Electrical Conductivity Spots Salty Soils

Electrical conductivity (EC) measures the amount of salt in a field as well as the field's composition—the amount of sand, clay, and organic matter. Farmers want to know the composition of their soil so that they can apply the correct number of seeds and chemicals to each section of their land. A farm with varying soil composition can be subdivided into sections according to EC data.

ARS agricultural engineer Hamid Farahani and his fellow researchers at the Water Management Laboratory in Fort Collins, Colorado, have shown that EC data can be a practical tool in determining how effective changes in irrigation water management practices have been in minimizing the buildup of salts in the crop root zone. In cases where there is no buildup of salts, any measured variability in EC would reflect changes in soil composition across the field.

Farahani uses a pickup truck to pull a machine that measures electrical conductivity of the soil. As the truck maneuvers over the field, two EC readings are taken every second: one that measures the top foot of the soil and another that measures the top three feet. A 140-acre field can be driven in 6 hours, giving about 14,000 data points.

A Global Positioning System (GPS) mounted on the truck links to satellites and tells a computer exactly where each data point is in the given field. This is similar to the devices found in some cars that can locate them if stolen.

Farahani puts the information collected into a special computer program to get an indication of the changes in salt loads across the fields. Different colors show the amount of EC within the field. With this machine, farmers can quickly get a map of their field's variability. Using the EC map as a guide, farmers only need to collect a few soil samples from each specific EC area to determine soil composition and decide whether or not to modify management.

Without this machine, it would take days to collect enough samples to make a similar map, and it would cost significantly more.

Farahani has gone back to survey farms 2 or 3 years later to find only small changes in EC, which indicates good irrigation practices. Drastic changes would have indicated problems in the overall management of the irrigation water.—By David Elstein, Agricultural Research Service Information Staff.

Hamid J. Farahani is in the USDA-ARS Water Management Research Unit, 3317 W. Vine Drive, Fort Collins, CO 80523-1325; phone (970) 491-8477, fax (970) 491-8247.

"Electrical Conductivity Spots Salty Soils" was published in the December 2002 issue of Agricultural Research magazine.