A slow-release nano calcium fertilizer and its preparation method and application

Abstract

The present invention relates to the technical field of calcium fertilizer, and in particular to a slow-release nano calcium fertilizer and a preparation method and application thereof, wherein the slow-release nano calcium fertilizer comprises a core and a slow-release layer coated on the outer surface of the core, wherein the core comprises the following raw materials by mass: 35-40 parts of nano suspended calcium, 10-15 parts of superphosphate, 15-20 parts of urea, 2-5 parts of slow-release agent, and 5-10 parts of sugar alcohol, and the slow-release layer comprises the following raw materials by mass: 20-25 parts of chitosan, 25-35 parts of sodium alginate, 15-20 parts of starch, 5-10 parts of lignin, and 30-35 parts of mineral potassium humate. The core in the fertilizer contains nano-sized calcium, which is more easily adsorbed by the root hairs or root tips of plants, and the slow-release layer can control the release rate of the calcium fertilizer, so that it can reduce the loss and waste of nutrients while meeting the needs of crops.

Description

Technical Field

[0001] The invention relates to the technical field of calcium fertilizers, and in particular to a slow-release nano calcium fertilizer and a preparation method and application thereof. Background Art

[0002] Calcium is an essential nutrient for plant growth and development, and has an important impact on crop growth and development and fruit quality. Although the soil is rich in calcium, it has problems such as poor solubility, easy calcification and precipitation, and element antagonism, which greatly hinder the absorption and utilization of plant calcium nutrition.

[0003] In recent years, nanomaterials have been widely studied due to their small size, easy preparation, green safety, and good biocompatibility. Among them, nanofertilizer, as one of the new nanotools, provides a potential new method for delivering nutrients to plants and shows excellent application prospects in agricultural production.

[0004] Therefore, how to use nanofertilizer as a calcium carrier to increase the calcium content of crops and improve crop quality is an urgent problem to be solved. Summary of the invention

[0005] In view of the defects mentioned in the background technology, the present invention provides a slow-release nano calcium fertilizer and a preparation method and application thereof. By nano-forming calcium, it is easier to be absorbed and utilized by crop roots.

[0006] In order to achieve the above-mentioned object of the invention, the technical solution of the present invention is as follows:

[0007] A slow-release nano calcium fertilizer comprises a core and a slow-release layer coated on the outer surface of the core; the core comprises the following raw materials in parts by mass: 35-40 parts of nano suspended calcium, 10-15 parts of superphosphate, 15-20 parts of urea, 3-5 parts of 3,4-dimethylpyrazole phosphate, 3-7 parts of gamma-polyglutamic acid, 2-5 parts of slow-release agent, and 5-10 parts of sugar alcohol; the slow-release layer comprises the following raw materials in parts by mass: 20-25 parts of chitosan, 25-35 parts of sodium alginate, 15-20 parts of starch, 5-10 parts of lignin, 30-35 parts of mineral potassium humate, 8-15 parts of urea formaldehyde resin, 7-12 parts of polyurethane, and 8-12 parts of claw bean resin glue.

[0008] The present invention also provides a method for preparing a slow-release nano calcium fertilizer, comprising the following steps:

[0009] S1, add 35-40 parts of nano suspended calcium, 10-15 parts of superphosphate, 15-20 parts of urea, 2-5 parts of sustained release agent, 3-5 parts of 3,4-dimethylpyrazole phosphate, 3-7 parts of γ-polyglutamic acid, and 5-10 parts of sugar alcohol into mixer A, heat the internal temperature of mixer A to 60-70° C. through a steam coil, stir for 0.5-1h, and obtain a mixed slurry;

[0010] S2, add 20-25 parts of chitosan, 25-35 parts of sodium alginate, 15-20 parts of starch, 5-10 parts of lignin, 30-35 parts of potassium humate from mineral sources, 8-15 parts of urea formaldehyde resin, 7-12 parts of polyurethane, 8-12 parts of claw bean resin glue, and 60-80 parts of water into mixer B, heat the internal temperature of mixer B to 60-70°C through a steam coil, stir for 0.5-1h, and obtain a coating slurry;

[0011] S3, sending the mixed slurry into a granulator for spray granulation to obtain a core with a particle size of 2 to 3 mm;

[0012] S4, the prepared core and coating slurry are coated into film by a drum coating machine, and the coating slurry is evenly coated on the outer surface of the core to form a slow-release layer. After coating and film formation, a slow-release nano calcium fertilizer is obtained.

