Coating method and application of hydroxy fatty acid-containing compound fertilizer

Abstract

The invention provides a coating method and application of a hydroxy fatty acid-containing compound fertilizer. Comprising the following steps: preparing a solution from a mixture of medium-chain potassium 3-hydroxyaliphatate, long-chain potassium aliphatate and sodium alginate, and spraying the solution on the surfaces of compound fertilizer particles to form a pre-coating layer; then spraying a metal salt solution containing CaCl2 or MgCl2, reacting and drying under the condition of 40-60 DEG C, so that the mixed potassium aliphatate is converted into Ca / Mg metal soap in situ on the surfaces of the particles, thereby forming a hydrophobic metal soap film layer. The obtained film layer is insoluble in the initial stage, is resistant to water corrosion and can effectively delay dissolution of nutritional ingredients in the compound fertilizer; after application, the metal soap film is exposed in a microorganism or CO2 environment, and the metal soap film is gradually degraded, so that the controlled release of the fertilizer and the coating film layer is realized. The method is convenient to use and suitable for large-scale production of the compound fertilizer. Medium-chain 3-hydroxy fatty acid has a unique effect of promoting growth of underground rhizomes, and the fatty acid metal soap coated compound fertilizer can be used for promoting growth of underground rhizome crops so as to achieve the purpose of increasing production and income.

Description

Technical Field

[0001] This invention belongs to the field of agricultural technology, specifically relating to a method for coating compound fertilizers containing hydroxy fatty acids and its application. Background Technology

[0002] Compound fertilizers, as commonly used high-efficiency fertilizers in agricultural production, are widely applied in the cultivation of grains, vegetables, and cash crops. Traditional compound fertilizers dissolve easily after application, releasing nutrients rapidly. The large release of nitrogen, phosphorus, potassium, and other nutrients in a short period often exceeds the crop's absorption capacity, leading not only to low fertilizer utilization but also potential environmental problems such as nutrient loss and groundwater pollution. Therefore, various slow-release or controlled-release compound fertilizer preparation technologies have been developed, among which surface coating is widely used due to its simple process and wide applicability. The coating layer of slow-release fertilizers mainly uses inorganic materials, organic polymers, and biodegradable materials. While polymers such as polyurethane, epoxy resin, and polylactic acid can effectively delay nutrient release, their raw material costs are high, their preparation processes are complex, and some materials are difficult to completely degrade in soil, posing environmental residue problems. Inorganic coating materials (such as sulfur or phosphates) have drawbacks such as easy cracking of the coating layer and unstable slow-release effects. In recent years, the preparation of biodegradable coatings from natural fatty acids or their salts has been found to be feasible and environmentally friendly. However, like other coating materials, they lack functional effects on plant growth.

[0003] In recent years, some polysaccharide and amino acid compounds have been widely used to regulate plant growth and resist abiotic stress, such as trehalose, fucoidan, γ-aminobutyric acid, 5-aminolevulinic acid, and γ-polyglutamic acid. Polysaccharide and amino acid compounds belong to the category of biostimulants, and their effects are milder and less likely to cause phytotoxicity. Since they are not biological pesticides, they can be used as additives in compound fertilizers to enhance fertilizer functionality. However, these biostimulants primarily promote the growth of the above-ground parts of crops. Unlike leafy and fruit-bearing crops, high yields of root-bearing crops (such as radishes, sweet potatoes, and potatoes) require nutrient transport to the underground parts to prevent excessive vegetative growth. While some pesticide formulations can be used to control excessive growth, such as paclobutrazol and calcium cyclohexane, their mechanisms of action are simple and prone to causing phytotoxicity. Biostimulants (non-pesticide) targeting the enlargement of underground roots and tubers are still relatively scarce.

[0004] Compared to polysaccharides and amino acid compounds, research on fatty acid biostimulants is relatively limited, and their mechanisms of action remain unclear. Medium-chain 3-hydroxy fatty acids (MCLAs) are a class of compounds widely found in nature, derived from microbial fermentation products or insect secretions, and are generally considered to be beneficial to both humans and the environment. Studies have found that extremely low concentrations (<1 mg / L) of MLAs can induce an immune response in Arabidopsis thaliana, but there are no reports of them promoting the enlargement of underground rhizomes, and they have not yet been used in compound fertilizer coating materials.

