Method for extracting salt ions with different occurrence forms in saline-alkali soil and evaluating corresponding salt tolerance threshold value of crops

Abstract

The invention belongs to the technical field of saline-alkali soil improvement, and discloses a method for extracting salt ions in different occurrence forms in saline-alkali soil and evaluating corresponding salt tolerance thresholds of crops. According to the method, saline-alkali soil types and differences of physicochemical properties of the saline-alkali soil are considered, and salt ions in three different occurrence forms, namely a soluble state, a combined state and a stubborn state, are obtained through a three-step physicochemical water extraction method. A crop germination experiment is simulated through sand culture, the germination potential and the germination rate of crops are investigated, and the saline-alkaline threshold values of the crops under the action of salt ions in different occurrence forms are comprehensively evaluated by combining indexes such as root activity of crop seedlings. The method is simple to operate, short in time consumption, low in cost and good in reproducibility, can accurately reflect the influence of the occurrence form of the salt ions in the soil on the salt tolerance threshold value of the crops, and has a certain reference value for risk management and control of the salt ions in the saline-alkali soil.

Description

Technical Field

[0001] The invention belongs to the technical field of saline-alkali land improvement and relates to a method for extracting salt ions in different occurrence forms in saline-alkali soil and evaluating the corresponding salt tolerance thresholds of crops. Background Art

[0002] my country has about 500 million mu of saline-alkali farmland of various types, of which 185 million mu can be reclaimed and utilized. These saline-alkali farmlands are important reserve farmland resources. How to rationally develop and utilize these saline-alkali farmland resources has become an important engine for sustained grain production increases. In the process of salinization control, physical concealed pipe salt drainage, chemical ion exchange, microbial salt-tolerant growth-promoting bacteria regulation, and halophyte bioremediation are usually used. The main principle of these methods is to control the movement of salt in the soil and reduce the salt content of the soil plow layer as much as possible, thereby improving the salt resistance of crops and achieving the purpose of promoting crop growth.

[0003] The salt ions in saline-alkali soil are mainly anions and cations with strong water solubility, including Na + , K + , Ca 2+ Mg 2+ , Cl - 、SO4 2- 、HCO3 - 、CO3 2- Most of these ions have good solubility and mobility in the soil and can migrate with water. However, it cannot be ignored that inorganic salt ions in the soil can also combine with organic components and clay minerals in the soil to form hydrogen bonding forces or ion binding forces, thereby weakening their migration ability in the soil and increasing the environmental stability of the salt ions themselves. When testing the salt ion content in the soil, the soil-water mixing method is generally used to extract the salt ions in the water phase. This method mainly measures the dissolved salt ions in the soil, which often ignores the content of other bound salt ions in the soil. In addition, salt ions in different forms may be released into the soil again as the soil microenvironment changes, and become part of the dissolved salt ions in the soil.

[0004] Organic acids in soil can affect the form of salt ions, and organic acids have little effect on the inorganic form of salt ions. As the concentration of organic acids in soil increases, the proportion of organically bound salt ions in soil increases. In the field of soil heavy metal remediation, the forms of heavy metals in soil can be divided into water-soluble / exchangeable, organically bound, iron-manganese oxide-bound, carbonate-bound, and residual forms. Among them, the water-soluble / exchangeable form is the most toxic to plants in the soil, followed by the organic bound, iron-manganese oxide-bound, and carbonate-bound forms, and the residual form is the weakest. Therefore, when conducting soil heavy metal remediation, it is usually preferred to reduce the proportion of water-soluble heavy metals in the soil and increase the proportion of bound or residual heavy metals through physical or chemical regulation methods, which can weaken the environmental migration capacity of heavy metals in the soil, prevent them from being absorbed by plant roots, and reduce the risk of heavy metal poisoning. At present, there is no clear classification and evaluation method for the extraction and determination of salt ion forms in soil. How to extract salt ions in different forms in soil has become the key to assessing the risk of soil salt ion toxicity.

