Straw charcoal-based slow-release organic fertilizer and preparation method thereof

Abstract

This invention relates to the field of slow-release fertilizer technology, specifically to straw-based slow-release organic fertilizer and its preparation method. The slow-release organic fertilizer includes an organic fertilizer core loaded with organic matter and nitrogen, phosphorus, and potassium nutrients using straw biochar as a carrier, and a slow-release coating covering the core. The slow-release coating is a cross-linked network formed by phytic acid cross-linking a hydroxyl polysaccharide substrate through phosphodiester bonds. Each phytic acid molecule forms an average of 1.5-3 phosphodiester bonds, with the remaining phosphate groups remaining as free phosphate groups and coordinating with iron and / or zinc. The phosphodiester bonds constitute the hydrolytic substrate of rhizosphere phosphatase. When crops are phosphorus deficient, the phosphatase secreted hydrolyzes the cross-linked bonds, causing the coating to uncross-link, thereby increasing the nutrient release rate with increasing phosphatase activity and simultaneously releasing trace elements such as iron and zinc. This allows nutrient release to adaptively match the crop's phosphorus deficiency requirements. The nitrogen release rate in still water over 28 days meets the national standard for slow-release fertilizers, and significantly improves the phosphorus accumulation and nitrogen fertilizer utilization rate in phosphorus-deficient soils.

Description

Technical Field

[0001] This invention relates to the field of slow-release fertilizer technology, specifically to straw-based slow-release organic fertilizer and its preparation method. Background Technology

[0002] The long-term and excessive use of fast-acting chemical fertilizers in agricultural production has led to the dual problems of low fertilizer utilization and environmental pollution. Traditional organic fertilizers and fast-acting compound fertilizers, once applied to the soil, have nutrient release rates that are difficult to match with the crop's absorption rhythm. In the early stages of crop growth, nutrient supply is excessive, causing ammonium nitrogen to be converted to nitrate nitrogen through nitrification and then leached into deeper layers by irrigation water, polluting groundwater and easily causing seedling burn. Conversely, in the middle and later stages of crop growth, nutrient supply is insufficient due to the rapid release and loss in the early stages, requiring multiple topdressings and increasing labor costs. To alleviate these contradictions, a slow-release fertilizer technology has been developed in this field, which uses a carrier to adsorb and load nutrients and supplements this with a polymer coating to control release.

[0003] Biochar, due to its well-developed porous structure, high specific surface area, and cation exchange capacity, is widely used as a nutrient carrier in slow-release fertilizers. Among the published technologies, CN102276357A discloses an organic matter-coated slow-release fertilizer and its preparation method, which coats organic matter onto the surface of fertilizer granules to delay nutrient release; CN101759496B discloses a biodegradable polymer-coated controlled-release fertilizer, using a biodegradable polymer membrane to control nutrient dissolution; CN101486614B discloses a coated degradable slow-release fertilizer; and CN101717317A discloses a long-acting slow-release diammonium phosphate and its preparation method, using nitrogen-controlling and phosphorus-promoting inhibitors to delay nitrogen conversion. In the field of biochar-based slow-release fertilizers, related research further explores granulation of urea, starch, and biochar, or coating biochar-based organic fertilizers with modified starch or polyvinyl alcohol to extend the nutrient release cycle.

[0004] However, the aforementioned technologies suffer from a long-overlooked common flaw. It is generally believed in the field that slow-release coatings should passively control nutrient release through water swelling or diffusion, with the nutrient release cycle pre-set by the coating's cross-linking degree and thickness during preparation, or triggered by non-specific environmental factors such as soil moisture, temperature, and pH. However, in reality, the timing of nutrient release should be anchored to the crop's own nutrient requirement rhythm, especially coupled with the crop's phosphorus demand status; existing slow-release coatings lack any biofeedback correlation with the crop's phosphorus stress level. This cognitive bias directly leads to a systematic mismatch between the release curve and the crop's nutrient requirement curve: before the crop reaches its peak nutrient demand, the coating passively releases nutrients due to water contact or changes in environmental pH, resulting in excessive accumulation and leaching of nutrients in the root zone; conversely, when the crop reaches its peak phosphorus demand but soluble phosphorus in the root zone is insufficient, the coating cannot sense this phosphorus deficiency signal and accelerates release accordingly, leading to insufficient nutrient supply during the peak demand period.

[0005] Furthermore, phosphorus deficiency in crops is often accompanied by a decrease in the availability of micronutrients such as iron and zinc, especially in calcareous soils where iron and zinc are easily fixed and difficult for crops to absorb. Existing slow-release fertilizer coatings only serve a physical barrier function; they cannot synergistically replenish micronutrients while releasing nutrients, nor can they reactivate insoluble phosphorus fixed by iron and aluminum oxides in the soil. Therefore, establishing a biofeedback relationship between the nutrient release rate of slow-release coatings and the degree of phosphorus stress in crops, and synergistically supplying micronutrients during the release process, is a pressing technical problem that needs to be solved in this field.

[0006] From the perspective of nutrient release triggering mechanisms, existing slow-release fertilizers can be broadly divided into two categories. The first category is water-swellable or diffusion-controlled, where the coating slowly swells upon contact with water in the soil, and nutrients diffuse outward through the swollen coating. The release rate is mainly determined by the thickness and density of the coating, and is fixed once prepared, unable to change according to the crop's nutrient requirement. The second category is environmental factor-responsive, where the coating responds to non-specific environmental factors such as soil moisture, temperature, or pH, accelerating release when temperature rises or pH changes. However, changes in soil temperature, moisture, and pH are mainly determined by climate, irrigation, and the properties of the soil itself, and are not necessarily related to the individual crop's phosphorus requirement. After irrigation or rainfall, regardless of whether the crop is at its peak nutrient requirement, this type of coating will accelerate release due to increased moisture, causing excessive release and leaching during non-nutrient-demand periods. Conversely, when the crop enters its peak phosphorus absorption period but soil temperature and humidity do not change significantly, this type of coating cannot accelerate release accordingly. It is evident that, whether passive swelling type or non-specific environmental response type, the release rhythm of existing slow-release coatings is detached from the crop's own nutrient requirement rhythm.

