial activation induction adjuvant is selected from at least one of glucose, sodium acetate, and sodium lactate.

[0023] In some embodiments of the present invention, the microbial activation induction adjuvant is used to assist in inducing the activation of iron-reducing bacteria in the soil.

[0024] In some embodiments of the present invention, the nutrient element salts include phosphate, inorganic selenium salt, and inorganic zinc salt.

[0025] In some embodiments of the present invention, the phosphate includes diammonium hydrogen phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, tripotassium phosphate, superphosphate, and monocalcium phosphate.

[0026] In some embodiments of the present invention, the inorganic selenium salts include sodium selenite, potassium selenate, and potassium selenite.

[0027] In some embodiments of the present invention, the inorganic zinc salts include zinc sulfate, zinc nitrate, zinc chloride, and zinc carbonate.

[0028] In some specific embodiments of the present invention, the nutrient element salt is potassium dihydrogen phosphate or sodium selenite.

[0029] In some embodiments of the present invention, the concentration of the alkali solution is 5 - 10 mol / L.

[0030] In some embodiments of the present invention, the alkali source of the alkali solution includes alkali metal hydroxides, alkali metal carbonates, and alkali metal bicarbonates.

[0031] In some embodiments of the present invention, the alkali source of the alkali solution is selected from at least one of sodium bicarbonate, sodium carbonate, potassium hydroxide, and sodium hydroxide.

[0032] In some embodiments of the present invention, the mass ratio of the porous carrier, binder, microbial activation induction adjuvant, nutrient element salt, ferrous salt to water is (400 - 600):(10 - 20):(20 - 30):(1 - 300):(40 - 60):1000.

[0033] In some specific embodiments of the present invention, the mass ratio of the porous carrier, binder, microbial activation induction adjuvant, nutrient element salt, ferrous salt to water is (450 - 550):(10 - 15):(22 - 28):(1 - 300):(45 - 55):1000.

[0034] In some specific embodiments of the present invention, when the nutrient element salt is phosphate, the mass ratio of the porous carrier, binder, microbial activation induction adjuvant, nutrient element salt, ferrous salt to water is (450 - 550):(10 - 15):(22 - 28):(220 - 290):(45 - 55):1000.

[0035] In some specific embodiments of the present invention, when the nutrient element salt is an inorganic selenium salt, the mass ratio of the porous carrier, the binder, the microbial activation induction auxiliary agent, the nutrient element salt, the ferrous salt to water is (450 - 550):(10 - 15):(22 - 28):(1 - 2):(45 - 55):1000.

[0036] The second aspect of the present invention provides a preparation method of the sustained-release and controlled-release fertilizer described in the first aspect of the present invention, comprising the following steps:

[0037] S1. Dissolve the nutrient element salt in water, add the binder and the microbial activation induction auxiliary agent, and then batch-add the porous carrier to obtain a porous carrier loaded with the nutrient element;

[0038] S2. Add the ferrous salt, adjust the pH of the system to 6.8 - 7.2 with an alkali solution, perform solid-liquid separation, and collect the solid phase to obtain the sustained-release and controlled-release fertilizer.

[0039] In some embodiments of the present invention, the temperature at which the nutrient element salt is dissolved in water is 20 - 30 °C.

[0040] In some embodiments of the present invention, when the nutrient element salt is a phosphate, it is necessary to dissolve the phosphate in water and make it in a supersaturated state, generate a precipitate and stir for 5 - 10 min without the precipitate disappearing.

[0041] In some embodiments of the present invention, the process of dissolving the nutrient element salt is assisted by stirring; the rotation speed of the stirring is 150 - 200 rpm.

[0042] In some embodiments of the present invention, the process of adding the binder and the microbial activation induction auxiliary agent is assisted by stirring; the rotation speed of the stirring is 150 - 200 rpm and the time is 8 - 15 min.

[0043] In some embodiments of the present invention, the process of adding the porous carrier is assisted by stirring; the rotation speed of the stirring is 80 - 120 rpm.

[0044] In some embodiments of the present invention, the batch addition amount of the porous material is 1 / 5 - 1 / 4 of the total amount.

[0045] In some embodiments of the present invention, the batch addition interval time of the porous material is 3 - 8 min.