[0005] At present, the method of evaluating plant salt tolerance is mainly to screen by hydroponics or adding water-soluble salts to the soil. However, it cannot be ignored that salt ions exist in different forms in the soil, which is determined by the complex components of the soil. The organic and inorganic components in the soil will adsorb or bind salt ions, affecting the activity of salt ions in the soil. Even if the total salt ion content in the soil is very high, some salt ions still exist as inert components, which are not easy to contact with plant roots, reducing the potential for plant root absorption. At the same time, soil physical and chemical properties, such as soil organic matter content and pH value, will affect the activity and binding form of salt ions. As the soil pH value increases, the solubility of salt ions decreases. The increase in soil organic matter content will enhance the soil's own buffering and retention capacity and prolong the residence time of salt in the soil. Therefore, when evaluating soil salt ion toxicity, it is necessary to give priority to the characteristics of the occurrence form of soil salt ions. This has important guiding significance for accurately evaluating the salt tolerance threshold of crops.

[0006] The present invention aims at the problem that the method for extracting the occurrence form of salt ions in saline-alkali soil and evaluating the corresponding salt tolerance threshold of crops is unclear. According to the extraction process of soil heavy metal occurrence form, the solubility of soil salt ions and the difference in soil physicochemical properties are considered, different types of salinized soils are collected, and three different forms of soil salt ions, including soluble, bound and stubborn, are extracted respectively by a three-step extraction method. At the same time, a sand culture simulation crop germination experiment is adopted to investigate the germination potential and germination rate of crops, and the root vitality of crop seedlings is combined with indicators such as the root vitality of crop seedlings to comprehensively evaluate the salt tolerance threshold of crops under different occurrence forms of salt ions, and a method for extracting salt ions with different occurrence forms in saline-alkali soil and evaluating the corresponding salt tolerance threshold of crops is established. The method is simple to operate, short in time and low in cost, has good effect stability, is suitable for extracting salt ions with different occurrence forms in saline-alkali soil, and helps to accurately evaluate the toxicity of salt ions in soil. Summary of the invention

[0007] The present invention aims at the problem that the method for extracting salt ions in different occurrence forms in saline-alkali soil and the evaluation of the corresponding salt tolerance threshold of crops are unclear, and a method for extracting salt ions in different occurrence forms in saline-alkali soil and evaluating the corresponding salt tolerance threshold of crops is established. The method takes into account the differences in soil types and physical and chemical properties of saline-alkali land, and obtains salt ions in three different occurrence forms, namely soluble, bound and recalcitrant, through a three-step physical and chemical water extraction method. A sand culture simulation crop germination experiment was adopted to investigate the germination potential and germination rate of crops, and the salinity threshold of crops under the action of salt ions in different occurrence forms was comprehensively evaluated in combination with indicators such as the root vitality of crop seedlings. The method is simple to operate, short in time and low in cost, has good reproducibility, and can accurately reflect the influence of the occurrence form of salt ions in the soil on the salt tolerance threshold of crops, which has a certain reference value for the risk control of salt ions in saline-alkali land.

[0008] The technical solution of the present invention:

[0009] A method for extracting salt ions in different forms in saline-alkali soil and evaluating the corresponding salt tolerance threshold of crops, the steps are as follows:

[0010] (1) Extraction of salt ions in different forms in saline-alkali soil

[0011] Extraction of soluble salt ions in saline-alkali soil: After sieving the soil sample, add deionized water at a solid-liquid ratio of 1:50; keep it at a temperature of 23℃-27℃ and an oscillation speed of 200r / min for 1-2h, and then centrifuge it at 4000r / min for 5-10min to separate the solid and liquid; retain the soil solid residue, filter the supernatant through a membrane, and shake it to a fixed volume to measure the concentration of soluble salt ions in the soil;

[0012] Extraction of bound salt ions in saline-alkali soil: add deionized water to the soil solid residue obtained after the extraction of soluble salt ions at a solid-liquid mass ratio of 1:3, react for 5 minutes at room temperature and a shaking speed of 200r / min, then centrifuge for 10 minutes at 4000r / min, discard the supernatant solution, and retain the residue. Repeat the above steps 2 to 3 times; add 1 mol / L magnesium chloride hexahydrate solution to the residue at a solid-liquid ratio of 1:(5-10), shake at 200r / min at room temperature, and then centrifuge for 10 minutes at 4000r / min. React for 1-2 hours, then add a mixed solution of 1 mol / L potassium acetate and 0.04 mol / L hydroxylamine hydrochloride at a solid-liquid ratio of 1: (5-10), then add deionized water at a solid-liquid ratio of 1: (20-30), shake at 200 r / min at room temperature for 5 minutes, then control the temperature at 85°C-95°C to react for 4 hours, and shake every 10 minutes during this period; centrifuge at 4000 r / min for 10 minutes, retain the residue, filter the supernatant through a membrane, and make up to volume and shake to measure the concentration of bound salt ions in the soil;

