Cd-reducing functional special fertilizer and preparation method thereof

Abstract

The invention discloses a functional special fertilizer for reducing Cd, which comprises a type A special fertilizer for treating acidic purple soil and a type B special fertilizer for treating yellow soil, the special fertilizers are scientific and reasonable in matching and comprehensive in nutrition, can meet the nutrition supply of plants, and can carry out corresponding treatment according to soil pollution of different causes. The method is high in pertinence and high in treatment efficiency, has multiple functions of adjusting the pH value of soil, supplementing salt-based ions, passivating heavy metals and improving the cadmium fixing capacity of roots, and has a good application prospect. The invention also discloses a preparation method of the functional special fertilizer for reducing Cd.

Description

Technical Field

[0001] The present invention relates to the technical field of soil treatment, and in particular to a functional special fertilizer for reducing Cd and a preparation method thereof. Background Art

[0002] The heavy metal cadmium (Cd) is one of the elements of heavy metal pollution in soil. Excessive cadmium levels in soil not only affect the soil's ecological functions but also inhibit the growth and development of crops, causing damage to them. It also accumulates in the edible parts of crops, enters the food chain, and threatens human health. Therefore, the remediation of cadmium-contaminated soil is of great significance in protecting soil ecological functions and ensuring food safety.

[0003] The treatment method for cadmium-contaminated soil is to apply soil conditioners. However, commonly used conditioners are limited in materials, have slow effects, are expensive, and have significant environmental impacts, making them unsuitable for widespread adoption. Furthermore, because the causes of heavy metal contamination in soil vary from region to region, and soil physical and chemical properties vary, using a single conditioner would lack specificity and could compromise soil remediation effectiveness.

[0004] The causes of heavy metal pollution in soil are generally divided into two categories. One is human factors, including mining, metal smelting, domestic sewage, traffic emissions and agricultural activities. This type of soil is mainly purple loam, which is rich in nitrogen, phosphorus and potassium and contains a certain amount of organic matter. However, due to human agricultural and industrial inputs, the content of heavy metals in the soil has increased, causing soil acidification. Soil acidification will further activate the activity of heavy metals in the soil, resulting in more serious soil heavy metal pollution; the other is natural factors, which are mainly affected by the high geological background. The soil in the high geological background area is mainly yellow loam, which has a low exchangeable base content, severe soil erosion, and poor soil. The heavy metal content of this type of soil itself is high, and the loss of soil base ions causes soil acidification. Soil acidification increases the bioavailability of heavy metals such as cadmium, which in turn increases the risks to agricultural products and the ecological environment. From this we can see that although the heavy metal contents in purple loam and yellow loam are both high, there are differences in the comprehensive soil properties of purple loam and yellow loam. In the process of soil management, we need to pay attention not only to the heavy metal content in the soil, but also to the effects of conditioners on the nutrient transformation of crops and the accumulation of heavy metals in the edible part of crops, so as to realize the multifunctional development of fertilizers.

[0005] Therefore, the present invention aims to provide a targeted, functional special fertilizer that simultaneously has the functions of reducing cadmium in agricultural products, achieving comprehensive fertilizer efficiency, and passivating heavy metals in soil and controlling acidification. Summary of the Invention

[0006] In order to overcome the shortcomings of the prior art, one of the purposes of the present invention is to provide a Cd-reducing functional special fertilizer to specifically treat cadmium-contaminated soil. The second purpose of the present invention is to provide a preparation method of a Cd-reducing functional special fertilizer.

[0007] The technical solutions adopted in the present invention are as follows: A functional special fertilizer for reducing Cd includes a type A special fertilizer for treating acidic purple soil, wherein the type A special fertilizer includes the following raw materials in parts by weight: 200-220 parts of urea, 170-190 parts of calcium oxide, 100-120 parts of biochar, 170-180 parts of calcium magnesium phosphate fertilizer, and 10-25 parts of polyaspartic acid; and also includes a type B special fertilizer for treating yellow loam, wherein the type B special fertilizer includes the following raw materials in parts by weight: 200-220 parts of urea, 120-140 parts of calcium oxide, 140-170 parts of biochar, 180-200 parts of sepiolite, and 40-50 parts of polyaspartic acid.

[0008] Compared with the existing technology, the present invention makes targeted treatment according to the different causes of heavy metal contaminated soil, and its specific beneficial effects and significant improvements are as follows: The present invention uses urea, calcium oxide, biochar and polyaspartic acid as core raw materials, and rationally matches calcium magnesium phosphate fertilizer or sepiolite according to the different physical and chemical properties of the soil to achieve targeted soil treatment. It has multiple functions such as regulating soil pH, replenishing base ions, passivating heavy metals, and improving the cadmium fixation ability of crop roots, thereby achieving the purpose of reducing the cadmium content in the edible part of crops and reducing food safety risks.

[0009] Among the core raw materials, urea can improve soil fertility and ensure the nutrient supply of crops; calcium oxide can adjust the soil pH, making it easier for heavy metals to form insoluble precipitates, and can also flocculate the clay part of the soil into more stable aggregates, thereby improving the physical structure of the soil; polyaspartic acid can directly chelate with cadmium and, under the action of transport proteins, fix the formed chelate in the root system, thereby reducing the transport of cadmium to the aboveground parts (stems, leaves, grains, etc.), thereby reducing the accumulation of cadmium in the grains; biochar can adjust the soil pH, adsorb free cadmium in the soil, effectively reducing the chance of cadmium being absorbed by plants, and can also use its alkaline substances to neutralize acidic soils, thereby reducing the activity of heavy metals in the soil and reducing the bioavailability of heavy metals; polyaspartic acid can form stable chelates with heavy metal ions, reduce the migration and bioavailability of heavy metals, and promote the absorption of PASP-heavy metal chelates by plant roots and bind to plant roots. By rationally combining the aforementioned substances, the present invention not only enables each substance to fully exert its own efficacy, but also further enhances its efficacy through the synergistic effect between the substances. Specifically, the synergistic effect between calcium oxide and biochar allows calcium oxide to be used to modify biochar, improving its phosphorus adsorption capacity and adsorbing phosphate and anionic heavy metals through chemical precipitation, ligand exchange, and surface electrostatic adsorption. The synergistic interaction between polyaspartic acid and biochar allows the organic matter in the biochar to form insoluble precipitates with heavy metals such as cadmium, and also to combine with the carboxyl and amino groups in the polyaspartic acid to form complexes. The resulting complexes have stronger adsorption than biochar alone, further enhancing the passivation effect of cadmium.

