Method for processing samples before total measurement of forest soil nutrients by microwave digestion instrument

Abstract

The invention relates to the technical field of agricultural science, and discloses a method for treating a sample before total measurement of forest soil nutrients by a microwave digestion instrument, which comprises the following steps: S1, weighing, screening and air-drying the forest soil sample, so that the sample has uniformity; s2, preparing a proper acid liquid mixture according to the characteristics of the soil sample; s3, putting the treated sample and an acid solution into a microwave digestion instrument, and performing digestion according to a set heating program; s4, the digested sample is filtered, concentrated and subjected to volume metering, and nutrient analysis is prepared; and S5, determining the nutrients of the digestion solution by using an inductively coupled plasma light emission spectrometer. By optimizing the combination of acid liquor proportion and microwave heating, nutrients in a soil sample can be released more comprehensively and more efficiently, and the microwave digestion technology can provide uniform heat in a short time, so that the acid liquor is rapidly heated and permeates into the sample, accurate release of all important elements is ensured, and the analysis accuracy is improved.

Description

Technical Field

[0001] The invention relates to the field of agricultural science and technology, and in particular to a sample processing method before full-quantity determination of forest soil nutrients by a microwave digestion instrument. Background Art

[0002] In the determination of soil nutrients, sample pretreatment is a key step to ensure the accuracy of analysis. Traditional soil digestion methods usually include wet digestion, dry digestion, etc. The wet digestion method usually uses strong acids (such as nitric acid, hydrochloric acid, and hydrofluoric acid) to extract nutrients from soil samples under heating conditions. The digestion process is relatively complicated, and the operation requires high temperature and high pressure conditions, and the digestion time is relatively long. The dry digestion method directly burns the sample at high temperature and is often used for soil samples with high organic matter content.

[0003] However, both wet and dry digestion methods have the disadvantages of complicated operation, long time, low efficiency and incomplete digestion. In addition, under high temperature conditions, some elements may be lost, affecting the accuracy of the measurement results. Summary of the invention

[0004] In view of the deficiencies in the prior art, the present invention provides a sample processing method before full determination of forest soil nutrients using a microwave digestion instrument, which solves the problem that traditional soil digestion methods may cause loss of certain elements under high temperature conditions, affecting the accuracy of the determination results.

[0005] To achieve the above purpose, the present invention is implemented by the following technical scheme: A processing method before the full determination of forest soil nutrients by a microwave digestion instrument comprises the following steps:

[0006] S1. Sample pretreatment

[0007] The forest soil samples were weighed, sieved and air-dried to make the samples homogeneous;

[0008] S2. Acid preparation

[0009] Prepare a suitable acid mixture according to the characteristics of the soil sample;

[0010] S3. Sample digestion

[0011] The treated sample and acid solution are placed in a microwave digestion instrument and digested according to the set temperature program;

[0012] S4. Post-processing

[0013] The digested samples were filtered, concentrated, and fixed to volume in preparation for nutrient analysis;

[0014] S5. Nutrient analysis

[0015] The nutrients in the digestate were determined using inductively coupled plasma optical emission spectrometry.

[0016] Preferably, the sample pretreatment in step S1 includes:

[0017] S101. Sieve the soil sample through a 100-mesh sieve, and take 0.1 g to 0.2 g of the sample after sieving to make the sample representative;

[0018] S102. Air-dry the soil sample to room temperature to remove moisture from the soil and make the soil particles dry evenly.

[0019] Preferably, the preparation of the acid solution in step S2 includes:

[0020] S201. Add 6 ml to 8 ml of concentrated nitric acid, 2 ml to 3 ml of hydrochloric acid and 1 ml to 2 ml of hydrofluoric acid to the digestion tank so that the acid solution fully covers the sample and activates the release of nutrients in the soil;

[0021] S202. Add the prepared acid solution into the digestion tank in proportion so that the volume of the acid solution is 2 to 5 times that of the soil sample.

