Method for green preparation of high-efficiency low-cost slow-release phosphate fertilizer from corn straw

Abstract

The invention discloses a method for preparing a high-efficiency low-cost slow-release phosphate fertilizer from corn straws in a green manner, which comprises the following steps: crushing crop straws, recovering and enriching crop straw phosphorus from a water extracting solution through a calcium / magnesium / iron and other mineral precipitation method, and finally collecting precipitates and drying to prepare the slow-release phosphate fertilizer. The method for recycling, enriching and utilizing crop straw phosphorus has the characteristics of greenness, high efficiency and low cost, and nearly 90% of straw phosphorus can be converted into straw-source calcium-based, magnesium-based and iron-based slow-release phosphate fertilizers; the prepared phosphate fertilizer has the characteristic of efficient slow release, the released phosphorus exists in soil in a form of high bioavailability and is not easy to fix by the soil, and the phosphorus absorption of wheat roots can be obviously promoted. In a word, the product and the preparation technology thereof realize green and efficient recycling of crop straw phosphorus; the prepared phosphate fertilizer has the characteristics of low cost, high fertilizer efficiency, slow release property, environment friendliness and the like.

Description

Technical Field

[0001] The present invention belongs to the field of organic fertilizer manufacturing, and relates to a method for green preparation of high-efficiency and low-cost slow-release phosphate fertilizer using corn straw. Background Art

[0002] In 2022, the crop straw output in China exceeded 860 million tons, mainly including corn, wheat and rice straws, among which corn straw exceeded 280 million tons, wheat straw exceeded 160 million tons, and rice straw exceeded 200 million tons (Liu Junjie et al., 2024), which are important phosphorus reservoir resources. For a long time, the high and stable yield of agriculture in China has mainly relied on the input of chemical phosphate fertilizers. However, because the phosphorus activity is high after the chemical fertilizers are applied to farmland and it is easy to be fixed by the soil, the total amount of soil phosphorus increases continuously but the phosphorus availability is not high, which not only increases the agricultural input cost but also aggravates the risk of farmland nutrient loss and its water pollution. Recently, due to the non-renewable phosphate rock resources, major phosphate rock countries such as the United States and Canada have begun to restrict phosphate fertilizer exports to ensure the future phosphate fertilizer demand of their own agriculture, and at the same time further affect the international phosphate fertilizer supply potential. Therefore, effectively recycling the phosphorus in crop straw resources and preparing slow-release phosphate fertilizers that are not easily fixed by the soil are of great significance for the green development of agriculture in China.

[0003] Currently, in the related technologies, straw is mostly used to produce slow-release fertilizers through carbonization or fermentation. For example, in the patent with the publication number CN 110423160 A, a preparation method of slow-release phosphate fertilizer with corn straw biochar as the carrier is disclosed. First, biochar is fired, and then the biochar is soaked in a saturated potassium dihydrogen phosphate solution and dried to obtain it; in the patent with the publication number CN 104909842 A, a method for producing slow-release phosphate fertilizer using crop straw is disclosed, which is obtained by mixing straw and phosphate rock powder and firing; in the patent with the publication number CN 116178078 A, a carbon-rich slow-release phosphate fertilizer and its preparation method are disclosed, which are obtained by co-hydrothermal reaction of straw and phosphate rock powder. However, the carbonization method often requires high-temperature or high-pressure conditions, which have high requirements for equipment and also consume a large amount of energy. For the fermentation approach, for example, in the patent with the publication number CN107721739 A, a fermented straw slow-release fertilizer is disclosed; in the patent with the publication number CN 107814622 A, a preparation method of a slow-release fertilizer for promoting straw returning to the field is disclosed. However, the production of slow-release fertilizers by fermentation often requires a series of complex technological steps, with a relatively long production cycle and high production costs, which may lead to untimely supply and inability to meet the seasonal demands of agricultural production. In addition, neither the carbonization nor the fermentation approach effectively enriches phosphorus from straw. Since the phosphorus content in straw itself is relatively low, the nutrient content of the produced fertilizers will still be difficult to meet the commercial demands. This technology can not only effectively recycle and enrich straw phosphorus resources and has the characteristics of environmental friendliness, but also 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 agriculture in China. Summary of the Invention