[0013] Extraction of stubborn salt ions in saline-alkali soil: add 10wt.% nitric acid solution to the residue obtained after the combined salt ion extraction for washing, and then heat and digest until the remaining amount of the nitric acid solution is 1 / 10 of the volume of the original nitric acid solution; add 10wt.% HNO3 solution, 40wt.% hydrofluoric acid solution, and 70wt.% perchloric acid solution in sequence according to the solid-liquid ratio of 1: (2-10), and shake gently, continue to heat and evaporate until the white smoke is gone, so that the soil sample becomes a white or light yellow colloid; if the digestion is not complete, add 40wt.% hydrofluoric acid 5-10 times the mass of the residue to continue digestion until complete; rinse the inner wall of the crucible with 0.5wt.% nitric acid solution, shake well and let it stand for 1h, filter the supernatant through a membrane, make it constant to volume and shake well to measure the stubborn salt ion concentration in the soil;

[0014] (2) Determination of salt tolerance thresholds of crops in different morphologies

[0015] Determination of water-soluble salt ion threshold: Add sand to the germination box, add a salt solution with a concentration of 10g / L to the sand according to the solid-liquid ratio (5-8): 1; put a number of crop seeds evenly at a density of 1 seed per cubic centimeter, and culture under constant temperature and humidity conditions for 7 days;

[0016] Starting from the first day of cultivation, the seed germination status was recorded regularly, and deionized water was added regularly to maintain the constant weight of the germination box. The germination potential was calculated based on the seed germination status on the 4th day, and the germination rate was calculated based on the seed germination status on the 7th day. Ten plants were randomly selected from each box to measure the plant height, root length, dry weight, and fresh weight. The germination index was calculated based on the number of germinations per day.

[0017] Germination potential = number of germinated seeds on the 4th day / total number of test seeds × 100%;

[0018] Germination rate = number of seeds germinated within 7 days / number of seeds tested × 100%;

[0019] Germination index = ΣGt / Dt, where Gt is the number of germinations at time t, and Dt is the corresponding number of germination days;

[0020] With salt ion content as the independent variable and seed germination rate at 7 days as the dependent variable, a regression equation was established through curve estimation; with the germination rate of crop seeds in soil without salt ions decreasing by 50% as the standard, the water-soluble salt ion thresholds of crops under different forms of salt ions were calculated respectively;

[0021] Determination of bound salt ion threshold: Based on the water-soluble salt ion threshold of the crop obtained above, a soil containing bound salt ions is set, wherein the concentration of bound salt ions in the soil is 1-10 times the water-soluble salt ion threshold of the crop; based on the content of bound salt ions per gram of soil, the bound salt ion soil is obtained according to the extraction method of salt ions in different occurrence forms in saline-alkali soil in step (1), and the obtained bound salt ion soil is added to the sand in the germination box, and deionized water is added according to the solid-liquid ratio of (5-8): 1, and stirred thoroughly; a number of crop seeds are evenly placed at a density of 1 seed per cubic centimeter, and cultured under constant temperature and humidity conditions for 7 days; the steps for testing the bound salt ion threshold are the same as the method for determining the water-soluble salt ion threshold;

[0022] Determination of the threshold value of stubborn salt ions: obtain soil containing stubborn salt ions according to the extraction method of salt ions in different forms in saline-alkali soil in step (1), add sand to the soil containing stubborn salt ions, the amount of stubborn salt ions added is 1-10 times the threshold value of water-soluble salt ions of crops, add deionized water according to the solid-liquid ratio of (5-8):1, and stir thoroughly; evenly put a number of crop seeds at a density of 1 seed per cubic centimeter, and culture for 7 days under constant temperature and humidity conditions; the steps for testing the threshold value of stubborn salt ions are the same as the method for determining the threshold value of water-soluble salt ions.

[0023] The soil is gray desert soil, brown desert soil, chernozem soil, aeolian sandy soil and loess soil.

[0024] The crops are cotton, corn, wheat and rapeseed.

[0025] The salt ion is Na + , K + , Ca 2+ Mg 2+ 、CO3 2- , Cl - 、SO4 2- .

