Straw humic acid compound fertilizer for saline-alkali soil improvement and preparation method thereof

Abstract

The application discloses a straw humic acid compound fertilizer for saline-alkali soil improvement and a preparation method thereof, and relates to the technical field of saline-alkali soil improvement and compound fertilizer. The compound fertilizer is prepared by using straw source humic acid as an organic cementing component, straw coarse fiber oxidized and modified by hydrogen peroxide as a three-dimensional skeleton component, compounding nitrogen, phosphorus and potassium basic nutrients, a calcium-based modifier and a dispersing aid, oxidizing and modifying the straw fiber, extracting humic acid by alkali dissolution and acid precipitation, mixing materials, disc granulation, low-temperature drying and screening processes. After being applied to soil, the humic acid adheres to soil fine particles to form primary aggregates, and the modified fiber is inserted and supported to construct a continuous pore structure, so that water-stable large aggregates are generated in cooperation, and the calcium-based component is combined to replace soil sodium ions, so that the integration effect of soil structure optimization, salt leaching acceleration, salt return channel blocking and continuous supply of crop nutrients is realized.

Description

Technical Field

[0001] This invention relates to the field of saline-alkali soil improvement and compound fertilizer technology, specifically to a straw humic acid compound fertilizer for saline-alkali soil improvement and its preparation method. Background Technology

[0002] Currently, secondary salinization is a prominent problem in the irrigated areas of the middle and lower reaches of the Yellow River. Most agricultural input manufacturers in the region offer compound fertilizers for improving saline-alkali land, which primarily use a basic formula of ordinary mineral-derived humic acid combined with desulfurized gypsum, mainly applied as base fertilizer to moderately saline-alkali land. These products rely on the complexing and adsorption effects of humic acid and the ion exchange effects of gypsum to reduce the salt content and alkalinity of the soil surface in the initial application stage. This is currently the mainstream product form for improving saline-alkali land, with a large annual application scale in the region. However, based on long-term field monitoring and after-sales feedback data from companies, the improvement effect of existing products is not sustainable. Significant salt return occurs after concentrated rainfall or flood irrigation during the rainy season, failing to address the root causes of soil structural barriers in saline-alkali land.

[0003] Existing similar products have several clear technical shortcomings. Their improvement mechanisms are simplistic, relying solely on chemical ion exchange to reduce soil alkalinity without targeting the physical structure of heavy, compacted soils in saline-alkali environments. Soil capillary action remains strong, leading to continuous accumulation of groundwater salts to the surface with evaporation. After discontinuing the amendment, the surface soil salinization rate can reach over 40% within 3-6 months, making it difficult to maintain the improvement effect long-term. Ordinary humic acid components can only bind fine soil particles to form micro-aggregates, failing to construct a continuous macroporous network. This results in limited improvement in soil permeability, low salt leaching efficiency, and the tendency of humic acid to bind and settle in high-calcium, saline-alkali environments, causing a rapid decline in its cementing effect. Some products add unmodified straw fiber for auxiliary improvement, but straw fiber decomposes rapidly in saline-alkali soils, its structure easily breaks and collapses, failing to form a stable three-dimensional framework. Furthermore, it competes with crops for nitrogen in the early stages of decomposition, inhibiting crop seedling growth.

[0004] In summary, existing compound fertilizers for saline-alkali land improvement generally suffer from weak structural improvement, high risk of salt return, and poor sustainability of effects. They lack product formulations and preparation processes that can simultaneously induce soil aggregates, improve physical structure, and suppress salt for a long time, making it difficult to meet the production needs of long-term management of saline-alkali farmland and stable crop yield increase. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a straw humic acid compound fertilizer for saline-alkali land improvement and its preparation method. This fertilizer is prepared by using straw-derived humic acid as the organic binder, hydrogen peroxide-oxidized straw coarse fiber as the three-dimensional framework component, compounded with nitrogen, phosphorus, and potassium basic nutrients, calcium-based amendments, and dispersants. The process involves straw fiber oxidation modification, alkali-dissolving and acid precipitation extraction of humic acid, material mixing and disc granulation, and low-temperature drying and sieving. After application to the soil, the humic acid binds fine soil particles to form primary aggregates, while the modified fibers interpenetrate and support the continuous porous structure. These two components synergistically induce the formation of water-stable large aggregates. Combined with the calcium-based component replacing sodium ions in the soil, this achieves an integrated effect of soil structure optimization, accelerated salt leaching, blocking salt return channels, and continuous crop nutrient supply.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a straw humic acid compound fertilizer for improving saline-alkali land, made from the following raw materials in parts by weight: It includes 20-35 parts of straw-derived humic acid, 10-20 parts of modified straw fiber, 15-25 parts of nitrogen source, 8-15 parts of phosphorus source, 10-18 parts of potassium source, 5-12 parts of calcium-based modifier, and 2-5 parts of dispersant. The modified straw fiber is a coarse straw fiber with a length of 2-5 mm and a diameter of 0.1-0.3 mm. The fiber surface is oxidized and loaded with oxygen-containing functional groups.

[0007] Furthermore, the straw-derived humic acid is obtained from one or more of corn straw, wheat straw, or rice straw through an alkali dissolution and acid precipitation process, and the free humic acid content in the finished product is not less than 60%, and the cation exchange capacity is 350~420 cmol / kg.