[0007] Crops exhibit distinct phases in phosphorus absorption, with peak demand occurring during critical periods such as the early reproductive growth stage. When soluble phosphorus in the root zone is insufficient, crops actively acquire phosphorus through a series of physiological responses. One phenomenon, well-recognized in the field but not yet used to regulate fertilizer release, is that phosphorus stress significantly increases the levels of acid phosphatase and phytase secreted by plant roots and rhizosphere phosphorus-solubilizing microorganisms, hydrolyzing soil organic phosphorus compounds and releasing absorbable inorganic phosphorus. In other words, the increased activity of rhizosphere phosphatase is itself a natural chemical signal that crops are in a phosphorus-deficient state and urgently need phosphorus supplementation. However, existing slow-release fertilizer technologies have never coupled this signal with the release of nutrient from the coating, missing the technological opportunity to use the crop's own phosphorus deficiency signal to reverse-regulate the nutrient release rate and achieve on-demand fertilization. This is precisely the core problem that this invention aims to solve. Summary of the Invention

[0008] To address the problems of existing slow-release fertilizers lacking a biological feedback correlation between release rate and crop phosphorus stress, resulting in a mismatch between fertilizer demand and supply timing, and failing to provide trace elements in conjunction with nutrient release, the present invention aims to provide straw-based slow-release organic fertilizer and its preparation method, so that the nutrient release rate is regulated by crop phosphorus deficiency signal feedback, and iron and zinc trace elements are supplied in conjunction with nutrient release during decrosslinking.

[0009] To achieve the above objectives, the technical solution of this invention is summarized as follows. This invention provides a straw-based slow-release organic fertilizer, comprising an organic fertilizer core loaded with organic matter and nitrogen, phosphorus, and potassium nutrients using straw biochar as a carrier, and a slow-release coating covering the organic fertilizer core. The core is that the slow-release coating does not rely on passive controlled release through physical swelling, but is composed of a cross-linked network formed by phytic acid cross-linking a hydroxyl-containing polysaccharide substrate through phosphodiester bonds. Each phytic acid molecule forms an average of 1.5 to 3 phosphodiester cross-links, with the remaining phosphate groups remaining as free phosphate groups. The phosphodiester bonds themselves constitute the hydrolytic substrate for acid phosphatase and / or phytase secreted by plant roots and rhizosphere microorganisms. When crops are under phosphorus stress, their roots and rhizosphere phosphorus-solubilizing microorganisms upregulate the secretion of phosphatases. Phosphatases specifically hydrolyze the phosphodiester bonds in the membrane cross-linking network, causing the cross-linking network to de-crosslink and the membrane to swell and degrade, thereby accelerating the release of nutrients from the core. The more severe the phosphorus stress in the crop, the more phosphatases are secreted, the faster the de-crosslinking and the greater the nutrient release rate. This establishes a biofeedback pathway between the nutrient release rate and the crop's phosphorus deficiency signal at the molecular level.

[0010] Furthermore, the free phosphate groups retained in phytic acid act as natural metal chelating sites to coordinate and load iron and / or zinc ions. During the stepwise hydrolysis of phytic acid phosphate groups by phosphatase, the chelated iron and zinc are released simultaneously, making the peak of phosphorus deficiency in crops a peak of micronutrient supply. Simultaneously, the organic acid radicals released from phytic acid and its hydrolysis products can activate insoluble phosphorus fixed by iron and aluminum oxides in the soil. Even further, the oligosaccharides generated by phosphatase hydrolysis of the polysaccharide cross-linking network serve as a carbon source for rhizosphere phosphorus-solubilizing microorganisms, and the released zinc ions act as a cofactor for phosphatase. Both work together to promote the proliferation of rhizosphere phosphorus-solubilizing microorganisms and their phosphatase secretion, which in turn accelerates the hydrolysis of phosphodiester bonds, forming a self-reinforcing positive feedback loop.

[0011] Compared with existing technologies, the beneficial effects of this invention are as follows: First, this invention designs the cross-linking bonds of the slow-release coating as phosphatase substrates, so that the nutrient release rate is regulated by the feedback of crop phosphorus deficiency signals, fundamentally changing the existing slow-release fertilizer paradigm that relies on preset cross-linking degree or passive controlled release by non-specific environmental factors, and realizing adaptive matching between the timing of fertilization and the crop's nutrient demand rhythm; Second, the phytic acid cross-linking agent has the functions of both a slow-release phosphorus source and a chelating carrier for iron and zinc trace elements. While releasing nitrogen, phosphorus, and potassium nutrients through decrosslinking, it synergistically supplies trace elements and activates fixed phosphorus, achieving multiple causes for one effect; Third, the decrosslinking products can promote the proliferation of rhizosphere phosphorus-solubilizing microorganisms and form a positive feedback, making the demand response sustainable; Fourth, the phytic acid, carboxymethyl starch, and straw biochar used are all natural, biodegradable, and environmentally friendly materials, which meet the requirements of green agriculture development. The slow-release organic fertilizer of this invention meets the requirements of the national standard for slow-release fertilizers under static water conditions with a cumulative nitrogen release rate of 28 days. In phosphorus-containing media or phosphorus-deficient soils, the release is significantly accelerated. In pot experiments on phosphorus-deficient soils, the crop phosphorus accumulation and nitrogen fertilizer utilization rate are significantly better than existing slow-release fertilizers.

[0012] Furthermore, the phytic acid used in this invention, namely inositol hexaphosphate, is a natural component widely found in plant seeds. As a chelating agent and antioxidant, it is also included in relevant food additive standards. After hydrolysis by phosphatase in the soil, it is ultimately converted into inorganic phosphorus and inositol that can be absorbed by plants, without introducing exogenous harmful substances. Carboxymethyl starch and straw biochar are derived from starch and crop straw, respectively, realizing the resource utilization of agricultural waste while ensuring the biodegradability and ecological safety of the product. The content of harmful elements such as arsenic, cadmium, lead, chromium, and mercury in the slow-release organic fertilizer obtained by this invention meets the relevant national standards for the limits of harmful elements in fertilizers. The technical solution of this invention is not only applicable to biochar cores prepared from crop straws such as corn straw and rice straw, but also not limited to carboxymethyl starch as a hydroxyl polysaccharide substrate. Any hydroxyl polysaccharide that can form a phosphate diester cross-linking network with phytic acid is applicable, thus possessing good raw material adaptability and promotional value. It is particularly suitable for precise fertilization on demand in low- and medium-yield fields, such as acidic red soil in the south and calcareous soil in the north, where phosphorus fixation and iron and zinc deficiencies are common problems. Attached Figure Description

[0013] Figure 1 This is a flowchart illustrating the preparation process of the straw-based slow-release organic fertilizer of the present invention. Figure 2 The cumulative nitrogen release curves of Example 1, Comparative Example 1, and Comparative Example 2 over 28 days in phosphatase-containing medium and enzyme-free still water are shown. Figure 3 This is a comparison chart of the phosphatase release increment in Example 1, Comparative Examples 1 and 4 in phosphorus-deficient soil. Detailed Implementation

[0014] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention. The phytic acid used in this invention is food-grade inositol hexaphosphate, which complies with the relevant provisions for antioxidants in GB 2760-2024, is of natural origin, and is biodegradable; the carboxymethyl starch used is food-grade sodium carboxymethyl starch; the slow-release organic fertilizer obtained, after testing, shows that its arsenic, cadmium, lead, chromium, and mercury content all meet the limits for harmful elements in fertilizers in GB / T 23349.