[0046] In some embodiments of the present invention, after the addition of the porous material is completed, it further includes a step of stirring for 25 - 35 min.

[0047] In some embodiments of the present invention, the porous material is pretreated before use, and the pretreatment includes the following steps: sieving the porous material to remove impurities, washing with deionized water to remove soluble ions, and filtering by suction to remove moisture.

[0048] In some embodiments of the present invention, the process of adding ferrous salt is assisted by stirring; the rotation speed of the stirring is 180 - 220 rpm.

[0049] In some embodiments of the present invention, the adding speed of the alkali solution is (5 - 15) mL / min.

[0050] In some embodiments of the present invention, after collecting the solid phase, the steps of suction filtration and drying are further included.

[0051] In some embodiments of the present invention, the drying temperature is greater than or equal to 25 °C.

[0052] The third aspect of the present invention provides the application of the sustained-release and controlled-release fertilizer described in the first aspect of the present invention in soil improvement.

[0053] In some embodiments of the present invention, the soil improvement includes supplementing nutrient elements, enhancing soil aeration and water retention.

[0054] The principle of soil improvement by the sustained-release and controlled-release fertilizer of the present invention is described as follows:

[0055] 1) The present invention uses porous materials such as diatomite as carriers, fixes nutrient elements required for plant growth such as selenium, zinc, phosphorus, and potassium in the pores and on the surface of the carriers, and forms an iron mineral film layer with the secondary mineral precipitate formed by ferrous oxidation to wrap the porous carrier loaded with nutrient elements to form a sustained-release and controlled-release fertilizer. The iron mineral film layer formed by ferrous oxidation can directionally regulate the nutrient element release rate according to the environmental moisture content. Specifically, in a high moisture content (50% - 100%), especially in a flooded anaerobic environment, the biological iron reduction process is enhanced, and the iron mineral film layer is gradually eroded by microorganisms to form a large number of pores, resulting in rapid release of nutrients in the sustained-release and controlled-release fertilizer; in a low moisture content (<50%), especially in the soil after drainage, the iron oxidation process is enhanced, and ferrous oxidation leads to the reformation of the iron mineral film layer and pore blockage, resulting in a significant decrease in nutrient release;

[0056] 2) The porous material surface has a three-dimensional pore structure, combined with silica gel with chelating and bonding effects, which can fix a large amount of nutrient elements, promoting the long-term release and persistent application of nutrient elements in the soil by the sustained-release and controlled-release fertilizer; in addition, the iron mineral film layer on the surface of the sustained-release and controlled-release fertilizer can also supplement iron elements to the soil during the dissolution process;

[0057] 3) The slow-release and controlled-release fertilizer uses porous materials such as diatomite as the core carrier, which has the advantages of maintaining soil moisture and increasing soil porosity. Therefore, during the use of this slow-release and controlled-release fertilizer, in addition to providing nutrient elements, it can also improve the physical and chemical properties of the soil and enhance soil vitality; that is, the slow-release and controlled-release fertilizer provided by the present invention can simultaneously achieve the directional regulated release of nutrient elements and the improvement of soil physical and chemical properties, realizing soil improvement.

[0058] In some embodiments of the present invention, the physical and chemical properties of the soil include at least one of the following:

[0059] 1) The pH is 4 - 8;

[0060] 2) The cation exchange capacity is 5 - 30 cmol / kg;

[0061] 3) The total organic matter content is 10 - 50 g / kg;

[0062] 4) The total iron content is 50 - 300 g / kg.

[0063] In some embodiments of the present invention, the soil is divided into high water content soil (water content 50% - 100%) and low water content soil (water content < 50%) according to the water content.

[0064] In some embodiments of the present invention, the dosage of the slow-release and controlled-release fertilizer is 10 - 50 kg / mu.

[0065] Compared with the prior art, the beneficial effects of the present invention are:

[0066] 1) For the slow-release and controlled-release fertilizer provided by the present invention, the nutrient elements are loaded on the porous carrier, and the outer surface of the carrier is coated with an iron mineral film layer. This slow-release and controlled-release fertilizer can combine with the soil moisture conditions, induce soil microorganisms to change the pore size of the iron mineral film layer on the material surface, thereby regulating the release rate of nutrient elements and promoting the efficient utilization of nutrient elements; at the same time, due to the porous carrier having the functions of maintaining soil moisture and increasing soil porosity, this slow-release and controlled-release fertilizer can also optimize the soil structure and enhance soil aeration and water retention.