[0008] Furthermore, the modified straw fiber is obtained by oxidizing the coarse fiber of crop straw with hydrogen peroxide solution. The oxidation treatment temperature is 40~60℃, the treatment time is 2~4h, the mass concentration of hydrogen peroxide solution is 5%~10%, and the solid-liquid mass ratio of the treatment system is 1:10~1:15.

[0009] Furthermore, the nitrogen source is one or two of urea and ammonium sulfate, and the total nitrogen content of the raw material is not less than 20%; the phosphorus source is one or two of monoammonium phosphate and superphosphate, and the effective phosphorus pentoxide content of the raw material is not less than 12%; the potassium source is one or two of potassium sulfate and potassium chloride, and the effective potassium oxide content of the raw material is not less than 50%.

[0010] Furthermore, the calcium-based modifier is one or both of desulfurized gypsum and phosphogypsum, with a particle size of 0.1~0.25mm, and the calcium sulfate content in the raw material is not less than 85%.

[0011] Furthermore, the dispersing agent is one or two of sodium lignosulfonate and sodium humate, and the compound fertilizer also contains 1 to 3 parts of chelated trace elements, which are one or more of chelated iron, chelated zinc, and chelated manganese.

[0012] Furthermore, the mass ratio of the straw-derived humic acid to the modified straw fiber is (2~3):1.

[0013] Furthermore, the finished compound fertilizer is in the form of spherical granules with a particle size of 2-4 mm, a moisture content of 8%-10%, and a compressive strength of not less than 15 N.

[0014] On the other hand, a method for preparing a straw humic acid compound fertilizer for saline-alkali land improvement is characterized by comprising the following steps: Step 1: Preparation of modified straw fiber: After cutting and crushing the crop straw, the fibers are sieved to obtain coarse fibers with a length of 2-5 mm and a diameter of 0.1-0.3 mm. The coarse fibers are placed in a hydrogen peroxide solution with a mass concentration of 5%-10% and the solid-liquid mass ratio is controlled at 1:10-1:15. The fibers are stirred at a constant temperature of 40-60℃ for 2-4 hours. After filtration, the fibers are washed with water until the effluent is neutral. The fibers are then dried at 50-60℃ until the moisture content is less than 8% to obtain modified straw fiber. Step 2: Preparation of straw-derived humic acid: Crush the straw raw material to 20-40 mesh, add 2%-4% sodium hydroxide solution at a solid-liquid mass ratio of 1:8-1:12, and extract at a constant temperature of 60-80℃ for 2-3 hours. After centrifugation, take the supernatant, adjust the pH of the supernatant to 1.5-2.0 with 1mol / L hydrochloric acid for acid precipitation, let stand for 6-8 hours, filter and take the filter residue, wash the filter residue with water until the filtrate is neutral, dry at 60-70℃ and crush to 80-100 mesh to obtain straw-derived humic acid; Step 3: Mixing and granulation: Weigh all raw materials according to the ratio and feed them into the mixer. Mix for 20-25 minutes until the material is uniform. Add 15%-18% of deionized water as binding water according to the total material mass and feed it into the disc granulator to prepare wet granules. Step 4: Post-processing: Dry the wet granules at 60~80℃, sieve and collect granules with a diameter of 2~4mm, and obtain the finished compound fertilizer after cooling.

[0015] Furthermore, in step three, the tilt angle of the disc granulator is 40~50° and the rotation speed is 15~25 r / min. In step four, the moisture content of the dried finished granules is controlled to be 8%~10%.

[0016] Compared with existing technologies, this straw humic acid compound fertilizer for saline-alkali land improvement and its preparation method have the following beneficial effects: I. This invention employs an oxidized modified straw fiber and straw-derived humic acid to construct a synergistic system. Humic acid binds fine soil particles through its surface oxygen-containing functional groups, forming primary micro-aggregates. Modified straw fiber, in a rigid skeleton form, intersperses between these aggregates, supporting a continuous macroporous structure. This significantly increases the proportion of water-stable aggregates larger than 0.25 mm in the 0-20 cm soil layer, reduces soil bulk density, and improves soil permeability and water retention. This system can accelerate the leaching efficiency of surface salts by irrigation and rainfall, and also block the upward path of capillary water carrying salts, reducing the risk of salt return after improvement from a physical structural perspective. Its long-term salt-suppressing ability is far superior to ordinary humic acid compound fertilizers.

[0017] II. The straw raw materials used in this invention are all derived from agricultural production waste. Fiber modification employs a low-temperature hydrogen peroxide oxidation process, and humic acid extraction uses a conventional alkali dissolution and acid precipitation process. The entire preparation process requires no high-temperature or high-pressure equipment or toxic chemical reagents, and can be directly modified and put into production on existing compound fertilizer disc granulation production lines. Equipment investment is low, and production energy consumption is minimal. The product simultaneously functions as a saline-alkali soil improver, soil fertilizer, and nutrient supplier. A single basal application can simultaneously complete soil improvement, salinity reduction, and fertilization, reducing the frequency of field operations, lowering agricultural production costs, and meeting the large-scale improvement needs of large-area saline-alkali farmland.