[0005] The references mentioned above are as follows: 1. Passos, MF et al. J MATE POLY SCI, 4(4):1-14. 2024; 2. Kutschera et al. Science 364, 178–181, 2019. Summary of the Invention

[0006] This invention aims to overcome the problems of existing compound fertilizer coating slow-release technology, such as the lack of plant growth regulation function of the coating layer, complex process, high cost, and insufficient biodegradability, and provides a compound fertilizer coating method containing hydroxy fatty acids and its application.

[0007] This method utilizes the combination of medium-chain 3-hydroxy fatty acid potassium, long-chain fatty acid potassium, and sodium alginate, along with a metal ion cross-linking reaction, to form a dense and hydrophobic metal soap film layer in situ on the surface of compound fertilizer granules, achieving controlled release of fertilizer nutrients. Simultaneously, medium-chain 3-hydroxy fatty acids have a physiological effect of promoting root and stem development, meaning that the coated fertilizer not only possesses slow-release properties but also significantly promotes the expansion and yield increase of underground root and stem crops.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: This invention provides a method for coating compound fertilizers containing hydroxy fatty acids, comprising the following steps: (1) Potassium 3-hydroxy fatty acids of medium chain and potassium 3-chain fatty acids are mixed at a mass ratio of 1:1 to 9 to form a mixed potassium fatty acid mixture, which is then prepared into an aqueous solution with sodium alginate; the mass fraction of the mixed potassium fatty acid mixture in the aqueous solution is 10 to 30%, and the mass fraction of sodium alginate in the aqueous solution is 1 to 5%. (2) Spray the aqueous solution onto the surface of the compound fertilizer granules to form a pre-coating; (3) Then spray the surface of the compound fertilizer granules with a metal salt solution containing CaCl2 with a molar concentration of 0.1~1.0 mol / L and / or MgCl2 with a molar concentration of 0.1–1.0 mol / L, so that the mixed fatty acid potassium / sodium alginate forms a metal ion cross-linking film. (4) The sprayed compound fertilizer granules are dried at 40–60 ℃ for 60 min and then cured to prepare coated compound fertilizer granules with a metal soap film layer.

[0009] Preferably, in step (1), the medium-chain 3-hydroxy fatty acid potassium is one or more of potassium 3-hydroxyoctanoate, potassium 3-hydroxydecanoate, or potassium 3-hydroxylaurate.

[0010] Preferably, in step (1), the long-chain fatty acid potassium is one or more of potassium oleate, potassium linoleate, potassium palmitate or potassium stearate.

[0011] Preferably, in step (2), the spraying amount is 2-10 kg of mixed fatty acid potassium per ton of compound fertilizer.

[0012] Preferably, in step (3), the metal salt solution contains CaCl2 with a molar concentration of 0.3-0.6 mol / L and / or MgCl2 with a molar concentration of 0.3-0.6 mol / L.

[0013] The present invention also provides a compound fertilizer prepared according to the above method.

[0014] The present invention also provides an application of the above-mentioned compound fertilizer in the cultivation of underground root and tuber crops.

[0015] This coating method is suitable for preparing slow-release compound fertilizers for underground root crops (including radishes, onions, sweet potatoes, potatoes, etc.), which can effectively improve fertilizer utilization, extend the nutrient release cycle, reduce nutrient loss, and promote root and stem enlargement and yield increase.

[0016] Compared with the prior art, the present invention has the following significant advantages: 1. The coating reaction of this invention occurs directly on the surface of fertilizer granules, without the need for additional polymeric film-forming agents or complex polymerization processes, simplifying the process and facilitating industrial implementation. The coating reaction is carried out directly on the surface of fertilizer granules by medium-chain 3-hydroxy fatty acid potassium and long-chain fatty acid potassium in Ca... 2+ / Mg 2+ The metallic soap film formed under these conditions possesses excellent hydrophobicity and structural stability, effectively preventing water penetration and nutrient ion diffusion, thereby achieving slow nutrient release. Sodium alginate and Ca... 2+ / Mg 2+ The insoluble gel formed during the process increases the flexibility and hydrophilicity of the metal soap film, making it less prone to breakage. The metal soap film can be gradually absorbed by microorganisms and plant roots in the soil environment, avoiding environmental residues and exhibiting good ecological compatibility. This method only requires conventional fertilizer coating equipment, operates under mild conditions, uses widely available raw materials, and has low cost, showing promising prospects for widespread application.