[0067] 2) The preparation method of the slow-release and controlled-release fertilizer provided by the present invention has easily available raw materials, low cost, is environmentally friendly, the preparation method is simple, has low energy consumption, does not produce solid waste, and is suitable for popularization and use;

[0068] 3) When the slow-release and controlled-release fertilizer provided by the present invention is applied to soil improvement, due to the water-responsive property of the mineral film layer, the release rate of nutrient elements can be regulated according to the water content in the soil, which can not only improve the utilization rate of nutrient elements, avoid over-fertilization, save resources, reduce the adverse effects of over-fertilization on the soil, but also enhance soil aeration and water retention by using the special structure of the porous carrier. Brief Description of the Drawings

[0069] Figure 1 Scanning electron micrographs of diatomite (a) and diatomite-iron mineral film composite material (b) in Example 1;

[0070] Figure 2 Graph of the change in ferrous iron concentration at different reaction stages in Example 1;

[0071] Figure 3 Scanning electron micrograph of the sustained-release fertilizer in Example 2;

[0072] Figure 4 Graph of the kinetic changes of dissolved selenium (a) and organically bound selenium (b) at different reaction stages in Example 2;

[0073] Figure 5 Scanning electron micrograph of the sustained-release fertilizer in Example 3;

[0074] Figure 6 Graph of the kinetic changes of hydrochloric acid extractable phosphorus (a) and available phosphorus (b) at different reaction stages in Example 3. Detailed implementation manners

[0075] The content of the present invention will be further described in detail through specific examples below. The raw materials, reagents or devices used in the examples can be obtained from conventional commercial channels or can be obtained by existing technical methods without special instructions. Unless otherwise specified, the test or measurement methods are conventional methods in the art.

[0076] Example 1

[0077] In this example, a diatomite-iron mineral film composite material without nutrient elements was prepared. By changing the system culture conditions, the periodic anaerobic-aerobic changes brought about by the changes in high and low moisture conditions in the soil were simulated. By measuring the change in ferrous iron concentration, the change of the iron mineral film was evaluated to determine whether the release rate of nutrients could be regulated by changing the moisture conditions. The specific steps are as follows:

[0078] (1) Cultivation of iron-reducing bacteria:

[0079] The iron-reducing bacterium Shewanella oneidensis MR-1 (abbreviated as MR-1) was purchased from the China Center for Marine Microbial Culture Collection Management, with the preservation number 1A01706, and stored in a -80 °C refrigerator.

[0080] Prepare 200 mL of LB medium, adjust the pH of the medium to 7.0 - 7.2, and then sterilize it at high temperature and high pressure using an autoclave.

[0081] Thaw the MR-1 bacterial strain stored in an -80°C refrigerator at room temperature. Place the LB medium and the bacterial strain in a sterile operating table, and use an inoculation needle to inoculate MR-1 onto the LB medium. Then, under the conditions of avoiding light and a constant temperature of 30°C, culture it on a shaker at a rotation speed of 180 rpm for 16 - 18 h to form a bacterial solution. After the culture is completed, centrifuge the bacterial solution using a centrifuge, and wash the solid bacterial precipitate 3 times with piperazine-1,4-diethanesulfonic acid with a pH of 7.0 and a concentration of 30 mmol / L, and adjust the OD 600 value to 0.1 for standby.

[0082] (2) Synthesis of diatomite-iron mineral film composite material:

[0083] Take 450 g of diatomite and place it on a 300-mesh sieve, and shake it at a rate of 50 rpm for 30 min to remove large particles and other impurities with larger particle sizes. After shaking, pour it into a beaker, wash it 2 - 3 times with deionized water, remove the soluble ions in the diatomite, and then use a suction filter and a 0.45-μm filter membrane to filter and remove the water. After filtration, keep the moisture content of the diatomite at about 60%.