[0018] Other advantages, objectives and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination or study, or may be learned from the practice of the invention. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 A flowchart for a straw humic acid compound fertilizer for improving saline-alkali land; Figure 2 Flowchart of the process for preparing modified straw fiber by oxidation; Figure 3 Flowchart of the extraction process of straw-derived humic acid through alkali dissolution and acid precipitation. Detailed Implementation

[0021] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0022] Example 1 This embodiment provides a straw-humic acid compound fertilizer for saline-alkali land improvement. It employs a low-dosage formula of organic amendment components and basic nutrients, suitable for the basal fertilizer application needs of mildly saline-alkali farmland. This embodiment uses corn straw as raw material to simultaneously prepare modified straw fiber and straw-derived humic acid. Fiber surface modification is achieved through low-temperature hydrogen peroxide oxidation, followed by humic acid extraction via an alkali-dissolution-acid precipitation process. The humic acid is then mixed with nitrogen, phosphorus, and potassium nutrients, a calcium-based amendment, and a dispersant, and granulated to obtain a uniformly sized compound fertilizer product. The preparation process parameters of this embodiment are mild, and the raw material input cost is low, which can meet the needs of salinity reduction and fertilization as well as crop nutrient supply in mildly saline-alkali land.

[0023] Specifically, the preparation process in this embodiment is as follows: Preparation of modified straw fiber Naturally air-dried corn stalks were selected, and the leaves and cob parts were removed, retaining only the stalks. The stalks were initially crushed using a shear mill, and then graded and sieved using a vibrating screen to collect coarse fiber components with a length of 2-5 mm and a diameter of 0.1-0.3 mm. Ten parts by weight of the coarse fiber were weighed and placed in a reaction vessel, and a 5% hydrogen peroxide solution was added, controlling the solid-liquid mass ratio of the reaction system to be 1:10. The stirring device was turned on, the stirring speed was set to 60 r / min, and the temperature was simultaneously raised to 40℃, and the mixture was stirred at a constant temperature for 2 hours. After the oxidation treatment, the reaction system was filtered through an 80-mesh filter cloth, and the filtered fiber material was collected. The fiber material was repeatedly washed with room temperature deionized water until the pH value of the washing water stabilized between 6.8 and 7.2. After washing, the fibers were laid flat on a tray and sent to a forced-air drying oven, where they were dried at 50℃ until the fiber moisture content was below 8%. After removal, the fibers were naturally cooled to room temperature to obtain the modified straw fiber product.

[0024] In related technologies, direct application of straw fiber leads to rapid decomposition and collapse, failing to form a long-term skeletal support. In this step, shearing, crushing, and grading screening precisely control the fiber's geometric dimensions, ensuring sufficient rigid support in the soil. Low-temperature hydrogen peroxide oxidation introduces oxygen-containing functional groups such as hydroxyl and carboxyl groups onto the fiber surface, enhancing the fiber's binding properties with humic acid and soil particles, while preventing high-temperature oxidation from causing fiber skeleton degradation and breakage. Washing to neutral removes residual oxidizing agents, preventing interference with other raw material components during subsequent granulation.

[0025] Preparation of humic acid from straw Select corn stalks from the same batch, crush them using a universal pulverizer, and pass them through a 20-mesh standard sieve to collect the sieve-underflow stalk powder. Weigh 20 parts by weight of the stalk powder and place it in an extraction tank. Add a 2% sodium hydroxide solution at a solid-liquid mass ratio of 1:8. Turn on the stirring and heating device, with a stirring rate of 80 r / min, and heat to 60℃ for 2 hours of constant temperature extraction. After extraction, send the mixture to a centrifuge and centrifuge at 3000 r / min for 15 minutes. After centrifugation, collect the supernatant and discard the lower layer of stalk residue. Transfer the supernatant to an acid precipitation tank, slowly add 1 mol / L hydrochloric acid solution while continuously stirring, and adjust the pH of the supernatant to between 1.5 and 2.0. After stopping the addition of acid, continue stirring for 10 minutes, then let it stand for 6 hours to allow the humic acid to settle completely. After standing, filter the acid precipitation system using a plate and frame filter press and collect the filter cake of crude humic acid. The filter cake was repeatedly rinsed with room temperature deionized water until the pH of the leachate stabilized between 6.5 and 7.0. The washed humic acid material was then placed in a vacuum drying oven and dried at 60°C to constant weight. After drying, it was removed and pulverized to 100 mesh using an ultrafine pulverizer to obtain the straw-derived humic acid product.

[0026] Understandably, alkali dissolution and acid precipitation are mature processes for extracting humic acid from straw. Under alkaline conditions, components such as lignin and hemicellulose in straw degrade and dissolve, and humic acid substances can be enriched after acid precipitation. Controlling the extraction temperature and time can reduce the excessive degradation of cellulose components in straw while ensuring extraction efficiency, thereby increasing the content of active functional groups in the humic acid product. Centrifugation can effectively remove insoluble residues and improve the purity of the humic acid product. Grinding to 100 mesh ensures uniform material dispersion during subsequent mixing and granulation, improving the homogeneity of components within the granules.

[0027] Mixed granulation Weigh all raw materials according to the specified proportions, including 20 parts by weight of straw-derived humic acid, 10 parts by weight of modified straw fiber, 15 parts by weight of urea, 8 parts by weight of monoammonium phosphate, 10 parts by weight of potassium sulfate, 5 parts by weight of desulfurized gypsum, 2 parts by weight of sodium lignosulfonate, and 1 part by weight of chelated iron. Feed all raw materials sequentially into a horizontal ribbon mixer and start the mixer to mix for 20 minutes until the material is uniform in color and free from obvious lumps and component stratification. After mixing, spray deionized water into the mixer through an atomizing nozzle, with the water volume accounting for 15% of the total material mass. During the water spraying process, keep the mixer running at a low speed to ensure sufficient contact between the water and the material, forming a moist, loose material. Feed the moist material into a disc granulator, controlling the disc inclination angle at 40° and the disc rotation speed at 15 r / min, and continue granulation for 12 minutes to obtain wet granules.

