To illustrate how soil electrical conductivity works, Craig Walters uses a science experiment from high school.
"Remember in high school when we wrapped nails with electrical wires, turned on a current from a battery, and watched iron filings line up in orderly rows as they became magnetized?
"The principal is the same," said the soil scientist for Pacer Corporation, a Colton, Washington, agricultural consulting firm.
Soil is an electrical conductor, made of clay particles that carry positive and negative charges, he said. Most of the positive cations are found in the middle of clay particles where micronutrients are held. Conduction of electricity takes place through the moisture-filled pores between individual soil particles.
Two techniques are used to measure soil electrical conductivity in the field. Electromagnetic induction, or EM, uses a sensing unit to measure the charge of the soil’s iron filings. When using electromagnetic induction, a current source passes over the soil surface but does not make physical contact. A sensor in the device measures the electromagnetic field that the current induces.
One tool, known as the EM 38, sends the current or pulse 38 centimeters from the sending unit to the sensing unit. EM units look like a carpenter’s level housed inside a specially designed cart. They are pulled down field rows by a lightweight vehicle, such as a small pickup an all-terrain vehicle.
The second technique uses contact electrode or direct current and measures the resistance of a direct current that is moving through the soil. Coulter-mounted electrodes penetrate the soil surface and measure the change in voltage between a source and sensor electrode. Veris Technologies makes the coulter-type device that reads electrical conductivity in the top three feet of soil.
Walters noted that metal interference in heavily trellised orchards and vineyards can be a drawback to using noncontact devices like the EM 38. The sensor must be kept away from metal objects to avoid interference, typically done by putting the device inside a plastic irrigation pipe and paying attention to the location of metal trellis supports and wires.