Organic water-soluble fertilizer suitable for crops and preparation method thereof

Abstract

The invention relates to the technical field of organic water-soluble fertilizers, in particular to an organic water-soluble fertilizer suitable for crops and a preparation method of the organic water-soluble fertilizer. The preparation method sequentially comprises the following steps: adjusting the pH value of an aqueous solution to 5.3-5.7, adding sodium polyaspartate, carboxyl polyethylene glycol carboxyl and dopamine hydrochloride, carrying out a condensation reaction in the presence of a condensing agent, and obtaining a solution A after the condensation reaction is ended; adding tannic acid, ferric nitrate and citrate, and keeping the pH value to obtain a solution B; adjusting the pH value to 6.6-7.0, adding sodium periodate for oxidation, and introducing taurine, histamine and ascorbic acid to obtain a solution C; and finally, adding carbonate, a large amount of ferric nitrate, zinc-magnesium salt and citrate, adjusting the pH value, and supplementing water to a certain amount to obtain a final product. By constructing a multidentate ligand and dynamic covalent network synergistic system, the long-term stability and anti-interference capability of metal ions are remarkably improved, hydrolytic precipitation is effectively inhibited, the low water-insoluble substance content is maintained, and good storage stability and field yield increasing effect are achieved.

Description

Technical Field

[0001] This invention relates to the field of organic water-soluble fertilizer technology, and in particular to an organic water-soluble fertilizer suitable for crops and its preparation method. Background Technology

[0002] Traditional organic water-soluble fertilizers often use small-molecule organic acids (such as citric acid and EDTA) as chelating agents to address the problem of insufficient micronutrient availability. However, these chelates are easily decomposed by microorganisms in the soil environment or undergo displacement reactions with competing anions such as carbonate and phosphate, leading to the redeposition and inactivation of metal ions such as iron and zinc in the rhizosphere. Especially in alkaline soils or soils with high bicarbonate content, the stability of these chelates is significantly reduced, and metal ions easily form hydroxide or carbonate precipitates, which not only reduces fertilizer utilization but may also clog drip irrigation systems.

[0003] Furthermore, the chelating capacity of a single ligand structure is limited, making it difficult to simultaneously ensure the synergistic stability of multiple trace elements. For example, ligands with a strong affinity for iron may exacerbate the precipitation of zinc and magnesium due to steric hindrance or charge repulsion. Some technologies attempt to extend the chelating chain segment using polymers (such as polyaspartic acid), but their rigid backbone has poor adaptability in dynamic soil environments and cannot effectively buffer ligand dissociation caused by pH fluctuations.

[0004] Other approaches introduce polyphenols (such as tannic acid) to coordinate with metals via phenolic hydroxyl groups. However, unmodified polyphenols are prone to self-polymerization or oxidative condensation, forming insoluble complexes that increase the content of water-insoluble matter. Currently, there is a lack of a dynamic coordination network system capable of maintaining the long-term dissolved state of multiple metal ions under neutral to weakly alkaline conditions and resisting interference from rhizosphere ions. Summary of the Invention

[0005] In view of this, the purpose of this invention is to propose an organic water-soluble fertilizer suitable for crops and its preparation method, so as to solve the problems that existing organic water-soluble fertilizers are prone to metal ion precipitation in environments with bicarbonate enrichment or pH fluctuations, resulting in decreased bioavailability of trace elements, blockage of drip irrigation systems, and insufficient long-term storage stability.

[0006] To achieve the above objectives, this invention provides a method for preparing an organic water-soluble fertilizer suitable for crops, comprising the following steps in sequence, with a final product total weight of 1000g:

[0007] S1, adjust the pH of the system to 5.3-5.7 in an aqueous solution, then add 15-22g of sodium polyaspartate, 1-3g of carboxylated polyethylene glycol carboxyl group and 3.6-6.0g of dopamine hydrochloride sequentially. A condensation reaction is carried out in the presence of 5.0-8.0g of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and 3.0-4.8g of N-hydroxysuccinimide, maintaining the pH at 5.3-5.7. Subsequently, add 0.8-1.2g of hydroxylamine hydrochloride to terminate the active intermediate, yielding solution A.