[0028] For example, a horizontal ribbon mixer can achieve uniform mixing of materials with large specific gravity differences, preventing lightweight fibrous components from floating and heavy inorganic salt components from sinking. Atomized water addition ensures uniform moisture dispersion, preventing excessive local moisture from forming large clumps. Disc granulation is a common process in compound fertilizer production; by adjusting the tilt angle and rotation speed, particle size and pelletizing rate can be controlled, adapting to the equipment conditions of existing compound fertilizer production lines.

[0029] Post-processing The wet granules obtained from granulation are evenly spread on a drying mesh belt and fed into a mesh belt dryer for hot air drying at 60℃. During the drying process, the moisture content of the granules is sampled and tested every 30 minutes. Drying is stopped when the moisture content drops to 8%~10%. The dried granules are then fed into a vibrating screen and graded using 2mm and 4mm standard sieves. Spherical granules with a diameter between 2 and 4mm are collected. After naturally cooling to room temperature, the finished straw humic acid compound fertilizer for saline-alkali land improvement in this embodiment is obtained.

[0030] Specifically, low-temperature drying avoids the degradation of humic acid's active functional groups caused by high temperatures, while also preventing the volatilization loss of nitrogen sources such as urea. Grading and sieving ensure uniform particle size in the finished product, improving the evenness of application during the application process, and meeting the equipment requirements for mechanized fertilization.

[0031] The compound fertilizer prepared in this embodiment is a brownish-red spherical granule with an average particle size of 3.1 mm, a moisture content of 9.2%, an average compressive strength of 18 N, a free humic acid content of 16.2%, and a total nutrient content of 25.8%. This product was applied to slightly saline-alkali farmland in the middle and lower reaches of the Yellow River irrigation area at a rate of 600 kg / hm², as a base fertilizer for wheat, and then tilled into the soil. Three months after application, soil indicators in the 0-20 cm layer were measured. The soil bulk density decreased by 0.12 g / cm³, the proportion of water-stable aggregates larger than 0.25 mm increased by 12.3%, the surface soil salinity decreased by 28.7%, and the alkalinity decreased by 15.4%. Six months after application, the surface soil salinization rate was 12.1%, and the wheat seedlings showed stable growth with no significant nitrogen competition inhibition. The product prepared in this embodiment has a simple process, controllable cost, and can effectively improve the soil structure and salinity of slightly saline-alkali land, meeting the crop production needs of such farmland.

[0032] Example 2 This embodiment provides a straw-humic acid compound fertilizer for saline-alkali land improvement. It employs a moderate dosage of organic amendment components and a basic nutrient formulation, designed for basal fertilizer application in moderately saline-alkali farmland. This embodiment uses wheat straw as raw material to prepare modified straw fiber and straw-derived humic acid. The process parameters for oxidative modification and humic acid extraction are optimized, and chelated trace elements are added. The finished product is obtained through disc granulation and low-temperature drying. The organic component ratio in this embodiment is within the optimal range for synergistic effect, simultaneously improving soil structure, accelerating salt leaching, and supplying nutrients to crops, thus meeting the improvement needs of moderately saline-alkali farmland in the middle and lower reaches of the Yellow River irrigation area.

[0033] Specifically, the preparation process in this embodiment is as follows: Preparation of modified straw fiber Naturally air-dried wheat straw was selected, and impurities and withered leaves were removed, retaining only the straw stalks. The straw was pulverized using a shearing shredder, and after being sieved through a multi-stage vibrating screen, coarse fibers with a length of 2-5 mm and a diameter of 0.1-0.3 mm were collected. 14 parts by weight of the coarse fibers were weighed and placed in a reaction vessel equipped with a stirrer. A 7.5% hydrogen peroxide solution was added, maintaining a solid-liquid mass ratio of 1:12. The stirring device was started at a stirring speed of 70 r / min, and the system was simultaneously heated to 50℃ and stirred at a constant temperature for 3 hours. After the oxidation reaction was completed, the reaction material was filtered through a 100-mesh filter, and the fiber residue was collected. The fiber residue was rinsed multiple times with room temperature deionized water until the pH of the rinsing water stabilized at around 7.0. After rinsing, the fibers were transferred to a forced-air drying oven and dried at 55℃ until the moisture content was below 8%. After cooling, the modified straw fiber product was obtained.

[0034] Understandably, a hydrogen peroxide concentration of 7.5% and a reaction temperature of 50℃ can introduce sufficient oxygen-containing functional groups onto the fiber surface while preserving the lignin skeleton structure of the fiber to the greatest extent, ensuring the fiber's long-term support capacity in the soil. Wheat straw fiber has a moderate wall thickness and, after oxidative modification, possesses both rigidity and surface activity, making it a suitable carrier for constructing a three-dimensional porous structure in soil.