[0015] The straw-based slow-release organic fertilizer of the present invention has a core-shell structure. The inner layer is an organic fertilizer core with straw biochar as a carrier and loaded with decomposed organic matter and nitrogen, phosphorus and potassium nutrients. The outer layer is a slow-release coating formed by phytic acid cross-linking carboxymethyl starch through phosphate diester bonds. Figure 1 The preparation process flow of the present invention is shown. The cumulative nitrogen release rate determination involved in the following examples was carried out in accordance with the static water extraction method specified in GB / T 23348-2009 "Slow-Release Fertilizers".

[0016] Example 1: This example describes the preparation of a phosphatase-responsive slow-release organic fertilizer using corn straw biochar as the core carrier, which is carried out in three stages: biochar core preparation, slow-release coating material preparation, and coating granulation.

[0017] The organic fertilizer core was prepared as follows: Air-dried corn stalks were crushed, sieved, and placed in an oxygen-limited pyrolysis furnace. Under nitrogen protection, the pyrolysis was carried out at 450°C for 2 hours to obtain corn stalk biochar. Its specific surface area was measured to be 218 m². 2 / g. The pyrolysis temperature is controlled within the range of 400~500°C, so that the biochar retains a rich microporous and mesoporous structure to provide nutrient adsorption sites and phosphatase adsorption and enrichment sites, while avoiding excessive temperature that would cause pore structure collapse and a decrease in specific surface area. The biochar is crushed and sieved to a particle size of 1~3 mm, and mixed with the mature organic liquid obtained from aerobic composting of livestock and poultry manure at a mass ratio of 1:1.5. It is then soaked at 40°C for 24 h to allow humic acid and soluble organic nutrients such as amino acids to be fully adsorbed into the pores of the biochar. After draining, organic matter-loaded biochar is obtained. It is then uniformly mixed with urea, diammonium phosphate and potassium sulfate at a target nitrogen-phosphorus-potassium mass ratio of 18:8:10. Bentonite, accounting for 3% of the material mass, is added as a binder in a disc granulator for granulation. After granulation, it is dried at 60°C until the moisture content is less than 5%, resulting in organic fertilizer cores with a particle size of 3~4 mm. In the later stages of nutrient release, the biochar core can also act as a secondary buffer due to its high specific surface area and cation exchange capacity, temporarily adsorbing and storing the ammonium nitrogen and metal ions released by decrosslinking, avoiding excessive release and leaching, and allowing the released nutrients to have a longer retention time in the root zone.

[0018] The preparation of the sustained-release coating material consisted of three steps: carboxymethylation, phytic acid crosslinking, and metal coordination loading. Carboxymethylation was carried out as follows: 100.0 g (0.6167 mol) of dry starch (based on dehydrated glucose units) was added to a mixed solvent of isopropanol and water in a volume ratio of 8:2. The mixture was alkalized at 30°C for 30 min. Subsequently, 58.28 g (0.6167 mol) of monochloroacetic acid was slowly added, and the mixture was heated to 60°C for etherification reaction for 2.5 h. The molar ratio of sodium hydroxide, monochloroacetic acid, and dehydrated glucose units in the reaction was 2.4:1.0:1.0. After the reaction, the mixture was neutralized with hydrochloric acid, washed with ethanol, and dried to obtain 107.77 g of carboxymethyl starch. The degree of carboxymethyl substitution was determined to be 0.40 by titration. Phytic acid crosslinking was carried out as follows: 15% of the above carboxymethyl starch was added with phytic acid (i.e., 16.16 g of phytic acid, 0.0245 mol), and 2.94 g of urea (0.0490 mol, molar ratio with phytic acid 2:1) was added as a catalyst. After thorough impregnation and mixing, the mixture was reacted under dry heat at 130°C for 1.5 h to allow the phosphate groups of phytic acid to undergo phosphorylation with the hydroxyl groups of carboxymethyl starch to form phosphate diester crosslinking bonds. The degree of reaction was controlled so that each molecule of phytic acid formed an average of about 2.0 phosphate diester crosslinking bonds, and the remaining phosphate groups were retained as free phosphate groups, resulting in 116.90 g of phytic acid crosslinked carboxymethyl starch. Metal coordination loading was carried out as follows: The above-mentioned phytic acid crosslinked carboxymethyl starch was dispersed in an aqueous phase at pH 4.5. According to the target iron content of 1.0% and zinc content of the finished product, 6.76 g (0.0250 mol) of ferric chloride hexahydrate and 3.07 g (0.0107 mol) of zinc sulfate heptahydrate were added to ensure loading efficiency. The mixture was subjected to coordination reaction at 40°C for 2 h, so that iron ions and zinc ions were chelated and coordinated onto the crosslinked network through the free phosphate groups of phytic acid. After filtration, washing with water and vacuum drying, 116.30 g of slow-release coating material was obtained.

[0019] The coating and granulation process is as follows: The above-mentioned slow-release coating material is prepared into an aqueous dispersion with a mass fraction of 15%, and uniformly sprayed onto the surface of the organic fertilizer core in a fluidized bed coating machine. The coating amount is controlled so that the slow-release coating accounts for 8% of the total mass of the slow-release organic fertilizer. The spraying temperature is 50°C, and after spraying, it is cured at 50°C for 30 min to obtain the straw-based slow-release organic fertilizer of this embodiment. Fourier transform infrared spectroscopy characterization shows that the obtained coating has a wavelength of approximately 1250 cm⁻¹. -1 With approximately 1000 cm -1Characteristic absorption of phosphodiester bonds was observed at the phytic acid molecule. Based on this, solid phosphorus nuclear magnetic resonance estimated that an average of about 2.0 phosphodiester crosslinks were formed per molecule of phytic acid. Inductively coupled plasma atomic emission spectrometry (ICP-AES) determined that the iron content in the finished product was 1.0% and the zinc content was 0.5%, which is consistent with the design target.