[0017] 2. Medium-chain 3-hydroxy fatty acids can regulate root growth and enhance the expansion capacity of underground parts, giving coated fertilizers a dual function of "slow release + root promotion." Hydroxy fatty acids have a special and significant effect on increasing the yield of radishes, potatoes, and sweet potatoes. The mechanism lies in the fact that medium-chain hydroxy fatty acids can promote the secretion of auxins by crops and gently inhibit gibberellins, thus having a certain effect on controlling excessive growth in the above-ground parts and directing nutrient supply towards the underground parts. Compared with chemically synthesized pesticide growth regulators, hydroxy fatty acids and long-chain fatty acids are natural fermentation products, containing no pesticide components, making them safe for humans and avoiding the toxicity and residue problems of chemical pesticides. Attached Figure Description

[0018] Figure 1 The graph shows the results of using compound fertilizer coated with hydroxy fatty acids as a base fertilizer to increase potato yield.

[0019] Figure 2 The results of using compound fertilizer coated with hydroxy fatty acids as base fertilizer to increase the yield of cherry radishes are shown in the figure. Detailed Implementation

[0020] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following embodiments are only for illustrating the present invention and should not be regarded as limiting the scope of the present invention.

[0021] The medium-chain hydroxy fatty acids used in the examples are all available from conventional reagent manufacturers: 3-hydroxydecanoic acid (CAS: 5561-87-5); 3-hydroxyoctanoic acid (CAS: 14292-27-4); and 3-hydroxylauric acid (CAS: 1883-13-2). The potassium salts of these fatty acids can be prepared by a simple neutralization method (dissolving the fatty acid in water at a 1:1 molar ratio with KOH, stirring, and then drying), or commercially available potassium salts of these fatty acids can be purchased directly. All other compounds are conventional commercial products. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer should be followed. Reagents or instruments used in the examples, unless otherwise specified by the manufacturer, are all conventional products that can be purchased commercially.

[0022] Example 1: Preparation of Mixed Fatty Acid Coated Compound Fertilizer Medium-chain 3-hydroxy fatty acid potassium and long-chain fatty acid potassium were mixed and dissolved in water, and then sodium alginate was added and stirred until dissolved. The proportions of each component are shown in Table 1, and spray solution I was prepared. Spray solution II was prepared by mixing CaCl2 and MgCl2 solutions with a molar volume ratio of 1:1 (each concentration of 0.3 mol / L).

[0023] Table 1. Raw material ratio of the coating solution

[0024] Subsequently, 10 kg of compound fertilizer granules (commercially available balanced fertilizer with N:P:K = 17:17:17, granule diameter 2-4 mm) were placed in a small horizontal drum coating machine and sprayed evenly with the above-mentioned mixed fatty acid potassium solution (spray solution I) at a speed of 30 rpm and a spraying rate of 10 mL / min to form a uniform wet film on the fertilizer surface. Then, while the drum was running, a CaCl2 / MgCl2 mixed solution (spray solution II) was sprayed at a spraying rate of 20 mL / min to allow the metal ions to undergo an in-situ saponification and cross-linking reaction with the carboxyl groups in the potassium fatty acid / sodium alginate.

[0025] The coated compound fertilizer granules are transferred to a hot air drying device and dried at 50 ℃ for 60 minutes to obtain compound fertilizer granules coated with a metallic soap film layer on the surface. The resulting product has a smooth appearance and uniform color.

[0026] Example 2: Comparison of coating effects under different fatty acid ratios The six coated compound fertilizers obtained in Example 1 (denoted as Compound Fertilizer 1#-5#, Comparative Example 1#) were subjected to a water-soluble nutrient release experiment (25 ℃ water bath shaking method), with the uncoated compound fertilizer as Comparative Example 2#. Specifically, 20 g of fertilizer sample was placed in a constant temperature water bath (25 ℃), 100 g of deionized water was added, and the mixture was continuously shaken. Samples were removed on days 3, 7, and 21, filtered or dried, and weighed to calculate the mass loss. The water-soluble nutrient release rate was expressed as the ratio of the difference between the initial mass and the remaining mass of the sample to the total initial nutrient content. The nutrient release rates of Compound Fertilizers 1#-5# are shown in Table 2. Table 2. Water-soluble nutrient release rate of compound fertilizer

[0027] Table 2 shows that among the compound fertilizers containing hydroxy fatty acids (1-5#), compound fertilizer #5# has the densest film layer and the most stable slow-release curve, indicating that increasing the long-chain component can improve hydrophobicity and slow-release properties. Comparative Example #1# contains only potassium oleate and lacks hydroxy fatty acids, resulting in even stronger film density and slower degradation. Comparative Example #2# has no coating layer and therefore no slow-release properties; nutrients are quickly and completely released.

