Overview

Soil pH is a measure of the hydrogen ion activity in soil solution. This measure is generally considered an index of the acid or base intensity of a soil. The pH reading is a product of complex electrode behavior with the soil suspension and differences in soil:solution ratio, electrolyte concentration of the soil suspension, and spatial placement of the electrode can affect this reading (Mclean 1982). Soil acidity has a direct effect on the plant and is a major factor affecting nutrient availability to plants.

References

Mclean, E.O. 1982. Soil pH and line requirements. In Methods of Soil Analysis, Part 2. Chemical and Microbiological Properties. Ag. Monograph No. 9, 2nd edition.

General Procedure

Field moist soil is weighed to 15g (14.98 to 15.02 g) into 2 replicate extraction cups. Cups are capped after weighing to avoid moisture loss. Using a repipet or graduated cylinder 30 ml deionized water is added to each cup, capped and shaken for a few seconds. The cap is removed to allow the solution to equilibrate with the atmosphere for at least 30 minutes. The pH meter is standardized at pH 7 and 4.

While gently swirling the slurry the electrode is placed into the slurry. pH is measured to the nearest 0.01. Between samples the electrode is rinsed with deionized water.

Materials

1. pH meter

2. Extraction cups and lids

3. Deionized water

Procedure

1. Weigh 15 g field moist soil into 2 extraction cups.

2. Place lids on cups.

3. Add 30 ml of deionized water.

4. Gently swirl cups to form soil slurry.

5. Remove caps at least 30 minutes before measuring pH.

6. Standardize pH meter using pH 7 and 4.

7. Gently swirl the soil slurry while taking measurement.

8. Record pH to the nearest 0.01.

9. Recheck standard buffer concentrations every 10 to 12 samples. Recalibrate meter as needed.

Calibration and Use of the pH Meter

1. Put pH buffer solutions in 50 mL beakers. Have four 100 ml beakers on hand for rinsing the electrode and a squirt bottle filled with nanopure water.

2. Remove the electrode from the storage buffer and remove the small cap from the fill hole (on the side of the electrode near the top). Rinse the tip of the electrode liberally with deionized water now and each time it is removed from a solution.

3. Press "MODE" to select pH and press "CAL" to start the calibration sequence. The prompt display will alternate between "< 1PT CAL" and "2PT CAL >".

4. Press the black button to the right of the display for 2-point calibration.

5. The prompt will read "CAL 1 >". If the displayed buffer value reads "7.00", press the right button indicated by the flashing ">". If not, enter "7.00" on the keypad, then press the indicated button. The readout will show the value of the first buffer and the prompt will read "CAL 1 = "

6. The prompt display will then read "CAL 2 >". Proceed as above (Step 4), entering "4.00". The readout will show the value of the second buffer and the prompt will read "CAL 2 =".
7. The prompt will then flash between quot;MEASURE" and "CAL 1". Rinse the electrode and place the electrode in the pH 7 buffer, and press the orange "=" button to start the measurement. Do not place electrode against the bottom of the cup; this will affect the reading.
8. The prompt will read "CAL 1". When the reading stabilizes, again press "=".