[0020] The release mechanism of the slow-release organic fertilizer obtained in this embodiment is as follows: Under conditions of sufficient phosphorus supply in the soil, rhizosphere phosphatase activity is low, the phytate phosphate diester cross-linking network remains dense, and nutrients are released slowly. When crops enter a state of phosphorus stress, root and rhizosphere phosphorus-solubilizing microorganisms upregulate the secretion of acid phosphatase and phytase, specifically hydrolyzing the phosphate diester bonds in the coating, causing the cross-linking network to de-crosslink, the coating to swell and degrade, and nutrients to be released at an accelerated pace. At the same time, iron and zinc chelated by the free phosphate groups of phytate are released synchronously with the hydrolysis of the phosphate groups. Phytate chelates ferric ions through its unique polyphosphate coordinating mechanism, forming a stable chelate structure that prevents iron ions from being easily lost from the soil. When phosphatase gradually removes the phosphate groups of phytate, the chelate structure disintegrates, and iron and zinc are released synchronously into the root zone for crop absorption during the peak period of crop phosphorus demand. At the same time, phytic acid and the inositol polyphosphate and organic phosphate released by its hydrolysis have strong metal coordination ability. They can compete with insoluble phosphorus fixed by soil iron and aluminum oxides for binding metal sites, thereby reactivating the originally fixed and ineffective phosphorus into a form that can be used by crops, and further improving the availability of phosphorus in phosphorus-deficient soils.

[0021] Example 2: This example prepares a phosphatase-responsive slow-release organic fertilizer with rice straw biochar as the core carrier. The process is basically the same as that in Example 1, except for the types of raw materials and some process parameters.

[0022] In the preparation of organic fertilizer cores, air-dried rice straw was used instead of corn straw. After being crushed and sieved, the rice straw biochar was obtained by pyrolysis at 420°C for 2 hours under nitrogen protection in an oxygen-limited pyrolysis furnace. The specific surface area was measured to be 176 m². 2 / g. The pore structure of rice straw biochar differs slightly from that of corn straw biochar, with a higher pore ratio. Its adsorption of soluble organic nutrients such as humic acid and amino acids is slightly lower than that of corn straw biochar. Therefore, the impregnation mass ratio of rice straw biochar to composted organic liquid was adjusted from 1:1.5 in Example 1 to 1:1.8. The impregnation temperature was the same at 40°C, and the time was the same at 24 h to ensure sufficient organic matter loading. After impregnation and draining, it was uniformly mixed with urea, diammonium phosphate, and potassium sulfate at a nitrogen-phosphorus-potassium mass ratio of 18:8:10. 3% bentonite was added by mass of the material for granulation and drying to obtain the rice straw biochar organic fertilizer core.

[0023] In the preparation of the sustained-release coating material, the molar ratio of sodium hydroxide, monochloroacetic acid, and dehydrated glucose units in the carboxymethylation step remained at 2.4:1.0:1.0. The mixture was first alkalized at 30°C for 30 min in a mixed solvent of isopropanol and water at a volume ratio of 8:2, followed by etherification at 60°C for 2.5 h. By appropriately extending the etherification time to 2.5 h and controlling the degree of reaction, the degree of substitution of the resulting carboxymethyl starch was 0.35. In the phytic acid crosslinking step, phytic acid was added at 20% of the mass of carboxymethyl starch, with the molar ratio of urea to phytic acid remaining at 2:1. The reaction was carried out under dry heat at 135°C for 1.5 h. Because the amount of phytic acid used was increased compared to Example 1, the crosslinking density increased accordingly. The average number of phosphate diester crosslinking bonds formed per molecule of phytic acid was controlled to be approximately 2.5, with the remaining phosphate groups remaining as free phosphate groups. In the metal coordination loading step, ferric chloride hexahydrate and zinc sulfate heptahydrate are added to an aqueous phase at pH 4.5 according to the target iron content of 1.5% and zinc content of the finished product. The metal salts are added in excess at 1.2 times the stoichiometric ratio. The coordination reaction is carried out at 40°C for 2 hours. After filtration, washing with water, and vacuum drying, a slow-release coating material is obtained. During coating granulation, the slow-release coating material is prepared as a 15% (w / w) aqueous dispersion and sprayed onto the surface of the rice straw biochar organic fertilizer core in a fluidized bed. The slow-release coating is controlled to account for 10% of the total mass of the slow-release organic fertilizer. After curing, the straw char-based slow-release organic fertilizer of this embodiment is obtained.

[0024] Testing showed that the slow-release organic fertilizer obtained in this embodiment had a cumulative nitrogen release rate of 70% in enzyme-free still water over 28 days, meeting the requirements of the national standard for slow-release fertilizers. In a medium containing phosphatase, the cumulative nitrogen release rate reached 87% over 28 days, with a phosphatase response release increment ΔR of 17%, indicating that a higher phytic acid crosslinking density and a higher iron-zinc loading are beneficial for enhancing phosphatase response sensitivity and micronutrient supply capacity. In a pot experiment on phosphorus-deficient soil, the aboveground phosphorus accumulation of maize in this embodiment was 44.1 mg / plant, grain yield was 39.2 g / plant, and nitrogen fertilizer utilization rate was 49.5%, all slightly better than in Example 1. This demonstrates that appropriately increasing the amount of phytic acid and the iron-zinc loading within the parameter range defined in this invention can further improve the crop's absorption and utilization of phosphorus and micronutrients.

[0025] Example 3: This example is a boundary parameter example, which examines the feasibility of the scheme under lower phytic acid dosage, higher carboxymethyl substitution degree and thinner coating, in order to define the lower boundary of the parameter window of the present invention.