[0010] For acidic purple soil, industrial waste, sewage discharge, agricultural sewage irrigation, and excessive use of pesticides and fertilizers will increase the accumulation of heavy metals in the soil and cause soil acidification. These activities directly or indirectly cause the excessive content of heavy metals in the soil. The Type A special fertilizer of this invention, by rationally combining calcium-magnesium-phosphate fertilizer with its core raw materials, can regulate soil pH and improve soil acidity. It provides cations such as calcium and magnesium, which compete with heavy metals for absorption sites in plant roots, reducing heavy metal absorption and thus reducing heavy metal bioactivity. Furthermore, elements such as calcium, magnesium, and phosphorus simultaneously promote the synthesis and expression of certain proteins in the root system, thereby inhibiting the migration of cadmium to aboveground parts (stems, leaves, seeds, etc.), significantly reducing cadmium accumulation in edible parts of crops (such as seeds), thereby minimizing food safety risks. Furthermore, the calcium-magnesium-phosphate fertilizer exhibits a synergistic interaction with polyaspartic acid. The specific reaction is speculated to be that the calcium and magnesium ions produced by the hydrolysis of the calcium-magnesium-phosphate fertilizer form coordination bonds with the carboxyl and amino groups of aspartic acid. This coordination structure may interact with cadmium, promoting the formation of more stable and insoluble cadmium compounds. This mechanism of action indirectly enhances the cadmium retention effect of crop roots.

[0011] Yellow loam soils are often associated with a high geological background of heavy metals, primarily due to the evolution of the parent material. During soil precipitation, elements such as cadmium, lead, and mercury in the rock accumulate strongly, resulting in soil concentrations of cadmium, lead, and mercury that are 10 or even 20 times higher than those in the parent rock. This ultimately leads to significant excess heavy metal levels in the soil. Furthermore, soil acidification is also affected by base ion loss and the impact of industrial and agricultural activities. The Type B special fertilizer of the present invention incorporates sepiolite in its core raw material. Sepiolite is a fibrous, hydrous magnesium silicate that provides a certain amount of magnesium, calcium, and potassium. Its porous structure allows it to adsorb heavy metals from the soil and fix the chelates or insoluble precipitates formed by the reaction between the core raw material and the heavy metals, resulting in improved long-term stability of heavy metal passivation. Furthermore, biochar, sepiolite, and polyaspartic acid, as chelating substances, exhibit synergistic adsorption. For soils with a high background of heavy metals, the combination of these three significantly improves the efficiency of heavy metal fixation. Furthermore, due to the low price of sepiolite, production costs are lower, significantly reducing raw material costs and facilitating widespread application.

[0012] In summary, the fertilizer of the present invention is scientifically and reasonably matched, and has comprehensive nutrition. It can meet the nutritional supply of plants and make appropriate treatment according to the different causes of soil pollution. It is not only highly targeted and efficient, but also can avoid applying unnecessary fertilization, disturbing the soil to the minimum extent, and is more conducive to the healthy and sustainable development of the soil. Moreover, the fertilizer of the present invention has multiple functions of regulating soil pH, supplementing base ions, passivating heavy metals, and improving the root cadmium fixation ability. It can improve the soil environment from many aspects, inhibit the activity of heavy metals, passivate soil heavy metals, and achieve the purpose of reducing cadmium in grains. It has good application prospects and is more in line with the development trend of multifunctional fertilizers in the future.

[0013] As a preferred embodiment of the present invention, the type A special fertilizer includes the following raw materials in parts by weight: 205-215 parts of urea, 175-185 parts of calcium oxide, 105-115 parts of biochar, 173-177 parts of calcium magnesium phosphate fertilizer, and 14-21 parts of polyaspartic acid; the type B special fertilizer includes the following raw materials in parts by weight: 205-215 parts of urea, 125-135 parts of calcium oxide, 147-162 parts of biochar, 185-195 parts of sepiolite, and 42-47 parts of polyaspartic acid.

[0014] As a preferred embodiment of the present invention, the type A special fertilizer includes the following raw materials in parts by weight: 210 parts of urea, 180 parts of calcium oxide, 110 parts of biochar, 175 parts of calcium magnesium phosphate fertilizer, and 17 parts of polyaspartic acid; the type B special fertilizer includes the following raw materials in parts by weight: 210 parts of urea, 130 parts of calcium oxide, 155 parts of biochar, 190 parts of sepiolite, and 45 parts of polyaspartic acid.

[0015] The present invention also discloses a method for preparing a Cd-reducing functional special fertilizer, comprising the following steps: S1: Material mixing stage: first, urea and polyaspartic acid are mixed in a blender, and then biochar, calcium magnesium phosphate or sepiolite, and calcium oxide are added in sequence and stirred thoroughly; S2: Granulation stage: The mixed materials are extruded and granulated using a rotary drum granulator; S3: Drying stage: The formed fertilizer granules are dried with hot air using a drum dryer; S4: Finished product stage: The fertilizer granules are screened and cooled before being bagged.

[0016] As a preferred embodiment of the present invention, in step S1, urea and polyaspartic acid are stirred at 110 to 125 degrees Celsius. After the stirring of urea and polyaspartic acid is completed, the same temperature is maintained, and biochar is continued to be added for stirring. After the stirring of urea, polyaspartic acid and biochar is completed, they are cooled to room temperature, and finally calcium magnesium phosphate or sepiolite and calcium oxide are added and stirred thoroughly.

[0017] As a preferred embodiment of the present invention, in step S3, the extruded particles are dried in a drum dryer with hot air at 110 to 125 degrees Celsius, with a drum speed of 20 to 30 r / min for 30 to 40 minutes.

[0018] As a preferred embodiment of the present invention, in step S2, the fertilizer particles formed by extrusion have a particle size of 1 to 5 mm.

[0019] As a preferred embodiment of the present invention, the stirring speed is 28 to 32 r / min, and the stirring time of urea and polyaspartic acid is greater than or equal to 0.5 h.

[0020] As a preferred embodiment of the present invention, the urea and polyaspartic acid are stirred at 120 degrees Celsius.

