How to Determine Soil PH Value.

Soil pH measurement is a common practice in agricultural research. The pH value of soil not only reflects the availability and form of nutrients but also has a close relationship with its physicochemical and biochemical properties. This article describes the currently used methods for soil pH measurement, both domestically and internationally, for reference purposes.

Dilution of Soil with Water or Solutions: Soil pH measurement methods require the use of one of the following three liquids to dilute the sample:

  • Water: with an electrical conductivity value less than 0.2 ms/m (at 25°C/77°F), and a pH value not lower than 5.6.
  • Potassium chloride solution: with a concentration of 1 mol/L.
  • Calcium chloride solution: with a concentration of 0.01 mol/L.

Sample Collection and Handling: Soil pH measurement methods have specific guidelines for sample collection:

  • Collect an adequate amount of fresh soil and place it indoors for air drying. Samples should be protected from direct sunlight and prevented from contamination by gases, acids, alkalis, dust, etc.
  • You can use a drying oven to dry the samples. Note: The set heating temperature of the drying oven should not exceed 40°C (104°F).
  • Extract an appropriate amount of the sample using the quartering method, removing any foreign matter other than soil, and filter it through a test sieve with a pore size of 0.84 mm.

Sample Preparation: Chinese and international soil pH measurement methods have different requirements for sample preparation:

  • Chinese standard: Weigh 10g (±0.1g) of the sample and place it in a 50mL container. Add 25mL of water (or solution), stir for 5 minutes, and then let it stand for 1 to 3 hours.
  • International standard: Weigh 10g (±0.1g) of the sample and place it in a 100mL container. Add 50mL of water (or solution), stir for 60 minutes, and then let it stand for 1 to 3 hours.

pH Measurement: Before conducting soil pH measurement, you need to calibrate the instrument. Once calibrated, place the prepared soil suspension on a stirrer and activate the stirring function.

  • Immerse the pH electrode (along with the temperature probe) into the sample liquid. Wait for the measurement value to stabilize, then record the reading.

Important Notes:

  • During calibration, the temperature of the standard buffer solution should not differ significantly from the sample liquid’s temperature, and the error should be kept within 1%.
  • Do not over-grind the sieved soil samples, as it may affect the measurement results.
  • The prepared sample liquid must settle for an adequate time to avoid affecting the distribution of hydrogen ions between the pH electrode diffusion layer and the free solution. If the pH of the current water extract is below 7, use a salt extract solution to determine the soil’s pH value, using either potassium chloride or calcium chloride solutions to prepare the soil sample.