Two subsamples (15 g each) of a sieved (4 mm sieve), field moist soil are placed into replicate centrifuge tubes or equivalent. Tubes are capped to avoid moisture loss and as necessary stored in the refrigerator until ready to analyze within a day. Using a repipet dispenser or graduated cylinder, 30 ml deionized water is added to each tube to achieve a soil: water ratio of 1:2. Tubes are capped and shaken for a few seconds; the cap is then removed and the slurry allowed to equilibrate with atmospheric CO2 and warm to room temperature for at least 30 minutes. The electrode of a pH meter, standardized at pH 7 and 4, is placed into the solution while gently swirling the slurry in the tube. pH is measured to the nearest 0.01. Between samples the electrode is rinsed with deionized water.

Materials

• pH meter (e.g., VWR sympHony)
• pH standard buffers of pH 7 and 4
• 50 mL centrifuge tubes and caps (or extraction cups and lids)
• 4 mm sieve
• Deionized water
• Squirt bottle with deionized water to rinse electrode
• Repipet dispenser or 50 mL graduated cylinder

Procedure

1. Weigh 15 g (14.98-15.02 g tolerance) of sieved, field-moist soil into each of 2 labeled 50 mL centrifuge tubes (or extraction cups).
2. Place caps on tubes to prevent drying if not immediately measuring pH.
3. Add 30 mL (for a soil:water of 1:2) of deionized water to each centrifuge tube. Replace cap.
4. Shake tubes to form soil slurry.
5. Remove caps and let stand for at least 30 minutes before measuring pH. This allows the soil to settle and the solution to equilibrate with atmospheric CO2.
6. Standardize pH meter using pH 7 and 4.
7. Gently swirl the soil slurry while taking pH measurement. Do not grind the electrode into the soil!
8. Record pH to the nearest 0.01.
9. Rinse the electrode between samples using the squirt bottle filled with deionized water.
10. Check pH calibration with pH buffer standards every 10 to 12 samples. Recalibrate meter as needed (follow user guide of pH meter used).

Calibration and Use of the pH Meter

1. Press the power key to turn on the meter.
2. Pour into separate 50 mL beakers enough of each pH buffer solution to cover the electrode tip (~10 mL).
3. Press the calibrate key.
4. Rinse the electrode with deionized water and blot dry with a lint-free tissue. Do not wipe or rub the electrode because static buildup will occur.
5. Insert the electrode into the first buffer and gently swirl beaker.
6. Wait for the pH icon to stop flashing and the arrow icon to start flashing. The meter should display the temperature-corrected pH buffer value.
7. If the displayed value is incorrect, then manually enter the value by pressing the up/down arrow keys to adjust each digit and the digits key to move to the next digit.
8. Press the calibrate key.
9. Rinse the electrode with deionized water and blot dry with a lint-free tissue. Do not wipe or rub the electrode.
   
10. Insert the electrode into the second buffer and gently swirl beaker.
11. Wait for the pH icon to stop flashing and the arrow icon to start flashing. The meter should display the temperature-corrected pH buffer value.
12. Press the calibrate key and then press the measure key to save and end the calibration. The slope will be displayed and then the meter will proceed to the measurement mode.