[0008] S2, add 4-8g of tannic acid to solution A, stir, then add 20-30g of ferric nitrate nonahydrate and 1.0-2.0g of trisodium citrate dihydrate, and maintain the pH at 5.4-5.8 to obtain solution B;

[0009] S3, adjust the pH of solution B to 6.6-7.0, add 0.4-0.8g sodium periodate, then add 1.0-1.5g taurine and 0.8-1.2g histamine, and finally add 0.6-1.0g L-ascorbic acid to obtain solution C;

[0010] S4. Add 5-7g sodium bicarbonate and 0.8-1.2g sodium carbonate to solution C, then add 100-140g ferric nitrate nonahydrate to adjust the pH to 6.2-6.6. Add 40-50g zinc sulfate heptahydrate and 25-32g magnesium sulfate monohydrate, then add 6-10g tripotassium citric acid monohydrate. Add deionized water to bring the total mass to 1000g to obtain an organic water-soluble fertilizer suitable for crops.

[0011] Preferably, the sodium polyaspartate has a weight-average molecular weight of 1000, and the carboxyl polyethylene glycol has a weight-average molecular weight of 2000.

[0012] Preferably, the pH is adjusted by reacting hydrochloric acid with sodium hydroxide.

[0013] Preferably, in step S3, sodium periodate is added in three equal portions at a temperature of 20°C.

[0014] Preferably, step S4 is performed under nitrogen protection and light-protected conditions.

[0015] Preferably, the temperature at which ferric nitrate nonahydrate is added in step S4 is 25°C.

[0016] Preferably, the aqueous solution in step S1 further contains 8.0-12.0 g of 2-morpholinoethanesulfonic acid hydrate.

[0017] Preferably, the amount of ferric nitrate nonahydrate added in step S4 is 3.3-7.0 times the amount of ferric nitrate nonahydrate added in step S2.

[0018] Preferably, step S4 further includes the addition of 0.6-1.2g of potassium sorbate.

[0019] Furthermore, the present invention also provides an organic water-soluble fertilizer suitable for crops, obtained by the above-described method for preparing an organic water-soluble fertilizer suitable for crops.

[0020] The beneficial effects of this invention are:

[0021] This invention significantly improves the adaptability of water-soluble fertilizers in complex soil environments by constructing a multiphase stable system with the synergistic effect of multidentate ligands and dynamic covalent networks. Sodium polyaspartate and carboxyl groups of polyethylene glycol form a flexible branched backbone. Through the synergistic hydration of carboxyl groups and ether bonds, a directional hydration layer is formed on the surface of metal ions, effectively inhibiting excessive interionic aggregation and heterogeneous nucleation, thereby reducing the formation of water-insoluble substances and maintaining the clarity of the system.

[0022] The o-quinone structure generated by in-situ oxidation of dopamine undergoes Michael addition and Schiff base reaction with the catechol unit in tannic acid to construct a pH-responsive dynamic covalent cross-linked network. This network can reversibly dissociate and recombine in the rhizosphere microdomain, which can buffer the effect of local pH fluctuations on the iron valence state and disperse the charge density through the electron delocalization of the polyphenol-iron complex, thereby reducing the substitution attack of bicarbonate on the central ion.

[0023] Citric acid derivatives, acting as auxiliary ligands, are embedded in the main network. Their ternary carboxyl groups form gradient coordination bonds with iron and zinc, creating a complementary protective layer around the polyphenol-iron network, further blocking the hydrolysis and precipitation pathway. The introduction of taurine and histamine enhances the system's hydrophilicity and complexation diversity. Sulfonic acid and imidazole groups, through strong hydration capabilities and nitrogen heterocycle coordination, respectively, improve the retention rates of zinc and magnesium under high ionic strength environments. The selective oxidation strategy using sodium periodate ensures the controllable formation of ortho-quinone structures, avoiding phase separation caused by polyphenol self-polymerization, while simultaneously promoting the covalent grafting of diamine / sulfonic acid groups onto the main chain, forming a three-dimensional ligand cage with a spatial buffering effect.