[0021] Compared to existing technologies, the present invention utilizes a rational preparation method that can improve fertilizer utilization. Studies have shown that the amino and carboxyl groups produced by the decomposition of polyaspartic acid at 110-125 degrees Celsius are highly reactive. By mixing urea with polyaspartic acid at high temperatures, the molecular motion of the substances is accelerated, the frequency of intermolecular collisions is increased, and a better reaction between the urea and polyaspartic acid molecules is promoted. This results in a product with improved sustained-release properties, thereby increasing fertilizer utilization. Biochar is then added at the same temperature. On the one hand, the biochar acts as a carrier, absorbing and stabilizing the resulting product within its pores, effectively "locking in" the efficacy of the drug. On the other hand, the high temperature promotes the binding of organic matter in the biochar with the carboxyl and amino groups of the polyaspartic acid, improving reaction efficiency. After urea, polyaspartic acid and biochar are evenly fused and mixed, they are cooled to form stable nutrient units. Calcium magnesium phosphate fertilizer or sepiolite and calcium oxide are then added and evenly mixed. They are then dried with hot air at the same temperature. This can re-stimulate the activity of some raw materials, further promote fusion, promote synergy, give full play to the fertilizer effect of the materials, improve the utilization rate of the fertilizer, and maximize economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 1 is a graph showing the efficiency of soil acidification control using type A special fertilizer according to an embodiment of the present invention; Figure 2 1 is a graph showing the efficiency of soil acidification control using type B special fertilizer according to an embodiment of the present invention; Figure 3 2 is a graph showing the Cd passivation efficiency of type A special fertilizer soil in an embodiment of the present invention; Figure 4 2. This is a graph showing the Cd passivation efficiency of type B special fertilizer soil in an embodiment of the present invention; Figure 5 This is a graph showing the efficiency of Cd fixation by the root system of type A special fertilizer according to an embodiment of the present invention; Figure 6 This is a graph showing the efficiency of Cd fixation by the root system of the type B special fertilizer according to an embodiment of the present invention; Figure 7 This is a graph showing the Cd reduction efficiency of type A special fertilizer in grains according to an embodiment of the present invention; Figure 8 This is a graph showing the Cd reduction efficiency of type B special fertilizer in grains according to an embodiment of the present invention; Figure 9 : is a correlation diagram of the Cd-fixing efficiency of the root system and the Cd-reducing efficiency of the grain of the type A special fertilizer in the embodiment of the present invention; Figure 10 3. This is a correlation diagram of the Cd-fixing efficiency of the root system and the Cd-reducing efficiency of the grain in the B-type special fertilizer according to the embodiment of the present invention. DETAILED DESCRIPTION

[0023] Typical embodiments that embody the features and advantages of the present invention are described in detail in the following description. It should be understood that the present invention is capable of various variations in different embodiments without departing from the scope of the present invention, and that the descriptions and illustrations are intended to be illustrative rather than limiting.

[0024] Polyaspartic acid (PASP) is a non-toxic, biodegradable, and environmentally friendly polymer. It contains a large number of free carboxyl and amide groups, and possesses excellent chelating and dispersibility. When used alone, PASP comes into contact with heavy metal ions, where its carboxyl and amino groups form chelated small molecules. This chelation effectively reduces the toxicity and mobility of heavy metal ions, thereby retaining them in the pectin of the cell walls. This effectively inhibits the transfer of heavy metals through the root system to the aboveground parts (stems, leaves, and other edible parts), reduces the accumulation of heavy metals in edible parts, and improves food safety.

[0025] Biochar is a carbon-rich porous material produced by heating under limited oxygen conditions. When used alone, its pore structure can adsorb free cadmium in the soil, reducing its free cadmium content and the risk of crop absorption. Biochar is also slightly alkaline, which can neutralize acidic soils and thus reduce the activity of heavy metals in the soil. However, the harsh production environment of biochar makes it expensive and its treatment effects are slow, which restricts its widespread application. Furthermore, when biochar is applied to acidic soils, the originally stable cadmium may also be activated, enhancing its bioavailability. However, in this solution, by adding polyaspartic acid, the polyaspartic acid and biochar can interact synergistically, further enhancing the passivation effect of cadmium and improving the long-term stability of the passivation.

[0026] Calcium magnesium phosphate fertilizer is a fertilizer rich in multiple elements such as phosphorus, calcium, and magnesium, and its main raw material is usually apatite. When used alone, although it can increase the various elements in the soil to meet the growth needs of crops, due to the rich phosphorus element in apatite, if applied excessively, its high concentration of phosphorus will produce an antagonistic effect with other nutrients, which will reduce the bioavailability of these elements, inhibit plant growth, and even cause poisoning to plants. In addition, the leaching of phosphorus will cause soil acidification, posing a risk of water pollution. After soil acidification, the activity of heavy metal elements will be enhanced, making heavy metals more easily absorbed by plants, affecting the edible safety of crops. However, in this solution, calcium oxide is added, which can neutralize the acidification caused by calcium magnesium phosphate fertilizer and inhibit the activity of heavy metal elements. In addition, polyaspartic acid is added in this solution. Polyaspartic acid can interact synergistically with calcium magnesium phosphate fertilizer, which can further improve the cadmium fixation ability of crop roots and the cadmium reduction ability of grains, reducing food safety risks.

[0027] Calcium oxide is an alkaline substance. When used alone, it not only neutralizes soil hydrogen ions, regulates soil pH, and makes it easier for heavy metals to form insoluble precipitates, but also flocculates soil clay particles into more stable aggregates, improving the soil's physical structure. In this solution, calcium oxide can also be combined with calcium magnesium phosphate fertilizers to alleviate soil acidification and inhibit the activity of heavy metal elements.

[0028] Sepiolite is a naturally porous mineral with excellent adsorption, heat resistance, and plasticity. It is commonly used in the manufacture of refractory materials, insulation materials, and mold materials in the foundry industry. In this solution, the pore structure of sepiolite can adsorb heavy metals and their chelates or insoluble substances, thereby improving the passivation efficiency of heavy metals and further enhancing the long-term stability of heavy metal passivation.

[0029] Example 1 This embodiment discloses a functional special fertilizer for reducing Cd, wherein type A special fertilizer for treating acidic purple soil includes the following raw materials in parts by weight: 200 parts of urea, 170 parts of calcium oxide, 100 parts of biochar, 170 parts of calcium magnesium phosphate fertilizer, and 10 parts of polyaspartic acid; type B special fertilizer for treating yellow loam includes the following raw materials in parts by weight: 200 parts of urea, 120 parts of calcium oxide, 140 parts of biochar, 180 parts of sepiolite, and 40 parts of polyaspartic acid.

[0030] Preparation method of functional special fertilizer for reducing Cd A method for preparing a Cd-reducing functional special fertilizer comprises the following steps: S1: Material mixing stage: (1) First, urea and polyaspartic acid are mixed and stirred in a mixer at 120 degrees Celsius, with a stirring speed of 28 to 32 r / min. The stirring time of urea and polyaspartic acid is greater than or equal to 0.5 h.

[0031] (2) After the urea and polyaspartic acid are stirred, maintain the same temperature and speed and continue to add biochar and stir.

[0032] (3) After the urea, polyaspartic acid and biochar are stirred, they are cooled to room temperature, and finally calcium magnesium phosphate or sepiolite and calcium oxide are added and stirred thoroughly.

[0033] S2: Granulation stage: The mixed materials are extruded and granulated using a rotary drum granulator. The size of the fertilizer particles formed by extrusion is 3 mm.