[0022] Preferably, the sample digestion in the S3 step includes digestion in a microwave digester, and the temperature program is set as: heating to 120°C in 7 to 10 minutes, heating to 150°C in 12 to 15 minutes, heating to 180°C in 15 to 20 minutes, and heating to 210°C in 20 to 25 minutes.

[0023] Preferably, the post-processing in step S4 includes:

[0024] S401. Pour the digestion solution into the crucible, rinse the digestion tank with a small amount of distilled water and transfer it to the crucible;

[0025] S402. The crucible is placed on a 200°C hot plate for acid treatment and evaporated to a remaining 1 ml to 2 ml;

[0026] S403. Transfer the remaining solution to a 25 ml volumetric flask and make up to volume with deionized water.

[0027] Preferably, the nutrient analysis in step S5 includes using an inductively coupled plasma optical emission spectrometer to determine the contents of total phosphorus, total potassium, Fe, Ca, Cd, Cr, Cu, Mg, Mn and Na elements in the sample solution; and performing correction of internal standard elements during the determination process.

[0028] Preferably, the digestion time in step S3 is automatically adjusted according to the properties of the sample. When the soil sample contains high organic matter components, the digestion time is 15 to 30 minutes, and the digestion time for samples with high mineral content is 10 to 20 minutes.

[0029] Preferably, the concentration and ratio of the acid solution are: nitric acid concentration is 68% to 72%, hydrochloric acid concentration is 36% to 38%, and hydrofluoric acid concentration is 40% to 42%.

[0030] Preferably, the digestion tank is made of high temperature and corrosion resistant Teflon or polytetrafluoroethylene.

[0031] The present invention provides a sample processing method before full determination of forest soil nutrients by a microwave digestion instrument. It has the following beneficial effects:

[0032] 1. The present invention optimizes the combination of acid ratio and microwave heating to release nutrients in soil samples more comprehensively and efficiently. Microwave digestion technology can provide uniform heat in a short time, so that the acid can be quickly heated and penetrated into the sample, promoting the decomposition of organic matter and minerals in the soil. This efficient release mechanism greatly improves the extraction efficiency of nutrients, especially for insoluble mineral elements in the soil, ensuring the accurate release of all important elements, thereby improving the accuracy of analysis.

[0033] 2. The precise temperature control and pressure control systems of the microwave digestion instrument of the present invention ensure that each soil sample is digested under the most appropriate conditions. These control systems reduce the errors and uncertainties in traditional digestion methods and avoid the problems of incomplete digestion or excessive digestion. The sufficient digestion of the sample can not only accurately release all nutrients in the soil, but also reduce the risk of sample contamination by reducing unnecessary chemical reactions, thereby ensuring the accuracy and reliability of the final nutrient determination results.

[0034] 3. Compared with traditional heating methods, the microwave digestion technology of the present invention significantly shortens the sample processing time. Microwaves can transfer heat quickly and evenly, shortening the digestion process from several hours to 20 to 30 minutes, greatly improving the experimental efficiency. This advantage is particularly prominent in large-scale sample analysis. A large number of soil samples can be processed quickly, significantly saving experimental time, and is suitable for high-throughput soil monitoring and agricultural research. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 The present invention is a flow chart of a sample processing method before full determination of forest soil nutrients by a microwave digestion instrument. DETAILED DESCRIPTION

[0036] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0037] Please see attached Figure 1 The embodiment of the present invention provides a method for processing samples before full determination of forest soil nutrients by a microwave digestion instrument, comprising the following steps:

[0038] S1. Sample pretreatment

[0039] The forest soil samples were weighed, sieved and air-dried to make the samples homogeneous;

[0040] S2. Acid preparation

[0041] Prepare a suitable acid mixture according to the characteristics of the soil sample;

[0042] S3. Sample digestion

[0043] The treated sample and acid solution are placed in a microwave digestion instrument and digested according to the set temperature program;

[0044] S4. Post-processing

[0045] The digested samples were filtered, concentrated, and fixed to volume in preparation for nutrient analysis;

[0046] S5. Nutrient analysis

[0047] The nutrients in the digestate were determined using inductively coupled plasma optical emission spectrometry.