[0028] Example 3: Comparison of different metal ion systems The optimal fatty acid ratio of formula 5# in the examples was adopted. At the same time, the three systems of CaCl2, MgCl2 and CaCl2+MgCl2 were compared. The total concentration in the system was 0.6 mol / L. In the CaCl2+MgCl2 system, the working concentration of CaCl2 was 0.3 mol / L and the working concentration of MgCl2 was 0.3 mol / L. The coating method described in Example 1 was used to prepare three coated compound fertilizers with different ion systems.

[0029] The results of implementing the water-soluble nutrient release experiment method described in section 2 are shown in Table 3.

[0030] Table 3. Water-soluble nutrient release rate of compound fertilizer

[0031] The results showed that: Ca alone 2+ The system is highly water-resistant but brittle, and the film layer is easily broken and detached, with a 3-day release rate of 45%; Mg alone 2+ The membrane layer of the system is flexible but porous, with poor hydrophobicity, and a release rate of 55% after 3 days; Ca 2+ / Mg 2+ The system (molar ratio 1:1) forms the most uniform film layer, has the best balance between sustained release and degradation performance, and has a 3-day release rate of 20% (consistent with the results measured in Example 2).

[0032] Example 4: Industrial Scale-up Test Based on the nutrient release rate results of Examples 2 and 3, Formula 5# showed the best slow-release effect. Therefore, it was selected as the optimal formula for preparing mixed fatty acid coated compound fertilizer and industrial scale-up experiments were conducted.

[0033] Formula 5# was applied to an industrial roller coating line for 1000 kg compound fertilizer, and operated under the following conditions: roller diameter 1.5 m, rotation speed 25 rpm; 45.35 kg of a mixed solution of potassium fatty acid / sodium alginate (spray solution I, prepared by adding 1 kg of potassium 3-hydroxydecanoate, 9 kg of potassium oleate, 0.35 kg of sodium alginate, and 35 L of water) was sprayed; 30 L of a mixed solution of CaCl2 / MgCl2 (spray solution II) was used, and the solution was sprayed evenly onto the surface of the rolling compound fertilizer using atomizing nozzles; after spraying, the fertilizer was dried with hot air at 45 ℃ for 90 min.

[0034] The results showed that the coating was uniform and strong, and could be continuously and stably applied, meeting the requirements for industrial slow-release compound fertilizer production. Water-soluble nutrient release experiments showed a release rate of 19% after 3 days and 42% after 7 days, which was close to the results of the small-scale test.

[0035] Example 5: Using mixed fatty acid-coated compound fertilizer as base fertilizer to increase potato yield. Experiment location: Fuyang District, Hangzhou City, Zhejiang Province Experimental period: End of January 2025, greenhouse cultivation Potato variety: Small potatoes saved from local farmers' own seeds Experimental agents: Compound fertilizers #4 and #5 prepared in Example 1 were used, with coated compound fertilizer (comparative example 1) and uncoated compound fertilizer (comparative example 2) serving as comparative examples. The compound fertilizer was applied as base fertilizer in furrows at a rate of 100 kg / mu. Additionally, 1000 kg / mu of organic fertilizer was added and evenly spread and covered with a thin layer of soil as base fertilizer.

[0036] Processing area: Each treatment area consists of 3 cells, each cell is 10 square meters. The cells are spaced 0.5 meters apart.

[0037] Treatment method: After cutting the seed potatoes into pieces, plant one potato per row with a spacing of 50 cm and a plant spacing of 30 cm. Perform routine field management.

[0038] Results Calculation: In early April 2025, 2 m were randomly selected from each community. 2 The number of newly grown potatoes in each region was counted and weighed. The results are shown in Table 4.