[0026] In the preparation of organic fertilizer cores, corn stalk biochar was obtained by pyrolyzing air-dried corn stalks at 400°C for 2 hours under nitrogen protection in an oxygen-limited pyrolysis furnace. This pyrolysis temperature is the lower limit of the temperature range specified in this invention, and its specific surface area was measured to be 152 m². 2 / g, still meeting the requirement of not less than 150 m 2 / g requirement; the remaining impregnation and loading of organic matter and nitrogen, phosphorus and potassium ingredients, granulation and drying processes are the same as in Example 1. In the preparation of the sustained-release coating material, the carboxymethylation step is improved by appropriately increasing the amount of monochloroacetic acid and extending the etherification time, so that the degree of substitution of the obtained carboxymethyl starch reaches 0.45. The higher degree of substitution improves the water solubility and film-forming properties of carboxymethyl starch. In the phytic acid crosslinking step, phytic acid is added at 10% of the mass of carboxymethyl starch. This amount is near the lower limit of the phytic acid amount range defined in this invention. The molar ratio of urea to phytic acid is still 2:1. The reaction is carried out at 120°C for 2 h under dry heat. Due to the low amount of phytic acid, the average number of phosphate diester crosslinking bonds formed per molecule of phytic acid is controlled to be about 1.8, and the remaining phosphate groups are retained as free phosphate groups. In the metal coordination loading step, the target iron content of 0.8% and zinc content of 0.4% in the finished product are added, and the reaction is carried out at 40°C for 2 h in an aqueous phase with pH 4.5. During the coating and granulation process, the slow-release coating is controlled to account for 6% of the total mass of the slow-release organic fertilizer. After solidification, the straw-based slow-release organic fertilizer of this embodiment is obtained.

[0027] Testing showed that the slow-release organic fertilizer obtained in this embodiment achieved a cumulative nitrogen release rate of 76% in enzyme-free still water over 28 days, with a first-day release rate of 7.2% and a differential dissolution rate of 1.5% / day. All slow-release indicators met the national standards for slow-release fertilizers. In a phosphatase-containing medium, the cumulative nitrogen release rate reached 90% over 28 days, with a phosphatase response release increment ΔR of 14%, maintaining significant phosphatase responsiveness. In a pot experiment on phosphorus-deficient soil, the phosphorus accumulation in maize was 41.0 mg / plant, the grain yield was 37.1 g / plant, and the nitrogen fertilizer utilization rate was 46.8%. This embodiment demonstrates that even under conditions where the phytic acid dosage and coating ratio are close to the lower limit of the window, the present invention can still achieve satisfactory slow-release performance and significant demand response, verifying the feasibility and robustness of the present invention within the stated parameter window.

[0028] To verify the independent contribution and synergistic effect of the essential technical features of this invention, the following five comparative examples are set up: comparative example 1 and comparative example 4 correspond to the verification of the underlying innovative features that distinguish this invention from the prior art, comparative example 3 corresponds to the verification of molecular design features, comparative example 2 corresponds to the closest to the prior art, and comparative example 5 corresponds to the verification of parameter criticality.

[0029] Comparative Example 1 used the same organic fertilizer core and carboxymethyl starch as Example 1, but citric acid was used instead of phytic acid as the cross-linking agent. Under the same conditions, the carboxymethyl starch was cross-linked to form a coating, with a coating amount of 8%. The cross-linking bond between citric acid and carboxymethyl starch is a carboxylic acid ester cross-link, not a phosphodiester bond, and therefore does not constitute a substrate for phosphatase hydrolysis. This comparative example is used to verify the necessity of the distinguishing feature of the phosphodiester cross-link as a phosphatase substrate in the unique features of this invention.

[0030] Comparative Example 2 did not have a slow-release coating. Ordinary biochar organic fertilizer granules were obtained by directly granulating the organic fertilizer core formula of Example 1. This corresponds to the closest existing technology to compounding biochar, urea and organic matter into granules.

[0031] Comparative Example 3 used the same phytic acid crosslinked carboxymethyl starch coating as Example 1, but omitted the metal coordination loading step in the preparation of the sustained-release coating material; that is, the coating was not loaded with iron and zinc. This comparative example was used to verify the independent contribution of the molecular design feature of phytic acid coordination loading of trace iron and zinc elements in the dependent claims of this invention.

[0032] Comparative Example 4 uses polyvinyl alcohol instead of phytic acid-crosslinked carboxymethyl starch as the coating material, forming a water-swellable physical coating at the same coating amount. This corresponds to the existing slow-release coating paradigm that relies on physical swelling for passive controlled release. This comparative example is used to verify the essential difference between the phosphatase-responsive release mechanism of this invention and passive controlled release.

[0033] Comparative Example 5 used the same phytic acid crosslinked carboxymethyl starch coating material as Example 1, but reduced the coating amount to 2% of the total mass of the slow-release coating as the slow-release organic fertilizer, which is lower than the lower limit of the coating ratio window defined by the present invention, and was used to verify the critical effect of the coating ratio parameter.

[0034] The test results of the above comparative examples are summarized in Table 1 below. In Comparative Example 1, the citric acid-crosslinked carboxymethyl starch coating showed a cumulative nitrogen release rate of 74% in enzyme-free still water after 28 days, comparable to Example 1, indicating that it also possesses good sustained-release performance. However, in a phosphatase-containing medium, its cumulative nitrogen release rate was only 76% after 28 days, with a phosphatase-responsive release increment ΔR of only 2%, indicating almost no phosphatase responsiveness. This is because the carboxylic acid ester crosslinking bond formed by citric acid and carboxymethyl starch cannot be hydrolyzed by phosphatase. In Comparative Example 4, the polyvinyl alcohol water-swellable coating showed cumulative nitrogen release rates of 75% and 76% in enzyme-free still water and a phosphatase-containing medium after 28 days, respectively, with a ΔR of only 1%, further confirming that non-phosphatase substrate-type sustained-release coatings cannot respond to phosphorus deficiency signals in crops. Although Comparative Example 3 omitted the iron-zinc loading, its phytate phosphate diester cross-linking network remained intact, thus maintaining a 16% phosphatase response release increment, comparable to Example 1. This indicates that the phosphatase responsiveness originates from the phytate phosphate diester cross-linking structure and is unrelated to the iron-zinc loading. However, the amount of phosphorus accumulation in maize in the pot experiment on phosphorus-deficient soil was significantly lower than in Example 1, highlighting the independent contribution of simultaneous iron and zinc micronutrient supply. Comparative Example 5, due to its excessively thin coating, experienced localized damage to the coating before sufficient decross-linking occurred. This resulted in a cumulative nitrogen release rate of 85% in 28 days of enzyme-free static water, exceeding the national standard limit for slow-release fertilizers, and its phosphatase response release increment decreased to 6%. This demonstrates that an excessively low coating ratio will simultaneously impair both slow-release performance and response sensitivity.