[0048] Specifically, by optimizing the combination of acid ratio and microwave digestion technology, the release efficiency of nutrients in soil samples can be significantly improved. Microwave digestion technology uses efficient and uniform heating to allow acid to quickly penetrate and activate organic and mineral components in the soil, ensuring the comprehensive release of nutrients. Precise temperature control and pressure regulation further improve the accuracy of sample digestion, reduce the phenomenon of incomplete or excessive digestion, and ensure the accuracy of nutrient determination. In addition, the microwave digestion instrument adopts a closed system, which reduces the emission of waste liquid and gas, greatly improves the safety and environmental protection of the experiment, and avoids the potential harm of acid to the experimental environment and personnel.

[0049] Sample pretreatment in step S1 includes:

[0050] S101. Sieve the soil sample through a 100-mesh sieve, and take 0.1 g to 0.2 g of the sample after sieving to make the sample representative;

[0051] S102. Air-dry the soil sample to room temperature to remove moisture from the soil and make the soil particles dry evenly.

[0052] Specifically, sample collection and preparation:

[0053] Randomly collect samples from multiple points in the forest soil and ensure that the samples at each sampling point are representative. The sampling equipment must be made of non-metallic materials (such as plastic shovels) to avoid contamination of soil samples by metal components.

[0054] Soil samples need to be marked according to different soil types, and the sampling time and location must be recorded for comparison in subsequent data analysis.

[0055] Air Dry:

[0056] The collected soil samples need to be air-dried in a cool, ventilated environment, away from direct sunlight. The air-drying process generally takes 24 to 48 hours, until the moisture content of the sample reaches a stable state. Avoid high-temperature air-drying to prevent changes in the organic matter in the sample.

[0057] Grinding and sieving:

[0058] The air-dried soil sample needs to be ground. Use a mortar and pestle to grind the sample into a fine powder. Usually grind the sample until it passes through a 100-mesh sieve to ensure the homogeneity of the sample. Too large particles may affect the efficiency of digestion, resulting in incomplete digestion of some components in the sample.

[0059] Beneficial effects:

[0060] Ensure representativeness: Ensure that each experimental sample accurately represents the characteristics of the entire soil area through appropriate sampling and mixing.

[0061] Avoid interference: The air-drying process removes moisture from the soil, preventing moisture from affecting the digestion effect and nutrient determination.

[0062] Improve digestion efficiency: Through grinding and screening, ensure that the sample is fine and uniform, which is conducive to the uniform action of the acid and the full release of nutrients.

[0063] The acid solution preparation in step S2 includes:

[0064] S201. Add 6 ml to 8 ml of concentrated nitric acid, 2 ml to 3 ml of hydrochloric acid and 1 ml to 2 ml of hydrofluoric acid to the digestion tank so that the acid solution fully covers the sample and activates the release of nutrients in the soil;

[0065] S202. Add the prepared acid solution into the digestion tank in proportion so that the volume of the acid solution is 2 to 5 times that of the soil sample.

[0066] Specifically, the acid composition is:

[0067] In this step, a mixture of nitric acid, hydrochloric acid and hydrofluoric acid is used. Different acid ratios can effectively decompose different components in the soil, such as organic matter, minerals, etc. The commonly used ratio is: 6 ml concentrated nitric acid, 2 ml hydrochloric acid, 1 ml hydrofluoric acid.

[0068] When preparing the acid solution, make sure the solution is of the appropriate concentration and stir the acid thoroughly to ensure uniformity.

[0069] Acid addition order:

[0070] After adding concentrated nitric acid to the digestion tank, add hydrochloric acid and hydrofluoric acid. This order helps dissolve minerals first and then decompose organic matter to ensure digestion efficiency.

