KBS028: Soil pH

Active

Replaces Soil pH

Abstract

Sampling Frequency: LTER Monthly 1989 to 1994, yearly 1995 to 1996, GLBRC annually, in the fall, in the Switchgrass Nitrogen Fertility/Harvest Experiment (other sites measured periodically)

Duplicate subsamples from composite soil samples taken in each plot are used to measure soil pH. Weighed soil samples (15 g fresh weight) are made into a soil suspension by the addition of deionized water in a soil:water ratio of 1:2. The glass electrode pH meter is standardized with standard buffer solutions of pH 7 and pH 4.

Protocol

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. Press the power key to turn on the meter.

  2. Put each pH buffer solutions in a 50 mL beaker. Have a squirt bottle filled with nanopure water available for rinsing the electrode.

  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 stir.

  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 stir.

  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.

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Datatables