[0004] The purpose of the present invention is to provide a simple and efficient method for enriching and recovering straw phosphorus and simultaneously producing slow-release phosphate fertilizer, including the following steps:

[0005] Step 1): Crush crop straw, extract crop straw by water oscillation, stir at 20 - 30 °C for 1 - 48 hours according to the ratio of 8 - 15 liters of water per kilogram of dry straw, and filter to obtain a straw extract

[0006] Step 2): Dilute the crop straw extract to 1 to 20 times, and add bicarbonate and metal salt per liter of the undiluted stock solution. When synthesizing straw-derived calcium-based slow-release phosphate fertilizer from corn straw, add at least 4.5 g of sodium bicarbonate and at least 5.9 g of calcium chloride; when synthesizing straw-derived magnesium-based slow-release phosphate fertilizer, add at least 1.8 g of sodium bicarbonate and at least 2.0 g of magnesium chloride; when synthesizing straw-derived iron-based slow-release phosphate fertilizer, add at least 2.5 g of ferric chloride. The dosage of the medicament for wheat and rice straw is the same: when synthesizing straw-derived calcium-based slow-release phosphate fertilizer, add at least 1.6 g of sodium bicarbonate and at least 2.0 g of calcium chloride; when synthesizing straw-derived magnesium-based slow-release phosphate fertilizer, add at least 1.5 g of sodium bicarbonate and at least 1.7 g of magnesium chloride; when synthesizing straw-derived iron-based slow-release phosphate fertilizer, add at least 0.72 g of ferric chloride. Step 3): Add an alkali solution to increase the pH of the mixture to form a precipitate. When synthesizing straw-derived calcium-based slow-release phosphate fertilizer, adjust the solution pH to above 8.5; when synthesizing straw-derived magnesium-based slow-release phosphate fertilizer, adjust the solution pH to above 10.5; when synthesizing straw-derived iron-based slow-release phosphate fertilizer, adjust the solution pH to above 3.0.

[0007] Step 4): After separating the precipitate and drying, the straw-derived slow-release phosphate fertilizer is obtained.

[0008] In a specific embodiment, in the step 1), stir at room temperature for 16 - 18 hours according to the ratio of 9 - 11 liters of water per kilogram of dry straw, and filter to obtain the straw extract.

[0009] In a specific embodiment, in the step 2), the properties of the same crop straw in different growth environments will be different, and the minimum input amounts of sodium bicarbonate and metal salt can be adjusted within the range of 50% to 200% to optimize the effect of the slow-release phosphate fertilizer.

[0010] In a specific embodiment, in the 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 - 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 - 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 - 3.7 to optimize the effect of the slow-release phosphate fertilizer.

[0011] In a specific embodiment, in the step 4), the method for separating the precipitate includes at least one of centrifugation, decantation, and filtration to achieve solid-liquid separation; the method for drying includes at least one of freeze-drying, air-drying, and oven-drying to achieve drying of the precipitate. If oven-dried, the temperature is 50 - 75°C.

[0012] It should be noted that in the step 1), the finer the crop straw is crushed, the more phosphorus can be extracted by subsequent water oscillation extraction. It can pass through a 1-mm sieve mesh, and in large-scale production, it can also pass through a 1-cm sieve mesh. When filtering the straw extract, gauze or a net bag can be used.

[0013] It should be noted that in the step 2), for the slow-release phosphate fertilizers prepared from different crop straws, the minimum input amounts of the sodium bicarbonate and metal salts are not only the most economical but also can make the performance of the synthesized slow-release phosphate fertilizers the best. The purpose of adding the bicarbonate and metal salts is to provide carbonate / hydrogen carbonate and metal ions, not limited to sodium bicarbonate and calcium chloride, magnesium chloride, iron chloride, including but not limited to raw materials containing and capable of generating hydrogen carbonate ions and calcium, magnesium, iron metal ions, such as sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium oxide, calcium hydroxide, magnesium hydroxide, magnesium sulfate, iron sulfate.

[0014] It should be noted that the straw comes from at least one of food crops, cash crops, oil crops, vegetable crops, fruit crops, forage 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.

[0015] The slow-release phosphate fertilizer prepared by the method of this application only recovers and utilizes the phosphorus resources in the crop straw and does not add exogenous mineral phosphate fertilizers. Compared with the prior art, this technology has the following beneficial effects:

[0016] (1) Green and efficient. This technology can recover and utilize 90% of the phosphorus in the straw and enrich and convert it into slow-release phosphate fertilizers, 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 having low subsequent treatment costs.

