A method for determining the organic matter content of soil

Abstract

This invention provides a method for determining the organic matter content in soil, belonging to the technical field of soil organic matter content determination. The method involves placing a soil sample into an Erlenmeyer flask, adding potassium dichromate and concentrated sulfuric acid, and heating the flask with a heating plate. Every minute, a new Erlenmeyer flask containing the soil sample is placed on the heating plate, and the boiling time of the first flask is recorded. This boiling time is maintained for 5 minutes, and so on, removing and placing the flask every minute for further digestion. A blank test is also performed. After the flasks have cooled, distilled water is added, followed by the addition of o-phenanthroline indicator. Titration is then performed with ferrous sulfate solution until the solution color in the flask changes from orange-yellow to blue-green and then to brick-red, marking the titration endpoint. The organic matter content is then calculated. The electric heating plate method of this invention provides more stable data, better data correlation, simpler experimental steps, and a safer and cleaner experimental process compared to oil bath or paraffin heating methods.

Description

Technical Field

[0001] This invention relates to the field of soil organic matter content determination technology, and in particular to a method for determining the organic matter content in soil. Background Technology

[0002] Soil organic matter is a crucial component of soil, playing a vital role in soil formation, fertility, environmental protection, and the sustainable development of agriculture and forestry. Soil organic matter content is a key indicator of soil fertility. It promotes soil structure formation, improves conditions for soil physical, chemical, and biological processes, and enhances soil absorption and buffering capacity. Furthermore, it contains various nutrients essential for plants, such as carbon, nitrogen, phosphorus, and sulfur. Therefore, determining soil organic matter content is essential for understanding soil fertility.

[0003] Methods for determining soil organic matter content include: potassium dichromate titration method (external heating method); visual colorimetry; ignition method (gravimetric method); photometric colorimetry; and the use of specialized soil testing instruments. The potassium dichromate titration method is the most commonly used and most accurate method in laboratories.

[0004] The potassium dichromate titration method (national standard method) uses oil bath or paraffin heating, which has too many drawbacks, including being cumbersome, time-consuming, labor-intensive, prone to environmental pollution, difficult to clean, and harmful to human health. Therefore, there is an urgent need to improve this method. Thus, improving a method for determining soil organic matter content is of paramount importance. Summary of the Invention

[0005] The purpose of this invention is to provide a method for determining the organic matter content in soil, in order to solve the problems of existing methods being singular, cumbersome, polluting the environment, and having poor data correlation.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for determining the organic matter content in soil, comprising the following steps: 1) Take 0.100~1.000g of soil sample to be tested into an Erlenmeyer flask, add 5ml of potassium dichromate solution and 5ml of concentrated sulfuric acid respectively, and heat the Erlenmeyer flask with a heating plate. Then, place an Erlenmeyer flask containing soil sample to be tested onto the heating plate every minute, record the boiling time of the first Erlenmeyer flask, and keep it boiling for 5 minutes. Remove one flask every minute and place another one for digestion. At the same time, perform a blank test. 2) After removing the Erlenmeyer flask and cooling it, add distilled water to the Erlenmeyer flask, then add o-phenanthroline indicator, and titrate with ferrous sulfate standard solution until the color of the solution in the Erlenmeyer flask changes from orange-yellow through blue-green to brick red, which is the titration endpoint. 3) Calculate the soil organic matter content according to the national standard method. The calculation formula is as follows: Soil organic matter content = 000； Where: soil organic matter content, in g / kg; V0—The volume of ferrous sulfate standard solution consumed in the blank test, in mL; V—The volume of ferrous sulfate standard solution consumed in the sample determination, in mL; c — Concentration of ferrous sulfate standard solution, in mol / L; m — the mass of air-dried soil taken, in grams; k——Moisture conversion coefficient for air-dried soil to oven-dried soil, k=1-moisture content of air-dried soil; if the sample is called oven-dried soil, then k=1.

[0007] Furthermore, the concentration of the potassium dichromate solution is 0.8000 mol / L.

[0008] Furthermore, the heating temperature is 180~200℃.

[0009] Further, distilled water is added to a volume of 60-70 mL.

[0010] Furthermore, the titration number of the o-phenanthroline indicator is 4 drops.

[0011] Furthermore, the concentration of the ferrous sulfate solution is 0.2 mol / L.