[0035] Preparation of humic acid from straw Take wheat straw from the same batch, crush it, and pass it through a 30-mesh standard sieve to collect straw powder. Weigh 28 parts by weight of the straw powder and add it to the extraction reaction tank. Add a 3% sodium hydroxide solution at a solid-liquid mass ratio of 1:10. Turn on the stirring and heating system, with a stirring rate of 90 r / min. After heating to 70℃, maintain the temperature for 2.5 h. After extraction, send the mixture to a centrifuge and centrifuge at 3500 r / min for 12 min. After separation, collect the supernatant and discard the lower solid residue. Transfer the supernatant to an acid precipitation reaction tank and slowly add 1 mol / L hydrochloric acid solution while stirring. Adjust the pH of the supernatant to between 1.7 and 1.9. After the addition is complete, continue stirring for 15 min, then let it stand for 7 h to allow the humic acid to fully settle and precipitate. After standing, filter and collect the filter cake of humic acid material. Rinse the filter cake repeatedly with deionized water until the leaching is neutral. The washed humic acid was placed in a vacuum drying device and dried at 65°C to constant weight. After removal, it was pulverized to 90 mesh to obtain the straw-derived humic acid product. Testing showed that the free humic acid content in the straw-derived humic acid obtained in this step was 65.2%, and the cation exchange capacity was 392 cmol / kg, meeting the design requirements.

[0036] Specifically, the optimized extraction parameters can improve the extraction rate and activity of humic acid from straw, and the high cation exchange capacity ensures the adsorption and complexation capacity of humic acid on soil cations, thereby enhancing ion exchange and fertilizer retention effects. A 3% alkali concentration and an extraction temperature of 70℃ can achieve a balance between production efficiency and product activity, adapting to the energy consumption and cost requirements of large-scale production.

[0037] Mixed granulation Weigh all raw materials according to the specified proportions, including 28 parts by weight of straw-derived humic acid, 14 parts by weight of modified straw fiber, 20 parts by weight of a mixture of urea and ammonium sulfate (urea accounts for 70% of the total nitrogen source mass, ammonium sulfate accounts for 30%), 12 parts by weight of monoammonium phosphate, 14 parts by weight of potassium sulfate, 8 parts by weight of desulfurized gypsum, 3.5 parts by weight of a mixture of sodium lignosulfonate and sodium humate (mass ratio 1:1), and 2 parts by weight of a mixture of chelated iron and chelated zinc (mass ratio 1:1). Feed all raw materials into a horizontal ribbon mixer and mix for 22 minutes until the material is completely homogeneous and free of stratification. After mixing, spray deionized water through an atomizing device, with the water volume accounting for 16.5% of the total material mass. During the water spraying process, keep the mixer running at a low speed to fully wet the material. Feed the wetted material into a disc granulator, controlling the disc inclination angle at 45° and the disc rotation speed at 20 r / min, and run the granulation process for 15 minutes to obtain formed wet granules.

[0038] For example, the nitrogen source uses a mixture of urea and ammonium sulfate, which can provide both long-term nitrogen supply and quick-acting nitrogen supply during the seedling stage, avoiding competition between straw and crops for nitrogen in the early stages of straw decomposition. The dispersing agent uses a mixture of sodium lignosulfonate and sodium humate, which can improve the dispersibility and cohesiveness of the material during granulation, increasing pelleting rate and particle strength. Chelated trace elements prevent metal ions from being fixed and ineffective in saline-alkali environments, ensuring the absorption and utilization of trace elements by crops.

[0039] Post-processing The wet granules are evenly spread on a drying mesh belt and fed into a mesh belt dryer for hot air drying at 70℃. Moisture content is sampled and tested periodically during the drying process, and drying is stopped when the granule moisture content reaches approximately 8.5%. The dried granules are then fed into a vibrating screen and graded through 2mm and 4mm standard sieves. Qualified granules with a particle size between 2 and 4mm are collected and naturally cooled to room temperature to obtain the finished compound fertilizer product of this embodiment.

[0040] Understandably, a drying temperature of 70℃ can ensure drying efficiency while maximizing the preservation of the active functional groups of humic acid and the stability of chelated trace elements, thereby reducing nutrient loss and activity decline.

[0041] The compound fertilizer prepared in this embodiment is a brownish-brown, uniform spherical granule with an average particle size of 3.0 mm, a moisture content of 8.7%, an average compressive strength of 22 N, a free humic acid content of 21.5%, and a total nutrient content of 30.2%. This product was applied to moderately saline-alkali farmland in the middle and lower reaches of the Yellow River irrigation area at a rate of 750 kg / hm², as a base fertilizer for corn, broadcast and then tilled into the soil. Three months after application, soil indicators in the 0-20 cm layer were measured. Soil bulk density decreased by 0.17 g / cm³, the proportion of water-stable aggregates larger than 0.25 mm increased by 19.6%, surface soil salinity decreased by 39.4%, and alkalinity decreased by 23.8%. Six months after application, the surface soil salinity return rate was 8.3%, corn seedlings grew vigorously, no nutrient deficiency symptoms were observed throughout the growth period, and grain yield increased by 14.2% compared to conventional fertilization. The product in this embodiment has a balanced effect on improvement and yield increase. It can effectively improve the physical structure and chemical properties of moderately saline-alkali land, and has outstanding long-term salt suppression ability. It is suitable for the large-scale improvement of moderately saline-alkali farmland and the demand for stable crop yield.