[0071] Acid preparation environment:

[0072] Acid preparation should be carried out in a well-ventilated laboratory, and protective equipment (such as gloves, goggles, and lab coats) should be worn during the preparation process to ensure the safety of operators.

[0073] Beneficial effects:

[0074] Efficient decomposition of soil components: Through a reasonable acid-liquid ratio, the organic matter and minerals in the soil can be effectively decomposed, fully releasing the nutrient elements required for analysis.

[0075] Safety: Acid preparation is performed in a fume hood and protective equipment is worn to reduce health risks caused by acid volatilization.

[0076] Optimized digestion effect: Accurately proportioned acid can maximize the digestion effect and avoid nutrient loss or incomplete digestion caused by too much or too little acid.

[0077] The sample digestion in step S3 includes digestion in a microwave digester, and the temperature program is set as follows: heating to 120°C for 7 to 10 minutes, heating to 150°C for 12 to 15 minutes, heating to 180°C for 15 to 20 minutes, and heating to 210°C for 20 to 25 minutes.

[0078] Specifically, the control of digestion temperature and time:

[0079] The microwave digestion instrument has an intelligent control system that can automatically adjust the temperature and pressure according to the characteristics of the soil sample (such as organic matter content, mineral content, etc.). The conventional digestion procedure is: 7 minutes to heat up to 120°C, 12 minutes to heat up to 150°C, 15 minutes to heat up to 180°C, and 20 minutes to heat up to 210°C.

[0080] The gradual increase in temperature and pressure helps to increase the solubility of various elements in the sample, thereby effectively releasing nutrients in the soil.

[0081] Monitoring of digestion process:

[0082] The built-in sensor in the microwave digester can monitor the temperature and pressure changes of the sample in real time to ensure the stability of the digestion process and avoid equipment damage or sample loss due to excessive pressure or temperature.

[0083] Beneficial effects:

[0084] Efficient digestion: Microwave digestion instrument can complete the digestion of soil samples in a shorter time, reducing the disadvantage of long-term heating in traditional digestion methods.

[0085] Automatic control: The intelligent control system ensures that the digestion process of each sample is carried out under the best conditions, improving the stability and accuracy of the experimental results.

[0086] Save time and energy: The optimized digestion program and temperature control settings can significantly save digestion time and energy consumption.

[0087] Post-processing in step S4 includes:

[0088] S401. Pour the digestion solution into the crucible, rinse the digestion tank with a small amount of distilled water and transfer it to the crucible;

[0089] S402. The crucible is placed on a 200°C hot plate for acid treatment and evaporated to a remaining 1 ml to 2 ml;

[0090] S403. Transfer the remaining solution to a 25 ml volumetric flask and make up to volume with deionized water.

[0091] Specifically, the acid removal process:

[0092] After the digestion solution is transferred to the crucible, the solution is heated to 200°C using a hot plate. Excess acid is removed by evaporation to ensure that the remaining acid concentration in the sample is suitable for subsequent analysis.

[0093] The acid-chasing operation requires precise control of temperature and evaporation time to avoid excessive evaporation leading to sample loss or uneven acidity.

[0094] Volume setting and filtration:

[0095] Add appropriate amount of deionized water and make up to 25 ml to ensure that the sample concentration is within the analytical range of the instrument.

[0096] The sample solution was filtered using a 0.45 μm filter membrane to remove insoluble matter and ensure the accuracy of the measurement results.

[0097] Beneficial effects:

[0098] Ensure analysis accuracy: Through precise volume determination and filtration processes, impurities in the digestion solution are removed to ensure the accuracy of nutrient element determination.

[0099] Improve instrument life: The acid removal process reduces the interference of acidic substances in the sample, which helps to protect the stability and service life of analytical instruments such as ICP-OES.

[0100] Improve the reliability of subsequent analysis: The solution after removing impurities is more suitable for analysis by high-precision instruments, avoiding the influence of impurities on the measurement results.