[0026] The solid-to-liquid ratios are all expressed as mass g: volume mL.

[0027] The supernatant was filtered through a 0.22 μm filter membrane.

[0028] The constant temperature and humidity conditions are as follows: the temperature is 25° C., and the humidity is 75%.

[0029] The beneficial effects of the present invention are as follows: the present invention takes into account factors such as the physical and chemical properties of saline-alkali soil and its salt ion content, obtains salt ions in three different forms in the soil by means of physical and chemical water extraction, optimizes the soil salt ion extraction method, and combines the salt ion storage form characteristics to construct a crop salt tolerance threshold assessment method based on the salt ion storage form. The method is simple to operate and has good reproducibility, can reflect the distribution characteristics of the soil salt ion storage form, and can accurately assess the salt tolerance threshold of crops in saline-alkali land. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Comparison of the effectiveness of salt ion form extraction test methods.

[0031] Figure 2 It is the salt tolerance threshold of cotton to the three forms of salt ions in saline-alkali soil. DETAILED DESCRIPTION

[0032] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions.

[0033] Example 1 Extraction of salt ions in different occurrence forms in saline-alkali soil.

[0034] Soils from different saline-alkali lands were collected from Xinjiang saline-alkali lands, and the basic physical and chemical properties of the soils are shown in Table 1. The extraction test of salt ions in different forms in soil saline-alkali lands was carried out according to the following method.

[0035] (1) Extraction of soluble salt ions in saline-alkali soil: Add 1.00 g (passed through a 0.15 mm sieve) of soil sample to a 50 mL centrifuge tube, add 10 mL of deionized water, and cover the tube. React for 2 h at a temperature of (25 ± 2) °C and a shaking speed of 200 times / min, centrifuge at 4000 rpm for 10 min, filter the supernatant into a (0.22 μm) 50 mL volumetric flask, make up to volume with deionized water, shake well for testing, and obtain the concentration of soluble salt ions in the soil.

[0036] (2) Extraction of bound salt ions in saline-alkali soil: Add about 40 mL of deionized water to the soil solid residue of step (1), shake at room temperature for 5 min (200 r / min), centrifuge at 4000 rpm for 5 min, discard the aqueous phase, leave the residue, repeat the above steps 3 times, add 8 mL of 1 mol / L magnesium chloride hexahydrate, shake at room temperature for 1 h (200 r / min), add 8 mL of 1 mol / L potassium acetate for extraction, adjust the pH to about 5.0 with acetic acid before extraction, shake for 8 h (200 r / min), add about 30 mL of deionized water, shake at room temperature for 5 min (200 r / min), add 20 mL The extraction was carried out with 25% (v / v) acetic acid solution of 0.04 mol / L hydroxylamine hydrochloride at 96±3°C for 4 h (shaken every 10 min), oscillated at room temperature for 3 min (200 r / min), centrifuged at 4000 rpm for 10 min, the supernatant was filtered into a (0.22 μm) 50 mL volumetric flask, made up to volume with deionized water, shaken for testing, and the concentration of bound salt ions in saline-alkali soil was obtained.

[0037] (3) Extraction of stubborn salt ions in saline-alkali soil: After discarding the aqueous phase in step (2), a soil solid residue is left. The residue treated in step (2) is digested and analyzed using a nitric acid-hydrofluoric acid-perchloric acid digestion method. Use 10% HNO3 to wash the residue in the centrifuge tube into the crucible, heat and digest until the solution remains about 3mL, add 10mL HNO3, 6.7ml HF (hydrofluoric acid), 3.4mL HCIO4 (perchloric acid), and shake gently, continue to heat and evaporate until the white smoke is gone, and finally the soil sample becomes a white or light yellow gelatinous block (if the digestion is incomplete, add 5ml HF to continue digestion until complete). Rinse the inner wall of the crucible with 0.5% dilute nitric acid, dissolve it in warm water, transfer it to a 50mL volumetric flask, make up to volume with deionized water, shake well and let it stand for 1h, then slowly transfer (0.22μm filter) to a 15mL centrifuge tube for testing. The salt ion content in saline-alkali soil was determined in turn by the traditional concentrated acid digestion method, and the results are attached. Figure 1 As shown, the results obtained by the traditional method and the method of this patent are in good consistency.