[0012] The beneficial effects of this invention are: The experimental data comparison shows that the electric heating plate external heating method of the present invention provides more stable data, better data correlation, simpler experimental steps, and a safer and cleaner experimental process than the oil bath or paraffin external heating method. Attached Figure Description

[0013] Figure 1 Figure showing the stability results of different methods for detecting organic matter content in soil; Figure 2 A graph showing the standard deviation of the organic matter content in soil detected by different methods. Detailed Implementation

[0014] This invention provides a method for determining the organic matter content in soil, comprising the following steps: 1) Take 0.100~1.000g of soil sample to be tested into an Erlenmeyer flask, add 5ml of potassium dichromate solution and 5ml of concentrated sulfuric acid respectively, and heat the Erlenmeyer flask with a heating plate. Then, place an Erlenmeyer flask containing soil sample to be tested onto the heating plate every minute, record the boiling time of the first Erlenmeyer flask, and keep it boiling for 5 minutes. Remove one flask every minute and place another one for digestion. At the same time, perform a blank test. 2) After removing the Erlenmeyer flask and cooling it, add distilled water to the Erlenmeyer flask, then add o-phenanthroline indicator, and titrate with ferrous sulfate standard solution until the color of the solution in the Erlenmeyer flask changes from orange-yellow through blue-green to brick red, which is the titration endpoint. 3) Calculate the soil organic matter content according to the national standard method. The calculation formula is as follows: Soil organic matter content = 000； Where: soil organic matter content, in g / kg; V0—The volume of ferrous sulfate standard solution consumed in the blank test, in mL; V—The volume of ferrous sulfate standard solution consumed in the sample determination, in mL; c — Concentration of ferrous sulfate standard solution, in mol / L; m — the mass of air-dried soil taken, in grams; k——Moisture conversion coefficient for air-dried soil to oven-dried soil, k=1-moisture content of air-dried soil; if the sample is called oven-dried soil, then k=1.

[0015] In this invention, the concentration of the potassium dichromate solution is 0.8000 mol / L.

[0016] In this invention, the heating temperature is 180~200℃, preferably 190℃.

[0017] In this invention, the distilled water is added to a volume of 60-70 mL, preferably 65 mL.

[0018] In this invention, the titration number of the o-phenanthroline indicator is 4 drops.

[0019] In this invention, the concentration of the ferrous sulfate solution is 0.2 mol / L.

[0020] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0021] The soil used in the following implementation cases and comparative examples is soil standard material ASA-10, with an organic matter content of 16-18 g / kg.

[0022] Example 1

[0023] Potassium dichromate heating plate external heating method: 1) Accurately weigh 0.3g of the air-dried sample through a 0.25mm sieve into an Erlenmeyer flask, add 5ml of 0.8000mol / L potassium dichromate solution and 5ml of concentrated sulfuric acid respectively, and heat the Erlenmeyer flask to 190℃ using a heating plate. Then, place an Erlenmeyer flask containing the soil sample to be tested onto the heating plate every minute, record the boiling time of the first Erlenmeyer flask, and maintain the boiling time for 5 minutes. Remove one flask every minute and place another one for digestion. At the same time, perform a blank test. 2) After removing the Erlenmeyer flask and cooling it, add 60 ml of distilled water to the Erlenmeyer flask, then add 4 drops of o-phenanthroline indicator, and titrate with 0.2 mol / L ferrous sulfate solution until the color of the solution in the Erlenmeyer flask changes abruptly from orange-yellow to blue-green to brick red, which is the titration endpoint; 3) Calculate the soil organic matter content according to the national standard method. The calculation formula is as follows: Soil organic matter content = 000； Where: soil organic matter content, in g / kg; V0—The volume of ferrous sulfate standard solution consumed in the blank test, in mL; V—The volume of ferrous sulfate standard solution consumed in the sample determination, in mL; c — Concentration of ferrous sulfate standard solution, in mol / L; m — the mass of air-dried soil taken, in grams; k——Moisture conversion coefficient for air-dried soil to oven-dried soil, k=1-moisture content of air-dried soil; if the sample is called oven-dried soil, then k=1.

[0024] Comparative Example 1

[0025] Potassium dichromate digestion furnace external heating method: Accurately weigh 0.3g of the air-dried sample, passed through a 0.25mm sieve, into a rigid test tube. Add 5ml of 0.8000mol / L potassium dichromate solution and 5ml of concentrated sulfuric acid. Set the temperature of the digestion furnace to 180℃. When the temperature rises to 160℃, place the weighed soil sample into the rigid test tube and perform a blank test simultaneously. Start timing when the liquid in the test tube begins to boil (the solution surface begins to churn, and large bubbles are generated). Slowly boil for 5 minutes, then remove the test tube. After the test tube cools, pour the solution into a 150ml Erlenmeyer flask. Rinse the inside of the test tube and the inside and outside of the small funnel with distilled water in small amounts several times, ensuring all the washings are rinsed into the Erlenmeyer flask. The final total volume is approximately 70ml. The method for calculating the organic matter content is the same as in Example 1.

