Soil pH

pH

Environmental Soils and Biogeochemistry Characterization Laboratory

These methods are adapted from the Soil Survey Laboratory Methods Manual (Soil Survey Staff, 2004) and Soil pH and Soil Acidity (Thomas, 1996). Some of the passages are freely copied from the Soil Survey Laboratory Methods Manual. The practitioner is admonished to review these sources for additional background information.

Reactivity: pH

An increase in the soil:water ratio or the presence of salts generally results in a decrease in the soil pH. The soluble salt content of the soil can be overcome by using dilute salt solutions, e.g., CaCl2 or KCl, instead of distilled water. The use of dilute salt solutions is a popular method for masking seasonal variation in soil pH. The pH readings are usually less with dilute salt solutions than with distilled water but may be equal to or greater in highly weathered tropical soils, i.e., soils with a high anion exchange capacity. When the pH values of various soils are compared, determination by the same method is important (Foth and Ellis, 1988).

Several pH determination methods are used and among a few are: NaF (1 N pH 7.5 to 7.8); saturated paste pH; oxidized pH; 1:1 water and 1:2 CaCl2 (final solution: 0.01 M CaCl2); 1 N KCl; and organic materials, CaCl2 (final solution ≈ 0.01 M CaCl2).

The 1:1 water pH and 1:2 0.01 M CaCl2 pH determinations are the routinely performed soil pH measurements in our laboratory. The CaCl2 is typically 0.5 to 1.5 pH units less than the 1:1 water pH.

Soil solution:

Two meter/electrode combinations are available in the laboratory:

An UltraBasic

Note that a PocketPal pH Tester (Hach Company, Loveland, CO.) or similar pen type with a 0.1 pH range of sensitivity can be used for field determinations, but in the laboratory we use an instrument with 0.01 range of sensitivity.

(Denver Industries, Bohemia, NY) or an accument AR20 (Fisher Scientific) benchtop meter and combination electrode with a 0.01 range of sensitivity.

The meter is first calibrated with pH 4, 7, & 10 standards and the slope of the calibration curve is recorded.

The pH is determined just above the soil sediment in the supernatant of 1:1 deionized water and 1:2 0.01 M CaCl2 solutions using 15g of soil (Soil Survey Staff, 204b). Clays may clog the KCl junction of the electrode and this slows the response. Clean the electrode. Rinse the electrode with distilled water and pat dry (do not wipe).

Do not store the electrode in deionized water.

Record the pH to the nearest 0.01 pH unit, but the pH is typically reported in written documents to the nearest 0.1 pH unit.

Procedure

  • Add 15g saved 2 mm, air dry soil to a medicine cup.
  • Add 15 mL of deionized water into the sample and stir.
  • Stir occasionally (measuring after 1-hr)
  • While waiting and stirring sample calibrate pH meter and record the calibration slope.
  • Stir the sample for 30 sec, wait 1 min after end of stirring and record the 1:1 water pH to the nearest 0.01 unit.
  • Next add 15 mL of 0.02 M CaCl2 to the sample.
  • Stir the sample for 30 sec, wait 1 min after end of stirring and record the 1:2 0.01 M CaCl2 pH to the nearest 0.01 unit.
  • Ensure that the calibration slope remains with the data set.
Note: Keep the pH probe in the supernatant above the soil. Using the meters bulb into the soil damages the probe (replacement costs of probes run $140 to $500). The ratio of soil to solution is important. Specially designated pH probe is used to measure in a saturated paste soil.

References

Soil Survey Staff. 2004b. 1:1 water pH. p. 213-215. In Rebecca Burt (ed.) Soil survey laboratory methods manual. Soil Surv. Invest. Rep. 42. Version 4.0. Available at https://nrcspad.sc.egov.usda.gov/DistributionCenter/pdf.aspx?productID=886

Thomas, G.W. 1996. Soil pH and soil acidity. p. 475-490. In D.L. Sparks (ed.) Methods of soil analysis. Part 3. Chemical Methods. SSSA Book Series no. 5. Madison, WI.

Foth, H.D., and B.G. Ellis. 1988. Soil fertility. John Wiley and Sons. New York, NY.

Last updated Tuesday, July 30, 2013

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