[0024] The stepwise addition of sodium bicarbonate and sodium carbonate, combined with light-protected and temperature-controlled operation, effectively suppressed the early hydrolysis tendency of iron ions, allowing zinc and magnesium ions to be orderly embedded in the stabilized network, ultimately achieving long-term uniform dispersion and anti-interference release of multi-metal ions. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0026] Example 1:

[0027] Step 1: Add 300g of deionized water and 8.0g of 2-morpholinoethanesulfonic acid hydrate to the reaction vessel, stir for 15min at room temperature, adjust the pH to 5.3 with hydrochloric acid, add 15g of sodium polyaspartate (weight average molecular weight 1000), 1g of carboxylated polyethylene glycol (weight average molecular weight 2000) and 3.6g of dopamine hydrochloride in sequence, stir for 30min, then add 5.0g of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and 3.0g of N-hydroxysuccinimide, maintain the pH at 5.3, stir for 2h, then add 0.8g of hydroxylamine hydrochloride to quench the O-acylurea intermediate, stir for 15min to obtain solution A;

[0028] Step 2: Add 4g of tannic acid to solution A obtained in step 1, stir for 5 minutes, then add 20g of ferric nitrate nonahydrate and 1.0g of trisodium citrate dihydrate, maintain the pH at 5.4, and continue stirring for 20 minutes to obtain solution B;

[0029] Step 3: Under light-protected conditions, adjust the pH of solution B obtained in step 2 to 6.6 with sodium hydroxide. Add 0.4g of sodium periodate in 3 batches at 20°C and stir for 10 min. Then add 1.0g of taurine and stir for 30 min. Next, add 0.8g of histamine and stir for 30 min. Finally, add 0.6g of L-ascorbic acid and stir for 60 min to obtain solution C.

[0030] Step 4: Under nitrogen protection and light-proof conditions, add 5g of sodium bicarbonate and 0.8g of sodium carbonate to solution C obtained in step 3, stir for 15 minutes, then add 100g of ferric nitrate nonahydrate at 25℃, stir for 40 minutes, then adjust the pH to 6.2 with hydrochloric acid, add 40g of zinc sulfate heptahydrate and 25g of magnesium sulfate monohydrate, stir for 30 minutes, then add 6g of tripotassium citric acid monohydrate, stir for 20 minutes, finally add 0.6g of potassium sorbate and add deionized water to a total mass of 1000g to obtain an organic water-soluble fertilizer suitable for crops.

[0031] Example 2:

[0032] Step 1: Add 300g of deionized water and 9.8g of 2-morpholinoethanesulfonic acid hydrate to the reaction vessel, stir for 15min at room temperature, adjust the pH to 5.5 with hydrochloric acid, add 18g of sodium polyaspartate (weight average molecular weight 1000), 2g of carboxylated polyethylene glycol (weight average molecular weight 2000) and 4.8g of dopamine hydrochloride in sequence, stir for 30min, then add 6.2g of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and 3.7g of N-hydroxysuccinimide, maintain the pH at 5.5, stir for 2h, then add 1g of hydroxylamine hydrochloride to quench the O-acylurea intermediate, stir for 15min to obtain solution A;

[0033] Step 2: Add 6g of tannic acid to solution A obtained in step 1, stir for 5 minutes, then add 25g of ferric nitrate nonahydrate and 1.2g of trisodium citrate dihydrate, maintain the pH at 5.6, and continue stirring for 20 minutes to obtain solution B;

[0034] Step 3: Under light-protected conditions, adjust the pH of the system of solution B obtained in step 2 to 6.8 with sodium hydroxide. Add 0.6g of sodium periodate in 3 batches at 20°C and stir for 10 min. Then add 1.2g of taurine and stir for 30 min. Next, add 1g of histamine and stir for 30 min. Finally, add 0.8g of L-ascorbic acid and stir for 60 min to obtain solution C.