[0017] (2) Slow-release and efficient. This product has excellent slow-release characteristics. In particular, the straw-derived calcium-based slow-release phosphate fertilizer is not easily fixed by the soil when applied to the soil, can maintain high phosphorus activity, and has a slow-release feature, which can effectively promote the phosphorus absorption of crops. At the same time, the performance of the phosphate fertilizer is better than that of the commonly used slow-release mineral phosphate fertilizer - calcium hydrogen phosphate. Compared with calcium hydrogen phosphate, the conversion amount of the active phosphorus released by this product to the available state Ca 2 -P is the same as that of calcium hydrogen phosphate, but the conversion amount to the slow-acting state Ca 8 -P with the second highest biological effectiveness is nearly twice that of calcium hydrogen phosphate and is less likely to be fixed by the soil.

[0018] (3) Simple and environmentally friendly. The equipment and reagents used are widely available, the process is short, and the whole process only takes 1-2 days, with low production costs. The raw materials used are completely composed of crop straw and calcium / magnesium / iron metal salts, bicarbonates, and sodium hydroxide that are non-toxic and harmless to the environment. Description of the Drawings

[0019] Figure 1 It is the effect diagram of the influence of the continuous stirring time after adjusting the pH in step 3).

[0020] Figure 2 It is the effect diagram of the influence of pH in step 2).

[0021] Figure 3 It is the effect diagram of the influence of the input amounts of calcium chloride, magnesium chloride, and ferric chloride in step 2).

[0022] Figure 4 It is the effect diagram of the influence of the input amounts of calcium chloride, magnesium chloride, and ferric chloride after the corn straw is respectively changed to wheat straw and rice straw in step 2).

[0023] Figure 5 It is the slow-release characteristics of straw-source calcium-based, magnesium-based, and iron-based slow-release fertilizers.

[0024] Figure 6 It is the evaluation diagram for supplementing the soil phosphorus pool and influencing phosphorus absorption in the wheat rhizosphere. Specific implementation manners

[0025] The present invention will be further described below in conjunction with embodiments. However, the embodiments of the present invention are only exemplary descriptions, and these implementation manners do not constitute any limitation to the present invention under any circumstances. The materials, reagents, etc. used, unless otherwise specified, can be obtained from commercial channels.

[0026] Dry straw: The water content of crop straw should be lower than 5%.

[0027] The straw mentioned above includes straws from different crop categories, selected from at least one of food crops, cash crops, oil crops, vegetable crops, fruit crops, forage 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.

[0028] Example 1

[0029] Synthesis of straw-source calcium-based, magnesium-based, and iron-based slow-release phosphate fertilizers

[0030] Step 1): The corn straw is crushed, sieved through a 1-mm sieve, stirred for 18 hours at a ratio of 10 liters of water per kilogram of corn straw, and filtered through a 38-um filter screen to obtain a straw extract.

[0031] Step 2), dilute the corn straw extract by 10.6 times; for the calcium-based slow-release phosphate fertilizer from straw, add 8.9 g of sodium bicarbonate and 11.8 g of calcium chloride per liter of the undiluted stock solution; for the magnesium-based slow-release phosphate fertilizer from straw, add 8.9 g of sodium bicarbonate and 10.1 g of magnesium chloride per liter of the undiluted stock solution; for the iron-based slow-release phosphate fertilizer from straw, add 17.2 g of ferric chloride per liter of the undiluted stock solution.

[0032] Step 3), for the calcium-based slow-release phosphate fertilizer from straw, use sodium hydroxide to adjust the pH to 8.5 and continue stirring for 4 h after adjusting the pH; for the magnesium-based slow-release phosphate fertilizer from straw, use sodium hydroxide to adjust the pH to 10.5 and continue stirring for 4 h after adjusting the pH; for the iron-based slow-release phosphate fertilizer from straw, use sodium hydroxide to adjust the pH to 4.5 and continue stirring for 4 h after adjusting the pH.

[0033] Step 4), centrifuge to collect the precipitate and dry it at 65 °C to obtain the slow-release phosphate fertilizer. The phosphorus precipitation rate is calculated as (1 - phosphorus concentration in the solution after reaction / phosphorus concentration before reaction).

