EC measurement and reporting

Initially calibrate the conductivity cell and meter in accordance with manufacturer s instructions. When calibrated, dip the conductivity cell into the supernatant, moving it up and down slightly without disturbing the settled soil. Take the reading with the cell stationary, when the system has stabilised. Rinse the cell with deionised water between samples and remove excess water. Complete EC measurements within three to four hours of obtaining the aqueous supernatant. Report EC (dS/m) at 25°C on an air-dry basis. No correction for moisture present in the sample is necessary. 

The pH of surface soils (0-100 mm) commonly range from 6.0 to 8.0, and it is useful to note that pHw values around 4.0 or less suggest the presence of sulfides, while levels in excess of 8.5 are indicative of the presence of significant quantities of exchangeable Na+. 

Measurement of pH involves detection of the potential of a glass-calomel electrode array (separate electrodes or in combination) using a pH or millivolt meter, standardised against buffer solutions of known pH. Values for these buffers vary with temperature (Bates 1962 Alvarez 1984). 

Procedures based on extraction with deionised water, dilute calcium chloride (0.01 M CaCl2) potassium chloride (1M KC1), saturated sodium fluoride (NaF) and hydrogen peroxide (H202) have been described. All procedures (except for H202 which ideally uses field-moist soil) are based on the use of air-dry soil of 2 mm particle size. No correction for water content is made when reporting pH results. The soil/solution ratio and the temperature of measurement should be noted on the report, as other soil/solution ratios are often used. These include saturation extracts and soil/solution ratios of 1 1 and 1 2.5. 

A 1 5 soil/water extract 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 0.5-0.6 of a pH unit (Baker etal. 1983) than those measured in the field by the mixed indicator/barium sulfate method of Raupach and Tucker (1959). The water should have a pH 6.5 but 7.5. If necessary, boil distilled or deionised water for 15 minutes and cool under C02-free conditions. EC should be 10 3dS/m. Standardisation of the equipment is undertaken using standard pH solutions usually pH 4.00, 7.0 and 9.183. Such solutions are normally purchased ready for use. 

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