[0035] Step 4: Under nitrogen protection and light-proof conditions, add 6g of sodium bicarbonate and 1g of sodium carbonate to solution C obtained in step 3, stir for 15 minutes, then add 120g of ferric nitrate nonahydrate at 25℃, stir for 40 minutes, then adjust the pH to 6.4 with hydrochloric acid, add 44g of zinc sulfate heptahydrate and 28.5g of magnesium sulfate monohydrate, stir for 30 minutes, then add 8g of tripotassium citric acid monohydrate, stir for 20 minutes, finally add 1g of potassium sorbate and add deionized water to a total mass of 1000g to obtain an organic water-soluble fertilizer suitable for crops.

[0036] Example 3:

[0037] Step 1: Add 300g of deionized water and 12.0g of 2-morpholinoethanesulfonic acid hydrate to the reaction vessel, stir for 15min at room temperature, adjust the pH to 5.7 with hydrochloric acid, add 22g of sodium polyaspartate (weight average molecular weight 1000), 3g of carboxylated polyethylene glycol (weight average molecular weight 2000) and 6.0g of dopamine hydrochloride in sequence, stir for 30min, then add 8.0g of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and 4.8g of N-hydroxysuccinimide, maintain the pH at 5.7, stir for 2h, then add 1.2g of hydroxylamine hydrochloride to quench the O-acylurea intermediate, stir for 15min to obtain solution A;

[0038] Step 2: Add 8g of tannic acid to solution A obtained in step 1, stir for 5 minutes, then add 30g of ferric nitrate nonahydrate and 2.0g of trisodium citrate dihydrate, maintain the pH at 5.8, and continue stirring for 20 minutes to obtain solution B;

[0039] Step 3: Under light-protected conditions, adjust the pH of solution B obtained in step 2 to 7.0 with sodium hydroxide. Add 0.8g of sodium periodate in three batches at 20°C and stir for 10 min. Then add 1.5g of taurine and stir for 30 min. Next, add 1.2g of histamine and stir for 30 min. Finally, add 1.0g of L-ascorbic acid and stir for 60 min to obtain solution C.

[0040] Step 4: Under nitrogen protection and light-proof conditions, add 7g of sodium bicarbonate and 1.2g of sodium carbonate to solution C obtained in step 3, stir for 15 minutes, then add 140g of ferric nitrate nonahydrate at 25℃, stir for 40 minutes, then adjust the pH to 6.6 with hydrochloric acid, add 50g of zinc sulfate heptahydrate and 32g of magnesium sulfate monohydrate, stir for 30 minutes, then add 10g of tripotassium citric acid monohydrate, stir for 20 minutes, and finally add 1.2g of potassium sorbate and add deionized water to a total mass of 1000g to obtain an organic water-soluble fertilizer suitable for crops.

[0041] Comparative Example 1:

[0042] The difference between Comparative Example 1 and Example 2 is that trisodium citrate dihydrate is not added in step 2, while the other conditions are the same as in Example 2.

[0043] Comparative Example 2:

[0044] The difference between Comparative Example 2 and Example 2 is that sodium periodate is not added in step 3, but taurine, histamine and L-ascorbic acid are added in sequence thereafter, and the other conditions are the same as in Example 2.

[0045] Comparative Example 3:

[0046] The difference between Comparative Example 3 and Example 2 is that carboxyl polyethylene glycol carboxyl groups are not added in step 1, while the other conditions are the same as in Example 2.

[0047] Comparative Example 4:

[0048] The difference between Comparative Example 4 and Example 2 is that the amount of tannic acid added in step 2 was adjusted from 6g to 1g, while other ingredients and pH control remained unchanged, and the remaining conditions were the same as in Example 2.

[0049] Comparative Example 5:

[0050] The difference between Comparative Example 5 and Example 2 is that step 3 is omitted, and 6g of sodium bicarbonate and 1g of sodium carbonate are added to solution B obtained in step 2. The other conditions are the same as in Example 2.