[0038] Table 1 Basic physical and chemical properties of soil samples collected from saline-alkali land in Xinjiang

[0039]

[0040]

[0041] Example 2 Analysis of the differences in extraction accuracy of various salt ions in soil.

[0042] Fresh soil samples were collected from saline-alkali land. The extraction test of salt ions in different forms in saline-alkali soil was carried out according to the following method.

[0043] (1) Extraction of soluble salt ions in saline-alkali soil: Add 1.00 g (passed through a 0.15 mm sieve) of soil sample to a 50 mL centrifuge tube, add 10 mL of deionized water, and cover the tube. Oscillate for 2 h on an oscillator at a temperature of (25 ± 2) °C and a vibration speed of 200 times / min. Centrifuge for 10 min at 4000 rpm. Filter the supernatant into a (0.22 μm) 50 mL volumetric flask, make up to volume with deionized water, shake well for testing, and obtain the soluble salt ion content in the saline-alkali soil.

[0044] (2) Extraction of bound salt ions in saline-alkali soil: About 30 mL of deionized water was added to the soil solid residue in step (1), and the mixture was shaken at room temperature for 5 min (200 rpm), centrifuged at 4000 rpm for 10 min, and the aqueous phase was discarded, leaving the residue. The above steps were repeated 3 times, and 10 mL of 1 mol / L magnesium chloride (MgCl2) was added. . 6H2O), shake at room temperature for 1h (200r / min), add 10mL 1mol / L potassium acetate for extraction, adjust the pH to about 5.0 with acetic acid before extraction, shake for 5h (200r / min), add about 30mL deionized water, shake at room temperature for 5min (200r / min), add 20mL 0.04mol / L hydroxylamine hydrochloride (NH2OHHCl) 25% (v / v) acetic acid (HAC) solution for extraction, the extraction temperature is 96±3℃, the time is 4h (shake once every 10 minutes during this period), shake at room temperature for 3min (200r / min), centrifuge at 4000 rpm for 10min, filter the supernatant into a (0.22μm) 50mL volumetric flask, make up to volume with deionized water, shake for measurement, and obtain the content of bound salt ions in saline-alkali soil.

[0045] (3) Extraction of stubborn salt ions in saline-alkali soil: After discarding the aqueous phase in step (2), the solid soil residue is left, which is digested and analyzed by nitric acid-hydrofluoric acid-perchloric acid digestion method. Wash the residue in the centrifuge tube into a crucible with 10% nitric acid, heat and digest until the solution remains about 3mL, add HNO310mL, HF (hydrofluoric acid) 6.7mL, HCIO43.4mL, and shake gently, continue to heat and evaporate until the white smoke disappears, and finally the soil sample becomes a white or light yellow gelatinous block (if the digestion is incomplete, add 5mL HF to continue digestion until complete). Rinse the inner wall of the crucible with 0.5% dilute nitric acid, dissolve it in warm water, transfer it to a 50mL volumetric flask, make up to volume with deionized water, shake well and let it stand for 1h, then slowly transfer (0.22μm filter) to a 15mL centrifuge tube for testing. The concentrated acid digestion method was used to determine the contents of different salt ion forms in saline-alkali soil in turn. The results showed that the contents of different salt ion forms measured by the traditional method and the method of this patent were both consistent.

[0046] Example 3 Determination of salt tolerance threshold of crops based on salt form

[0047] Select the Xinlu Zao 80 cotton seeds of uniform size, round and full, and free of pests and diseases, and conduct germination experiments by sand culture. Disinfect the germination box, cotton seeds, and sand in advance, wipe the germination box washed with deionized water and dried with anhydrous ethanol (C2H6O), soak the cotton seeds with 5% sodium hypochlorite (NaClO) for 5 minutes, and then rinse the seeds with deionized water 5 times, pass the sand through a 2mm sieve, rinse it repeatedly with water, and then dry it at high temperature (120℃) for 8 hours.

[0048] Determination of water-soluble salt ion threshold: 1000g of fine sand was accurately weighed in each germination box, and 180mL of pre-prepared NaCl solution was used in each box for sand mixing treatment. Six NaCl concentration gradients (0mmol / L (CK), 100mmoL / L, 200mmoL / L, 250mmoL / L, 300mmoL / L, 350mmoL / L, 400mmoL / L) were set, and each treatment was repeated 3 times. Thirty seeds (5×6) were evenly placed in each germination box, and the treatment time was 7d.