[0034] S3: Drying stage: The formed fertilizer particles are dried with hot air using a drum dryer, wherein the drum dryer performs drying with hot air at 120 degrees Celsius, the drum speed is 20-30 r / min, and the drying time is 30-40 minutes.

[0035] S4: Finished product stage: The fertilizer granules are screened and cooled before being bagged.

[0036] Example 2 This embodiment discloses a functional fertilizer for reducing Cd. Type A fertilizer for treating acidic purple soil comprises the following raw materials in parts by weight: 220 parts urea, 190 parts calcium oxide, 120 parts biochar, 180 parts calcium magnesium phosphate, and 25 parts polyaspartic acid; and Type B fertilizer for treating yellow loam soil comprises the following raw materials in parts by weight: 220 parts urea, 140 parts calcium oxide, 170 parts biochar, 200 parts sepiolite, and 50 parts polyaspartic acid. The preparation method of this fertilizer is the same as that of Example 1.

[0037] Example 3 This embodiment discloses a functional fertilizer for reducing Cd. Type A fertilizer for treating acidic purple soil comprises the following raw materials in parts by weight: 210 parts urea, 180 parts calcium oxide, 110 parts biochar, 175 parts calcium magnesium phosphate, and 17 parts polyaspartic acid; and Type B fertilizer for treating yellow loam soil comprises the following raw materials in parts by weight: 210 parts urea, 130 parts calcium oxide, 155 parts biochar, 190 parts sepiolite, and 45 parts polyaspartic acid. The preparation method of this fertilizer is the same as that of Example 1.

[0038] Comparative Example 1 The organic fertilizer purchased on the market is decomposed farmyard manure, the organic fertilizer from Chengdu Gerges Biotechnology Co., Ltd.

[0039] Comparative Example 2 The calcium magnesium phosphate fertilizer purchased on the market is a product of Hubei Yuhui Chemical Co., Ltd.

[0040] Comparative Example 3 The calcareous soil conditioner purchased on the market has lime as its main ingredient and is a product of Environmental Bridge (Hunan) Ecological Environment Engineering Co., Ltd.

[0041] 1. Comparative Analysis of the Special Fertilizer of the Present Invention and Comparative Examples 1-3 The present invention conducted experiments in two regions: yellow loam and acidic purple soil. Rice was the research subject in the acidic purple soil region, and corn was the research subject in the yellow loam region. Both rice and corn varieties were selected from common local varieties. A quadratic orthogonal regression rotation center combination test method was used in both regions. The treatment effects of the fertilizers were compared with those of existing conditioners (Comparative Examples 1-3) using soil acidification treatment efficiency, soil Cd passivation efficiency, root Cd fixation efficiency, and crop grain Cd reduction efficiency as response indicators. Type A fertilizer was used to treat the acidic purple soil region, with calcium oxide, biochar, calcium magnesium phosphate fertilizer, and polyaspartic acid as factor variables. The specific scheme is shown in Table 1-1. Type B fertilizer was used to treat the yellow loam region, with calcium oxide, biochar, sepiolite, and polyaspartic acid as factor variables. The specific scheme is shown in Table 2-1.

[0042] Table 1-1 Factor level coding table Table 2-1 Factor level coding table 1. Soil acidification control efficiency A: If Figure 1 As shown in the data, the existing conditioners (organic fertilizer, calcium magnesium phosphate fertilizer and calcareous soil conditioner) were spread on the acidic purple soil area before plowing. The acidification control efficiency of organic fertilizer, calcium magnesium phosphate fertilizer and calcareous soil conditioner were 4.11%, 7.14% and 3.8% respectively. The acidification control efficiency of the treatment group with type A special fertilizer in the acidic purple soil ranged from 13.93% to 36.18%, which were better than the existing conditioners, indicating that type A special fertilizer is more effective in treating acidic soil.

[0043] B: If Figure 2 As shown in the data, when existing conditioners (organic fertilizer, calcium magnesium phosphate fertilizer and calcareous soil conditioner) were applied to yellow loam, the acidification control efficiency of yellow loam was 3.78%, 20% and 2.89% respectively. When the treatment group with type B special fertilizer was applied to yellow loam, the acidification control efficiency ranged from 2.23% to 31.88%, with an average efficiency of 11.29%, which was better than calcium magnesium phosphate fertilizer and organic fertilizer, indicating that type B special fertilizer was effective in treating acidic soil.

[0044] In summary, the special fertilizer of the present invention is more effective in treating acidified soil than existing conditioners.

[0045] , soil Cd passivation efficiency (1) Soil Cd passivation efficiency of type A and type B special fertilizers A: If Figure 3As shown, when existing conditioners (organic fertilizer, calcium magnesium phosphate fertilizer and calcareous soil conditioner) were applied to acidic purple soil, the Cd passivation efficiency of the acidic purple soil was 21.88%, 18.93% and 6.63%, respectively. The Cd passivation efficiency of the treatment group applying type A special fertilizer varied in the range of 28.46% to 78.06%, with an average efficiency of 53%, which was better than that of existing conditioners, indicating that the soil cadmium passivation effect of the type A special fertilizer of the present invention is more significant.

[0046] B: If Figure 4 As shown, applying existing conditioners (organic fertilizer, calcium magnesium phosphate fertilizer, and calcareous soil conditioner) to yellow loam soil resulted in Cd passivation efficiencies of 15.42%, 10%, and -10.6% in purple loam soil, respectively. The Cd passivation efficiency in the treatment group treated with Type B special fertilizer ranged from 28.54% to 74.36%, with an average efficiency of 53.40%, outperforming existing conditioners and demonstrating that the Type B special fertilizer of the present invention is more effective in soil cadmium passivation.

[0047] In summary, the special fertilizer of the present invention has a more significant effect on the passivation efficiency of Cd in soil compared with existing conditioners.

[0048] (2) Orthogonal test variance analysis of soil Cd passivation efficiency of type A and type B special fertilizers The present invention uses an orthogonal test variance analysis method to optimize the proportion of special fertilizer raw materials under the indicator of soil Cd passivation efficiency.

[0049] A: The orthogonal test variance analysis of soil Cd passivation efficiency of type A special fertilizer, as shown in Table 1-2, shows that the quadratic orthogonal regression model with calcium oxide, biochar, calcium magnesium phosphate fertilizer, and PASP as experimental factors and soil Cd passivation efficiency as response factor is significant, with an F value of 4.67, P < 0.01. The model is statistically significant and the optimal solution of the formula can be analyzed using the soil Cd passivation efficiency index.

