This method uses dilute calcium chloride (0.01 M CaCl2) at a 1:5 soil/solution ratio. Electrodes are positioned in the unstirred supernatant during measurement. Results are commonly from 0.5 to 1.0 pH unit lower than those obtained by Method 4A1 (e.g. Conyers and Davey 1988), although smaller differences are possible when soils contain high levels of soluble salts (naturally and/or from fertiliser additions) and/or when soil colloids exhibit variable surface charge characteristics. Fluctuations in soil soluble salts, due to seasonal conditions and fertiliser inputs, cause less variation of pH values measured in 0.01 M CaCl2 than those measured in water (White 1969).
0.01 M Calcium Chloride
Dissolve 1.470 g calcium chloride dihydrate (CaCl2.2H2O) and make to 1.0 L with deionised water; this water to be in equilibrium with the atmosphere.
pH Buffer Solutions
pH buffer solutions are as described for Method 4A1.
Standardise the pH meter as described in Method 4A1.
Proceed as in Method 4A1, except use 0.01 M CaCl2 solution for preparing the soil suspension instead of deionised water. Allow the suspension to settle for around 20–30 min and make all measurements on the day of extraction, ideally within 4 h. Position the electrodes in the unstirred supernatant and record the pH value when the meter reading appears steady.
Report pH (1:5 soil/0.01 M CaCl2) on an air-dry basis.