A method for green preparation of high-efficiency and low-cost slow-release phosphate fertilizer using crop straw
By extracting phosphorus from crop straw through crushing and water oscillation, and combining it with the use of bicarbonate and metal salts, a high-efficiency, low-cost slow-release phosphate fertilizer is prepared. This solves the problems of high equipment, high energy consumption, and long cycles in the existing technology, achieves effective phosphorus enrichment and slow-release effects, and reduces production costs and environmental pollution risks.
Patent Information
- Application Number
- CN202510222444.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2025-02-27
- Publication Date
- 2025-09-30
- Estimated Expiration
- 2045-02-27
AI Technical Summary
The existing technology for preparing slow-release phosphate fertilizers has problems such as high equipment requirements, high energy consumption, long production cycle, high cost, and difficulty in effectively enriching phosphorus, resulting in the inability to meet seasonal agricultural production needs. In addition, chemical fertilizers are easily fixed in the soil, leading to nutrient loss and water pollution risks.
By crushing crop straw, extracting phosphorus by water oscillation, adding bicarbonate and metal salts, adjusting the pH value to form a precipitate, separating and drying, a slow-release phosphate fertilizer is produced. The process is simple and only takes 1-2 days. Equipment and reagents are easily available, and the production cost is low.
It achieves efficient recycling and utilization of phosphorus in crop straw, and prepares slow-release phosphorus fertilizer that is not easily fixed by soil and has high biological effectiveness, which reduces production costs, simplifies the process, reduces waste disposal costs, and meets agricultural production needs.
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Figure CN120058421B_ABST
Abstract
Description
Technical Field
[0001] The invention belongs to the field of organic fertilizer manufacturing and relates to a method for greenly preparing high-efficiency and low-cost slow-release phosphate fertilizer by utilizing crop straw. Background Art
[0002] For a long time, high and stable agricultural yields in my country have relied primarily on chemical phosphate fertilizers. However, because chemical fertilizers are highly reactive and easily fixed in the soil after application, the total amount of soil phosphorus continues to increase, but its effectiveness is low. This increases agricultural input costs while exacerbating nutrient loss from farmland and the risk of water pollution. Therefore, effectively recycling and utilizing crop straw phosphorus resources and preparing slow-release phosphate fertilizers that are not easily fixed in the soil are of great significance to the green development of my country's agriculture.
[0003] Current technologies often utilize straw to produce slow-release fertilizers through carbonization or fermentation. For example, patent application publication number CN 110423160 A discloses a method for preparing a slow-release phosphate fertilizer using corn straw biochar as a carrier. The method involves first burning the biochar, then soaking it in a saturated potassium dihydrogen phosphate solution, and then drying it. Patent application publication number CN 104909842 A discloses a method for producing a slow-release phosphate fertilizer using crop straw, which is produced by co-burning straw with phosphate rock powder. Patent application publication number CN 116178078 A discloses a carbon-rich slow-release phosphate fertilizer and its preparation method, which is produced by co-hydrothermal reaction of straw and phosphate rock powder. However, carbonization methods often require high temperatures or pressures, placing high demands on equipment and consuming significant amounts of energy. For the fermentation approach, for example, a fermented straw slow-release fertilizer is disclosed in the patent with application publication number CN107721739 A; and a method for preparing a slow-release fertilizer that promotes straw return to the field is disclosed in the patent with application publication number CN 107814622 A. However, fermentation to produce slow-release fertilizers often requires a series of complex process steps, a relatively long production cycle, and high production costs, which may lead to untimely supply and inability to meet the seasonal needs of agricultural production. In addition, neither the carbonization nor the fermentation approach effectively enriches phosphorus from the straw. Because the phosphorus content in the straw itself is relatively low, the nutrient content of the produced fertilizer will still be difficult to meet commodity demand. This technology can not only effectively recycle and enrich straw phosphorus resources and has green and environmentally friendly characteristics, but the prepared phosphate fertilizer is not easily fixed by the soil, has high biological effectiveness, and has the characteristics of slow release and high efficiency, which is of great significance for promoting the green and high-quality development of my country's agriculture. Summary of the Invention