[0035] The cumulative nitrogen release rate was determined according to the static water extraction method of GB / T 23348-2009. 10.0 g of sample was placed in 250 mL of deionized water and allowed to stand at 25°C. The total nitrogen content of the extract was measured on days 1, 3, 7, 14, 21, and 28. The cumulative nitrogen release rate was calculated as the mass fraction of cumulative dissolved nitrogen relative to the total nitrogen in the sample. Simultaneously, a phosphatase-containing medium group was set up, in which acid phosphatase and phytase (total enzyme activity 50 U / mL) were added to the extract to simulate a high-phosphatase environment in the rhizosphere. The cumulative nitrogen release rate was measured at the same time points. The phosphatase response release increment ΔR was defined as the difference between the cumulative nitrogen release rate of the same sample on day 28 in the phosphatase-containing medium and in enzyme-free static water, used to characterize the sample's response sensitivity to phosphatase signals. The differential dissolution rate was calculated according to GB / T23348-2009. The pot experiment on phosphorus-deficient soil used phosphorus-deficient red soil as the substrate and maize as the test crop. Each pot was treated with the same amount of nitrogen for each sample. At maturity, the phosphorus accumulation in the aboveground parts of maize, grain yield, and nitrogen fertilizer utilization rate were measured. On day 30 after fertilization, the number of phosphorus-solubilizing microorganisms and the activity of phosphatase in the rhizosphere soil were measured. The number of phosphorus-solubilizing microorganisms was counted by the dilution plate method, and the phosphatase activity was determined by the disodium p-nitrophenyl phosphate method. The results were expressed as the amount of p-nitrophenol released per gram of dry soil per hour (μmol / (g·h)). Each experiment was repeated three times.

[0036] To characterize the formation of phosphate diester crosslinks between phytic acid and carboxymethyl starch, Fourier transform infrared spectroscopy was used to compare and analyze carboxymethyl starch and phytic acid-crosslinked carboxymethyl starch: the crosslinked sample at approximately 1250 cm⁻¹... -1 The characteristic absorption peak at approximately 1000 cm⁻¹ is attributed to the stretching vibration of the phosphorus-oxygen double bond in the phosphodiester bond. -1 The presence of an absorption peak at the point of absorption indicates that the phosphate group of phytic acid has formed a phosphodiester bond with the hydroxyl group of carboxymethyl starch. Solid-state phosphorus nuclear magnetic resonance (SMR) further confirmed this, showing a clear resonance signal in the phosphodiester region of the cross-linked sample, allowing estimation of the average number of phosphodiester cross-links formed per molecule of phytic acid. The loading of iron and zinc in the coating was determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). The mass fraction of iron and zinc in the sample was determined after nitric acid digestion. The mass fraction of the slow-release organic fertilizer containing the coating was calculated from the mass increase of the granules before and after coating. The degree of substitution of carboxymethyl starch was determined by potentiometric titration after acid washing.

[0037] To simulate the difference in nutrient release under phosphorus deficiency and phosphorus-sufficient conditions, two potted plant groups were established: one in phosphorus-deficient soil and one in phosphorus-rich soil. The rhizosphere phosphatase activity and corresponding nutrient release rates were measured in both groups to verify the positive correlation between nutrient release rate and rhizosphere phosphatase activity. All quantitative data are expressed as the mean of three replicates. Differences between groups were tested using analysis of variance to ensure the reliability and comparability of the data. Key detection data for each example and comparative example are summarized in Table 1.

[0038] Table 1 Key detection data for each embodiment and comparative example

[0039] Combined with Table 1 and Figure 2 , Figure 3 The technical effects of this invention and the synergistic relationship of its various features can be quantitatively analyzed as follows. For example... Figure 2 As shown, Example 1 achieved a cumulative nitrogen release rate of 73% in enzyme-free still water over 28 days, meeting the requirements of GB / T 23348-2009 for slow-release fertilizers with a cumulative nutrient release rate of no more than 80% over 28 days. Its first-day release rate of 6.5% was no higher than the standard requirement of 15%, and its differential dissolution rate of 1.3% / day also fell within the acceptable range of 0.25-2.5% / day. The corresponding indicators of Examples 2 and 3 were also qualified. However, Comparative Example 2, lacking a slow-release coating, achieved a first-day release rate of 34% and a differential dissolution rate of 8.5% / day, with a 28-day release rate of 95%, which did not meet the slow-release fertilizer standard. Comparative Example 5, with a coating ratio of only 2%, which was lower than the window specified in this invention, experienced coating damage due to insufficient mechanical strength before decrosslinking, resulting in a 28-day release rate of 85%, exceeding the standard limit. This demonstrates the critical significance of the lower limit of the coating ratio.

[0040] The core effect of this invention, which distinguishes it from existing technologies, lies in the incremental release ΔR of the phosphatase response. For example... Figure 3 As shown, the ΔR values ​​for Examples 1, 2, and 3 are 16%, 17%, and 14%, respectively, indicating significantly accelerated release in phosphatase-containing media compared to enzyme-free static water. In contrast, Comparative Example 1, using a citrate carboxylic acid ester crosslinking membrane, has a ΔR of only 2%; Comparative Example 4, using a polyvinyl alcohol water-swellable membrane, has a ΔR of only 1%; and Comparative Example 2, without a membrane, has a ΔR of only 1%. Comparative Examples 1 and 4 also possess complete sustained-release membranes but exhibit almost no phosphatase responsiveness. The only difference lies in the fact that their membrane crosslinking bonds are not phosphatase substrates. This quantitatively demonstrates that the phosphatase responsiveness of this invention does not originate from the sustained-release membrane itself but from the distinctive technical feature of phytate phosphate diester crosslinking bonds acting as phosphatase substrates. The ΔR jumps from approximately 1-2% to 14-17%, a nonlinear emergent effect that cannot be achieved by a simple sustained-release membrane structure.

[0041] In pot experiments on phosphorus-deficient soils, the independent contributions and synergistic relationships of various characteristics became more apparent. Taking phosphorus accumulation in the aboveground parts of maize as an example, Comparative Example 2, which is closest to the existing technology, had a phosphorus accumulation of only 28.5 mg / plant due to rapid nutrient loss in the early stages. Although Comparative Examples 1 and 4, which used non-responsive slow-release coatings, had slow-release properties, their phosphorus accumulation only increased slightly to 31.2 and 30.1 mg / plant, respectively. This indicates that simply extending the release period without matching it with the crop's nutrient requirement rhythm has limited effect on improving phosphorus absorption. Comparative Example 3, which exhibits phosphatase response but is not loaded with iron and zinc, saw an increase in phosphorus accumulation to 35.8 mg / plant, a net increase of approximately 4.6 mg / plant compared to Comparative Example 1. This is attributed to the contribution of fertilization timing matching resulting from the cross-linking response characteristics of phytate phosphate diester. Meanwhile, Example 1, loaded with both iron and zinc, further increased phosphorus accumulation to 42.6 mg / plant, a net increase of approximately 6.8 mg / plant compared to Comparative Example 3. This is attributed to the molecular design contribution of phytate free phosphate groups coordinating with and loading iron and zinc, simultaneously releasing trace elements with phosphatase hydrolysis, and activating fixed phosphorus. It is noteworthy that the coordination loading of iron and zinc must be attached to the phytate phosphate diester cross-linking network to be achieved; the two are structurally inseparable, thus the effects of responsive release and simultaneous supply of trace elements are coupled and symbiotic. In summary, Example 1 showed an approximately 49.5% increase in phosphorus accumulation compared to Comparative Example 2, an increase in grain yield from 27.8 g / plant to 38.5 g / plant, and an increase in nitrogen fertilizer utilization rate from 33.5% to 48.2%.