[0084] Take a 2-L beaker and add 1000 mL of deionized water. Insert a stirrer into the water and stir at a rate of 180 rpm. Add 13 g of chromatography silica gel (mass fraction is about 1%) and 25 g of glucose (mass fraction is about 2%), and continue to stir for 10 min until the solution becomes uniformly translucent. Reduce the stirring rate to 100 rpm, add 100 g of wet diatomite, stir for 5 min and then continue to add 100 g of wet diatomite. Repeat the steps until 500 g of wet diatomite is added, and continue to stir for 30 min.

[0085] After adding the diatomite, increase the rotation speed of the stirrer to 200 rpm, and add 50 g of ferrous sulfate heptahydrate. After mixing evenly, gradually add a 5.0 mmol / L sodium hydroxide solution at a rate of 10 mL / min using a syringe, and at the same time use a pH meter to monitor the pH of the system in real time to control the final pH of the system between 6.8 - 7.2. After the pH stabilizes, stop stirring and let it stand for 30 min. A large amount of yellowish-brown precipitate accumulates at the bottom of the solution, indicating that the diatomite-iron mineral film composite material is initially formed.

[0086] Discard the supernatant, and use a suction filter and a 0.45-μm filter membrane to filter and remove the water from the remaining solid. Then spread the wet diatomite-iron mineral film composite material sample flat in the room to dry. The room should be cool and well-ventilated, and keep the temperature not lower than 25°C. At the same time, break up and stir the material once every 12 h to accelerate drying and promote material dispersion to avoid caking. After drying for 48 h, obtain the diatomite-iron mineral film composite material.

[0087] Figure 1The scanning electron microscope images of diatomite (a) and diatomite-iron mineral film composite material (b) in Example 1 are shown in FIG. Figure 1 It can be seen that the original diatom surface is smooth, has no attachments, and contains many voids; in the composite material prepared in Example 1, the iron mineral is coated on the surface of the diatomite, and the pores on the surface of the diatomite are significantly reduced, indicating that the iron mineral coating is formed.

[0088] (3) Evaluation of the change of iron mineral film layer with water content

[0089] Add 50 mL of OD to a 120 mL vial. 600 To a 0.1% MR-1 bacterial solution, 0.5 mL of a 1000 mmol / L sodium lactate solution and 0.5 g of a diatomaceous earth-iron mineral film composite material were added. The initial concentration of the composite material was about 10 g / L.

[0090] First, the solution was aerated with nitrogen for 40 minutes to remove oxygen, and the mouth of the vial was sealed with a rubber stopper and an aluminum cap to simulate the anaerobic environment when the soil had high moisture content. The reaction time was 96 hours. After the high-moisture soil simulation condition was completed, the rubber stopper and aluminum cap of the vial were removed, and the bottle was sealed with a 0.22μm sterile breathable membrane to prevent other bacteria from entering. The vial was placed on a shaker and cultured at a rate of 180rpm. At this time, the water in the vial came into contact with the outside atmosphere, and the water gradually evaporated into the low-moisture soil simulation environment, entering the low-moisture soil simulation condition, and the reaction time was 54 hours.

[0091] Figure 2 is a graph showing the change in ferrous iron concentration at different reaction stages in Example 1, Figure 2 It can be seen that in the high-moisture soil simulation environment, the total concentration of ferrous iron gradually increased, indicating that the iron reduction process occurred, indicating that in the high-moisture environment, the iron mineral film layer was gradually eroded, which was conducive to the formation of pores on the surface of the material. As the reaction time increased, the accumulation of ferrous iron gradually increased, reaching 75 mg / L in 96 hours. This result shows that the added material can adapt to the high-moisture soil simulation environment for a long time, induce iron-reducing microorganisms to carry out iron reduction process, and thus increase the release rate of nutrients.

[0092] After the cultivation conditions changed from a high-moisture soil simulation environment to a low-moisture soil simulation environment, the ferrous iron concentration decreased rapidly. It decreased from 75 mg / L to 1.3 mg / L at 105 h (i.e., the 9th hour after changing the reaction conditions), and decreased to approximately 0 mg / L during the subsequent reaction time. The rapid decrease in the ferrous iron concentration indicates that the reduced iron elements on the iron mineral film layer are oxidized again, transformed from the dissolved state to the solid state, indicating the regeneration of the iron mineral film layer. This result shows that the added material can quickly adapt to the low-moisture soil simulation environment, induce the regeneration of the iron mineral film layer, and thus inhibit the release rate of nutrient elements. The results of the high-moisture and low-moisture soil environment simulation experiments in Example 1 show that the iron mineral film layer has water-responsive characteristics, and its structure can change according to the change of water content.