[0034]

[0035] (According to 1 kg of dry straw: 10 L of water)

[0036] Table 1

[0037] According to Table 1, during the water oscillation extraction process, 92.1% of the phosphorus enters the extract from the corn straw. The phosphorus precipitation rates of the calcium-based, magnesium-based, and iron-based are 97.2%, 98.1%, and 99.7% respectively. Therefore, the conversion rates of phosphorus from corn straw to slow-release phosphate fertilizers are 90%, 90%, and 92% respectively.

[0038] Example 2

[0039] Investigate the influence of the stirring time after adjusting the pH in Step 3)

[0040] The stirring time after adjusting the pH in Step 3) is as Figure 1 . Others are the same as in Example 1. The phosphorus precipitation rate is calculated as (1 - phosphorus concentration in the solution after reaction / phosphorus concentration before reaction).

[0041] According to Figure 1 It can be seen that the synthesis reactions of the calcium-based, magnesium-based, and iron-based slow-release phosphate fertilizers from straw reach equilibrium immediately after adjusting the pH and can remain stable within 6 h, which is sufficient time to complete operations such as solid-liquid separation.

[0042] Example 3

[0043] Investigate the influence of the pH in Step 3)

[0044] The pH in Step 2) is as Figure 2The rest is the same as in Example 1. The phosphorus precipitation rate is calculated as (1 - phosphorus concentration in the solution after reaction / phosphorus concentration before reaction).

[0045] According to Figure 2 , for the synthesis reactions of calcium-based, magnesium-based, and iron-based slow-release phosphate fertilizers from straw, the highest phosphorus precipitation rate can be achieved when the pH reaches 8.5, 10.5, and 3.0 respectively. However, for the synthesis of calcium-based slow-release phosphate fertilizer from straw, it is found in actual operation that it is difficult to reach equilibrium at pH 8.5 - 9.0, and the pH needs to be adjusted multiple times for a long time to maintain the pH. Therefore, pH 9.0 - 9.5 is selected as the optimal pH, with a slight excess of alkali solution to ensure the highest phosphorus recovery rate; for iron-based slow-release phosphate fertilizer from straw, although pH 3.0 can precipitate phosphorus to the greatest extent, it will result in too small particle size of the ferrihydrite precipitate formed and difficult to separate. Therefore, pH 3.5 is selected as the optimal pH. In summary, the optimal pH values for synthesizing calcium-based, magnesium-based, and iron-based slow-release phosphate fertilizers from straw are 9.0 - 9.5, 10.5 - 11.0, and 3.5 - 3.7 respectively.

[0046] Example 4

[0047] Investigate the influence of the input amounts of sodium bicarbonate, calcium chloride, magnesium chloride, and iron chloride in step 2)

[0048] The input amounts of calcium chloride, magnesium chloride, and iron chloride in step 2) are as Figure 3 . For calcium-based and magnesium-based slow-release phosphate fertilizers from straw, adjust the amount of sodium bicarbonate added to keep the mass ratio with calcium chloride or magnesium chloride unchanged all the time. The rest is the same as in Example 1. The phosphorus precipitation rate is calculated as 1 - phosphorus concentration in the solution after reaction / phosphorus concentration before reaction, and the phosphorus availability of the precipitate is calculated as phosphorus extracted by 0.5M NaHCO 3 solution in the precipitate / total phosphorus.

[0049] According to Figure 3 , for the corn straw extract, aiming at the highest precipitation rate, the least raw material input, and the highest phosphorus availability of the precipitate, the optimal input amounts of raw materials required for synthesizing calcium-based, magnesium-based, and iron-based slow-release phosphate fertilizers from straw are 5.9 g / L of calcium chloride, 2.0 g / L of magnesium chloride, and 2.5 g / L of iron chloride respectively.

[0050] Example 5

[0051] Investigate the influence of the input amounts of sodium bicarbonate, calcium chloride, magnesium chloride, and iron chloride after replacing corn straw with wheat straw and rice straw respectively in step 2).