[0004] The object of the present invention is to provide a simple and efficient method for enriching and recovering straw phosphorus and simultaneously generating slow-release phosphate fertilizer, comprising the following steps:
[0005] Step 1) Crush the crop straw, extract the crop straw using water oscillation, add 8-15 liters of water per kilogram of dry straw, stir at 20-30°C for 1-48 hours, filter, and obtain a straw extract;
[0006] Step 2), diluting the crop straw extract to 1 to 20 times, and adding bicarbonate and metal salt per liter of the original solution before dilution; for corn straw, when synthesizing straw-derived calcium-based slow-release phosphate fertilizer, add at least 4.5 grams of sodium bicarbonate and at least 5.9 grams of calcium chloride; when synthesizing straw-derived magnesium-based slow-release phosphate fertilizer, add at least 1.8 grams of sodium bicarbonate and at least 2.0 grams of magnesium chloride; when synthesizing straw-derived iron-based slow-release phosphate fertilizer, add at least 2.5 grams of ferric chloride; for wheat and rice straw, the amount of agent added is the same: when synthesizing straw-derived calcium-based slow-release phosphate fertilizer, add at least 1.6 grams of sodium bicarbonate and at least 2.0 grams of calcium chloride; when synthesizing straw-derived magnesium-based slow-release phosphate fertilizer, add at least 1.5 grams of sodium bicarbonate and at least 1.7 grams of magnesium chloride; when synthesizing straw-derived iron-based slow-release phosphate fertilizer, add at least 0.72 grams of ferric chloride;
[0007] Step 3), adding alkali solution to increase the pH of the mixed solution until a precipitate is formed. When synthesizing straw-derived calcium-based slow-release phosphate fertilizer, the pH of the solution is adjusted to above 8.5; when synthesizing straw-derived magnesium-based slow-release phosphate fertilizer, the pH of the solution is adjusted to above 10.5; when synthesizing straw-derived iron-based slow-release phosphate fertilizer, the pH of the solution is adjusted to above 3.0;
[0008] Step 4) After separating and drying the precipitate, the straw-derived slow-release phosphate fertilizer is obtained.
[0009] In one embodiment, in step 1), 9-11 liters of water are added to each kilogram of dry straw, stirred at room temperature for 16-18 hours, and filtered to obtain a straw extract.
[0010] In one embodiment, in step 2), the properties of the same crop straw may be different in different growth environments, and the minimum input amount of sodium bicarbonate and metal salts may be adjusted within the range of 50% to 200% to optimize the effect of the slow-release phosphate fertilizer.
[0011] In one specific embodiment, in step 3), when synthesizing straw-derived calcium-based slow-release phosphate fertilizer, the pH value of the solution should be adjusted to between 9.0 and 9.5; when synthesizing straw-derived magnesium-based slow-release phosphate fertilizer, the pH value of the solution should be adjusted to between 10.5 and 11.0; when synthesizing straw-derived iron-based slow-release phosphate fertilizer, the pH value of the solution should be adjusted to between 3.5 and 3.7; to optimize the effect of the slow-release phosphate fertilizer.
[0012] In one embodiment, in step 4), the method of separating the precipitate comprises at least one of centrifugation, decantation, and filtration to achieve solid-liquid separation; the method of drying comprises at least one of freeze-drying, air-drying, and oven-drying to achieve drying of the precipitate. If oven-drying is used, the temperature is 50-75°C.
[0013] It should be noted that in step 1), the finer the crop straw is pulverized, the better. More phosphorus can be extracted by subsequent water oscillation extraction. The straw can pass through a 1 mm sieve. In mass production, it can also pass through a 1 cm sieve. When filtering the straw extract, gauze or a net bag can be used.