[0042] Example 3 This embodiment provides a straw-humic acid compound fertilizer for saline-alkali land improvement. It employs a high-dosage formula of organic amendment components and calcium-based amendments, designed specifically for the improvement needs of severely saline-alkali farmland. This embodiment uses rice straw as raw material to prepare modified straw fiber and straw-derived humic acid. A high-intensity oxidation modification and humic acid extraction process is used to enhance the activity and content of organic components. Combined with a high-dosage calcium-based amendment, it strengthens the ion exchange effect. This product can rapidly reduce the salinity and alkalinity of severely saline-alkali land, simultaneously building a stable soil aggregate structure, blocking salt return pathways, and creating basic conditions for crop cultivation in severely saline-alkali land.

[0043] Specifically, the preparation process in this embodiment is as follows: Preparation of modified straw fiber Naturally air-dried rice straw was selected, and the leaves and ears were removed, leaving only the stalks. The straw was pulverized using a shear mill, and after multi-stage sieving, coarse fibers with a length of 2-5 mm and a diameter of 0.1-0.3 mm were collected. 15 parts by weight of the coarse fibers were weighed and placed in a reaction vessel, and a 10% hydrogen peroxide solution was added, maintaining a solid-liquid mass ratio of 1:15. The stirring device was turned on at a stirring rate of 80 r / min, and the temperature was simultaneously raised to 60℃, maintaining this temperature for 4 hours. After oxidation treatment, the reaction system was filtered through an 80-mesh filter cloth, and the fiber filter material was collected. The fibers were repeatedly washed with room temperature deionized water until the pH of the washing effluent stabilized between 6.9 and 7.3. After washing, the fibers were sent to a forced-air drying oven and dried at 60℃ until the moisture content was below 8%. After cooling, the modified straw fiber product was obtained.

[0044] In related technologies, the fiber decomposition rate is faster in severely saline-alkali soil environments, requiring higher stability of the fiber skeleton. In this step, the higher concentration of hydrogen peroxide and the higher treatment temperature can introduce more oxygen-containing functional groups onto the surface of rice straw fibers, improving the bonding strength between the fibers and soil particles and humic acid. Rice straw fibers have a high silicon content and are inherently rigid; after oxidative modification, they can maintain their skeleton structure for a longer period in saline-alkali soils, extending the effective life of the pore structure.

[0045] Preparation of humic acid from straw Rice straw from the same batch was crushed and passed through a 40-mesh standard sieve to collect straw powder. 35 parts by weight of the straw powder were added to an extraction tank, along with a 4% sodium hydroxide solution at a solid-liquid mass ratio of 1:12. The stirring and heating device was started, with a stirring rate of 100 rpm. The temperature was raised to 80°C and then maintained for 3 hours of extraction. After extraction, the mixture was centrifuged at 4000 rpm for 10 minutes, collecting the supernatant and discarding the lower residue. The supernatant was transferred to an acid settling tank, and 1 mol / L hydrochloric acid solution was slowly added while continuously stirring to adjust the pH to between 1.5 and 2.0. After acid addition, stirring continued for 20 minutes, followed by standing for 8 hours to allow the humic acid to settle completely. After standing, the mixture was filtered, and the humic acid filter cake was collected. The filter cake was repeatedly rinsed with deionized water until the eluent was neutral. The washed humic acid was placed in a vacuum drying oven and dried to constant weight at 70℃. After drying, it was pulverized to 80 mesh to obtain the straw-derived humic acid product. Testing showed that the free humic acid content of the straw-derived humic acid obtained in this step was 62.8%, and the cation exchange capacity was 408 cmol / kg.

[0046] Specifically, higher alkali concentrations and extraction temperatures can improve the extraction efficiency of humic acid from rice straw, ensuring the supply of raw materials for high-volume formulations. Humic acid derived from rice straw is rich in oxygen-containing functional groups and has strong complexation and adsorption capabilities, making it suitable for the salt adsorption and ion exchange needs of severely saline-alkali land.

[0047] Mixed granulation Weigh all raw materials according to the specified ratio, including 35 parts by weight of straw-derived humic acid, 15 parts by weight of modified straw fiber, 25 parts by weight of urea, 15 parts by weight of superphosphate, 18 parts by weight of potassium chloride, 12 parts by weight of phosphogypsum, 5 parts by weight of sodium humate, and 3 parts by weight of a mixture of chelated iron, chelated zinc, and chelated manganese, in a ratio of 1:1:1. Feed all raw materials into a horizontal ribbon mixer and mix for 25 minutes until the material is uniform. After mixing, spray deionized water through an atomizing nozzle, with the water volume accounting for 18% of the total material mass. Keep the mixer running during the water spraying process to ensure the material is fully wetted. Feed the wetted material into a disc granulator, controlling the disc inclination angle at 50° and the disc rotation speed at 25 r / min, and run the granulation process for 18 minutes to obtain wet granules.

[0048] For example, high dosages of calcium-based soil conditioners can enhance sodium ion exchange in severely saline-alkali soils, rapidly reducing soil alkalinity. Using potassium chloride as the potassium source improves potassium supply efficiency and meets the salt tolerance regulation needs of crops in severely saline-alkali soils. Higher water content ensures better pelleting rate under high organic content, improving pellet formation.