[0052] The input amounts of calcium chloride, magnesium chloride, and iron chloride after changing corn straw to wheat straw and rice straw respectively in step 2) are as Figure 4。In step 2), for the calcium-based and magnesium-based slow-release phosphate fertilizers from straw, always adjust the amount of sodium bicarbonate added so that its mass ratio to calcium chloride or magnesium chloride remains unchanged. The dilution factor of the wheat straw and rice straw extracts is changed to 3.5 times, the pH of the calcium-based straw fertilizer is changed to 9.25, and the pH of the iron-based fertilizer is changed to 3.5. The rest is the same as in Example 1. The phosphorus precipitation rate is calculated as 1 - phosphorus concentration in the solution after reaction / phosphorus concentration before reaction.

[0053] According to Figure 4 , for the wheat and rice straw extracts, aiming at the highest precipitation rate, the least raw material input, and the highest phosphorus availability of the precipitate, the optimal raw material input amounts required for the synthesis of calcium-based, magnesium-based, and iron-based slow-release phosphate fertilizers from the two straw extracts are the same, which are 2.0 g / L of calcium chloride, 1.7 g / L of magnesium chloride, and 0.72 g / L of ferric chloride respectively.

[0054] Example 6

[0055] Evaluation of the slow-release properties of calcium-based, magnesium-based, and iron-based slow-release phosphate fertilizers from straw - Phosphorus release test

[0056] First, comprehensively consider Examples 1 - 5 to determine the optimal synthesis conditions. Taking the corn straw extract as an example, in step 2), for the calcium-based slow-release phosphate fertilizer from straw, add 4.5 g of sodium bicarbonate and 5.9 g of calcium chloride per liter of the undiluted stock solution, then use sodium hydroxide to adjust the pH to 9.25, and continue stirring for 4 h after adjusting the pH; for the magnesium-based slow-release phosphate fertilizer from straw, add 1.8 g of sodium bicarbonate and 2.0 g of magnesium chloride per liter of the undiluted stock solution, then use sodium hydroxide to adjust the pH to 10.5, and continue stirring for 4 h after adjusting the pH; for the iron-based slow-release phosphate fertilizer from straw, add 2.5 g of ferric chloride per liter of the undiluted stock solution, then use sodium hydroxide to adjust the pH to 3.5, and continue stirring for 4 h after adjusting the pH. The rest is the same as in Example 1. Calcium-based slow-release phosphate fertilizer from straw Ca#, magnesium-based slow-release phosphate fertilizer from straw Mg#, and iron-based slow-release phosphate fertilizer from straw Fe# are obtained respectively.

[0057] For the phosphorus release test, add Ca#, Mg#, Fe#, calcium hydrogen phosphate, and diammonium hydrogen phosphate to 200 mL of deionized water, and let it stand and incubate at 25°C. The sampling time points are as Figure 5 . The phosphorus concentration is measured using ICP-OES.

[0058] According to Figure 5 , the calcium-based, magnesium-based, and iron-based slow-release fertilizers from straw all have good slow-release characteristics, specifically manifested as follows: I: The release rate of nutrients in the fertilizer within 24 h does not exceed 15%; II: The slow-release time is 10 - 20 days; III: The maximum release rate of the calcium-based and magnesium-based slow-release phosphate fertilizers from straw is higher than 90%, and the maximum release rate of the iron-based slow-release phosphate fertilizer from straw 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] Pot experiment of calcium-based slow-release phosphate fertilizer from straw

[0061] The soil type used in the experiment was cinnamon soil. Plant residues, intrusions such as stones, and neoforms were carefully removed and air-dried, then passed through a 2-mm sieve. CK without applying phosphate fertilizer, applying calcium hydrogen phosphate, and applying Ca# were set. Phosphorus application P: 34 mg P / kg air-dried soil, urea 214 mg / kg air-dried soil, potassium chloride 158 mg / kg air-dried soil. The dry weight of the plants in each pot was measured. After the whole plant was pulverized, the phosphorus content in the plants was determined by digestion method. The phosphorus uptake of the plants = dry weight of the plants * phosphorus content in the plants, as shown in Table 1. At the same time, the inorganic phosphorus fractionation method of calcareous soil by Jiang Baifan and Gu Yichu was used to determine the phosphorus form distribution in non-rhizosphere and rhizosphere soils after pot cultivation, and to evaluate how this product replenishes the soil phosphorus pool and affects phosphorus uptake in the wheat rhizosphere, see Figure 6 .