[0039] Table 4 Potato yield of each group

[0040] Statistical analysis: a, b, c, d represent differences between groups, P < 0.05.

[0041] As shown in Table 3, using compound fertilizer coated with mixed fatty acids as base fertilizer significantly increased potato yield, with an increase rate between 17.7% and 25.0%. It also increased the proportion of potatoes over 100 g, indicating its ability to promote potato enlargement. Comparative Example 1 shows that while fatty acid coating alone (without hydroxy fatty acids) also had a certain yield-increasing effect, it was far less than that of compound fertilizer groups 4 and 5, demonstrating that hydroxy fatty acids play a significant role in increasing potato yield. Three potato plants were randomly selected from compound fertilizer group 5 and comparative example 2. The dug potatoes were arranged as follows... Figure 1 As shown.

[0042] Example 6: Using mixed fatty acid-coated compound fertilizer as base fertilizer to increase sweet potato yield. Experiment location: Wuxing District, Huzhou City, Zhejiang Province Experiment time: May 2024 Sweet potato variety: Yanshu 25 Experimental agents: Compound fertilizers #4 and #5 prepared in Example 1 were used, with coated compound fertilizer (Comparative Example 1) and uncoated compound fertilizer (Comparative Example 2) serving as comparative examples. The compound fertilizer was applied as base fertilizer in furrows at a rate of 100 kg / mu. Additionally, 1000 kg / mu of organic fertilizer was added and evenly spread and covered with a thin layer of soil as base fertilizer.

[0043] Processing area: Each treatment area consists of 3 cells, each cell is 10 square meters. The cells are spaced 0.5 meters apart.

[0044] Treatment method: Raise seed potatoes in seedbeds. When the seedlings grow to 15-20 cm, cut them and transplant them. Plant them with a row spacing of 70 cm and a plant spacing of 30 cm. Perform routine field management.

[0045] Results calculation: In October 2024, all sweet potatoes in each plot were dug up, the quantity was calculated, and the weight was measured.

[0046] The results are shown in Table 5.

[0047] Table 5. Sweet potato yield of each group

[0048] Statistical analysis: a, b, c, d represent differences between groups, P < 0.05.

[0049] It is evident that using compound fertilizer coated with mixed fatty acids as base fertilizer can significantly increase sweet potato yield, with an increase rate between 23.3% and 36.7%, indicating that it can promote sweet potato yield. While fatty acid coating alone (without hydroxy fatty acids) also has a certain yield-increasing effect, it is not as effective as compound fertilizer groups 4 and 5. This shows that hydroxy fatty acids also play a major role in increasing sweet potato yield.

[0050] Example 7: Using mixed fatty acid-coated compound fertilizer as base fertilizer to increase cherry radish yield. Experiment location: Wuxing District, Huzhou City, Zhejiang Province Experiment date: December 4, 2024, greenhouse cultivation Radish variety: Carroll cherry radish Experimental agents: Compound fertilizers 4# and 5# prepared in Example 1 were used, with coated compound fertilizer (comparative example 1#) without hydroxy fatty acids and uncoated compound fertilizer (comparative example 2#) serving as comparative examples. 50 kg / mu of compound fertilizer and 1000 kg / mu of organic fertilizer were evenly spread and covered with a thin layer of soil as base fertilizer.

[0051] Processing area: Each treatment area consists of 3 cells, each cell is 10 square meters. The cells are spaced 0.5 meters apart.

[0052] Treatment method: Sow radish seeds in holes with a row spacing of 30 cm and a plant spacing of 20 cm. After emergence, thin out the seedlings, leaving one strong seedling per hole. Perform routine field management.

[0053] Results calculated: On January 22, 2025, all radishes in each community were harvested and weighed.

[0054] The results are shown in Table 6. The harvested radishes are as follows: Figure 2 As shown.

[0055] Table 6. Cherry Radish Yields in Each Group

[0056] Statistical analysis: a and b represent differences between groups, P<0.05.