[0013] Preferably, the preparation method of the nano suspended calcium comprises the following steps:

[0014] S11, calcining calcite at high temperature to obtain calcium oxide, and mixing calcium oxide with water in a container at a weight ratio of 1:5 to obtain Ca(OH) 2 Serum;

[0015] S12, Ca(OH) 2 The slurry and crystal reaction controller are added to the reactor together, and CO is continuously introduced into the reactor. 2 , carbonizing at a temperature of 50 to 65° C. to obtain a non-crystalline nano-calcium carbonate slurry;

[0016] S13, adding the long-chain molecular amphiphilic polymer, the chelating agent, and the lignin into the reaction kettle in sequence and mixing them with the nano calcium carbonate slurry, and sending the mixed nano calcium carbonate slurry to an ultrasonic disperser for dispersion to obtain nano suspended calcium.

[0017] Preferably, in the step S4, the core is sent to the coating chamber of the drum coating machine, and the prepared coating slurry is evenly sprayed onto the surface of the core by the spraying system of the drum coating machine, and the spraying time is controlled at 3 to 5 minutes. After the spraying is completed, hot air at 70 to 80° C. is blown into the coating chamber, and the chamber is ventilated and cured for 5 to 10 minutes. After the curing is completed, the spraying system is started again, and the spraying and curing are repeated until the coating slurry is sprayed. After the coating slurry is sprayed, hot air at 60 to 65° C. is blown into the coating chamber, and the chamber is ventilated and cured for 30 to 35 minutes, and then cooled to room temperature to obtain a slow-release nano calcium fertilizer.

[0018] Preferably, the sustained-release agent is NBPT.

[0019] Preferably, the crystal reaction controller is a mixture of bisphosphonate, polyethylene glycol and sodium dodecylbenzene sulfonate, the weight ratio of bisphosphonate, polyethylene glycol and sodium dodecylbenzene sulfonate is 2:2:33, and the addition amount of the crystal reaction controller accounts for 1:1 of Ca(OH) 2 2-2.5% of the slurry weight.

[0020] Preferably, the long-chain molecular amphiphilic polymer is PEO-b-PS or PS-g-PAA, and the addition amount of the long-chain molecular amphiphilic polymer accounts for 1 to 1.5% of the weight of the nano-calcium carbonate slurry.

[0021] Preferably, the chelating agent is EDTA or DTPA, and the added amount of the chelating agent accounts for 1.5-2% of the weight of the nano-calcium carbonate slurry.

[0022] Preferably, the CO 2 The kiln gas is obtained by purifying the flue gas decomposed from calcined calcite. 2 The volume concentration is 25-40%.

[0023] The present invention also provides an application of a slow-release nano calcium fertilizer, comprising the following steps:

[0024] Mix air-dried livestock manure, slow-release nano-calcium fertilizer, fermented earthworm feces, fermented bacteria, humic acid, and fertilization-enhancing bacteria and spread them evenly into the soil. Use a rotary tiller to mix them with the tilled layer of soil. The application amount is 100 to 250 per mu.

[0025] After adopting the above technical solution, the beneficial effects of the present invention are:

[0026] The core of the fertilizer contains nano-calcium, which has a large surface area and many surface active atoms. The unsaturated atoms have greater binding energy, are not easily calcified, and are more easily adsorbed by the root hairs or root tips of plants. The slow-release layer can control the release rate of calcium fertilizer, so that it can reduce nutrient loss and waste while meeting crop needs. DETAILED DESCRIPTION

[0027] The features and exemplary embodiments of various aspects of the present invention will be described in detail below. In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only configured to explain the present invention and are not configured to limit the present invention. For those skilled in the art, the present invention can be implemented without the need for some of these specific details. The following description of the embodiments is only to provide a better understanding of the present invention by illustrating examples of the present invention.

