Electrical Conductivity – EC
Tarleton State University
Soil Characterization Laboratory
These methods are adapted from the Soil Survey Laboratory Methods Manual (Soil Survey Staff, 2004) and Soil pH and Soil Acidity (Thomas, 1996). Some of the passages are freely copied from the Soil Survey Laboratory Methods Manual. The practitioner is admonished to review these sources for additional background information.
Salt prediction is used not only to predict which soils have measurable amounts of soluble salts but also to predict the quantity and the appropriate dilutions for salt analyses of those soils. If salt prediction or conductivity is less than 1 dS cm-1 soils are considered non-salty, and generally, no other salt analyses are performed.
Summary of Method
A soil sample is mixed with water and allowed to stand overnight. The electrical conductivity (EC) of the mixture is measured using an electronic bridge. The EC by this method is used to indicate the presence of soluble salts (U.S. Salinity Laboratory Staff, 1954).
Reverse osmosis deionized water is used to zero and flush the conductivity cell. The extract temperature is assumed to be 25°C. If the temperature deviates significantly, a correction may be required.
Provide airtight storage of KCl solution and samples to prevent soil release of alkali-earth cations. Exposure to air can cause gains and losses of water and dissolved gases significantly affecting EC readings.
At lest three meters are available in the laboratory:
1) A Handheld DiST®4 EC tester with ATC, a 0 to 19.99 mS/cm range, and a resolution of 0.01mS/cm (Hanna Instruments, Woonsocket, RI).
2) Traceable Expanded-Range Conductivity Meter (Control Company, Friendswood, TX.)
Both meters are to be calibrated to premixed KCl 447 microSiemens solution (Oakton Instruments, Vernon Hills IL.).
3) Multi-Function Conductivity Meter Model 3082 Series (Amber Science Inc., Eugene, OR)
Review the information manual for the meter in use.
Measurements are made in the supernatant of a 1:2 (soil:deionized-H2O) mixture (Soil Survey Staff, 2004a)
- Weigh 10.0 g of soil
- Add 20 mL of RO water to sample.
- Swirl to mix, cover, and allow to stand overnight.
- Standardize the conductivity bridge using RO water (blank) and 0.010 N KCl (1.41 mmhos cm-1).
- Read conductance of supernatant solution directly from the bridge.
- Record conductance to 0.01 mmhos cm-1.
Soil Survey Staff. 2004a. Electrical conductivity. p. 277-278. In Rebecca Burt (ed.) Soil survey laboratory methods manual. Soil Surv. Invest. Rep. 42. Version 4.0. Available at http://soils.usda.gov/technical/lmm/ (accessed 20 May 2007; verified 14 July 2011). Natl. Soil Survey Ctr., Lincolin, NE.
Foth, H.D., and B.G. Ellis. 1988. Soil fertility. John Wiley and Sons. New York, NY.
U.S. Salinity Laboratory Staff. 1954. L.A. Richards (ed.) Diagnosis and improvement of saline and alkali soils. 160 p. USDA Handb. 60. U.S. Govt. Print. Office, Washington, DC.