[0093] Example 2

[0094] In this example, a slow-release fertilizer containing selenium element was prepared. By changing the system cultivation conditions, the periodic change of anaerobic-aerobic caused by the change of high-moisture and low-moisture conditions in the soil was simulated. By measuring the change of selenium concentration, the change of the iron mineral film layer was evaluated to evaluate whether the release rate of nutrients can be regulated by changing the water conditions:

[0095] (1) Preparation of slow-release fertilizer, the steps are as follows:

[0096] S11. Take 550 g of diatomite and place it on a 300-mesh sieve, shake it at a rate of 50 rpm for 30 min to remove large particles and other impurities with larger particle sizes. After shaking, pour it into a beaker, wash it 3 times with deionized water to remove the dissolved ions in the diatomite, and then use a suction filter and a 0.45-μm filter membrane to filter and remove the water. The moisture content of the filtered diatomite is about 65%.

[0097] S12. Take a 2-L beaker and add 1000 mL of deionized water, insert a stirrer into the water and stir at a rate of 180 rpm. Add 1.5 g of sodium selenate until it is completely dissolved. After the solution is stable, add 13 g of chromatography silica gel (mass fraction is about 1%) and 25 g of glucose (mass fraction is about 2%), and continue to stir for 10 min until the solution becomes uniformly translucent. Reduce the stirring rate to 100 rpm, add 100 g of wet diatomite, stir for 5 min and then continue to add 100 g of wet diatomite. Repeat the steps until 500 g of wet diatomite is added, and continue to stir for 30 min;

[0098] S21. After adding diatomite, increase the rotation speed of the blender to 200 rpm, and add 50 g of ferrous sulfate heptahydrate. After mixing evenly, gradually add a 5.0 mmol / L sodium hydroxide solution at a rate of 10 mL / min using a syringe, and simultaneously use a pH meter to monitor the pH of the system in real time to control the final pH of the system between 6.8 and 7.2. After the pH stabilizes, stop stirring and let it stand for 30 min. A large amount of yellowish-brown precipitate accumulates at the bottom of the solution, indicating the preliminary formation of the slow-release fertilizer;

[0099] S22. Discard the supernatant, and use a suction filter and a 0.45 μm filter membrane to filter the remaining solid to remove moisture. Then spread the wet sample flat in the room to dry. The room should be cool and well-ventilated, and the temperature should be kept not lower than 25 °C. At the same time, break up and stir the material once every 12 h to accelerate drying and promote material dispersion to avoid caking. After drying for 48 h, the finally synthesized sample is obtained.

[0100] Figure 3 is the scanning electron micrograph of the slow-release fertilizer in Example 2. It can be seen from Figure 3 that a large amount of iron minerals cover the surface of diatomite, and the pores are significantly reduced, indicating that the selenium element loaded into the pores and on the surface of diatomite is tightly wrapped by the iron mineral coating. The release rate of the selenium element in the synthesized iron mineral-coated selenium-rich diatomite can change with the change of the iron mineral coating.

[0101] (2) Evaluate the release of selenium element in the slow-release fertilizer with the change of soil moisture content:

[0102] The soil sample was collected from Taishan City, Jiangmen City, Guangdong Province. The soil-grown crop is rice, and the surface soil sample of 0-20 cm was collected. After the soil sample was collected, it was placed in the laboratory, dried in a cool and well-ventilated place, and large rhizospheres, stones and other impurities were removed. After passing through a 200-mesh sieve, it was reserved for use. The basic physical and chemical properties of the soil are as follows: pH = 4.49, cation exchange capacity is 13.6 cmol / kg, total organic matter content is 46.2 g / kg, and total iron content is 40.56 g / kg;