[0050] Among them, the P values ​​corresponding to calcium oxide, calcium magnesium phosphate fertilizer, and PASP were 0.0032, 0.0027, and 0.0269, respectively, and the P values ​​were all less than 0.05, indicating that calcium oxide, calcium magnesium phosphate fertilizer, and PASP all significantly affected the soil Cd passivation efficiency; and the P value of the BD source in Table 1-2 was 0.0375, and BD was the combined effect of biochar and PASP. Since the P value of the BD source was less than 0.05, it can be further verified that there is an interaction between biochar and PASP, and the interaction between the two significantly affects the soil Cd passivation efficiency.

[0051] Table 1-2 Variance analysis of Cd passivation efficiency in acidic purple soil The equation of the Cd passivation efficiency model of acidic purple soil obtained by orthogonal test variance analysis is: (Y1)=4.79A+0.46B+4.91C+3.36D+0.64AB-1.16AC+0.81AD+2.61BC-3.83BD+2.75CD-2.68A²-2.76B²-0.2C²+4.21D²+54.13 Under this model, the range of soil Cd passivation efficiency of the treatment group of type A special fertilizer was further optimized, and the data of the first ten groups were taken. The soil Cd passivation efficiency of the first ten groups ranged from 62.1% to 77.7%. At this time, the ranges of calcium oxide, biochar, calcium magnesium phosphate fertilizer and PASP were 17.21-20 kg, 10-14.9 kg, 14.23-17.73 kg and 1-5 kg, respectively. The special fertilizer raw materials under the above ratio had a better Cd passivation effect on acidic purple soil.

[0052] B: The orthogonal test variance analysis of soil Cd passivation efficiency of type B special fertilizer, as shown in Table 2-2, shows that the quadratic orthogonal regression model with calcium oxide, biochar, sepiolite, and polyaspartic acid (PASP) as experimental factors and soil Cd passivation efficiency as response factor is significant, with an F value of 5.57, P < 0.01, and the model is statistically significant.

[0053] Among them, the P values ​​of the four factors of calcium oxide, biochar, sepiolite, and PASP were all less than 0.05, indicating that calcium oxide, biochar, sepiolite, and PASP all significantly affected the soil Cd passivation efficiency, and the P value of the BD source in Table 2-2 was 0.0493. BD is the combined effect of biochar and PASP. Since the P value of the BD source is less than 0.05, it can be further verified that there is an interaction between biochar and PASP, and the interaction between the two significantly affects the soil Cd passivation efficiency.

[0054] Table 2-2 Variance analysis of Cd passivation efficiency in yellow soil source sum of squares degrees of freedom mean square F-number P-value Significance Model 3281.25 14 234.37 5.57 0.001 significant A-CaO 630.7 1 630.7 15 0.0015 B-Biochar 270.35 1 270.35 6.43 0.0228 C-Sepiolite 751.19 1 751.19 17.87 0.0007 D-PASP 789.86 1 789.86 18.79 0.0006 AB 28.81 1 28.81 0.6853 0.4208 AC 6.22 1 6.22 0.1478 0.706 AD 0.0826 1 0.0826 0.002 0.9652 BC 18.72 1 18.72 0.4451 0.5148 BD 192.33 1 192.33 4.57 0.0493 CD 49.02 1 49.02 1.17 0.2973 A² 205.21 1 205.21 4.88 0.0431 B² 231.64 1 231.64 5.51 0.0331 C² 5.39 1 5.39 0.1281 0.7254 D² 61.56 1 61.56 1.46 0.245 residuals 630.69 15 42.05 Lack of Fit 518.91 10 51.89 2.32 0.1826 not significant Natural error 111.78 5 22.36 total 3911.94 29 The equation of the Cd passivation efficiency model in yellow soil obtained by orthogonal test variance analysis is: (Y2)=5.13A+3.36B+5.59C+5.74D-1.34AB-0.62AC-0.072AD-1.08BC+3.47BD-1.75CD-2.74A²-2.91B²+0.44C²+1.5D²+57.03 Under this model, the range of soil Cd passivation efficiency of the treatment group of type B special fertilizer was further optimized according to its soil Cd passivation efficiency, and the data of the first ten groups were taken. The soil Cd passivation efficiency of the first ten groups ranged from 46.51% to 68.1%. At this time, the ranges of calcium oxide, biochar, sepiolite and PASP were 11.73-14.94 kg, 12.18-18.44 kg, 11.4-19.7 kg and 3.25-4.98 kg, respectively. The special fertilizer raw materials under the above ratio had a better Cd passivation effect on yellow loam soil.

[0055] , root system Cd fixation efficiency (1) Root-fixing Cd efficiency of type A and type B fertilizers Figure 5 The root system Cd fixation efficiency data of type A special fertilizer is shown in the figure below. Figure 6 The data of root system Cd fixation efficiency of the treatment group of type B special fertilizer are as follows: Figure 5 and Figure 6 All of them are wavy, and the data size patterns vary greatly, indicating that the effect of the special fertilizer of the present invention on root Cd fixation is affected by the proportion of certain raw materials.

[0056] (2) Variance analysis of orthogonal experiment on root Cd fixation efficiency of type A and type B special fertilizers The orthogonal experiment variance analysis method was used to optimize the ratio of special fertilizer raw materials based on the root system Cd fixation efficiency.

[0057] A: The orthogonal test variance analysis of the root-fixing Cd efficiency of type A special fertilizer, as shown in Table 1-3, showed that the quadratic orthogonal regression model with calcium oxide, biochar, calcium magnesium phosphate fertilizer, and PASP as experimental factors and root-fixing Cd efficiency as the response factor was significant, with an F value of 3.29, P < 0.05. The model was statistically significant and the root-fixing Cd efficiency index could be used to analyze the optimal solution for the formula.

[0058] Among them, the P values ​​of calcium magnesium phosphate and PASP were 0.0026 and 0.0233, respectively, and the P values ​​were both less than 0.05, indicating that these two factors significantly affected the efficiency of root Cd fixation. The P value of the CD source in Table 1-3 was 0.0136, P < 0.05, and CD was the combined effect of calcium magnesium phosphate and PASP. This further verified that there was an interaction between calcium magnesium phosphate and PASP, and the interaction between the two also significantly affected the efficiency of root Cd fixation.