[0101] The nutrient analysis in step S5 includes using an inductively coupled plasma optical emission spectrometer to determine the contents of total phosphorus, total potassium, Fe, Ca, Cd, Cr, Cu, Mg, Mn and Na in the sample solution; and performing correction for internal standard elements during the determination process.

[0102] Specifically, the analysis method is selected:

[0103] Inductively coupled plasma optical emission spectrometer (ICP-OES) was used to simultaneously determine the total phosphorus, total potassium, Fe, Ca, Cd, Cr, Cu, Mg, Mn, Na and other elements in the samples.

[0104] ICP-OES is an efficient multi-element analysis technique that can detect the concentration of multiple elements simultaneously with high sensitivity and accuracy.

[0105] Use of standard solutions and internal standard elements:

[0106] Before elemental analysis, calibration is performed using standard solutions to ensure the accuracy and consistency of the analysis results.

[0107] Internal standard elements (such as yttrium and germanium) can be used to eliminate matrix effects and improve measurement accuracy.

[0108] Beneficial effects:

[0109] High-throughput analysis: The simultaneous determination of multiple elements by ICP-OES greatly improves the analysis efficiency and enables the determination results of multiple nutrient elements to be obtained in one experiment.

[0110] Precision and sensitivity: The high precision and high sensitivity of ICP-OES technology can accurately measure trace elements in soil samples to meet the needs of soil nutrient assessment.

[0111] Data stability: Data stability and reliability are ensured through calibration with standard solutions and the use of internal standard elements.

[0112] The digestion time in step S3 is automatically adjusted according to the nature of the sample. When the soil sample contains high organic matter components, the digestion time is 15 to 30 minutes, and the digestion time for samples with high mineral content is 10 to 20 minutes.

[0113] Specifically, improve digestion efficiency: By automatically adjusting the digestion time according to the difference in organic matter and mineral content in the soil sample, it can be ensured that each sample can be digested under optimal conditions. For soils with higher organic matter content, longer digestion time (15 to 30 minutes) helps to thoroughly decompose organic matter and release more nutrients. For soils with higher mineral content, shorter digestion time (10 to 20 minutes) can effectively prevent over-digestion, avoid unnecessary element loss or overly acidic solutions, and improve sample processing efficiency.

[0114] Save time and energy: Using different digestion times for different samples not only ensures the digestion effect, but also avoids unnecessary long digestion time, which can save experimental time and energy costs and improve experimental efficiency.

[0115] Improve the processing ability of sample diversity: This design of automatically adjusting the digestion time enables the method to handle various soil types, thereby improving the versatility and adaptability of the method.

[0116] The concentration and proportion of the acid solution are: nitric acid concentration is 68% to 72%, hydrochloric acid concentration is 36% to 38%, and hydrofluoric acid concentration is 40% to 42%.

[0117] Specifically, efficient decomposition of soil components: The precise acid ratio ensures that organic matter and minerals in soil samples are effectively decomposed. Nitric acid is mainly used to oxidize organic matter, hydrochloric acid helps to decompose carbonates and dissolve metal elements in the soil, and hydrofluoric acid helps to destroy silicate minerals in the soil. By properly controlling the concentration and ratio of these acids, the release of nutrients can be maximized to ensure the integrity and accuracy of the analysis results.

[0118] Reduced sample-to-sample variability: By controlling the precise ratio of acid concentration, different soil types (whether high in organic matter or mineral content) can achieve optimal digestion results, reducing the risk of elements being dissolved due to too strong or too weak acid, thereby improving experimental consistency.

[0119] Protect the environment and equipment: Appropriate acid concentration helps avoid excessive corrosion of equipment and elements in soil samples, reduces acid usage, reduces environmental pollution and extends the service life of experimental equipment.

[0120] The material used for the digestion tank is high temperature resistant and corrosion resistant Teflon or polytetrafluoroethylene.