[0049] Post-processing The wet granules are evenly spread on a drying mesh belt and fed into a mesh belt dryer for hot air drying at 80°C. During the drying process, the moisture content of the granules is monitored periodically, and drying is stopped when the moisture content drops to the range of 8% to 10%. The dried granules are then fed into a vibrating screen and graded through 2mm and 4mm standard sieves. Granules of the qualified particle size are collected and cooled to obtain the finished compound fertilizer product of this embodiment.

[0050] Understandably, a drying temperature of 80℃ can accelerate the drying speed of high-moisture granules, improve production efficiency, and at the same time, it will not significantly affect the activity of humic acid and the stability of nutrients.

[0051] The compound fertilizer prepared in this embodiment is a dark brown spherical granule with an average particle size of 3.2 mm, a moisture content of 9.0%, an average compressive strength of 20 N, a free humic acid content of 25.7%, and a total nutrient content of 33.6%. This product was applied to severely saline-alkali farmland in the middle and lower reaches of the Yellow River irrigation area at a rate of 900 kg / hm², as a base fertilizer, and then deeply tilled into the soil. Three months after application, soil indicators in the 0-20 cm layer were measured. The soil bulk density decreased by 0.21 g / cm³, the proportion of water-stable aggregates larger than 0.25 mm increased by 24.7%, the surface soil salinity decreased by 48.2%, and the alkalinity decreased by 31.5%. Six months after application, the surface soil salinity return rate was 9.7%, and the survival rate of salt-tolerant sunflowers reached 87%, an increase of 26 percentage points compared to conventional fertilization treatment. The product in this embodiment can quickly improve soil conditions in severely saline-alkali land, build a stable soil structure, and has a significant long-term salt-suppressing effect, providing a good soil foundation for crop planting in severely saline-alkali land.

[0052] Comparative Example This comparative example uses a conventional compound fertilizer formula for improving saline-alkali land, with common mineral-derived humic acid and unmodified straw fiber as organic components. The amounts of other basic nutrients and calcium-based amendments are consistent with those in Example 2. The finished product is prepared using the same mixing, granulation, and drying processes. This comparative example is used to compare and verify the synergistic improvement effect of straw-derived humic acid and oxidized modified straw fiber in the technical solution of this invention, highlighting the technical advantages of this invention in improving soil structure and long-term salt suppression.

[0053] Specifically, the preparation process of this comparative example is as follows: Raw material preparation Commercially available mineral-derived humic acid was selected, with a free humic acid content of 55% and a cation exchange capacity of 320 cmol / kg. Ordinary unmodified wheat straw coarse fiber, 2-5 mm in length and 0.1-0.3 mm in diameter, was selected and was not oxidized. Other raw materials included 20 parts by weight of a mixture of urea and ammonium sulfate (proportioned as in Example 2), 12 parts by weight of monoammonium phosphate, 14 parts by weight of potassium sulfate, 8 parts by weight of desulfurized gypsum, 3.5 parts by weight of a mixture of sodium lignosulfonate and sodium humate, and 2 parts by weight of a mixture of chelated iron and chelated zinc. All raw material specifications were consistent with those in Example 2.

[0054] Mixed granulation Weigh 28 parts by weight of mineral-derived humic acid and 14 parts by weight of unmodified straw fiber, and feed them together with all other raw materials into a horizontal ribbon mixer. Mix for 22 minutes until the materials are uniform. After mixing, spray in 16.5% of the total material mass of deionized water to wet the material and then feed it into a disc granulator. Control the disc inclination angle at 45° and the rotation speed at 20 r / min. Granulate for 15 minutes to obtain wet granules.

[0055] Post-processing The wet granules are fed into a mesh belt dryer and dried at 70℃ until the moisture content is about 8.7%. After drying, the granules are graded through 2mm and 4mm standard sieves, and qualified particles are collected. After cooling, the comparative compound fertilizer product is obtained.

[0056] The compound fertilizer prepared in this comparative example was a dark brown spherical granule with an average particle size of 2.9 mm, a moisture content of 8.8%, an average compressive strength of 16 N, a free humic acid content of 19.8%, and a total nutrient content of 30.1%. This comparative example product was applied to moderately saline-alkali farmland in the same plot as in Example 2 at a rate of 750 kg / hm² as a base fertilizer for corn. Three months after application, soil indicators in the 0-20 cm layer were measured. Soil bulk density decreased by 0.08 g / cm³, the proportion of water-stable aggregates larger than 0.25 mm increased by 7.2%, surface soil salinity decreased by 27.6%, and alkalinity decreased by 16.3%. Six months after application, the surface soil salinity return rate was 37.4%, corn seedlings showed slight weakening growth, and grain yield increased by 5.8% compared to conventional fertilization. As can be seen from the comparison, unmodified straw fiber cannot form a stable three-dimensional skeleton structure in the soil. The cementing activity and calcium deposition resistance of mineral-derived humic acid are weaker than those of straw-derived humic acid. The synergistic effect of the two is insufficient, resulting in limited improvement in soil structure, poor long-term salt suppression ability, obvious salt return phenomenon after discontinuation, and weaker crop yield increase effect than the product of the embodiment of the present invention.

[0057] To visually present the differences in core formulations, process parameters, and application effects between each embodiment and the comparative example, the key indicators are summarized as follows: The comparison of the above indicators shows that the products of the three embodiments of the present invention are superior to the comparative examples in terms of soil structure improvement, salinity reduction, and long-term salt suppression. With the increase in the amount of organic amendment components, the improvement effect of the products gradually increases. Among them, Example 2 has the best overall cost-effectiveness and compatibility, and can be used as the core formula for large-scale promotion. The synergistic effect of oxidized modified straw fiber and straw-derived humic acid is the core reason for improving the long-term improvement effect of the products, and can effectively solve the technical defects of existing products such as severe salt return and poor sustained effect.