[0049] Determination of the threshold value of stubborn salt ions: Set 5 stubborn sodium ion content gradients (0g (CK), 0.4g, 0.8g, 1.2g, 1.6g). According to the fact that there are about 0.015g stubborn sodium ions in 1.0g of soil, soils containing 5 stubborn sodium ion content gradients were prepared according to the method of extracting salt ions in saline-alkali land. Add the soil to the sand, keep the total weight at 1000g, add 180mL of deionized water, and stir thoroughly. 16 seeds are evenly placed in each germination box, and the treatment time is 7d.

[0050] Determination of inorganic salt bound state threshold: Set 5 inorganic salt bound state sodium ion and stubborn state sodium ion total content gradients (0g (CK), 1.2g, 2.4g, 3.6g, 4.8g). According to the fact that there are about 0.005g inorganic salt bound state sodium ions in 1.0g soil, soils containing 5 inorganic salt bound state sodium ion content gradients were prepared according to the method of extracting the occurrence form of salt ions in saline-alkali land. Add the soil to the sand to keep the total weight at 1000g, add 180mL of deionized water, and stir thoroughly.

[0051] At the same time, 5 gradients containing only stubborn sodium ions (0g (CK), 0.8g, 1.6g, 2.4g, 3.2g) were set up to eliminate the influence of the residue state in the soil system on crop germination. According to the fact that there are about 0.015g stubborn sodium ions in 1.0g soil, soils containing 5 stubborn sodium ion content gradients were prepared according to the method of extracting salt ions in saline-alkali land. The soil was mixed with sand to keep its total weight at 1000g, 180mL of deionized water was added, and it was stirred thoroughly. 16 seeds were evenly placed in each germination box, and the treatment time was 7d.

[0052] Calculation of salt tolerance threshold: Starting from the first day of cultivation, record the germination status every day, weigh and add deionized water to keep the germination box constant. Calculate the germination potential based on the germination status on the 4th day, and the germination rate based on the germination status on the 7th day. The results are shown in the attached Figure 2 As shown, the water-soluble salt ion threshold, inorganic salt-bound threshold, and stubborn salt ion threshold of Xinlu Zao 80 cotton were 235mmoL / L, 472mmoL / L, and 716mmoL / L, respectively. Xinlu Zao 80 cotton has a higher stubborn salt tolerance threshold.

[0053] Table 1 Record of seed germination process under water-soluble salt ion conditions (taking Xinlu Zao 80 as an example)

[0054]

[0055]