[0051] Comparative Example 6:

[0052] The difference between Comparative Example 6 and Example 2 is that 120g of ferric nitrate nonhydrate in step 4 was added together with 25g of ferric nitrate nonhydrate in step 2, while the other conditions were the same as in Example 2.

[0053] Performance testing:

[0054] Test Item 1: pH and Water-Insoluble Matter Content

[0055] The results were determined according to NY / T 1973-2021 and are shown in Table 1.

[0056] Test item two: Iron, manganese, and zinc content

[0057] The results were determined according to NY / T 1974-2010 and are shown in Table 1.

[0058] Test item 3: Resistance to bicarbonate precipitation and metal retention rate

[0059] Iron, zinc, and magnesium were measured before and after the intervention according to NY / T 1974-2010; pH and water-insoluble matter were measured according to NY / T 1973-2021. 100 mL of sample was taken and, at 25°C, sodium bicarbonate solution was added to bring the final sodium bicarbonate concentration to 10 mmol·L⁻¹. -1 The pH was adjusted to 7.5, kept at a constant temperature for 24 hours, filtered through a 0.4 μm filter, and the concentrations of iron, zinc, and magnesium in the filtrate were measured. The retention rate % was calculated as (concentration after intervention / concentration before intervention) × 100. The amount of water-insoluble matter was recorded, n = 3, and the average value was reported. The results are shown in Table 1.

[0060] Test item four: Storage stability

[0061] Component determination was performed according to NY / T 1974-2010, and pH and water-insoluble matter were determined according to NY / T 1973-2021. Each sample was collected in a sealed, light-proof glass bottle, 200 mL in total. Conditions: 54℃ incubator for 14 days. Iron, zinc, magnesium, pH, and water-insoluble matter were measured before and after the experiment. The retention rate of components and the change in water-insoluble matter were calculated. n=3. The average values ​​were reported. Results are shown in Table 1.

[0062] Test item five: Effect of application to potted plants

[0063] A small-scale pot experiment was conducted in accordance with the design and statistical principles of NY / T 3552-2020. A uniform wheat variety was used, and the pot soil was neutral loam. The pot diameter was 25 cm, and the soil dry weight was 8.0 kg. The application regimen was as follows: 200 mL of water was diluted 1:11.5 and applied in a circular trench along the pot wall for root irrigation. The application was repeated twice, once on the 7th day after sowing and again at the jointing stage. The control group received the same amount of water directly. The yield was measured at maturity, and the dry yield per ear was compared with the control group to calculate the yield increase rate.

[0064] Table 1 Performance Test Results

[0065] Sample Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Comparative Example 6 pH 6.2 6.4 6.6 6.4 6.4 6.4 6.4 6.3 6.5 Water insolubles / g L -1 ]] 0.12 0.08 0.10 0.38 0.22 0.18 0.26 0.42 0.47 Fe / L -1 ]] 16.59 20.04 23.50 19.30 19.98 20.06 19.97 19.89 18.53 <![CDATA[Zinc / g·L -1 > 9.09 10.02 11.37 9.78 10.05 10.00 9.92 9.95 9.71 <![CDATA[magnesium / g·L -1 > 4.39 5.01 5.62 5.03 5.02 4.99 5.01 4.98 4.97 <![CDATA[Iron retention rate (HCO3 - ) / %]]> 91.2 93.8 92.7 78.5 84.6 86.3 82.4 80.3 76.9 <![CDATA[Zinc retention rate (HCO3 - ) / %]]> 90.1 91.6 90.8 88.7 89.7 89.2 90.6 88.9 87.3 <![CDATA[Magnesium retention rate (HCO3 - ) / %]]> 97.4 97.8 97.6 97.2 97.4 97.9 97.5 96.9 97.2 Iron retention rate after accelerated storage / % 95.3 96.7 94.9 89.4 92.1 91.6 90.8 88.6 87.9 <![CDATA[Insoluble matter in water after accelerated storage / g·L -1 > 0.18 0.11 0.16 0.71 0.29 0.35 0.46 0.82 0.95 Potted plant yield increase rate / % 10.7 13.8 12.6 7.1 8.6 8.1 7.7 6.4 4.9