[0121] Specifically, high temperature resistance and strong corrosion resistance: Teflon and polytetrafluoroethylene as digestion tank materials have strong resistance to high temperature and acid and alkali corrosion, and can withstand the high temperature and high pressure environment generated during microwave digestion without reaction or damage. This can ensure the long-term stable use of the digestion tank and reduce the occurrence of equipment failures.

[0122] Improve the safety of the digestion process: Under high temperature and high pressure conditions, choosing suitable high temperature and corrosion resistant materials can avoid leakage or contamination caused by container aging or reaction, and ensure the safety of experimental operations.

[0123] Avoid sample contamination: The digestion tank uses Teflon or polytetrafluoroethylene to avoid possible contamination of the sample by metal materials, ensuring the purity of the sample during the digestion process, thereby ensuring the accuracy of the final nutrient determination results.

[0124] Improve experimental durability: The stability of this material allows the digestion tank to be used multiple times for a long time, reducing the maintenance frequency and material replacement costs, and reducing economic expenses during the experiment.

[0125] The following is an explanation of soil nutrient elements:

[0126] Total Phosphorus:

[0127] Phosphorus is one of the essential macroelements for plant growth, and is mainly involved in processes such as energy transfer, cell division, and photosynthesis. Total phosphorus in the soil includes inorganic phosphorus and organic phosphorus, and is an indicator of the total phosphorus content in the soil. Too much or too little phosphorus will affect plant growth.

[0128] Total Potassium:

[0129] Potassium is another macronutrient required by plants, mainly used for processes such as water regulation, enzyme activity, and sugar and protein synthesis. Total potassium in the soil refers to all forms of potassium, including exchangeable potassium, fixed potassium, etc. Potassium plays an important role in plant stress resistance (such as drought resistance and disease resistance).

[0130] Fe(Iron):

[0131] Iron is one of the trace elements of plants and is involved in the electron transfer reaction in plant photosynthesis and the composition of certain enzymes. Iron in the soil usually exists in the form of iron oxide (such as iron oxide), which affects the availability of soil and the absorption of plants.

[0132] Ca(Calcium):

[0133] Calcium is one of the indispensable elements for plant growth. Its main function is to enhance the stability of cell walls, participate in the structural formation of plants, and affect the growth and development of roots. Calcium in soil mainly comes from minerals such as calcium carbonate.

[0134] Cd (Cadmium):

[0135] Cadmium is one of the harmful elements in plants and is a heavy metal. Although it has almost no essential effect on plants, excessive cadmium content will affect plant growth and pose potential hazards to the environment and human health. Cadmium in soil usually comes from industrial pollution, pesticides, etc.

[0136] Cr(Chromium):

[0137] Chromium is also one of the heavy metals. Excessive chromium can be toxic to plants and microorganisms in the soil, affecting soil quality and plant growth. The source of chromium in the soil is usually industrial wastewater discharge or certain agricultural fertilizers.

[0138] Cu(Copper):

[0139] Copper is one of the essential trace elements for plants and is involved in many physiological processes of plants, especially the synthesis of enzymes related to respiration and photosynthesis. Too high or too low copper concentration in the soil will affect the healthy growth of plants.

[0140] Mg(Magnesium):

[0141] Magnesium is an important element for plant growth. It is mainly involved in the synthesis of chlorophyll, which is an important component of photosynthesis. In addition, magnesium is also involved in the energy metabolism of plant cells. Magnesium in the soil usually comes from minerals and is essential for plant health.

[0142] Mn(Manganese):

[0143] Manganese is a trace element for plants and is involved in photosynthesis, respiration, nitrogen metabolism and other physiological processes. Manganese deficiency can stunt plant growth, especially affecting root development. Excessive manganese can be toxic to plants.

[0144] Na (Sodium, sodium):

[0145] Sodium is a trace element in plants. Although it is not an essential nutrient for plants, it can be a necessary nutrient source in some special plants, such as halophytes. In soil, sodium usually comes from a high-salt environment. High concentrations of sodium can have a negative impact on soil structure and lead to salinization.