[0028] Example 1

[0029] A method for preparing a slow-release nano calcium fertilizer comprises the following steps:

[0030] Step 1: Preparation of mixed slurry

[0031] According to the specified weight proportions, 35 parts of nano-suspended calcium, 10 parts of superphosphate, 15 parts of urea, 2 parts of NBPT, 3 parts of γ-polyglutamic acid, 3 parts of 3,4-dimethylpyrazole phosphate and 5 parts of sugar alcohol are accurately weighed, and these raw materials are put into mixer A together, and the stirring device of mixer A is started to stir. While stirring, mixer A is heated to 60-70°C by a steam coil. After 0.5-1 hour of mixing and stirring, a uniform and fine mixed slurry is obtained.

[0032] Nano-suspended calcium can provide the calcium required by crops. Nanoparticles have a large specific surface area, which can enhance the contact area with plant roots or soil, promote the rapid dissolution and penetration of nutrients, and can be more efficiently absorbed and utilized by plants, thereby improving the utilization rate of fertilizers.

[0033] Superphosphate provides plants with phosphorus, which plays an important role in plant root development, flower bud differentiation and fruit ripening.

[0034] Urea, as a nitrogen source in fertilizers, provides plants with the necessary nitrogen elements to support the growth of green leaves and protein synthesis of plants.

[0035] NBPT, the full name is N-(n-butyl) thiophosphoric triamide. It is used as a slow-release agent to improve the utilization rate of urea. It slows down the decomposition of urea into ammonia and carbon dioxide by inhibiting the urease activity in the soil, thereby extending the fertilizer effect period of urea, reducing ammonia volatilization losses, and improving the utilization rate of nitrogen fertilizer.

[0036] γ-Polyglutamic acid can adjust the pH value, effectively balance the soil pH value, and avoid soil acidification, salinization and hardening caused by long-term use of chemical fertilizers. γ-Polyglutamic acid also has multiple anions, which can block sulfate, phosphate and other trace elements such as calcium and magnesium from combining to form low-soluble salts, thereby improving fertilizer utilization.

[0037] 3,4-Dimethylpyrazole phosphate is a nitrification inhibitor. Its role in fertilizer is mainly to inhibit the activity of nitrifying bacteria in the soil, thereby slowing down the growth of ammonium nitrogen (NH 4 + ) to nitrate nitrogen (NO 3 - ) conversion process, prolonging the retention time of ammonium nitrogen in the soil, allowing plants to have more time to absorb and utilize nitrogen, thereby improving the utilization rate of nitrogen fertilizer. At the same time, 3,4-dimethylpyrazole phosphate can reduce nitrogen volatilization losses and nitrogen oxide emissions.

[0038] As a natural nutrient osmotic agent, sugar alcohol can improve the permeability of plant cell membranes and promote the absorption of calcium by plants. Sugar alcohol can also enhance the resistance of plants to stress, such as drought and cold, and improve the overall growth of plants.

[0039] Step 2: Preparation of coating slurry

[0040] According to the specified weight proportions, accurately weigh 20 parts of chitosan, 25 parts of sodium alginate, 15 parts of starch, 5 parts of lignin, 30 parts of potassium humate from mineral sources, 8 parts of urea formaldehyde resin, 7 parts of polyurethane, 8 parts of claw bean resin glue and 60 parts of water, put these raw materials into mixer B, start the stirring device of mixer B for stirring. While stirring, heat mixer B to 60-70°C through a steam coil. After 0.5-1 hour of mixing and stirring, a uniform and delicate coating slurry is obtained.