[0103] Prepare 3 50-mL vials, each adding 5.0 g of soil, 0.5 g of slow / controlled-release fertilizer and 50 mL of deionized water. Seal the vials with rubber stoppers and aluminum caps, place them on a shaker and shake at a rate of 240 rpm for 30 min to mix the soil, slow / controlled-release fertilizer and water evenly; place the vials in a constant temperature incubator at 30 °C in the dark. At this time, the vials are isolated from the outside atmosphere and water cannot be lost, so as to simulate the anaerobic environment under high soil moisture content, and the reaction time is 40 days. After the high soil moisture simulation condition is over, remove the rubber stoppers and aluminum caps of the vials, seal the bottles with 0.22-μm sterile breathable membranes to prevent other bacteria from entering, and place them on a shaker to culture at a rate of 180 rpm. Air enters the vials and water gradually evaporates, entering the low soil moisture simulation condition, and the reaction time is 2 days.

[0104] Figure 4 It is the kinetic change diagram of dissolved selenium (a) and organically bound selenium (b) at different reaction stages in Example 2. It can be seen from Figure 4 that in the high soil moisture simulation environment, the total concentration of dissolved selenium gradually increases and reaches 0.86 μg / L on the 40th day, indicating that the selenium in the slow / controlled-release fertilizer is released into the soil. The selenium concentration in the reaction system does not increase rapidly in a short time, but increases slowly, indicating that selenium is slowly released and gradually accumulates in the soil system, further indicating that the iron mineral film layer can adapt to the soil environment and achieve the purpose of slow release of nutrient elements in the high soil moisture environment; the change trend of organically bound selenium in the high soil moisture simulation environment is similar to that of dissolved selenium, and it gradually increases with the change of culture time and reaches 6.3 μg / L on the 40th day; both dissolved and organically bound selenium are forms that are easily absorbed by crops, indicating that the sodium selenate added to the slow / controlled-release fertilizer can be converted into various beneficial forms, which is conducive to the full absorption and utilization of crops.

[0105] After the culture condition enters the low soil moisture simulation environment from the high soil moisture simulation environment, the concentrations of both dissolved selenium and organically bound selenium decrease. On the 42nd day of the total reaction (i.e., the 2nd day after changing the reaction condition), the concentration of dissolved selenium decreases to 0.55 μg / L, and the concentration of organically bound selenium decreases to 5.14 μg / L. The decrease in the concentrations of dissolved and organically bound selenium is because the oxygen content is sufficient in the low soil moisture environment and the iron mineral film layer regenerates. On the one hand, it restricts the further release of selenium, resulting in no further accumulation of selenium, and on the other hand, the regeneration of the iron mineral film layer re-fixes a part of the selenium that has been released, resulting in a decrease in the selenium concentration.

[0106] In summary, the sustained-release fertilizer in the present invention can precisely control the release of selenium in the soil environment by changing the moisture conditions. In a high-moisture environment conducive to plant nutrient absorption, it promotes the release of nutrient elements, while in a low-moisture environment unfavorable for plant nutrient absorption, it reduces the release of selenium in the soil environment and fixes part of the released selenium to achieve the effect of nutrient preservation.

[0107] Example 3

[0108] In this example, a phosphorus-containing sustained-release fertilizer was prepared, and by changing the system culture conditions, the periodic anaerobic-aerobic changes brought about by the changes in high and low moisture conditions in the soil were simulated. By measuring the changes in phosphorus concentration, the changes in the iron mineral film layer were evaluated to determine whether the release rate of nutrients could be regulated by changing the moisture conditions:

[0109] (1) The preparation of the sustained-release fertilizer is as follows:

[0110] S11. Take 530 g of diatomaceous earth and place it on a 300-mesh sieve, shake it at a rate of 50 rpm for 25 min to remove large particles and other impurities with larger particle sizes. After shaking, pour it into a beaker, wash it 3 times with deionized water to remove the dissolved ions in the diatomaceous earth, and then use a suction filter and a 0.45-μm filter membrane to filter out the moisture. The moisture content of the filtered diatomaceous earth is about 65%.