[0146] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for processing samples before full determination of forest soil nutrients by microwave digestion instrument, characterized in that: The following steps are involved: S1. Sample pretreatment The forest soil samples were weighed, sieved and air-dried to make the samples homogeneous; S2. Acid preparation Prepare a suitable acid mixture according to the characteristics of the soil sample; S3. Sample digestion The treated sample and acid solution are placed in a microwave digestion instrument and digested according to the set temperature program; S4. Post-processing The digested samples were filtered, concentrated, and fixed to volume in preparation for nutrient analysis; S5. Nutrient analysis The nutrients in the digestate were determined using inductively coupled plasma optical emission spectrometry.

2. The method for processing samples before the full determination of forest soil nutrients by microwave digestion instrument according to claim 1, characterized in that: The sample pretreatment in step S1 includes: S101. Sieve the soil sample through a 100-mesh sieve, and take 0.1 g to 0.2 g of the sample after sieving to make the sample representative; S102. Air-dry the soil sample to room temperature to remove moisture from the soil and make the soil particles dry evenly.

3. The processing method before the microwave digestion instrument forest soil nutrient full amount determination sample according to claim 1 is characterized in that, The acid solution preparation in step S2 includes: S201. Add 6 ml to 8 ml of concentrated nitric acid, 2 ml to 3 ml of hydrochloric acid and 1 ml to 2 ml of hydrofluoric acid to the digestion tank so that the acid solution fully covers the sample and activates the release of nutrients in the soil; S202. Add the prepared acid solution into the digestion tank in proportion so that the volume of the acid solution is 2 to 5 times that of the soil sample.

4. The method for processing samples before the full determination of forest soil nutrients by microwave digestion instrument according to claim 1, characterized in that: The sample digestion in step S3 includes digestion in a microwave digestion instrument, and the temperature program is set as follows: heating to 120°C in 7 to 10 minutes, heating to 150°C in 12 to 15 minutes, heating to 180°C in 15 to 20 minutes, and heating to 210°C in 20 to 25 minutes.

5. The method for processing samples before the full determination of forest soil nutrients by microwave digestion instrument according to claim 1, characterized in that: The post-processing in step S4 includes: S401. Pour the digestion solution into the crucible, rinse the digestion tank with a small amount of distilled water and transfer it to the crucible; S402. The crucible is placed on a 200°C hot plate for acid treatment and evaporated to a remaining 1 ml to 2 ml; S403. Transfer the remaining solution to a 25 ml volumetric flask and make up to volume with deionized water.

6. The method for processing samples before the full determination of forest soil nutrients by microwave digestion instrument according to claim 1, characterized in that: The nutrient analysis in step S5 includes using an inductively coupled plasma optical emission spectrometer to determine the contents of total phosphorus, total potassium, Fe, Ca, Cd, Cr, Cu, Mg, Mn and Na in the sample solution; and performing correction for internal standard elements during the determination process.

7. The method for processing samples before the microwave digestion instrument forest soil nutrient full determination according to claim 1 is characterized in that: The digestion time in step S3 is automatically adjusted according to the nature of the sample. When the soil sample contains high organic matter components, the digestion time is 15 to 30 minutes, and the digestion time for samples with high mineral content is 10 to 20 minutes.

8. The method for processing samples before the full determination of forest soil nutrients by microwave digestion instrument according to claim 1, characterized in that: The concentration and proportion of the acid solution are as follows: nitric acid concentration is 68% to 72%, hydrochloric acid concentration is 36% to 38%, and hydrofluoric acid concentration is 40% to 42%.

9. The method for processing samples before the full determination of forest soil nutrients by microwave digestion instrument according to claim 3, characterized in that: The digestion tank is made of high temperature resistant and corrosion resistant Teflon or polytetrafluoroethylene.

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