3A1 Electrical conductivity (EC) of 1:5 soil/water extract

This method, updated from Rayment and Higginson (1992), is based on a 1:5 w/v soil/water extract with air-dry (40°C) soil. This soil/water ratio has been widely used in Australia and considerable data have accumulated. While it is realised sparingly soluble salts will contribute to a greater extent at this ratio than at more concentrated ratios, the values of EC are satisfactory for most purposes. When the same suspension is to be used for the determination of Cl^– and pH, the EC should be determined first so there is no risk of Cl^– contamination from the calomel reference electrode (or similar). Air-dry soil is preferred to oven-dry (105°C) soil, as the latter may convert at least part of any gypsum present to plaster of paris, which has higher water solubility (Rhoades 1982).

If soils contain more than about 1% of gypsum, the soil suspension will approach saturation and have an EC of about 2.2 dS/m. When much gypsum is present it will not be dissolved completely in a 1:5 soil/water suspension. A precise indication of soluble salts, however, loses significance in such soils.

Table 3.2. Summary detaÕs of laboratory and field methods described in this chapter.

Code	Technology	Test method	Notes
3A1	Conductivity cell and conductivity meter in conjunction with a 1:5 soil/water suspension.	Electrical conductivity (EC) of 1:5 soil/water extract.	Common, cheap and valuable soil test for all soil types. Method is based on use of air-dry soils.
3B1	Calculation based on EC_1:5 (dS/m) × 0.336.	Estimated soluble salt concentration.	Best estimates of soluble salts when Cl^– are dominant.
3C1	Calculation based on EC_1:5 and a relationship published by Gillman and Bell (1978).	Estimated soil ionic strength.	Estimates soil ionic strength as mM at 0.1 bar soil moisture. Particularly suited to highly weathered soils.
3D1	In-field measurement of the potential difference between a Pt indicator electrode and a Ag/AgCl reference electrode, both connected to a millivoltmeter.	Redox potential (Eh; field).	Not recommended for use on dry soils. Soil Eh values usually vary with depth and have most relevance in sub-soils.

EC values increase with increasing temperature and must be corrected if not measured at 25°C. An approximate correction can be made by increasing the values by 2% for each degree that the ambient temperature is below 25°C, and decreasing them by a similar percentage when the temperature is above 25°C. The EC_1:5 is reported on an air-dry basis because the conversion to an oven-dry basis cannot be readily calculated. See Method 14B1 for the determination of EC_/SE.

Reagents

Deionised Water

The water is to have an EC of <10^-4 dS/m, and have a CO₂ concentration no more than that in equilibrium with the atmosphere (refer to Note 1).

Acid-Dichromate Cleaning Solution

To 32mL of a saturated water solution of sodium dichromate (Na₂Cr₂O₇) add 1.0 L sulfuric acid (H₂SO₄; 18 M). Handle with caution and care as this solution is both corrosive and a strong oxidant.

0.01 M Potassium Chloride Reference Solution

Dissolve 0.7455 g potassium chloride (KCl; previously dried at 110°C for 2 h) and make volume to 1.0 L with deionised water that is free of CO₂. This solution has an EC of 1.413 dS/m at 25°C.

Procedure

Prepare a 1:5 w/v soil/water suspension. For example, weigh 20.0 g air-dry soil into a suitable bottle or jar and add 100 mL deionised water. Mechanically shake (end-over-end preferred), at 25°C in a closed system for 1 h to dissolve soluble salts. Allow around 20–30 min minimum for the soil to settle.

Calibrate the conductivity cell and meter in accordance with manufacturer’s instructions, using the KCl reference solution at the temperature of the suspensions.

Dip the conductivity cell into the settled supernatant, moving it up and down slightly without disturbing the settled soil. Take the reading with the cell stationary when the system has stabilized (see Notes 2 and 3). Rinse the EC cell with deionised water between samples and remove excess water. Complete EC measurements within 3–4 h of obtaining the aqueous supernatant. Reference soils should be included in each batch of unknown samples.

Report EC (dS/m) at 25°C on an air-dry (40°C) basis.

Notes

1. High quality RODI that contains inconsequential traces of soluble OM is preferred for reagents and standard solutions. This equates to ASTM Type 1 grade of reagent water.

2. If EC readings become erratic, clean the EC electrode by soaking it in Acid-Dichromate Cleaning Solution overnight, followed by thorough rinsing with deionised water. If the platinum black has flaked, recoat according to the procedure outlined in APHA (1998). Rinse electrodes thoroughly and keep immersed in water when not in use.

3. The depth of insertion of the EC electrode should be checked against the 0.01 M KCl Reference Solution to determine locations where no effect on the correct reading occurs. With unshielded electrodes, small containers may be unsuitable for use.