[0062] Fertilizer type Dry weight of plant (g) Phosphorus uptake of plant (mg / pot) CK 3.65±0.16b 5.17±0.12b Calcium hydrogen phosphate 5.13±0.25a 7.49±0.80a Straw-based calcium slow-release phosphate fertilizer 5.52±0.25a 7.77±0.58a

[0063] For the same column of data, the same letter behind indicates no significant difference. LSD test was used, α = 0.05

[0064] Table 2

[0065] According to Table 2, the calcium-based slow-release phosphate fertilizer from straw can effectively increase the dry weight of plants and the phosphorus uptake of plants, and the promotion effect has no significant difference from that of the slow-release mineral phosphate fertilizer calcium hydrogen phosphate.

[0066] The released phosphorus is stored in the soil in a form with high biological availability. According to Figure 6 , by analyzing the phosphorus form distribution in non-rhizosphere soil, it was found that after being applied to cinnamon soil, compared with calcium hydrogen phosphate, more of the phosphorus released by the calcium-based slow-release phosphate fertilizer from straw is stored in cinnamon soil in a form with high biological availability. Specifically, it is more converted into the Ca 8 -P fraction with the second highest biological availability than calcium hydrogen phosphate, and further quantitative analysis found that it converts 72 - 80% of the phosphorus into the Ca 8 -P fraction, which is about twice the conversion amount of calcium hydrogen phosphate (29% - 48%). At the same time, its conversion rate to the Ca 2 -P phosphorus fraction with the highest biological availability is (22% - 25%), which is the same as that of calcium hydrogen phosphate (22% - 24%).

[0067] By analyzing the phosphorus form distribution in rhizosphere soil, it was found that the calcium-based slow-release phosphate fertilizer from straw can promote phosphorus uptake in the wheat rhizosphere more than calcium hydrogen phosphate. Specifically, after applying this fertilizer, the Ca 2 -P fraction in the rhizosphere is lower than that in the calcium hydrogen phosphate treatment, and the Ca 2-P, as the component with the highest biological availability, its changes in rhizosphere soil can be attributed to fertilizer transformation and wheat rhizosphere absorption. However, there is no significant difference in the conversion amounts of the two to this component in non-rhizosphere soil, so the factor of fertilizer transformation can be excluded. Therefore, after applying straw-based calcium slow-release phosphate fertilizer, the reason why the Ca 2 -P component is lower than that of the calcium hydrogen phosphate treatment is that it can promote phosphorus absorption 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), crushing the crop straw, extracting the crop straw by water oscillation, adding 8-15 liters of water per kilogram of dry straw, stirring at 20-30°C for 1 hour to 48 hours, filtering, and obtaining the straw extraction stock solution; Step 2), diluting the crop straw extract solution to 1 to 20 times; adding 4.5-17.8 grams of sodium bicarbonate and 5.9-23.6 grams of calcium chloride per liter of crop straw extract solution; Step 3), adding alkali solution to increase the pH of the mixed solution until a precipitate is formed; Step 4), after the precipitate is separated and dried, the straw-derived slow-release phosphate fertilizer is obtained.

2. The method according to claim 1, characterized in that In step 2), carbonate (bicarbonate) and metal salt are added.

3. The method according to claim 1, characterized in that In step 1), the crop straw is crushed, and the crop straw is extracted by water oscillation, with 9-11 liters of water per kilogram of dry straw, stirred at 25° C. for 16-18 hours; filtered to obtain the straw extraction stock solution.

4. The method according to claim 1, characterized in that: In step 2), the properties of the same crop straw in different growth environments may be different, 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.

5. The method according to claim 2, characterized in that: 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; in order to optimize the effect of the slow-release phosphate fertilizer.

6. The method according to claim 1, characterized in that In step 4), the method of separating the precipitate comprises at least one of centrifugation, decantation and filtration to achieve solid-liquid separation.

7. The method according to claim 1, characterized in that 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.

8. The method according to claim 2, characterized in that: The carbonate (bicarbonate) and metal salt are one or more of magnesium chloride, ferric chloride, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium oxide, calcium hydroxide, magnesium hydroxide, magnesium sulfate or ferric sulfate.

9. The method according to claim 1, characterized in that: The crop straws include, but are not limited to, corn straws, wheat straws, rice straws, soybean straws, sorghum straws, sugarcane straws, cotton straws, sweet potato straws, oat straws, and alfalfa straws.

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