[0059] Table 1-3 Variance analysis of Cd fixation efficiency of roots in acidic purple soil source sum of squares degrees of freedom mean square F-number P-value Significance Model 40097.56 14 2864.11 3.29 0.0143 significant A-CaO 2806.88 1 2806.88 3.23 0.0925 B-Biochar 1864.83 1 1864.83 2.14 0.1637 C-Calcium magnesium phosphate fertilizer 11326.42 1 11326.42 13.03 0.0026 D-PASP 5549.09 1 5549.09 6.38 0.0233 AB 337.96 1 337.96 0.3887 0.5424 AC 149.92 1 149.92 0.1724 0.6839 AD 386.98 1 386.98 0.4451 0.5148 BC 2725.83 1 2725.83 3.13 0.0969 BD 134.3 1 134.3 0.1545 0.6998 CD 6797.45 1 6797.45 7.82 0.0136 A² 2561.92 1 2561.92 2.95 0.1066 B² 29.19 1 29.19 0.0336 0.8571 C² 2189.2 1 2189.2 2.52 0.1334 D² 2237.09 1 2237.09 2.57 0.1296 residuals 13042.63 15 869.51 Lack of Fit 9881.4 10 988.14 1.56 0.3249 not significant Natural error 3161.23 5 632.25 total 53140.19 29 The equation of the Cd fixation efficiency model of the root system in acidic purple soil obtained by orthogonal test variance analysis is: (Y3)=10.81A-8.81B+21.72C+15.21D-4.6AB-3.06AC+4.92AD-13.05BC-2.9BD-20.61CD+9.66A²+1.03B²-8.93C²-9.03D²+117.78 Under this model, the range of root Cd fixation efficiency of the treatment group of type A special fertilizer was further optimized according to its root Cd fixation efficiency, and the data of the first ten groups were taken. The root Cd fixation efficiency of the first ten groups ranged from 148% to 188.6%. At this time, the ranges of calcium oxide, biochar, calcium magnesium phosphate fertilizer and PASP were 15.35-25 kg, 10-12.2 kg, 13.1-19.92 kg and 1.48-4.61 kg, respectively. The special fertilizer raw materials under the above ratio had a better effect on plant root Cd fixation.

[0060] B: The variance analysis of the orthogonal experiment on the root-fixing Cd efficiency in yellow loam soil with type B special fertilizer is shown in Table 2-3. The quadratic orthogonal regression model with calcium oxide, biochar, sepiolite, and polyaspartic acid (PASP) as experimental factors and root-fixing Cd efficiency as response factor was significant, with an F value of 3.58, P < 0.01, and the model was statistically significant.

[0061] Among them, the P values ​​of sepiolite and PASP were 0.0321 and 0.0001, respectively, and the P values ​​were both less than 0.05, indicating that both sepiolite and PASP significantly affected the Cd fixation efficiency of roots in yellow loam soil.

[0062] Table 2-3 Variance analysis of Cd fixation efficiency of roots in yellow loam soil source sum of squares degrees of freedom mean square F-number P-value Significance Model 30596.08 14 2185.43 3.58 0.0098 significant A-CaO 25.65 1 25.65 0.0421 0.8403 B-Biochar 5.65 1 5.65 0.0093 0.9246 C-Sepiolite 3403.89 1 3403.89 5.58 0.0321 D-PASP 22963.54 1 22963.54 37.65 <0.0001 AB 400.02 1 400.02 0.6559 0.4307 AC 1.75 1 1.75 0.0029 0.958 AD 1686.19 1 1686.19 2.76 0.1171 BC 114.14 1 114.14 0.1872 0.6714 BD 31.27 1 31.27 0.0513 0.8239 CD 12.66 1 12.66 0.0208 0.8874 A² 646.39 1 646.39 1.06 0.3196 B² 830.6 1 830.6 1.36 0.2614 C² 194.88 1 194.88 0.3196 0.5802 D² 6.97 1 6.97 0.0114 0.9163 residuals 9147.84 15 609.86 Lack of Fit 6333.59 10 633.36 1.13 0.4766 not significant Natural error 2814.25 5 562.85 total 39743.92 29 The equation of the Cd fixation efficiency model of yellow soil roots obtained by orthogonal test variance analysis is: (Y4) =1.03A+0.49B+11.91C+30.93D-5AB-0.33AC-10.27AD-2.67BC-1.4BD-0.89CD-4.85A²+5.5B²-2.67C²+0.5D²+66.36 Under this model, the treatment groups of type B special fertilizer were further optimized according to their root Cd fixation efficiency, and the data of the first ten groups were taken. When the root Cd fixation efficiency of the first ten groups ranged from 171.31% to 237.37%, the ranges of calcium oxide, biochar, sepiolite and PASP were 8.55-16.43 kg, 13.52-19.47 kg, 10.24-18.34 kg and 3.96-4.9 kg, respectively. Under the above ratio of special fertilizer raw materials, the plant root fixation effect of Cd was better.

[0063] , Cd reduction efficiency of crop grains (1) Cd reduction efficiency of type A and type B fertilizers in crop grains A: If Figure 7 As shown, the Cd-reducing efficiency of existing conditioners (organic fertilizer, calcium magnesium phosphate fertilizer, and calcareous soil conditioner) in crop grains was 50.59%, 46.3%, and 32.07%, respectively. The Cd-reducing efficiency of the Type A special fertilizer treatment group ranged from 31.43% to 93.1%, with an average efficiency of 62.1%, outperforming existing conditioners and demonstrating that the Type A special fertilizer of the present invention is more efficient in reducing Cd in crop grains.

[0064] B: If Figure 8 As shown, the Cd-reducing efficiency of existing conditioners (organic fertilizer, calcium magnesium phosphate fertilizer, and calcareous soil conditioner) in crop grains was 38.18%, 19.49%, and 38.69%, respectively. The Cd-reducing efficiency of the Type B special fertilizer in the treatment group ranged from 39.47% to 75.69%, with an average efficiency of 62.93% and an optimal efficiency of 75.69%, outperforming conventional conditioners. This indicates that the Type B special fertilizer of the present invention has a more significant Cd-reducing efficiency in crop grains.

[0065] In summary, the Cd-reducing efficiency of the crop seeds of the special fertilizer of the present invention is more significant than that of existing conditioners.

[0066] (2) Orthogonal analysis of Cd reduction efficiency in grains using type A and type B fertilizers The orthogonal experiment variance analysis method was used to optimize the ratio of special fertilizer raw materials based on the indicator of grain Cd reduction efficiency.

[0067] A: The orthogonal test variance analysis of the Cd reduction efficiency of crop grains using type A special fertilizer, as shown in Table 1-4, showed that the quadratic orthogonal regression model with calcium oxide, biochar, calcium magnesium phosphate fertilizer, and polyaspartic acid (PASP) as experimental factors and the cadmium reduction efficiency of acidic purple soil grains as the response factor was significant, with an F value of 3.48, P < 0.05. The model was statistically significant, and the optimal solution for the formula could be analyzed using the Cd reduction efficiency index of crop grains.