[0014] It should be noted that in step 2), for the slow-release phosphate fertilizer prepared from different crop straws, the minimum input amount of sodium bicarbonate and metal salt is not only the most economical, but also enables the synthesized slow-release phosphate fertilizer to have the best performance. The purpose of the added bicarbonate and metal salt is to provide carbonate / bicarbonate and metal ions, and is not limited to sodium bicarbonate and calcium chloride, magnesium chloride, and ferric chloride, and includes but is not limited to raw materials containing or capable of generating bicarbonate ions and calcium, magnesium, and iron metal ions, such as sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium oxide, calcium hydroxide, magnesium hydroxide, magnesium sulfate, and ferric sulfate.
[0015] It should be pointed out that the straw comes from at least one of food crops, cash crops, oil crops, vegetable crops, fruit crops, feed crops, and medicinal crops, specifically including but not limited to corn straw, wheat straw, rice straw, soybean straw, sorghum straw, sugarcane straw, cotton straw, sweet potato straw, oat straw, and alfalfa straw.
[0016] The slow-release phosphate fertilizer prepared by the method of the present application only recycles the phosphorus resources in the crop straw without adding exogenous mineral phosphate fertilizer; compared with the existing technology, this technology has the following beneficial effects:
[0017] (1) Green and efficient. This technology can recycle 90% of the phosphorus in straw and enrich and convert it into slow-release phosphate fertilizer, turning waste into treasure. At the same time, the straw residue after water extraction can still be directly returned to the field, generating less waste and low subsequent processing costs.
[0018] (2) Slow release and high efficiency. This product has excellent slow release characteristics. In particular, straw-derived calcium-based slow release phosphate fertilizer is not easily fixed by the soil after being applied to the soil. It can maintain high phosphorus activity and has slow release characteristics, which can effectively promote crop phosphorus absorption. At the same time, the performance of phosphate fertilizer is better than the commonly used slow release mineral phosphate fertilizer - calcium hydrogen phosphate. Compared with calcium hydrogen phosphate, the amount of active phosphorus released by this product converted to the fast-acting state Ca2-P is consistent with that of calcium hydrogen phosphate, but the amount converted to the slow-acting state Ca8-P with the second highest bioavailability is nearly 2 times that of calcium hydrogen phosphate, which is less likely to be fixed by the soil.
[0019] (3) Simple and environmentally friendly. The equipment and reagents used are widely available, the process is simple, and the entire process takes only 1-2 days, with low production costs. The raw materials used are entirely composed of crop straw and environmentally friendly calcium / magnesium / iron metal salts, bicarbonate, and sodium hydroxide. BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Figure 1 This is a diagram showing the effect of continued stirring time after pH adjustment in step 3);
[0021] Figure 2 This is the pH effect diagram in step 2);
[0022] Figure 3 This is a diagram showing the effect of the amount of calcium chloride, magnesium chloride, and ferric chloride added in step 2);
[0023] Figure 4 Graph showing the effects of calcium chloride, magnesium chloride, and ferric chloride input after corn straw in step 2) is replaced with wheat straw and rice straw, respectively;
[0024] Figure 5 The slow-release characteristics of straw-derived calcium-based, magnesium-based, and iron-based slow-release fertilizers;
[0025] Figure 6 The present invention provides an evaluation diagram for replenishing the soil phosphorus pool and affecting the phosphorus absorption in the wheat rhizosphere.
[0026] The present invention will be further described below with reference to the following examples. However, the examples are for illustrative purposes only and are not intended to limit the present invention in any way. The materials and reagents used are all commercially available unless otherwise specified.
[0027] Dry straw: The moisture content of crop straw should be less than 5%.
[0028] The straw includes straw from different crop categories, selected from at least one of food crops, cash crops, oil crops, vegetable crops, fruit crops, feed crops, and medicinal crops, specifically including but not limited to corn straw, wheat straw, rice straw, soybean straw, sorghum straw, sugarcane straw, cotton straw, sweet potato straw, oat straw, and alfalfa straw.