[0042] The positive feedback loop effect can be corroborated by rhizosphere microecological indicators. On day 30 after fertilization, the number of phosphorus-solubilizing microorganisms in the rhizosphere of Example 1 reached 8.5 × 10⁻⁶. 6 The CFU / g and phosphatase activity reached 2.8 μmol / (g·h); the comparative example 3, which was not loaded with iron and zinc, lacked zinc as a cofactor for phosphatase, and its number of phosphate-solubilizing microorganisms and phosphatase activity decreased to 5.1 × 10⁻⁶. 6 The CFU / g was 2.0 μmol / (g·h); in Comparative Example 4, which used polyvinyl alcohol coating and an unreactive cross-linked oligosaccharide carbon source, the CFU / g was further reduced to 3.2 × 10⁻⁶. 6 CFU / g and 1.5 μmol / (g·h). This indicates that the polysaccharides and zinc ions released by decrosslinking can indeed promote the proliferation of rhizosphere phosphate-solubilizing microorganisms and their phosphatase secretion, thereby accelerating the hydrolysis of crosslinks, forming a self-reinforcing positive feedback loop, and enabling sustained demand response.

[0043] The control experiment between phosphorus-deficient and phosphorus-rich soils further confirmed the positive correlation between nutrient release rate and rhizosphere phosphatase activity. In phosphorus-deficient soils, maize rhizosphere phosphatase activity was high, and the nutrient release rate of Example 1 was significantly accelerated under these conditions, with the cumulative nitrogen release rate reaching levels close to that of the phosphatase-containing medium group in the first 28 days. In contrast, in phosphorus-rich soils, maize rhizosphere phosphatase activity was low, and the nutrient release rate of Example 1 slowed down accordingly, with the cumulative nitrogen release rate approaching that of the enzyme-free still water group. In contrast, the release rates of Comparative Examples 1 and 4 were almost identical in phosphorus-deficient and phosphorus-rich soils, and did not change with the difference in rhizosphere phosphatase activity. This control result directly proves that the biofeedback pathway between nutrient release rate and crop phosphorus deficiency signal established in this invention is real and effective. That is, the more phosphorus-deficient the crop, the higher the rhizosphere phosphatase activity, and the faster the slow-release organic fertilizer of this invention releases nutrients, thereby achieving an adaptive match between nutrient supply and demand in time. Based on the above data on static water release, enzyme-containing medium release, phosphorus-deficient and phosphorus-rich soil control, and pot experiment, the data from each group corroborate each other and are logically consistent, collectively demonstrating the significant progress of this invention in on-demand fertilization compared to existing slow-release fertilizers.

[0044] The technical effectiveness of this invention stems from the synergistic coupling of fundamental mechanism innovation and molecular design. At the fundamental mechanism level, this invention breaks through the traditional understanding that slow-release coatings rely on passive controlled release through physical swelling and have a fixed preset release cycle. It discovers that phosphatases upregulated by roots and rhizosphere phosphate-solubilizing microorganisms during crop phosphorus deficiency can serve as a natural chemical signal characterizing the degree of phosphorus stress in crops. Accordingly, this invention designs the cross-linking bonds of the slow-release coating itself as a phosphatases substrate, allowing the rate of decrosslinking of the coating to be directly regulated by the activity of rhizosphere phosphatases, thereby establishing a biofeedback pathway between nutrient release rate and crop phosphorus deficiency signals at the molecular level. The more severe the crop phosphorus stress, the more rhizosphere phosphatases are secreted, the faster the phosphodiester cross-linking bonds are hydrolyzed, and the faster the coating decrosslinking and nutrient release. Conversely, when phosphorus supply is sufficient and phosphatases activity is low, the cross-linking network remains dense, and nutrients are released slowly. This achieves an adaptive match between the timing of fertilization and the crop's nutrient requirement rhythm, which is something that non-responsive slow-release coatings such as Comparative Example 1 and Comparative Example 4 cannot achieve.

[0045] At the molecular design level, this invention uses phytic acid as a cross-linking agent, which has multiple synergistic mechanisms. First, phytic acid is inositol hexaphosphate, with each molecule containing six phosphate groups. This invention controls an average of 1.5 to 3 phosphate groups to form phosphodiester cross-linking bonds with the hydroxyl groups of polysaccharides, while the rest remain as free phosphate groups. This ensures both the film-forming properties and mechanical strength of the coating, and allows the free phosphate groups to serve as the initial adsorption and recognition sites for phosphatases, accelerating enzymatic hydrolysis. Simultaneously, the hydrolyzed free phosphate groups become a slow-release phosphorus source. Second, the free phosphate groups of phytic acid are natural iron and zinc chelating sites. During the gradual removal of phosphate groups by phosphatases, the chelated iron and zinc are released simultaneously, making the peak of phosphorus deficiency in crops coincide with the peak of micronutrient supply. This synchronicity cannot be achieved by passively controlled-release coatings and external micronutrient addition. Furthermore, the organic acid anions released by phytic acid and its hydrolysis can compete for coordination with insoluble phosphorus fixed by iron and aluminum oxides in the soil, activating the fixed phosphorus. Third, the oligosaccharides generated by phosphatase hydrolysis of the polysaccharide cross-linking network serve as carbon sources for rhizosphere phosphate-solubilizing microorganisms, and the released zinc ions act as cofactors for phosphatase. Both work together to promote the proliferation of phosphate-solubilizing microorganisms and the secretion of phosphatase, which in turn accelerates the hydrolysis of cross-linking bonds, forming a positive feedback loop that sustainably amplifies the demand response. The aforementioned underlying mechanisms and molecular design are mutually defined and inseparable: the demand for phosphatase response stems from a fundamental understanding of crop nutrient requirement rhythms, and the phytate phosphate diester cross-linking structure is precisely the molecular solution to meet this demand; the simultaneous supply of iron and zinc must rely on the phytate cross-linking network to be achieved. The bidirectional verification of Comparative Examples 3 and 4 in Table 1, and the non-linear improvement in phosphorus accumulation, yield, and nitrogen fertilizer utilization rate of the examples relative to the comparative examples, quantitatively demonstrate the synergistic effect of underlying innovation and molecular design.