[0066] Data Analysis:

[0067] As can be seen from the data in Examples 1-3 in Table 1, the product prepared by this invention maintains low water-insoluble matter and stable pH. Iron and zinc retention is stable under bicarbonate intervention, and the composition and clarity are well maintained after accelerated storage. Potted plant applications show a high yield increase. This indicates that the dopamine-polyaspartic acid / carboxylated polyethylene glycol backbone and the tannic acid-iron / citric acid dual network work synergistically. Combined with the dynamic covalent bonds of taurine and histamine introduced after activation with sodium periodate, this can buffer valence fluctuations in the rhizosphere, disperse local charges, and provide multidentate coordination. Therefore, it is speculated that the system's resistance to hydrolysis and salting out comes from the superposition of multiple ligand densities and weak interactions, which both inhibits the rapid polymerization and hydrolysis of iron and reduces the probability of zinc and magnesium being precipitated by bicarbonate. Simultaneously, the hydration layer of the carboxylated polyethylene glycol segments reduces interparticle interactions, maintaining long-term clarity and sprinkler-friendly application.

[0068] As can be seen from the data in Table 1 for Example 2 and Comparative Example 1, when trisodium citric acid is omitted, the retention and post-storage stability of iron under bicarbonate intervention significantly decreases, while the amount of water-insoluble matter increases. The main reason is that the tricarboxylic acid group can both competitively coordinate iron ions and provide a buffer ligand outside the polyphenol-iron network. Without this group, iron is more prone to hydrolytic polymerization near neutrality, which also affects the solubility of zinc. Therefore, citric acid and the polyphenol-iron network exhibit complementary synergy, significantly reducing the risk of bicarbonate-induced precipitation.

[0069] As can be seen from the data in Example 2 and Comparative Example 2 in Table 1, directly introducing taurine and histamine without sodium periodate oxidation resulted in a deterioration in the system's resistance to salting out and its thermal storage stability. The main reason for this is the failure to form an ortho-quinone structure, leading to insufficient subsequent Michaelis addition and Schiff base crosslinking, insufficient dynamic covalent network density, and decreased charge dispersion and valence buffering capacity. Therefore, selective oxidation followed by diamine / sulfonic acid grafting is crucial for achieving dynamic stability.

[0070] As can be seen from the data in Example 2 and Comparative Example 3 in Table 1, when the carboxyl polyethylene glycol segment is missing, the water-insoluble matter increases while the magnesium retention is slightly higher. This is mainly because magnesium has a large hydration radius and easily forms outer spherical complexes. The missing segment reduces the iron-polyphenol bridging effect, thus having a smaller impact on magnesium; however, the poor wetting and dispersibility leads to increased micro-flocculation. Therefore, it is evident that carboxyl polyethylene glycol has a significant synergistic effect with the main framework in improving clarification and sprinkler irrigation compatibility.

[0071] As can be seen from the data in Example 2 and Comparative Example 4 in Table 1, when the addition of tannic acid is insufficient, the bicarbonate resistance of iron decreases significantly, while the retention of zinc slightly improves. The main reason is the insufficient density of the polyphenol-iron network, making iron more susceptible to hydrolysis. However, the lower polyphenol bridging reduces the co-precipitation entanglement with zinc. Therefore, there are upper and lower limits to the addition of tannic acid, which need to be optimized in conjunction with the ratio of citric acid.

[0072] As can be seen from the data in Example 2 and Comparative Example 5 in Table 1, after removing the entire oxidative-addition crosslinking step, the resistance to salt precipitation and thermal storage performance significantly declined, and the clarity deteriorated significantly. The main reason is the absence of a dynamic covalent network; the system relies solely on the carboxyl groups of the main chain for coordination, making it more prone to salt precipitation when local charge density is high and ionic strength fluctuates. Therefore, the synergistic effect of dynamic covalent and main chain coordination (1+1>2) is crucial for maintaining long-term stability.