[0041] Urea formaldehyde resin and claw bean resin glue have good bonding properties and can be tightly combined with chitosan, sodium alginate, starch, etc. to form a solid coating layer. This coating layer can effectively wrap fertilizer particles, prevent the rapid release of nutrients, and achieve a slow release or controlled release effect of fertilizers. Urea formaldehyde resin will release a certain amount of nitrogen and other organic substances during the decomposition process. These substances can increase the organic matter content in the soil, improve the soil's ability to retain water and fertilizer, improve soil structure, and promote soil microbial activity, which is beneficial to the growth and development of crops.

[0042] Polyurethane coating can reduce the volatilization and leaching of nitrogen, phosphorus and other elements in fertilizers, and is biodegradable, thereby reducing pollution to groundwater and the surrounding environment. At the same time, polyurethane coating can be designed to have specific water permeability and air permeability to control the release rate of nutrients in fertilizers. This controlled release property helps to reduce the initial loss of nutrients and ensure a continuous supply of nutrients during the crop growing season.

[0043] Humic acid generated by the degradation of lignin in the soil can improve soil permeability, prevent soil compaction, and improve the soil's ability to retain water and fertilizer. At the same time, the three-dimensional network structure of lignin can absorb free calcium that is not absorbed by crops, and gradually release calcium as it degrades, thereby extending the effective period of calcium fertilizer.

[0044] Mineral potassium humate can provide a variety of trace elements required by plants. Its chelating ability can improve the effectiveness of nutrients in fertilizers, and the absorption and utilization rate of nutrients such as nitrogen, phosphorus, and potassium can reach more than 50%, thereby greatly enhancing the nutrition of plants themselves. Mineral potassium humate is alkaline, which can neutralize excess acidic substances in the soil, adjust the soil pH, and provide a suitable growth environment for crops.

[0045] Step 3: Core spraying granulation

[0046] The mixed slurry is sent to the granulator for spray granulation. During the spray granulation process, the mixed slurry is evenly sprayed into the granulator. As the granulator rotates or vibrates, the water in the slurry gradually evaporates, and the material particles gradually condense and form under the action of mechanical force to obtain a core with a particle size of 2 to 3 mm.

[0047] Step 4: Coating film

[0048] In the drum coating machine, the weight ratio of coating slurry to core is 3:7 (i.e., coating slurry accounts for 30% and core accounts for 70%). The cores with a particle size of 2-3 mm are sent into the coating chamber of the drum coating machine. The prepared coating slurry is evenly sprayed onto the surface of the core through the spraying system of the drum coating machine. The spraying time is controlled within 3-5 minutes to ensure that the slurry can fully wet the surface of the core and initially form a coating layer.

[0049] After spraying, hot air at 70-80℃ is blown into the coating room, and the ventilation curing time is controlled at 5-10 minutes. The function of hot air is to accelerate the evaporation of water in the coating slurry and promote the initial curing of the coating layer. After the initial spraying and curing, the spray system is started again to spray the core for the second time. Repeat the above spraying and curing process until the coating slurry is completely sprayed.

[0050] After the coating slurry is sprayed, hot air at 60-65°C is blown into the coating room for final ventilation and curing. This curing time is relatively long, generally 30-35 minutes, to ensure that the coating layer is completely cured and has a certain mechanical strength. After the curing is completed, the product is allowed to cool naturally to room temperature to obtain a slow-release nano calcium fertilizer.

[0051] Chitosan and sodium alginate have excellent film-forming properties. Under the action of starch as an adhesive, the three can be well coated with the core, forming one or more slow-release layers on the surface of the core. This slow-release layer can control the release rate of nutrients in calcium fertilizer, achieve long-term slow release, and improve the utilization rate of fertilizers and the absorption efficiency of crops.

[0052] Example 2

[0053] The difference from Example 1 is that:

[0054] In mixer A, 37 parts of nano-suspended calcium, 12 parts of superphosphate, 16 parts of urea, 2.5 parts of NBPT, 3.5 parts of 3,4-dimethylpyrazole phosphate, 4 parts of γ-polyglutamic acid, and 6 parts of sugar alcohol;

[0055] In mixer B, 22 parts of chitosan, 30 parts of sodium alginate, 18 parts of starch, 10 parts of urea formaldehyde resin, 8 parts of polyurethane, 8.5 parts of claw bean resin glue, 8 parts of lignin, 32 parts of mineral potassium humate and 65 parts of water;

[0056] In the rotary drum coating machine, the weight ratio of coating slurry to core is 1:3 (i.e., coating slurry accounts for 25% and core accounts for 75%).