[0046] In summary, the substantial advancements of this invention can be summarized from the perspective of a paradigm shift. Regardless of the coating material used, the regulatory variables for nutrient release in existing slow-release fertilizers are always the physical parameters of the coating itself or non-specific environmental factors of the soil, with crops always in a passive position during this release process. However, this invention, by designing the coating cross-links as specific substrates for crop phosphorus deficiency signal molecules (phosphatases), transforms crops from passive recipients to active regulators of nutrient release. The more severe the phosphorus deficiency in the crop, the more phosphatases it secretes, and the faster the nutrient release of the slow-release organic fertilizer of this invention. This achieves a paradigm shift from fertilizer-centric passive controlled release to crop-centric on-demand fertilization. This paradigm shift cannot be achieved through simple material replacement or parameter optimization, but is based on a fundamental understanding of the physiological signals of crop phosphorus stress and the precise molecular design of phytate phosphate diester cross-linking. These two aspects are mutually defined and inseparable, constituting the core of the invention's inventiveness that distinguishes it from existing technologies. Although Comparative Examples 1 and 4 in Table 1 above also have complete sustained-release membranes, they have almost no phosphatase responsiveness, which confirms the irreplaceability of this inventive core from both sides.

[0047] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A straw-based slow-release organic fertilizer, comprising an organic fertilizer core loaded with organic matter and nitrogen, phosphorus, and potassium nutrients using straw biochar as a carrier, and a slow-release coating covering the organic fertilizer core, characterized in that: The slow-release coating is a cross-linked network formed by phytic acid cross-linking a hydroxy polysaccharide substrate through phosphodiester bonds. The slow-release coating accounts for 3% to 15% of the mass of the slow-release organic fertilizer. Each molecule of phytic acid forms an average of 1.5 to 3 phosphodiester bonds, with the remaining phosphate groups remaining as free phosphate groups. The phosphodiester bonds constitute the hydrolytic substrates of acid phosphatases and / or phytases secreted by plant roots and rhizosphere microorganisms. When the crop is under phosphorus stress and secretes the phosphatases, the phosphatases hydrolyze the phosphodiester bonds, causing the cross-linked network to de-crosslink, thereby releasing the nutrients in the core of the organic fertilizer through the de-crosslinked coating. The nutrient release rate increases with the increase of the phosphatase activity.

2. The straw-based slow-release organic fertilizer according to claim 1, characterized in that: The hydroxyl-containing polysaccharide substrate is carboxymethyl starch, and the degree of carboxymethyl substitution of the carboxymethyl starch is 0.3~0.

5.

3. The straw-based slow-release organic fertilizer according to claim 1, characterized in that: The phytic acid retains free phosphate groups that are coordinated and loaded with iron ions and / or zinc ions. Based on the mass of the slow-release organic fertilizer, the iron content is 0.5%~3.0% and the zinc content is 0.2%~1.5%. The iron ions and / or zinc ions are released simultaneously as the phosphatase hydrolyzes the phytic acid.

4. The straw-based slow-release organic fertilizer according to claim 1, characterized in that: The straw biochar is obtained by pyrolysis of corn stalks or rice stalks under limited oxygen conditions at 400-500°C, and its specific surface area is not less than 150 m². 2 / g; The nitrogen, phosphorus, and potassium nutrients in the organic fertilizer core are provided by urea, diammonium phosphate, and potassium sulfate.

5. The straw-based slow-release organic fertilizer according to claim 1, characterized in that: The slow-release coating accounts for 5% to 12% of the mass of the slow-release organic fertilizer.

6. The straw-based slow-release organic fertilizer according to claim 1, characterized in that: The polysaccharide-degraded oligosaccharides generated by the phosphatase hydrolyzing the cross-linked network, as well as the released zinc ions, serve as carbon sources and phosphatase cofactors for rhizosphere phosphate-solubilizing microorganisms, respectively, promoting the proliferation of rhizosphere phosphate-solubilizing microorganisms and their phosphatase secretion, thereby accelerating the hydrolysis of the phosphodiester bonds.

7. A method for preparing the straw-based slow-release organic fertilizer according to any one of claims 1 to 6, characterized in that, The process includes the following steps: (1) hydroxyl polysaccharide is etherified with monochloroacetic acid under alkaline conditions to obtain carboxymethylated polysaccharide; (2) the carboxymethylated polysaccharide is mixed with phytic acid and heated in the presence of a catalyst to undergo phosphorylation crosslinking to obtain phytic acid crosslinked polysaccharide; (3) the phytic acid crosslinked polysaccharide is coordinated and loaded with iron salt and / or zinc salt in an acidic aqueous phase to obtain a slow-release coating material; (4) the slow-release coating material is coated onto the core of an organic fertilizer that uses straw biochar as a carrier and is loaded with organic matter and nitrogen, phosphorus and potassium nutrients to obtain the straw char-based slow-release organic fertilizer.

8. The method according to claim 7, characterized in that: In step (1), the molar ratio of the hydroxy polysaccharide, monochloroacetic acid and sodium hydroxide is 1.0:1.0:2.4 based on dehydrated glucose units. The reaction solvent is isopropanol and water in a volume ratio of 8:

2. The reaction is first alkalized at 30°C for 30 min and then etherified at 60°C for 2.5 h. The degree of substitution of the resulting carboxymethylated polysaccharide is 0.3~0.

5.

9. The method according to claim 7, characterized in that: In step (2), the amount of phytic acid used is 5% to 30% of the mass of the carboxymethylated polysaccharide, the catalyst is urea and the molar ratio of urea to phytic acid is 2:1, the heating temperature is 120 to 140°C and the time is 1 to 2 h.

10. The method according to claim 7, characterized in that: In step (3), the iron salt is ferric chloride hexahydrate and the zinc salt is zinc sulfate heptahydrate, which are coordinated and loaded in an aqueous phase with pH 4~5 and carried out at 40°C for 2 h; in step (4), the coating amount of the slow-release coating material is such that the slow-release coating accounts for 3%~15% of the mass of the slow-release organic fertilizer.

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