This method is similar to Method 4A1, except it is based on a 1:2.5 soil/water suspension, with the calomel reference electrode positioned during measurement in the settled, unstirred supernatant, approximately 1 cm higher than the centre point of the sphere of the glass electrode (separately or as a combination electrode). The test derives from New Zealand (Blakemore et al. 1987), where it was used for research studies and for land use surveys. This method is commonly unsuitable for peats and soils high in OM, because of difficulties in obtaining a workable slurry. Raupach (1954) and Blakemore et al. (1987) provide more details on such things as positioning of electrodes, the degree of stirring, and the ratio of soil to suspension medium.
All reagents, including pH buffer solutions, are as described for Method 4A1.
Weigh 10 g air-dry soil (<2 mm) into a 100 mL beaker and add 25 mL deionised water. Stir vigorously with a homogeniser or high speed stirrer and then leave to stand overnight. Calibrate the pH meter and use thoroughly cleaned glass and calomel electrodes for all measurements. Place the electrodes as indicated in the above preamble and without stirring, measure and record pH. Replicate determinations should give results within 0.1 pH unit.
Report pH (1:2.5 soil/water) on an air-dry basis.