[0073] As can be seen from the data in Example 2 and Comparative Example 6 in Table 1, adding all ferric iron in advance significantly increases the initial hydrolysis and alkalization micro-regions, making it easier for displacement-co-precipitation to occur when zinc and magnesium subsequently enter the system. The main reason is that the instantaneously high local pH and iron concentration lead to the formation of ferric hydroxide or ferric carbonate micronuclei, which become the cores for heterogeneous precipitation. Therefore, it is clear that stepwise iron addition and pH control under bicarbonate addition conditions are key to avoiding early nucleation and subsequent co-precipitation.

[0074] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A method for preparing an organic water-soluble fertilizer for crops, characterized by, The preparation method comprises the following steps in sequence: S1, adjusting pH of the system to 5.3-5.7 in an aqueous solution, sequentially adding 15-22 g of sodium polyaspartate, 1-3 g of carboxyl polyethylene glycol carboxyl and 3.6-6.0 g of dopamine hydrochloride, and performing condensation reaction in the presence of 5.0-8.0 g of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride and 3.0-4.8 g of N-hydroxysuccinimide, maintaining pH at 5.3-5.7, and then adding 0.8-1.2 g of hydroxylamine hydrochloride to terminate active intermediates to obtain solution A; S2, adding 4-8 g of tannic acid to solution A, stirring, and then adding 20-30 g of iron nitrate nonahydrate and 1.0-2.0 g of trisodium citrate dihydrate, maintaining pH at 5.4-5.8 to obtain solution B; S3, adjusting pH of solution B to 6.6-7.0, adding 0.4-0.8 g of sodium periodate, then adding 1.0-1.5 g of taurine and 0.8-1.2 g of histamine, and finally adding 0.6-1.0 g of L-ascorbic acid to obtain solution C; S4, adding 5-7 g of sodium bicarbonate and 0.8-1.2 g of sodium carbonate to solution C, then adding 100-140 g of iron nitrate nonahydrate, adjusting pH to 6.2-6.6, adding 40-50 g of zinc sulfate heptahydrate and 25-32 g of magnesium sulfate monohydrate, and then adding 6-10 g of tripotassium citrate monohydrate, and adding deionized water to a total mass of 1000 g to obtain the organic water-soluble fertilizer suitable for crops.

2. The method of preparing organic water soluble fertilizer suitable for crops as claimed in claim 1, wherein, The weight average molecular weight of the sodium polyaspartate is 1000, and the weight average molecular weight of the carboxyl polyethylene glycol carboxyl is 2000.

3. The method of preparing organic water soluble fertilizer suitable for crops as claimed in claim 1, wherein, The pH adjustment is performed by using hydrochloric acid and sodium hydroxide.

4. The method of preparing organic water soluble fertilizer suitable for crops as claimed in claim 1, wherein, The sodium periodate in step S3 is added in three equal portions, and the addition temperature is 20°C.

5. The method of preparing organic water soluble fertilizer suitable for crops as claimed in claim 1, wherein, The step S4 is performed under the conditions of nitrogen protection and light protection.

6. The method of preparing organic water soluble fertilizer suitable for crops as claimed in claim 1, wherein, The aqueous solution in step S1 further contains 8.0-12.0 g of 2-morpholinoethanesulfonic acid hydrate.

7. The method of preparing organic water soluble fertilizer suitable for crops as claimed in claim 1, wherein, The addition amount of the iron nitrate nonahydrate in step S4 is 3.3-7.0 times the addition amount of the iron nitrate nonahydrate in step S2.

8. The method of preparing organic water soluble fertilizer suitable for crops as claimed in claim 1, wherein, The step S4 further adds 0.6-1.2 g of potassium sorbate.

9. The method of preparing organic water soluble fertilizer suitable for crops as claimed in claim 1, wherein, The addition temperature of the iron nitrate nonahydrate in step S4 is 25°C.

10. An organic water soluble fertilizer suitable for use in agriculture, characterized in that, The preparation method of the organic water-soluble fertilizer suitable for crops is the preparation method according to any one of claims 1-9.