[0029] Example 1
[0030] Synthesis of straw-derived calcium-, magnesium- and iron-based slow-release phosphate fertilizers
[0031] Step 1): The corn straw is crushed, sieved to 1 mm, and stirred at a ratio of 10 liters of water per kilogram of corn straw for 18 hours, and filtered through a 38 μm filter to obtain a straw extract.
[0032] Step 2), dilute the corn straw extract 10.6 times; for straw-derived calcium-based slow-release phosphate fertilizer, add 8.9g sodium bicarbonate and 11.8g calcium chloride per liter of stock solution before dilution; for straw-derived magnesium-based slow-release phosphate fertilizer, add 8.9g sodium bicarbonate and 10.1g magnesium chloride per liter of stock solution before dilution; for straw-derived iron-based slow-release phosphate fertilizer, add 17.2g ferric chloride per liter of stock solution before dilution.
[0033] Step 3), for straw-derived calcium-based slow-release phosphate fertilizer, use sodium hydroxide to adjust the pH to 8.5, and continue stirring for 4 hours after the pH is adjusted; for straw-derived magnesium-based slow-release phosphate fertilizer, use sodium hydroxide to adjust the pH to 10.5, and continue stirring for 4 hours after the pH is adjusted; for straw-derived iron-based slow-release phosphate fertilizer, use sodium hydroxide to adjust the pH to 4.5, and continue stirring for 4 hours after the pH is adjusted.
[0034] Step 4) Collect the precipitate by centrifugation and dry it at 65℃ to obtain slow-release phosphate fertilizer. The phosphorus precipitation rate is calculated as (1-phosphorus concentration of solution after reaction / phosphorus concentration before reaction). (Based on 1kg dry straw: 10L water)
[0035] Straw type Carbon concentration of extract (mg / L) Phosphorus concentration of extract (mg / L) Total phosphorus content of straw (mg / kg) Phosphorus extraction rate (%) corn stalks 5599 176.2 1914.6±45.5 92.1±2.2
[0036] Table 1
[0037] According to Table 1, during the water oscillation extraction process, 92.1% of the phosphorus from the corn stover was converted into the extract. The precipitation rates of calcium-based, magnesium-based, and iron-based phosphorus were 97.2%, 98.1%, and 99.7%, respectively. Therefore, the conversion rates of phosphorus from corn stover to slow-release phosphate fertilizer were 90%, 90%, and 92%, respectively.
[0038] Example 2
[0039] Investigate the effect of continued stirring time after pH adjustment in step 3)
[0040] After adjusting the pH in step 3), continue stirring for as long as Figure 1 Other conditions were the same as those in Example 1. The phosphorus precipitation rate was calculated as (1-phosphorus concentration of the solution after the reaction / phosphorus concentration before the reaction).
[0041] according to Figure 1 It can be seen that the synthesis reaction of straw-derived calcium-based, magnesium-based, and iron-based slow-release phosphate fertilizers reaches equilibrium immediately after the pH is adjusted, and can remain stable within 6 hours, which is enough time to complete operations such as solid-liquid separation.
[0042] Example 3
[0043] Investigating the effect of pH in step 3)
[0044] The pH in step 2) is as follows Figure 2Other conditions were the same as those in Example 1. The phosphorus precipitation rate was calculated as (1-phosphorus concentration of the solution after the reaction / phosphorus concentration before the reaction).