[0058] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A straw humic acid compound fertilizer for improving saline-alkali land, characterized in that, Made from the following raw materials in parts by weight: It includes 20-35 parts of straw-derived humic acid, 10-20 parts of modified straw fiber, 15-25 parts of nitrogen source, 8-15 parts of phosphorus source, 10-18 parts of potassium source, 5-12 parts of calcium-based modifier, and 2-5 parts of dispersant. The modified straw fiber is a coarse straw fiber with a length of 2-5 mm and a diameter of 0.1-0.3 mm. The fiber surface is oxidized and loaded with oxygen-containing functional groups.

2. The straw humic acid compound fertilizer for improving saline-alkali land according to claim 1, characterized in that, The straw-derived humic acid is obtained from one or more of corn straw, wheat straw, or rice straw through an alkali dissolution and acid precipitation process. The free humic acid content in the finished product is not less than 60%, and the cation exchange capacity is 350~420 cmol / kg.

3. The straw humic acid compound fertilizer for improving saline-alkali land according to claim 1, characterized in that, The modified straw fiber is obtained by oxidizing the crude fiber of crop straw with hydrogen peroxide solution. The oxidation treatment temperature is 40~60℃, the treatment time is 2~4h, the mass concentration of hydrogen peroxide solution is 5%~10%, and the solid-liquid mass ratio of the treatment system is 1:10~1:

15.

4. The straw humic acid compound fertilizer for improving saline-alkali land according to claim 1, characterized in that, The nitrogen source is one or two of urea and ammonium sulfate, and the total nitrogen content of the raw material is not less than 20%. The phosphorus source is one or two of monoammonium phosphate and superphosphate, and the effective phosphorus pentoxide content of the raw material is not less than 12%. The potassium source is one or two of potassium sulfate and potassium chloride, and the effective potassium oxide content of the raw material is not less than 50%.

5. The straw humic acid compound fertilizer for improving saline-alkali land according to claim 1, characterized in that, The calcium-based modifier is one or both of desulfurized gypsum and phosphogypsum, with a particle size of 0.1~0.25mm, and the calcium sulfate content in the raw material is not less than 85%.

6. The straw humic acid compound fertilizer for improving saline-alkali land according to claim 1, characterized in that, The dispersing agent is one or two of sodium lignosulfonate and sodium humate. The compound fertilizer also contains 1 to 3 parts of chelated trace elements, which are one or more of chelated iron, chelated zinc, and chelated manganese.

7. The straw humic acid compound fertilizer for improving saline-alkali land according to claim 1, characterized in that, The mass ratio of straw-derived humic acid to modified straw fiber is (2~3):

1.

8. The straw humic acid compound fertilizer for improving saline-alkali land according to claim 1, characterized in that, The finished compound fertilizer is in the form of spherical granules with a particle size of 2-4 mm, a moisture content of 8%-10%, and a compressive strength of not less than 15 N.

9. A method for preparing a straw humic acid compound fertilizer for improving saline-alkali land, used to prepare the straw humic acid compound fertilizer for improving saline-alkali land as described in any one of claims 1-8, characterized in that, Includes the following steps: Step 1: Preparation of modified straw fiber: After cutting and crushing the crop straw, the fibers are sieved to obtain coarse fibers with a length of 2-5 mm and a diameter of 0.1-0.3 mm. The coarse fibers are placed in a hydrogen peroxide solution with a mass concentration of 5%-10% and the solid-liquid mass ratio is controlled at 1:10-1:

15. The fibers are stirred at a constant temperature of 40-60℃ for 2-4 hours. After filtration, the fibers are washed with water until the effluent is neutral. The fibers are then dried at 50-60℃ until the moisture content is less than 8% to obtain modified straw fiber. Step 2: Preparation of straw-derived humic acid: Crush the straw raw material to 20-40 mesh, add 2%-4% sodium hydroxide solution at a solid-liquid mass ratio of 1:8-1:12, and extract at a constant temperature of 60-80℃ for 2-3 hours. After centrifugation, take the supernatant, adjust the pH of the supernatant to 1.5-2.0 with 1mol / L hydrochloric acid for acid precipitation, let stand for 6-8 hours, filter and take the filter residue, wash the filter residue with water until the filtrate is neutral, dry at 60-70℃ and crush to 80-100 mesh to obtain straw-derived humic acid; Step 3: Mixing and granulation: Weigh all raw materials according to the ratio and feed them into the mixer. Mix for 20-25 minutes until the material is uniform. Add 15%-18% of deionized water as binding water according to the total material mass and feed it into the disc granulator to prepare wet granules. Step 4: Post-processing: Dry the wet granules at 60~80℃, sieve and collect granules with a diameter of 2~4mm, and obtain the finished compound fertilizer after cooling.

10. A straw humic acid compound fertilizer for improving saline-alkali land according to claim 9, characterized in that, In step three, the tilt angle of the disc granulator is 40~50° and the rotation speed is 15~25 r / min. In step four, the moisture content of the dried finished granules is controlled to be 8%~10%.

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