[0111] S12. Take a 2-L beaker and add 1000 mL of deionized water, insert a stirrer into the water and stir at a rate of 180 rpm. Control the dissolution temperature at about 30°C, add 281 g of potassium dihydrogen phosphate. If the temperature is too high, the amount of potassium dihydrogen phosphate required will gradually decrease to make the potassium dihydrogen phosphate solution supersaturated, with a small amount of white precipitate at the bottom. Continue stirring for 5 min until the precipitate does not disappear. After the solution is stable, add 13 g of chromatography silica gel (mass fraction about 1%) and 25 g of glucose (mass fraction about 2%), and continue stirring for 10 min until the solution becomes uniformly translucent. Reduce the stirring rate to 100 rpm, add 100 g of wet diatomaceous earth, stir for 5 min and then continue to add 100 g of wet diatomaceous earth. Repeat the steps until 500 g of wet diatomaceous earth is added, and continue stirring for 30 min;

[0112] S21. After adding the diatomaceous earth, increase the mixer speed to 200 rpm and add 50 g of ferrous sulfate heptahydrate. After mixing evenly, use a syringe to gradually add a 5.0 mmol / L sodium hydroxide solution at a rate of 10 mL / min, and at the same time use a pH meter to monitor the pH of the system in real time to control the final pH of the system between 7.0. After the pH is stable, stop stirring and let it stand for 30 min. A large amount of yellow-brown precipitate accumulates at the bottom of the solution, indicating that the sustained-release fertilizer is initially formed;

[0113] S22. Discard the supernatant, and use a suction filter and a 0.45 μm filter membrane to filter the remaining solid to remove moisture. Then spread the wet sample flat in the room to dry. The room should be cool and well-ventilated, with the temperature not lower than 25 °C. At the same time, break up and stir the material every 12 h to accelerate drying and promote material dispersion, avoiding caking. After drying for 48 h, the finally synthesized sample is obtained.

[0114] Figure 5 It is the scanning electron micrograph of the sustained-release and controlled-release fertilizer in Example 3. It can be seen from Figure 5 that a large amount of iron minerals cover the surface of diatomite, and the pores are significantly reduced, indicating that the phosphorus loaded into the pores and on the surface of diatomite is tightly wrapped by the iron mineral coating. The release rate of phosphorus in the synthesized iron mineral-coated phosphorus-rich diatomite can change with the change of the iron mineral coating..

[0115] (2) Evaluate the release of phosphorus in the sustained-release and controlled-release fertilizer with the change of soil moisture content:

[0116] The soil samples were collected from Yingtan City, Jiangxi Province. The crops planted in the soil were rice, and the surface soil samples of 0-20 cm were collected. After the soil samples were collected, they were placed in a greenhouse and dried in a cool and well-ventilated place. Then large rhizospheres, stones and other impurities were picked out. After passing through a 10-mesh sieve, they were reserved for use. The basic physical and chemical properties of the soil are as follows: pH = 4.40, total organic matter content is 44.8 g / kg, and total iron content is 17.6 g / kg;

[0117] Prepare 3 10-L flowerpots, add 5.0 kg of soil and 250 g of the sustained-release and controlled-release fertilizer and mix well. Add deionized water until the accumulated water on the soil surface is about 5 cm, and place it in a ventilated place in the greenhouse for cultivation. Water once every 2 days to keep the accumulated water on the soil surface at about 5 cm. At this time, the soil is isolated from the external atmosphere to form an anaerobic environment of high-moisture soil, and the reaction lasts for 40 days. After the high-moisture reaction of the soil ends, no more water is added continuously, and the water in the flowerpot is allowed to evaporate naturally. At this time, the soil environment enters the aerobic condition of low-moisture soil, and the reaction time is 20 days.

[0118] Figure 6 It is the kinetic change diagram of hydrochloric acid-extractable phosphorus (a) and available phosphorus (b) at different reaction stages in Example 3. It can be seen from Figure 6It can be seen that the initial phosphorus accumulation in the reaction system is the background value of soil phosphorus. Under the reaction conditions of high-moisture soil, the concentration of hydrochloric acid-extractable phosphorus gradually increased, reaching 110.4 mg / kg on the 40th day, and the phosphorus concentration did not increase rapidly in a short period of time, but increased slowly, indicating that the phosphorus in the slow-release fertilizer was slowly released in the soil and gradually accumulated, further indicating that the iron mineral film layer can adapt to the soil environment and achieve the purpose of slow release of nutrients in the high-moisture soil environment. The change trend of available phosphorus in high-moisture soil conditions is similar to that of hydrochloric acid-extractable phosphorus, which gradually increased with the change of incubation time and reached 239.6 mg / L on the 40th day. Both hydrochloric acid-extractable phosphorus and available phosphorus are highly active phosphorus elements in the soil, which are easily absorbed and converted by crops, indicating that potassium dihydrogen phosphate added to slow-release fertilizers can be converted into a variety of beneficial forms, which is conducive to the full absorption and utilization of crops.

