Soil pH—Glass Electrode Method
Standards
- Buffer solution 4.00 (Fisher # SB101-500)
- Buffer solution 5.00 (Fisher # SB102-500)
- Buffer solution 7.00 (Fisher # SB108-500)
- Deionized (DI) water (503 Porter)
Materials
- pH meter with glass electrode
- 125 ml sample cup
- Graduated cylinder (25 ml) or repipettor
- Orbital shaker
Procedure
Add the following to 125 ml sample cup
- 10 g Oven-dried equivalent, sieved soil (can be air dried or field fresh)
- 20 ml Deionized (DI) water—A 1:1 dilution (10 g soil to 10 ml DI water) is also acceptable. Do not exceed a 1:5 dilution.
Place on orbital shaker (150 rpm) for 30 minutes.
Let stand for 10 minutes to equilibrate with atmospheric CO2 and then swirl gently.
Calibrate pH meter with pH 4.0, 5.0, and 7.0 buffer solutions.
- Note that buffer solutions are standardized to specific temperatures. You may have to adjust buffer pH when room temperature differs greatly from 25°C.
Insert pH electrode into soil slurry and read the pH after display has stabilized on a calibrated pH meter.
Rinse pH electrode with DI water to remove any soil or organic matter before moving onto next sample. Please do not blot with a tissue (i.e., Kimwipe).
Check calibration after every 15 samples to nearest pH buffer of soil pH. Recalibrate if necessary.
What to Report
- Soil slurry dilution (1:2)
- Water extracted (as opposed to 0.01 CaCl2 or 1 M KCl solution)
- Air dried or field fresh soil
- pH units to at least 0.1
Created: 01/27/09; Revised: 10/10/11