[0068] Among them, the P values ​​of biochar and calcium magnesium phosphate were 0.0358 and 0.018, respectively, and the P values ​​were both less than 0.05, indicating that both biochar and calcium magnesium phosphate significantly affected the Cd reduction efficiency of crop grains. In addition, the P value of CD source in Tables 1-4 was 0.0133, P < 0.05, further indicating that there was an interaction between calcium magnesium phosphate and PASP, and the interaction between the two also significantly affected the Cd reduction efficiency of crop grains.

[0069] Table 1-4 Variance analysis of Cd reduction efficiency in acidic purple soil grains source sum of squares degrees of freedom mean square F-number P-value Significance Model 5990.350 14 427.88 3.48 0.0112 significant A-CaO 69.200 1 69.2 0.5625 0.4648 B-Biochar 654.580 1 654.58 5.32 0.0358 C-Calcium magnesium phosphate fertilizer 866.680 1 866.68 7.04 0.018 D-PASP 52.280 1 52.28 0.425 0.5243 AB 1.410 1 1.41 0.0115 0.9161 AC 55.630 1 55.63 0.4522 0.5115 AD 13.510 1 13.51 0.1098 0.745 BC 46.850 1 46.85 0.3808 0.5464 BD 69.570 1 69.57 0.5655 0.4637 CD 968.730 1 968.73 7.87 0.0133 A² 140.360 1 140.36 1.14 0.3024 B² 1085.270 1 1085.27 8.82 0.0095 C² 2361.760 1 2361.76 19.2 0.0005 D² 1.110 1 1.11 0.009 0.9256 residuals 1845.400 15 123.03 Lack of Fit 1424.420 10 142.44 1.69 0.2922 not significant Natural error 420.980 5 84.2 total 7835.750 29 The equation of the Cd reduction efficiency model of acidic purple soil grains obtained by orthogonal test variance analysis is: (Y4)=-1.7A+5.22B+6.01C+1.48D-0.3AB+1.86AC+0.92AD+1.71BC-2.09BD+7.78CD+2.26A²+6.29B²+9.28C²-0.2D²+47.99 Under this model, the range of Cd-reducing efficiency of crop grains in the treatment groups of type A special fertilizer was further optimized according to their data, and the data of the first ten groups were taken. The Cd-reducing efficiency of crop grains in the first ten groups ranged from 95.25% to 119.31%. At this time, the ranges of calcium oxide, biochar, calcium magnesium phosphate fertilizer and PASP were 15-59-24.85 kg, 6.24-14.21 kg, 10.53-20 kg and 1.1-4.1 kg, respectively. The special fertilizer raw materials under the above ratio had a better Cd-reducing efficiency in crop grains.

[0070] B: The orthogonal test variance analysis of the Cd reduction efficiency of crop grains using type B special fertilizer was conducted. As shown in Table 2-4, the quadratic orthogonal regression model with calcium oxide, biochar, sepiolite, and polyaspartic acid (PASP) as experimental factors and the Cd reduction efficiency of yellow loam grains as the response factor was significant, with an F value of 3.74, P < 0.01, and the model was statistically significant.

[0071] Among them, the P values ​​of sepiolite and polyaspartic acid were 0.0105 and 0.0001, respectively, and the P values ​​were both less than 0.05, indicating that sepiolite and polyaspartic acid significantly affected the Cd reduction efficiency of crop seeds.

[0072] Table 2-4 Variance analysis of Cd reduction efficiency in yellow loam grains source sum of squares degrees of freedom mean square F-number P-value Significance Model 1924.46 14 137.46 3.74 0.008 significant A-CaO 0.025 1 0.025 0.0007 0.9796 B-Biochar 9.84 1 9.84 0.2676 0.6125 C-Sepiolite 314.29 1 314.29 8.55 0.0105 D-PASP 1095.08 1 1095.08 29.78 <0.0001 AB 31.62 1 31.62 0.8598 0.3685 AC 44.34 1 44.34 1.21 0.2895 AD 1.89 1 1.89 0.0514 0.8237 BC 1.71 1 1.71 0.0466 0.832 BD 31.96 1 31.96 0.8692 0.3659 CD 233.7 1 233.7 6.36 0.0235 A² 6.32 1 6.32 0.172 0.6842 B² 47.43 1 47.43 1.29 0.2739 C² 45.3 1 45.3 1.23 0.2845 D² 47.18 1 47.18 1.28 0.2751 residuals 551.61 15 36.77 Lack of Fit 453.47 10 45.35 2.31 0.184 not significant Natural error 98.14 5 19.63 total 2476.07 29 The equation of the Cd reduction efficiency model of yellow loam grains obtained by orthogonal test variance analysis is: (Y6)=0.03A-0.64B+3.62C+6.75D-1.41AB-1.66AC+0.34AD-0.33BC+1.41BD-3.82CD-0.48A²-1.32B²+1.29C²-1.31D²+64.39 Under this model, the range of Cd-reducing efficiency of crop grains in the treatment groups of type B special fertilizer was further optimized according to their data. The data of the first ten groups were taken. The Cd-reducing efficiency of crop grains in the first ten groups ranged from 76.32% to 84.46%. At this time, the amounts of calcium oxide, biochar, sepiolite, and polyaspartic acid ranged from 9.3 to 14.92 kg, 10.43 to 18.33 kg, 17.7 to 20 kg, and 1.5 to 4.89 kg, respectively. The special fertilizer raw materials under the above ratio had a better Cd-reducing efficiency in crop grains.

[0073] (3) Correlation analysis between soil Cd passivation efficiency, root Cd fixation efficiency, and crop grain Cd reduction efficiency Depend on Figure 9 The equation for the correlation between the root-fixing Cd efficiency and the grain-reducing Cd efficiency of type A special fertilizer is: (Y)=0.1057X+51.181,R=0.256,N=30 Depend on Figure 10 The equation for the correlation between the Cd fixation efficiency of the root system and the Cd reduction efficiency of the B-type special fertilizer is as follows: (Y)=0.1528X+52.977,R=0.612,N=30 In the formula, R represents the correlation coefficient and N represents the sample size.

[0074] Depend on Figure 9 、 Figure 10 The results show that the Cd efficiency of rice roots is positively correlated with the Cd inactivation efficiency in soil and the Cd reduction efficiency in grains, while the Cd efficiency of corn roots is positively correlated with the Cd inactivation efficiency in soil and the Cd reduction efficiency in grains. This indicates that higher soil heavy metal content leads to greater Cd sequestration by crop roots after application of the special fertilizer, and correspondingly, lower Cd uptake by grains. This means that even in soils with extremely high heavy metal content, the special fertilizer of this invention can effectively reduce Cd accumulation in grains, ensuring food safety.