[0119] After the culture conditions were changed from high-moisture soil reaction conditions to low-moisture soil reaction conditions, the concentrations of hydrochloric acid-extractable phosphorus and available phosphorus decreased. On the 60th day of the total reaction (i.e., the 20th day after the reaction conditions were changed), the concentration of hydrochloric acid-extractable phosphorus decreased to 70.0 mg / kg, and the concentration of available phosphorus decreased to 175.6 mg / L. The decrease in the concentrations of hydrochloric acid-extractable phosphorus and available phosphorus is due to the sufficient oxygen content in the low-moisture soil environment and the regeneration of the dissolved iron mineral film in the high-moisture soil environment. On the one hand, it limits the further release of phosphorus, resulting in no more increase in phosphorus. On the other hand, the regeneration of the iron mineral film re-fixes part of the released phosphorus, resulting in a decrease in the phosphorus concentration.

[0120] In summary, the slow-release fertilizer in the present invention can accurately control the release of phosphorus in the soil environment by changing the moisture conditions, promote the release of nutrients in a high-moisture environment that is beneficial to plant nutrient absorption, and reduce the release of phosphorus in the soil environment in a low-moisture environment that is not conducive to plant nutrient absorption, and fix part of the released phosphorus to achieve the effect of nutrient preservation.

Claims

1. A slow-release fertilizer, characterized in that: The slow-release fertilizer is a core-shell structure, wherein the core is a porous carrier loaded with nutrient elements, and the core-shell is an iron mineral membrane layer.

2. The slow-release fertilizer according to claim 1, characterized in that The porous carrier is selected from at least one of diatomaceous earth, bentonite, zeolite, montmorillonite, expanded vermiculite, sepiolite or attapulgite; and / or, the particle size of the porous carrier is less than 50 μm; And / or, the water content of the porous carrier is 60%-80%.

3. The slow-release fertilizer according to claim 1, characterized in that The nutrient elements include at least one of essential nutrient elements and beneficial nutrient elements required for plant growth.

4. The slow-release fertilizer according to any one of claims 1 to 3, characterized in that: The slow-release fertilizer comprises the following preparation raw materials: porous carrier, ferrous salt, adhesive, microorganism activation induction auxiliary agent, nutrient element salt, water and alkali solution.

5. The slow-release fertilizer according to claim 4, characterized in that The adhesive is selected from at least one of chromatography silica gel and silica sol; And / or, the microorganism activation induction auxiliary agent is selected from at least one of glucose, sodium acetate and sodium lactate; And / or, the alkali source of the alkali solution includes alkali metal hydroxide, alkali metal carbonate, and alkali metal bicarbonate.

6. The slow-release fertilizer according to claim 4, characterized in that: The mass ratio of the porous carrier, adhesive, microbial activation induction auxiliary agent, nutrient element salt, ferrous salt and water is (400-600):(10-20):(20-30):(1-300):(40-60):1000.

7. The method for preparing the slow-release fertilizer according to any one of claims 4 to 6, characterized in that: The following steps are involved: S1, dissolving nutrient element salt in water, adding adhesive and microbial activation induction auxiliary agent, and then adding porous carriers in batches to obtain porous carriers loaded with nutrient elements; S2, adding ferrous salt, adding alkali solution to adjust the pH of the system to 6.8-7.2, separating the solid and liquid, collecting the solid phase, and obtaining the slow-release fertilizer.

8. Use of the slow-release fertilizer according to any one of claims 1 to 6 in soil improvement.

9. The use according to claim 8, characterized in that: The physical and chemical properties of the soil include at least one of the following: 1) pH 4-8; 2) Cation exchange capacity is 5-30 cmol / kg; 3) Total organic matter content is 10-50g / kg; 4) The total iron content is 50-300g / kg.

10. The use according to claim 8, characterized in that: The dosage of the slow-release fertilizer is 10-50 kg / mu.

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