[0057] It is evident that using compound fertilizer coated with mixed fatty acids as base fertilizer can significantly increase the yield of cherry radishes, with radish weight increasing by 11.55-22.34%. Comparative Example 1 shows that fatty acid coating alone (without hydroxy fatty acids) had almost no yield-increasing effect, indicating that hydroxy fatty acids are the main factor in increasing radish yield. The difference between cherry radishes and potatoes and sweet potatoes may lie in the shorter growth cycle of cherry radishes, thus the impact of slow-release fertilizer on their yield is not significant. Hydroxy fatty acids, however, act through a mechanism that stimulates plant growth, thereby determining the radish's enlargement.

[0058] Example 8: Using mixed fatty acid-coated compound fertilizer as base fertilizer and top dressing to increase garlic yield. Experiment location: Wuxing District, Huzhou City, Zhejiang Province Experiment date: December 26, 2024, greenhouse cultivation Variety: Local white-skinned garlic Experimental agents: Compound fertilizers #4 and #5 prepared in Example 1 were used, with coated compound fertilizer (comparative example 1) and uncoated compound fertilizer (comparative example 2) serving as comparative examples. 50 kg / mu of compound fertilizer and 1000 kg / mu of organic fertilizer were evenly spread and covered with a thin layer of soil as base fertilizer. A top dressing of 5 kg / mu was applied in early March, using the same fertilizer type as the base fertilizer.

[0059] Processing area: Each treatment area consists of 3 cells, each cell is 10 square meters. The cells are spaced 0.5 meters apart.

[0060] Treatment method: Sow radish seeds in holes with a row spacing of 30 cm and a plant spacing of 20 cm. After emergence, thin out the seedlings, leaving one strong seedling per hole. Perform routine field management.

[0061] Results: On January 22, 2025, all radishes in each community were harvested and weighed. The results are shown in Table 7.

[0062] Table 7 Garlic yield of each group

[0063] Statistical analysis: a and b represent differences between groups, P<0.05.

[0064] It is evident that using compound fertilizer coated with mixed fatty acids as base fertilizer can significantly increase garlic yield. Comparative Example 1# shows that fatty acid coating alone (without hydroxy fatty acids) also has a certain yield-increasing effect. This is because garlic has a long growth cycle, so the slow release of fertilizer can play a role. However, hydroxy fatty acids play a decisive role in increasing yield.

[0065] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for coating a hydroxyl-containing fatty acid-containing fertilizer, characterized by, The method comprises the following steps: 1) mixing medium-chain 3-hydroxy fatty acid potassium with long-chain fatty acid potassium at a mass ratio of 1:1-9 to form a mixed fatty acid potassium, and then mixing the mixed fatty acid potassium with sodium alginate to prepare an aqueous solution; the mass fraction of the mixed fatty acid potassium in the aqueous solution is 10-30%, and the mass fraction of the sodium alginate in the aqueous solution is 1-5%; 2) spraying the aqueous solution on the surface of the compound fertilizer particles to form a pre-coating layer; 3) then spraying a metal salt solution containing CaCl2 at a molar concentration of 0.1-1.0 mol / L and / or MgCl2 at a molar concentration of 0.1-1.0 mol / L on the surface of the compound fertilizer particles, so that the mixed fatty acid potassium / sodium alginate forms a metal ion cross-linked film; 4) drying the sprayed compound fertilizer particles at 40-60 ℃ for 60 min to prepare coated compound fertilizer particles with a metal soap film layer.

2. The method of claim 1, wherein, In the step 1), the medium-chain 3-hydroxy fatty acid potassium is selected from one or more of 3-hydroxyoctanoic acid potassium, 3-hydroxydecanoic acid potassium and 3-hydroxylauric acid potassium.

3. The method of claim 1, wherein, In the step 1), the long-chain fatty acid potassium is selected from one or more of potassium oleate, potassium linoleate, potassium palmitate and potassium stearate.

4. The method of claim 1, wherein, In the step 2), the spraying amount is 2-10 kg of the mixed fatty acid potassium per ton of compound fertilizer.

5. The method of claim 1, wherein, In the step 3), the metal salt solution preferably contains CaCl2 at a molar concentration of 0.3-0.6 mol / L and / or MgCl2 at a molar concentration of 0.3-0.6 mol / L.

6. A compound fertilizer prepared by the method according to any one of claims 1-5.

7. Use of the compound fertilizer according to claim 6 in cultivating underground rhizome crops.

8. Use according to claim 7, characterized in that, The use is specifically for promoting the rhizome enlargement of radishes, onions, sweet potatoes and potatoes.

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