[0045] according to Figure 2 The synthesis reactions of straw-derived calcium-, magnesium-, and iron-based slow-release phosphate fertilizers achieve the highest phosphorus precipitation rates at pH values of 8.5, 10.5, and 3.0, respectively. However, in the actual synthesis of straw-derived calcium-, magnesium-, and iron-based slow-release phosphate fertilizers, it was found that equilibrium was difficult to achieve at pH values of 8.5-9.0, requiring repeated pH adjustments over a long period of time to maintain the pH. Therefore, a pH of 9.0-9.5 was selected as the optimal pH, allowing for a slight excess of alkali solution to ensure the highest phosphorus recovery rate. For straw-derived iron-based slow-release phosphate fertilizers, although a pH of 3.0 maximized phosphorus precipitation, the resulting ferrihydrite precipitate was too small to be easily separated, thus selecting a pH of 3.5 as the optimal pH. In summary, the optimal pH values for synthesizing straw-derived calcium-, magnesium-, and iron-based slow-release phosphate fertilizers are 9.0-9.5, 10.5-11.0, and 3.5-3.7, respectively.
[0046] Example 4
[0047] Investigate the impact of the amount of sodium bicarbonate, calcium chloride, magnesium chloride, and ferric chloride in step 2)
[0048] The amount of calcium chloride, magnesium chloride and ferric chloride in step 2) is as follows: Figure 3 For straw-derived calcium-based and magnesium-based slow-release phosphate fertilizers, the amount of sodium bicarbonate added was adjusted so that its mass ratio with calcium chloride or magnesium chloride remained constant. Other conditions were the same as in Example 1. The phosphorus precipitation rate was calculated as 1-phosphorus concentration of the solution after reaction / phosphorus concentration before reaction, and the phosphorus availability of the precipitate was calculated as phosphorus extracted from a pH 8.5 0.5 M NaHCO3 solution / total phosphorus.
[0049] according to Figure 3 For corn straw extract, with the highest precipitation rate, the least raw material input, and the highest precipitate phosphorus effectiveness as the goal, the optimal raw material input required for the synthesis of straw-derived calcium-based, magnesium-based, and iron-based slow-release phosphate fertilizers are 5.9 g / L calcium chloride, 2.0 g / L magnesium chloride, and 2.5 g / L iron chloride, respectively.
[0050] Example 5
[0051] The effects of the input amounts of sodium bicarbonate, calcium chloride, magnesium chloride, and ferric chloride were investigated after the corn straw in step 2) was replaced with wheat straw and rice straw, respectively.
[0052] In step 2), the amount of calcium chloride, magnesium chloride and ferric chloride added after corn straw is replaced with wheat straw and rice straw is as follows: Figure 4In step 2), for straw-derived calcium-based and magnesium-based slow-release phosphate fertilizers, the amount of sodium bicarbonate added was always adjusted to keep its mass ratio with calcium chloride or magnesium chloride unchanged. The dilution multiple of the wheat straw and rice straw extracts was changed to 3.5 times, the pH of the straw-derived calcium-based fertilizer was changed to 9.25, and the pH of the iron-based fertilizer was changed to 3.5. Other changes were the same as in Example 1. The phosphorus precipitation rate was calculated as 1-phosphorus concentration of the solution after reaction / phosphorus concentration before reaction.
[0053] according to Figure 4 For wheat and rice straw extracts, with the highest precipitation rate, minimum raw material input, and highest precipitate phosphorus effectiveness as the goals, the optimal raw material input required for the synthesis of straw-derived calcium-based, magnesium-based, and iron-based slow-release phosphate fertilizers of the two straw extracts is the same, namely 2.0 g / L of calcium chloride, 1.7 g / L of magnesium chloride, and 0.72 g / L of iron chloride.
