This method, the most widely used in Australia, is based on a soil/water ratio of 1:5 at 25°C. The pH is determined after shaking for 1 h. The suspension is mechanically stirred during measurement to minimise changes in electrode potential associated with suspension effects and positioning of electrodes. Results by this procedure are commonly higher by about 0.5 to 0.6 of a pH unit (Baker et al. 1983) than those measured in the field by the mixed indicator/barium sulfate method of Raupach and Tucker (1959).
Distilled/deionised water-soil extracts
This water is to be in equilibrium with atmosphere with respect to CO2 concentration, and should have an EC of <1.5 × 10-3 dS/m.
Distilled/deionised water-buffer solutions
This water should have a pH ≥6.5 but ≤7.5, which can be obtained by boiling distilled or deionised H2O for 15 min and cooling under CO2-free conditions. Its EC should be <10-3 dS/m (Alvarez 1984).
pH 4.00 Buffer (25°C)
Prepare a 0.0496 M solution by dissolving 10.12 g potassium hydrogen phthalate (KHC8H4O4; previously dried for 2 h at 110°C) and make volume to 1.0 L with water described for use with buffer solutions (Alvarez 1984). Elaborate conditions to exclude CO2 are unnecessary, but protect against evaporation and contamination. Store for up to one month but replace solution if mould appears.
pH 6.86 Buffer (25°C)
Dissolve 3.387 g potassium dihydrogen phosphate (KH2PO4) and 3.533 g disodium phosphate (Na2HPO4) in deionised water and make volume to 1.0 L with deionised water described for use with buffer solutions. Dry the chemicals for 2 h at 110–130°C before use. Store for up to one month in chemical-resistant glass and protect from CO2, evaporation, and contamination but replace solution if mould appears.
pH 7.0 Buffer (25°C)
Use a purchased solution. Alternatively, dissolve 2.721 g potassium dihydrogen phosphate (KH2PO4, previously dried at 130°C for 2 h) and 3.904 g anhydrous disodium hydrogen phosphate (Na2HPO4; previously dried at 130°C for 2 h) and make to 1.0 L with deionised water, as described for use with buffer solutions. These correspond to 0.020 M KH2PO4 and 0.0275 M Na2HPO4. Protect solution from CO2, evaporation, and contamination. Store for up to one month but replace solution if mould appears.
pH 9.183 Buffer (25°C)
Dissolve 3.80 g sodium tetraborate (Na2B4O7.10H2O, stored in a desiccator over a saturated aqueous solution of NaCl and sucrose) and make to 1.0 L with deionised water described for use with buffer solutions. (Note that Na2B4O7.10H2O may lose water of crystallisation during long storage in a chemical store.) Add a small crystal of thymol (C10H14O) to prevent growth of microorganisms. Storage of the bulk buffer in a sealed container for up to one month should be possible. Prevent absorption of atmospheric CO2 and use within 10 min of pouring into an open container.
Prepare a 1:5 soil/water suspension, which is the same as defined for Method 3A1. For example, weigh 20.0 g air-dry soil (<2 mm) into a suitable bottle or jar and add 100 mL deionised water. Mechanically shake, end-over-end, at 25°C in a closed system for 1 h. Allow around 20–30 min for the soil to settle and make all measurements on the day of extraction, ideally within 4 h. When soil EC (method 3A1), water soluble Cl– (method 5A1; 5A2), and/or water soluble NO3– (Method 7B1) is/are also required, EC should be measured first and aliquots taken prior to measurement for pH, water soluble Cl– and NO3–.
Standardise the pH meter according to manufacturer’s instructions using the buffer at pH 6.86 or pH 7.0, and either the 4.0 or 9.183 buffer depending on the expected values for the soils. The use of three buffers during calibration provides a check on the linearity of electrode response. When soil pH values >10 are expected, use a glass electrode designed for highly alkaline conditions.
Stir these buffer solutions with a mechanical stirrer during measurement. Occasionally confirm there is adequate leakage of KCl from the calomel electrode, otherwise inaccurate readings may be obtained. This is achieved by placing the calomel electrode in 10 mL of deionised water for 1 min before testing for presence of Cl– with AgNO3. Thoroughly wash electrodes between the measurement of buffer solutions and between buffer solutions and soil solutions/extracts.
When measuring pH of soil suspensions, ensure electrodes are well immersed. Record the pH value obtained when the meter appears steady while the suspension is being mechanically stirred. Replicate determinations should give results within 0.1 pH unit.
Report pH (1:5 soil/water) on an air-dry basis.