[0057] Example 3

[0058] The difference from Example 1 is that:

[0059] In mixer A, 38.5 parts of nano-suspended calcium, 14 parts of superphosphate, 16.5 parts of urea, 4 parts of NBPT, 4.5 parts of 3,4-dimethylpyrazole phosphate, 5 parts of γ-polyglutamic acid, and 8 parts of sugar alcohol;

[0060] In mixer B, 24 parts of chitosan, 32 parts of sodium alginate, 18.5 parts of starch, 12 parts of urea formaldehyde resin, 10 parts of polyurethane, 10 parts of claw bean resin glue, 8.5 parts of lignin, 34 parts of mineral potassium humate and 70 parts of water;

[0061] In the drum coating machine, the weight ratio of coating slurry to core is 1:4 (i.e., coating slurry accounts for 20% and core accounts for 80%).

[0062] Example 4

[0063] The difference from Example 1 is that:

[0064] In mixer A, 40 parts of nano-suspended calcium, 15 parts of superphosphate, 20 parts of urea, 5 parts of NBPT, 5 parts of 3,4-dimethylpyrazole phosphate, 7 parts of γ-polyglutamic acid and 10 parts of sugar alcohol;

[0065] In mixer B, 25 parts of chitosan, 35 parts of sodium alginate, 20 parts of starch, 15 parts of urea formaldehyde resin, 12 parts of polyurethane, 12 parts of claw bean resin glue, 10 parts of lignin, 35 parts of mineral potassium humate and 80 parts of water;

[0066] In the drum coating machine, the weight ratio of coating slurry to core is 3:17 (i.e., coating slurry accounts for 15% and core accounts for 85%).

[0067] In the above embodiments, the preparation method of the nano suspended calcium comprises the following steps:

[0068] Step 1: Preparation of Ca(OH) 2 Serum

[0069] Calcining calcite (mainly calcium carbonate) at high temperature, the temperature is controlled at 850-950℃, calcite decomposes at high temperature to generate calcium oxide and carbon dioxide, the obtained calcium oxide and water are mixed in a container at a weight ratio of 1:5 to obtain Ca(OH) 2 Serum.

[0070] Step 2: Carbonization reaction

[0071] Ca(OH) 2 The slurry and crystal reaction controller are added to the reactor together. The amount of crystal reaction controller added accounts for 10% of Ca(OH) 2 The solid content of the slurry is 5-10% on a dry basis. Start the stirring device and continuously introduce CO into the reactor. 2 , CO 2 The flow rate into the reactor is 3m³ / h.

[0072] In order to improve resource utilization, CO 2 The source of the kiln gas can be the flue gas decomposed from the calcined calcite and purified to obtain the kiln gas, but it is necessary to ensure that the CO 2 The volume concentration is 25-40%.

[0073] The raw materials are carbonized at a temperature of 50-65°C. When the pH monitoring system measures the pH of the slurry to be ≤8.5, the kiln gas or CO is stopped. 2 , obtaining a non-crystalline nano calcium carbonate slurry.

[0074] The crystal reaction controller is a mixture of bisphosphonate, polyethylene glycol and sodium dodecylbenzene sulfonate, and the weight ratio of bisphosphonate, polyethylene glycol and sodium dodecylbenzene sulfonate is 2:2:3.

[0075] Nano calcium carbonate in non-crystalline form is also called amorphous nano calcium carbonate. Amorphous nano calcium carbonate can significantly promote the growth and development of crops, increase the dry matter yield of crops, and improve the growth conditions of crops due to its advantages such as small particles, large specific surface area, high solubility and good biocompatibility.