[0054] Example 6
[0055] Evaluation of slow-release properties of straw-derived calcium-based, magnesium-based, and iron-based slow-release phosphate fertilizers - Phosphorus release test
[0056] First, based on Examples 1-5, optimal synthesis conditions were determined. Taking corn straw extract as an example, in step 2), for straw-derived calcium-based slow-release phosphate fertilizer, 4.5g sodium bicarbonate and 5.9g calcium chloride were added per liter of pre-dilution solution. The pH was then adjusted to 9.25 with sodium hydroxide, and stirring was continued for 4 hours after pH adjustment. For straw-derived magnesium-based slow-release phosphate fertilizer, 1.8g sodium bicarbonate and 2.0g magnesium chloride were added per liter of pre-dilution solution. The pH was then adjusted to 10.5 with sodium hydroxide, and stirring was continued for 4 hours after pH adjustment. For straw-derived iron-based slow-release phosphate fertilizer, 2.5g ferric chloride was added per liter of pre-dilution solution. The pH was then adjusted to 3.5 with sodium hydroxide, and stirring was continued for 4 hours after pH adjustment. Other conditions were the same as in Example 1. These yielded straw-derived calcium-based slow-release phosphate fertilizers Ca#, Mg#, and Fe#, respectively.
[0057] For the phosphorus release test, Ca#, Mg#, Fe#, calcium hydrogen phosphate, and diammonium hydrogen phosphate were added to 200 mL of deionized water and incubated at 25°C. The samples were taken at the following time points: Figure 5 The phosphorus concentration was determined using ICP-OES.
[0058] according to Figure 5 Straw-derived calcium-based, magnesium-based, and iron-based slow-release fertilizers all have good slow-release characteristics, specifically: Ⅰ: the release rate of nutrients in the fertilizer within 24 hours does not exceed 15%; Ⅱ: the slow-release time is 10-20 days; Ⅲ: the maximum release rate of straw-derived calcium- and magnesium-based slow-release phosphate fertilizers is higher than 90%, and that of straw-derived iron-based slow-release phosphate fertilizer is higher than 70%. The maximum release rates of the three slow-release phosphate fertilizers are all higher than that of calcium hydrogen phosphate.
[0059] Example 7
[0060] Potted experiment on straw-derived calcium-based slow-release phosphate fertilizer
[0061] The soil type used in the experiment was damp brown soil. Plant residues, stones and other intrusive bodies and new bodies were carefully removed and air-dried, and passed through a 2mm sieve. CK without phosphorus fertilizer was set up, calcium hydrogen phosphate was applied, and Ca# was applied. Phosphorus P was applied: 34 mg P / kg air-dried soil, 214 mg / kg air-dried soil, and 158 mg / kg air-dried soil. The dry weight of the plant in each pot was measured, and the phosphorus content of the plant was determined by the digestion method after the whole plant was crushed. The phosphorus absorption amount of the plant = plant dry weight * plant phosphorus content, see Table 1. At the same time, the calcareous soil inorganic phosphorus grading method of Jiang Baifan and Gu Yichu was used to measure the distribution of phosphorus forms in the non-rhizosphere and rhizosphere soils after potted planting, and to evaluate how this product replenishes the soil phosphorus pool and affects the phosphorus absorption of wheat rhizosphere, see. Figure 6 The same letters after the data in the same column indicate no significant difference. LSD test was used with α=0.05.
[0062] Fertilizer type Plant dry weight (g) Phosphorus absorption by plants (mg / pot) CK 3.65±0.16 b 5.17±0.12 b Calcium hydrogen phosphate 5.13±0.25 a 7.49±0.80 a Straw-derived calcium-based slow-release phosphate fertilizer 5.52±0.25 a 7.77±0.58 a
[0063] Table 2
[0064] According to Table 2, straw-derived calcium-based slow-release phosphate fertilizer can effectively increase plant dry weight and plant phosphorus uptake, and the improvement effect is not significantly different from that of slow-release mineral phosphate fertilizer calcium hydrogen phosphate.
[0065] The released phosphorus is stored in the soil in a form with high bioavailability. Figure 6 Analyzing the distribution of phosphorus forms in non-rhizosphere soils revealed that, after application to damp brown soil, straw-derived calcium-based slow-release phosphate fertilizers released more phosphorus in the more bioavailable form than did dicalcium phosphate. Specifically, straw-derived calcium-based slow-release phosphate fertilizers converted more phosphorus into the second-most bioavailable Ca8-P fraction than dicalcium phosphate. Further quantitative analysis revealed that straw-derived calcium-based slow-release phosphate fertilizers converted 72-80% of phosphorus into the Ca8-P fraction, roughly double the conversion rate of dicalcium phosphate (29-48%). Furthermore, the conversion rate to the most bioavailable Ca2-P fraction (22-25%) was comparable to dicalcium phosphate (22-24%).