[0026] Comparative Example 2

[0027] The external heating method for potassium dichromate includes the potassium dichromate oil bath method (national standard method): Accurately weigh 0.3g of the air-dried sample, passed through a 0.25mm sieve, into a test tube. Add 5ml of 0.8000mol / L potassium dichromate solution and 5ml of concentrated sulfuric acid. Place a wire cage containing 10 test tubes (each cage should have 2 test tubes for a blank test, using ignited soil instead of soil samples; all other procedures are the same) into an oil bath at 190℃. The temperature in the oil bath should be lowered to approximately 170℃, and then maintained at 170℃. Start timing when the liquid in the test tubes begins to boil (the solution surface begins to churn, and large bubbles are generated). Slowly boil for 5 minutes. Remove the wire cage, allow it to cool slightly, and wipe the outside of the test tubes clean with paper. After the test tubes have cooled, pour the solution into a 150ml Erlenmeyer flask. Rinse the inside of the test tubes and the inside and outside of the small funnel with distilled water in small amounts several times, ensuring all the washings are poured into the Erlenmeyer flask. The final total volume should be approximately 70ml. The method for calculating the organic matter content is the same as in Example 1.

[0028] The above examples and comparative examples were repeated 10 times to determine the organic matter content in soil using different heating methods. The results are shown in Table 1 below: Table 1. Soil organic matter content values ​​determined by different heating methods for ASA-10

[0029] Comparative analysis of the experimental data shows that, compared with the oil bath or paraffin external heating method (national standard method), the experimental data obtained by the digestion furnace external heating method and the electric hot plate external heating method are more stable and have better correlation. At the same time, the experimental steps are simpler and the experimental process is safer and cleaner.

[0030] As shown in Figure 1, the curve of the hot plate method is the flattest, followed by the digestion furnace method, while the curve of the oil bath method fluctuates the most. In Figure 2, the mean value measured by the hot plate method is closest to the true value, and the standard deviation is also the smallest among the three. The core improvement of the experimental method of this invention lies in the different heating devices. The improved device can more accurately control the digestion time of organic matter in each soil sample; at the same time, the hot plate method can more directly and clearly observe the boiling state of the digestion liquid, and the control of the boiling time is more precise, thus ensuring the accuracy and consistency of the experimental data; furthermore, the hot plate method has the simplest experimental steps and the experimental process is safer and cleaner.

[0031] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for determining the organic matter content in soil, characterized in that, Includes the following steps: 1) Take 0.100~1.000g of soil sample to be tested into an Erlenmeyer flask, add 5ml of potassium dichromate solution and 5ml of concentrated sulfuric acid respectively, and heat the Erlenmeyer flask with a heating plate. Then, place an Erlenmeyer flask containing soil sample to be tested onto the heating plate every minute, record the boiling time of the first Erlenmeyer flask, and keep it boiling for 5 minutes. Remove one flask every minute and place another one for digestion. At the same time, perform a blank test. 2) After removing the Erlenmeyer flask and cooling it, add distilled water to the Erlenmeyer flask, then add o-phenanthroline indicator, and titrate with ferrous sulfate standard solution until the color of the solution in the Erlenmeyer flask changes from orange-yellow through blue-green to brick red, which is the titration endpoint; 3) Calculate the soil organic matter content according to the national standard method. The calculation formula is as follows: Soil organic matter content = 000； Where: soil organic matter content, in g / kg; V0—The volume of ferrous sulfate standard solution consumed in the blank test, in mL; V—The volume of ferrous sulfate standard solution consumed in the sample determination, in mL; c — Concentration of ferrous sulfate standard solution, in mol / L; m — the mass of air-dried soil taken, in grams; k——Moisture conversion coefficient for air-dried soil to oven-dried soil, k=1-moisture content of air-dried soil; if the sample is called oven-dried soil, then k=1.

2. The method for determining the organic matter content in soil according to claim 1, characterized in that, The concentration of the potassium dichromate solution is 0.8000 mol / L.

3. The method for determining the organic matter content in soil according to claim 1, characterized in that, The heating temperature is 180~200℃.

4. The method for determining the organic matter content in soil according to claim 1, characterized in that, Add distilled water to a final volume of 60-70 mL.

5. The method for determining the organic matter content in soil according to claim 1, characterized in that, The titration number of the o-phenanthroline indicator is 4 drops.

6. The method for determining the organic matter content in soil according to claim 1, characterized in that, The concentration of the ferrous sulfate solution is 0.2 mol / L.

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