[0076] Step 3: Modification

[0077] The long-chain molecular amphiphilic polymer, chelating agent and lignin are sequentially added into a reaction kettle and mixed with the nano calcium carbonate slurry. The mixed nano calcium carbonate slurry is sent to an ultrasonic disperser for dispersion to obtain nano suspended calcium.

[0078] The amount of lignin added is 2-2.5% of the weight of the nano calcium carbonate slurry. The long-chain molecular amphiphilic polymer is PEO-b-PS (polyethylene oxide-b-polystyrene) or PS-g-PAA (polystyrene-graft-polyacrylic acid), and its amount of addition is 1-1.5% of the weight of the nano calcium carbonate slurry. The chelating agent is EDTA or DTPA, and its amount of addition is 1.5-2% of the weight of the nano calcium carbonate slurry.

[0079] Prepare 6 flower pots filled with equal amounts of soil, sow 2-3 tomato seeds in each flower pot, and sow 1-2 cm below the soil surface. Take samples of the finished products of Examples 1-4, and apply equal amounts of slow-release nano calcium fertilizer to each group of flower pots, four of which are experimental groups, and the remaining two groups are control groups (the control group is applied with ordinary fertilizer).

[0080] Use a watering tool to water all the pots thoroughly, keep the soil moist but not too wet, and all pots receive the same light time and intensity. After all the seeds have germinated, observe the growth of the tomatoes in each pot. Pull out the weaker or overcrowded seedlings to ensure that only one of the strongest tomato plants is retained in each pot. During the growth of the tomatoes, continue to observe their growth (for a period of 120 days), and measure growth parameters including plant height, stem diameter, germination period (the process from seed sowing to the first true leaf breaking through the heart), seedling period (from the first true leaf breaking through the heart to the beginning of large buds), and fruit setting period (from the appearance of large buds to the first bunch of fruits setting). The results are as follows:

[0081]

[0082] As can be seen from the table, the yield per plant, plant height, stem diameter, etc. of the plants treated with slow-release nano-calcium fertilizer were significantly better than those of the control group.

[0083] An embodiment of the present invention also provides an application of a slow-release nano-calcium fertilizer in soil improvement. Air-dried livestock manure, slow-release nano-calcium fertilizer, fermented earthworm feces, fermentation bacteria, humic acid, and fertilization and synergistic bacteria are mixed in a weight ratio of 80:48:40:1:30:1 and evenly spread into the soil. The fermentation bacteria are a composite bacterial powder of Bacillus subtilis, Aspergillus niger and Streptomyces tenuifolius, and the fertilization and synergistic bacteria are a composite powder of lactic acid bacteria, acetic acid bacteria, Bifidobacterium, Actinomycetemcomitans and photosynthetic bacteria.

[0084] The above raw materials (slow-release nano calcium fertilizer is prepared by the method of Example 1) are mixed with the tillage layer soil with a rotary tiller, and the application amount is 100-250 kg / mu. The results are as follows:

[0085]

[0086] As can be seen from the table above, the soil conductivity has been significantly reduced and the organic matter content has been greatly increased. When the soil conductivity is too high, it means that the salt content in the soil may exceed the standard, leading to crop physiological disorders, such as obstructed water and nutrient absorption, which in turn affects crop yields. This application reduces the salt content of the soil, thereby reducing the conductivity of the soil. At the same time, the increase in organic matter content also indicates that the soil quality has been improved. This improvement helps crops grow healthily, improves the efficiency of water and nutrient absorption of crops, and thus promotes an increase in crop yields.