[0066] Analysis of phosphorus forms in rhizosphere soil revealed that straw-derived calcium-based slow-release phosphate fertilizer promoted phosphorus uptake in the wheat rhizosphere more effectively than calcium phosphate (DCP). Specifically, the Ca2-P fraction in the rhizosphere after application of this fertilizer was lower than that in the DCP-treated group. Changes in Ca2-P, the most bioavailable fraction, in the rhizosphere soil can be attributed to fertilizer conversion and wheat rhizosphere uptake. However, there was no significant difference in the conversion of Ca2-P to this fraction in the non-rhizosphere soil, ruling out fertilizer conversion as a factor. Therefore, the lower Ca2-P fraction after application of straw-derived calcium-based slow-release phosphate fertilizer relative to the DCP-treated group is likely due to its ability to promote phosphorus uptake in the wheat rhizosphere.
Claims
1. A method for green preparation of high-efficiency and low-cost slow-release phosphate fertilizer using crop straw, characterized in that: The following steps are involved: Step 1) Crush the crop straw and extract the crop straw using water oscillation, adding 8-15 liters of water per kilogram of dry straw, stirring at 20-30° C. for 16-48 hours, and filtering to obtain a straw extract solution; Step 2), diluting the crop straw extract stock solution to 1 to 20 times; adding bicarbonate and metal salt per liter of the stock solution before dilution; when synthesizing straw-derived calcium-based slow-release phosphate fertilizer, adding at least 4.5 grams of sodium bicarbonate and at least 5.9 grams of calcium chloride; when synthesizing straw-derived magnesium-based slow-release phosphate fertilizer, adding at least 1.8 grams of sodium bicarbonate and at least 2.0 grams of magnesium chloride; when synthesizing straw-derived iron-based slow-release phosphate fertilizer, adding at least 2.5 grams of ferric chloride; for wheat and rice straw, when synthesizing straw-derived calcium-based slow-release phosphate fertilizer, adding at least 1.6 grams of sodium bicarbonate and at least 2.0 grams of calcium chloride; when synthesizing straw-derived magnesium-based slow-release phosphate fertilizer, adding at least 1.5 grams of sodium bicarbonate and at least 1.7 grams of magnesium chloride; when synthesizing straw-derived iron-based slow-release phosphate fertilizer, adding at least 0.72 grams of ferric chloride; Step 3), adding alkali solution to increase the pH of the mixed solution to generate a precipitate; when synthesizing straw-derived calcium-based slow-release phosphate fertilizer, the pH value of the solution is adjusted to between 9.0 and 9.5; when synthesizing straw-derived magnesium-based slow-release phosphate fertilizer, the pH value of the solution is adjusted to between 10.5 and 11.0; when synthesizing straw-derived iron-based slow-release phosphate fertilizer, the pH value of the solution is adjusted to between 3.5 and 3.7; Step 4) After separating and drying the precipitate, the straw-derived slow-release phosphate fertilizer is obtained.
2. The method according to claim 1, characterized in that In step 1), the crop straw is crushed and extracted by water oscillation, with 9-11 liters of water per kilogram of dry straw, stirred at 25° C. for 16-18 hours, and filtered to obtain a straw extract solution.
3. The method according to claim 1, characterized in that In step 4), the method of separating the precipitate includes at least one of centrifugation, decantation, and filtration to achieve solid-liquid separation.
4. The method according to claim 1, wherein In step 4), the drying method includes at least one of freeze drying, air drying, and oven drying to dry the precipitate, and the drying temperature is 50-75°C.