Claims

1. A method for extracting salt ions in different forms in saline-alkali soil and evaluating the corresponding salt tolerance threshold of crops, characterized in that: Here are the steps: (1) Extraction of salt ions in different forms in saline-alkali soil Extraction of soluble salt ions in saline-alkali soil: After sieving the soil sample, add deionized water at a solid-liquid ratio of 1:50; keep it at a temperature of 23℃-27℃ and an oscillation speed of 200r / min for 1-2h, and then centrifuge it at 4000r / min for 5-10min to separate the solid and liquid; retain the soil solid residue, filter the supernatant through a membrane, and shake it well to measure the concentration of soluble salt ions in the soil; Extraction of bound salt ions in saline-alkali soil: add deionized water to the soil solid residue obtained after the extraction of soluble salt ions according to the solid-liquid mass ratio of 1:3, react for 5 minutes at room temperature and at a shaking speed of 200r / min, then centrifuge for 10 minutes at 4000r / min, discard the supernatant solution, and retain the residue. Repeat the above steps 2 to 3 times; add 1 mol / L magnesium chloride hexahydrate solution to the residue according to the solid-liquid ratio of 1: (5-10), shake at 200r / min at room temperature, and then centrifuge for 10 minutes at 4000r / min. Swing for 1-2 hours, then add 1 mol / L potassium acetate and 0.04 mol / L hydroxylamine hydrochloride mixed solution according to the solid-liquid ratio of 1: (5-10), then add deionized water according to the solid-liquid ratio of 1: (20-30), oscillate at 200 r / min for 5 minutes at room temperature, then control the temperature at 85℃-95℃ to react for 4 hours, and shake every 10 minutes during this period; centrifuge at 4000 r / min for 10 minutes, retain the residue, filter the supernatant through a membrane, shake well to measure the concentration of bound salt ions in the soil; Extraction of stubborn salt ions in saline-alkali soil: add 10wt.% nitric acid solution to the residue obtained after the combined salt ion extraction for washing, and then heat and digest until the remaining amount of the nitric acid solution is 1 / 10 of the volume of the original nitric acid solution; add 10wt.% HNO3 solution, 40wt.% hydrofluoric acid solution, and 70wt.% perchloric acid solution in sequence according to the solid-liquid ratio of 1: (2-10), and shake gently, continue to heat and evaporate until the white smoke disappears, so that the soil sample becomes a white or light yellow gel block; if the digestion is not complete, add 40wt.% hydrofluoric acid 5-10 times the mass of the residue to continue digestion until complete; rinse the inner wall of the crucible with 0.5wt.% nitric acid solution, shake well and let it stand for 1h, filter the supernatant through a membrane, shake well and measure the stubborn salt ion concentration in the soil; (2) Determination of salt tolerance thresholds of crops in different morphologies Determination of water-soluble salt ion threshold: Add sand to the germination box, add a salt solution with a concentration of 10g / L to the sand according to the solid-liquid ratio (5-8): 1; put a number of crop seeds evenly at a density of 1 seed per cubic centimeter, and culture under constant temperature and humidity conditions for 7 days; Starting from the first day of cultivation, the seed germination status was recorded regularly, and deionized water was added regularly to maintain the constant weight of the germination box. The germination potential was calculated based on the seed germination status on the 4th day, and the germination rate was calculated based on the seed germination status on the 7th day. Ten plants were randomly selected from each box to measure the plant height, root length, dry weight, and fresh weight. The germination index was calculated based on the number of germinations per day. Germination potential = number of germinated seeds on the 4th day / total number of test seeds × 100%; Germination rate = number of seeds germinated within 7 days / number of seeds tested × 100%; Germination index = ΣGt / Dt, where Gt is the number of germinations at time t, and Dt is the corresponding number of germination days; With salt ion content as the independent variable and seed germination rate at 7 days as the dependent variable, a regression equation was established through curve estimation; with the germination rate of crop seeds in soil without salt ions decreasing by 50% as the standard, the water-soluble salt ion thresholds of crops under different forms of salt ions were calculated respectively; Determination of bound salt ion threshold: Based on the water-soluble salt ion threshold of the crop obtained above, a soil containing bound salt ions is set, wherein the concentration of bound salt ions in the soil is 1-10 times the water-soluble salt ion threshold of the crop; based on the content of bound salt ions per gram of soil, the bound salt ion soil is obtained according to the extraction method of salt ions in different occurrence forms in saline-alkali soil in step (1), and the obtained bound salt ion soil is added to the sand in the germination box, and deionized water is added according to the solid-liquid ratio of (5-8): 1, and stirred thoroughly; a number of crop seeds are evenly placed at a density of 1 seed per cubic centimeter, and cultured under constant temperature and humidity conditions for 7 days; the steps for testing the bound salt ion threshold are the same as the method for determining the water-soluble salt ion threshold; Determination of the threshold value of stubborn salt ions: obtain soil containing stubborn salt ions according to the extraction method of salt ions in different forms in saline-alkali soil in step (1), add sand to the soil containing stubborn salt ions, the amount of stubborn salt ions added is 1-10 times the threshold value of water-soluble salt ions of crops, add deionized water according to the solid-liquid ratio of (5-8):1, and stir thoroughly; evenly put a number of crop seeds at a density of 1 seed per cubic centimeter, and culture for 7 days under constant temperature and humidity conditions; the steps for testing the threshold value of stubborn salt ions are the same as the method for determining the threshold value of water-soluble salt ions.

2. The method according to claim 1, characterized in that The soil is gray desert soil, brown desert soil, chernozem soil, aeolian sandy soil and loess soil.

3. The method according to claim 1, characterized in that The crops are cotton, corn, wheat and rapeseed.

4. The method according to claim 1, characterized in that: The salt ion is Na + , K + , Ca 2+ Mg 2+ 、CO3 2- , Cl - 、SO4 2- .

5. The method according to claim 1, characterized in that The solid-to-liquid ratios are all expressed as mass g: volume mL.

6. The method according to claim 1, characterized in that The supernatant was filtered through a 0.22 μm filter membrane.

7. The method according to claim 1, characterized in that The constant temperature and humidity conditions are as follows: the temperature is 25° C., and the humidity is 75%.

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