Method 58a Measurement of pH

Reagents.

• Buffer capsules/tablets - dissolve to make solutions of pH 4.0, 7.0 and 9.0.

Procedure. Calibrate the pH meter according to the manufacturer's instructions using buffers to cover the pH range of the soil samples. Transfer a 10ml scoop of sieved (^2 mm) air-dry soil (struck off level without tapping) into a flat-bottom plastic vending cup, add 25 ml water and a magnetic PTFE-encased stirrer bar. Place on a multi-position electronic stirrer unit (e.g. 15-place Variomag) and stir for 15 min. Tilt the cup, if necessary, to ensure the pH electrode is sufficiently immersed (the soil suspension should reach the porous plug liquid junction on the side of a combination glass electrode). The electrode should not be abraded by the abrasive soil at the bottom of the cup. Swirl a couple of times and allow the drift in pH to stabilize (about 30 s) before taking the reading. Rinsing between samples is not necessary unless soils have widely differing pH values. Recalibrate the meter hourly. If required, retain the suspension for the determination of lime requirement.

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