[0087] According to the embodiments of the present invention as described above, these embodiments do not describe all the details in detail, nor do they limit the invention to specific embodiments. Obviously, many modifications and changes can be made based on the above description. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can make good use of the present invention and the modified use based on the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a slow-release nano calcium fertilizer, characterized in that: The steps include: S1. According to the weight ratio, 35-40 parts of nano suspended calcium, 10-15 parts of superphosphate, 15-20 parts of urea, 3-5 parts of 3,4-dimethylpyrazole phosphate, 2-5 parts of NBPT, 5-10 parts of sugar alcohol, and 3-7 parts of γ-polyglutamic acid are added to the mixer A, wherein the nano suspended calcium is in a non-crystalline form. The internal temperature of the mixer A is heated to 60-70° C. by a steam coil, and stirred for 0.5-1 h to obtain a mixed slurry; S2, according to the weight parts, add 20-25 parts of chitosan, 25-35 parts of sodium alginate, 15-20 parts of starch, 5-10 parts of lignin, 30-35 parts of mineral potassium humate, 8-15 parts of urea formaldehyde resin, 7-12 parts of polyurethane, 8-12 parts of claw bean resin glue, and 60-80 parts of water into the mixer B, heat the internal temperature of the mixer B to 60-70 ° C through the steam coil, stir for 0.5-1h, and obtain the coating slurry; S3, sending the mixed slurry into a granulator for spray granulation to obtain a core with a particle size of 2 to 3 mm; S4, the core and the coating slurry are sent to the drum coating machine according to the weight ratio of 7:

3. In the coating chamber of the drum coating machine, the prepared coating slurry is evenly sprayed onto the surface of the core through the spraying system of the drum coating machine. The spraying time is controlled within 3 to 5 minutes. After the spraying is completed, hot air at 70 to 80° C. is blown into the coating chamber. The ventilation curing time is controlled within 5 to 10 minutes. The above spraying and curing process is repeated until the coating slurry is completely sprayed. S5. After the coating slurry is sprayed, hot air at 60-65° C. is blown into the coating chamber for ventilation and curing. The curing time is controlled at 30-35 minutes. After the curing is completed, it is naturally cooled to room temperature to obtain a slow-release nano calcium fertilizer. In step S1, the method for preparing nano suspended calcium comprises the following steps: S11, calcining calcite at high temperature to obtain calcium oxide, and mixing the calcium oxide with water in a container at a weight ratio of 1:5 to obtain a Ca(OH)2 slurry; S12, adding Ca(OH)2 slurry and a crystal reaction controller into a reactor, continuously introducing CO2 into the reactor, and carbonizing at a temperature of 50 to 65° C. to obtain a non-crystalline nano calcium carbonate slurry; The crystal reaction controller is a mixture of bisphosphonate, polyethylene glycol and sodium dodecylbenzene sulfonate, the weight ratio of the bisphosphonate, polyethylene glycol and sodium dodecylbenzene sulfonate is 2:2:3, and the addition amount of the crystal reaction controller accounts for 2-2.5% of the weight of the Ca(OH)2 slurry; S13, adding a long-chain molecular amphiphilic polymer, a chelating agent, and lignin to a reactor in sequence and mixing with the nano calcium carbonate slurry, and sending the mixed nano calcium carbonate slurry to an ultrasonic disperser for dispersion to obtain a non-crystalline nano suspended calcium; The long-chain molecular amphiphilic polymer is PEO-b-PS or PS-g-PAA, and the addition amount of the long-chain molecular amphiphilic polymer accounts for 1-1.5% of the weight of the nano calcium carbonate slurry.

2. The method for preparing the slow-release nano calcium fertilizer according to claim 1, characterized in that: The chelating agent is EDTA or DTPA, and the added amount of the chelating agent accounts for 1.5-2% of the weight of the nano calcium carbonate slurry.

3. The method for preparing the slow-release nano calcium fertilizer according to claim 1, characterized in that: The CO2 is kiln gas obtained by purifying flue gas decomposed from calcined calcite, and the volume concentration of CO2 in the kiln gas is 25-40%.

4. An application of the slow-release nano calcium fertilizer prepared by the preparation method as claimed in claim 1 in soil improvement, characterized in that: The method comprises the following steps: mixing air-dried livestock manure, slow-release nano calcium fertilizer, fermented earthworm manure, fermented bacteria, humic acid and fertilization enhancing bacteria and spreading the mixture evenly into the soil, and mixing the mixture evenly with the tillage layer soil with a rotary tiller, with an application amount of 100-250kg / mu.

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