[0075] 2. Conclusion Compared with existing conditioners, the special fertilizer of the present invention has better control effects in terms of soil acidification control efficiency, soil cadmium passivation efficiency, plant root cadmium fixation efficiency, crop seed Cd reduction efficiency, and soil fertilizer efficiency. In addition, type A special fertilizer and type B special fertilizer are respectively provided according to soils with different physical and chemical properties, which is more targeted in actual use and can greatly improve control efficiency.

[0076] Among them, acidic purple soil is affected by industry and agriculture, which leads to the accumulation of heavy metals in the soil and causes soil acidification, thus causing the heavy metal content in the soil to exceed the standard. Type A special fertilizer is used to treat acidic purple soil. The calcium oxide, biochar, and calcium magnesium phosphate fertilizer in Type A special fertilizer all have the function of controlling soil acidification, and the synergistic effect between different raw materials further improves its soil heavy metal control ability, thereby realizing the three-step strategy of acid adjustment, soil cadmium passivation, and root fixation of soil heavy metals, reducing the absorption and accumulation of soil heavy metals in the edible parts of crops, and realizing the safe production of agricultural products.

[0077] The high geological background of heavy metals in yellow loam is more prominent, and soil acidification is mostly caused by the loss of base ions. Therefore, type B special fertilizer is used to treat yellow loam. Type B special fertilizer can absorb base ions in the soil to a certain extent and prevent loss. Most of the raw materials are alkaline substances that can regulate the soil pH well. The application of PASP enhances the crop roots' ability to hold heavy metals, and can effectively deal with the problem of crop pollution caused by high geological background of heavy metals.

[0078] 3. Optimization of the raw material ratio of the special fertilizer of the present invention Based on the comprehensive consideration of crops' requirements for comprehensive fertilizer effect and grain cadmium reduction, the special fertilizer of the present invention is optimized by taking the intersection of the optimal ratio ranges of each group for each indicator screened out to optimize the raw material range, and the optimal ratio range of raw materials for type A special fertilizer is obtained as follows: 200-220 parts of urea, 170-190 parts of calcium oxide, 100-120 parts of biochar, 170-180 parts of calcium magnesium phosphate, and 10-25 parts of polyaspartic acid; the raw material ratio range for type B special fertilizer is obtained as follows: 200-220 parts of urea, 120-140 parts of calcium oxide, 140-170 parts of biochar, 180-200 parts of sepiolite, and 40-50 parts of polyaspartic acid.

[0079] The orthogonal test method was used for further optimization within the optimal ratio range of the above raw materials, and the results showed that when the type A special fertilizer was in the following weight parts: 210 parts of urea, 180 parts of calcium oxide, 110 parts of biochar, 175 parts of calcium magnesium phosphate, and 17 parts of polyaspartic acid; and when the type B special fertilizer was in the following weight parts: 210 parts of urea, 130 parts of calcium oxide, 155 parts of biochar, 190 parts of sepiolite, and 45 parts of polyaspartic acid, the cadmium reduction effect of crop grains was optimal, which was the optimal ratio.

[0080] The above embodiments are only preferred embodiments of the present invention and cannot be used to limit the scope of protection of the present invention. Any non-substantial changes and replacements made by technicians in this field on the basis of the present invention fall within the scope of protection required by the present invention.

Claims

1. A functional special fertilizer for reducing Cd, characterized in that: It includes a type A special fertilizer for treating acidic purple soil, which includes the following raw materials in parts by weight: 200-220 parts of urea, 170-190 parts of calcium oxide, 100-120 parts of biochar, 170-180 parts of calcium magnesium phosphate, and 10-25 parts of polyaspartic acid; it also includes a type B special fertilizer for treating yellow loam, which includes the following raw materials in parts by weight: 200-220 parts of urea, 120-140 parts of calcium oxide, 140-170 parts of biochar, 180-200 parts of sepiolite, and 40-50 parts of polyaspartic acid.

2. The Cd-reducing functional special fertilizer according to claim 1, characterized in that: The type A special fertilizer includes the following raw materials in parts by weight: 205-215 parts of urea, 175-185 parts of calcium oxide, 105-115 parts of biochar, 173-177 parts of calcium magnesium phosphate, and 14-21 parts of polyaspartic acid; the type B special fertilizer includes the following raw materials in parts by weight: 205-215 parts of urea, 125-135 parts of calcium oxide, 147-162 parts of biochar, 185-195 parts of sepiolite, and 42-47 parts of polyaspartic acid.

3. The Cd-reducing functional special fertilizer according to claim 1, characterized in that: The type A special fertilizer includes the following raw materials in parts by weight: 210 parts of urea, 180 parts of calcium oxide, 110 parts of biochar, 175 parts of calcium magnesium phosphate, and 17 parts of polyaspartic acid; the type B special fertilizer includes the following raw materials in parts by weight: 210 parts of urea, 130 parts of calcium oxide, 155 parts of biochar, 190 parts of sepiolite, and 45 parts of polyaspartic acid.

4. A method for preparing a Cd-reducing functional special fertilizer, characterized in that: The following steps are involved: S1: Material mixing stage: first, urea and polyaspartic acid are mixed in a mixer, and then biochar, calcium magnesium phosphate or sepiolite, and calcium oxide are added in sequence and fully mixed; S2: Granulation stage: the mixed materials are extruded and granulated using a rotary drum granulator; S3: Drying stage: using a drum dryer to dry the formed fertilizer particles with hot air; S4: Finished product stage: The fertilizer granules are screened and cooled before being bagged.

5. The method for preparing the Cd-reducing functional special fertilizer according to claim 4, characterized in that: In step S1, urea and polyaspartic acid are stirred at 110-125 degrees Celsius. After the stirring of urea and polyaspartic acid is completed, the same temperature is maintained, and biochar is continuously added for stirring. After the stirring of urea, polyaspartic acid and biochar is completed, they are cooled to room temperature, and finally calcium magnesium phosphate or sepiolite and calcium oxide are added for sufficient stirring.

6. The method for preparing the Cd-reducing functional special fertilizer according to claim 4, characterized in that: In the step S3, the extruded particles are dried in a drum dryer with hot air at 110 to 125 degrees Celsius, the drum speed is 20 to 30 r / min, and the drying time is 30 to 40 minutes.

7. The method for preparing the Cd-reducing functional special fertilizer according to claim 4, characterized in that: In the step S2, the fertilizer particles formed by extrusion have a particle size of 1 to 5 mm.

8. The method for preparing the Cd-reducing functional special fertilizer according to claim 5, characterized in that: The stirring speed is 28-32 r / min, and the stirring time of urea and polyaspartic acid is greater than or equal to 0.5 h.

9. The method for preparing the Cd-reducing functional special fertilizer according to claim 6, characterized in that: The urea and polyaspartic acid were kept stirring at 120 degrees Celsius.

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