Mapping electrical conductivity in soil shows soil-to-yield relationship.

If you're optimistic about the theory of variable rate inputs based on grids/soil types, you may want to check out the latest technology for finite soil measuring, which identifies soil electrical conductivity (EC). Precision farming practitioners and their consultants are looking to soil EC as a way to more precisely define soil type and patterns across fields.

"Soil EC readings are light years ahead of the soil survey manuals," says Dan Kaminsky, sales agronomist with Helena Chemical. "The data give us a better way to draw in soil boundaries and create management zones by soil type."

One of his customers, John Nidlinger, Decatur, IN, has completed 1-acre grid sampling on the majority of his 2,200 tillable acres but sees value in soil EC data. "Measuring soil EC...has given us a much more accurate measurement of changes in our soil types," he says. "When we compared our soil EC maps to four years of yield maps, we found a relationship between high EC readings and lower yields. This is another major link in understanding our soils. It's an important part of the precision ag information package."

What is soil EC? Electrical conductivity is the ability of a material to conduct an electrical current. Soil EC has no direct effect on crop growth or yield. "The utility of EC mapping comes from the relationships that frequently exist between EC and a variety of other soil properties that arehighly related to crop productivity," says Tom Doerge, precision farming agronomist for Pioneer Hi-Bred International. These properties include water-holding capacity, topsoil depth, cation exchange capacity (CEC), soil drainage, organic matter level, soil nutrient levels and subsoil characteristics.

EC readings correlate strongly to soil grain size or texture. Sand has low conductivity, silts have a medium conductivity and clays have a high conductivity.

Induction or direct contact methods The two primary methods of measuring soil conductivity are by electromagnetic induction (EMI) or by direct contact. A contact method from Veris Technologies uses at least four electrodes that are in physical contact with the soil to inject a current and measure the voltage that results. An EMI sensor does not make contact but instead uses a transmitter coil to induce a magnetic field into the soil and a receiver coil to measure the response. The two methods give similar results. Soil EC measurements typically need only be taken once in a field.

The Veris 3100 Soil EC Mapping System uses six heavy-duty spring-loaded coulter electrodes mounted on a steel frame sensor cart that can be pulled across the field for on-the-go readings of soil EC at two depths simultaneously: 1 ft. and 3 ft. The cost of the basic Veris 3100 is $11,500. Some co-ops and ag retailers have purchased the units and offer soil EC mapping for fees of $4 to $8/acre. For more information, contact Veris Technologies, a division of Gioprobe Systems, Dept. FIN, 601 N. Broadway, Salina, KS 67401, 785/825-1978.

The EM38 by Geonics Limited is a lightweight, handheld EMI device powered by a 9v battery. It also can be mounted on a trailer. Geotechnical and environmental engineers, archaeologists, crop consultants and agricultural researchers are primary users of EM38. It takes soil EC readings at 2 ft. or 5 ft. These readings can be taken simultaneously with the new EM38-DD. For more information, contact Geonics Ltd., Dept. FIN, 1745 Meyerside Dr., Unit 8, Mississauga, Ontario, Canada L5T 1C6, 905/670-9580.

Field experience "Soil EC defines soil differences much better than a soil survey map, which has inclusions [other soil types found within the boundaries of the main soil type] of up to several acres within a general soil type," Doerge explains. "Soil EC readings can be taken every second resulting in 50 to 100 measuring points per acre. These readings identify soil inclusions that are one-fourth of an acre in size. This provides a good basis for a directed soil sampling program." The information when combined with other precision farming data can help growers with management decisions including variable rate prescriptions.

Dan Kaminsky's firm has mapped 600 acres since it purchased a Veris machine in December. Helena charges $4.50 to $5/acre for the soil EC mapping. Kaminsky has compared the Veris maps to customers' precision yield maps. "The Veris readings correlated better to yield than did soil survey maps or nutrient maps," he says.

Tim Godfrey, Jonesville, MI, purchased a Veris machine two years ago and has mapped soil EC on his 2,800 acres. "I wasn't finding a high correlation between my 2_1/2-acre grid soil samples and the yields I was getting," Godfrey says. "The conductivity maps help me break my large soil types into smaller management zones." The cost of his soil sampling has been a third of the cost of his grid sampling scheme, and his lime and potash bill has been reduced as well.

Interpreting EC readings Like John Nidlinger, Godfrey found a correlation between his EC maps and his yield maps. However, the correlation was the opposite of the one Nidlinger found: Godfrey discovered that higher EC readings correlated to higher yields.

The relationship between conductivity and yield is not constant or necessarily linear. For example, in some areas, higher conductivity indicates higher clay and CEC, resulting in higher yields on those sites. That was true for Godfrey whose southern Michigan land is generally quite sandy. There, higher EC indicated more productive silt and clay soils. In other regions, the higher conductivity could indicate excessive clay, which may limit production by impeding root growth and water and nutrient uptake. In Nidlinger's case, the high EC readings indicated heavy clay soils that were poorly drained. In other situations, the relationship between soil EC and yields produces a bell-shaped curve, with the highest yields from the medium conductivity soils.

"The correlation between yield and conductivity depends on whether the parent material of the soil is glacial, alluvial [river-deposited] or loess [wind-deposited]," says Bob Gunzenhauser, Premier Crops Systems, Urbandale, IA. "It really shows us the texture of the soil and how internal drainage affects yield."

"The readings are telling you different things in different locations," Doerge says. "To understand the readings you need to understand your soil and agronomic conditions." An inexpensive aerial bare-soil photograph and a GPS topography map can help distinguish soil properties and drainage characteristics. Doerge recommends that growers consider hiring a crop consultant or mapping center to help with analysis of the EC data and other precision farming layers.

Better management zones Although EC readings don't have universal meaning from one geographic area to the next, the relative differences of conductivity within a field can be used to create management zones when combined with other precision farming information.

"Soil EC shows the soil's water-holding capacity and nutrient-supplying capacity," says Martin Rosek, CCA, precision services manager for Stratfield in St. Charles, MI. Rosek correlates the EC readings with CEC to create a formula that converts the shallow Veris readings into a detailed CEC map. "I now have about 5,000 points instead of 15 points of CEC information in a field. I use this very accurate CEC layer for variable rate prescriptions of lime and potash and to determine nutrient removal and replacement levels," Rosek says. "This takes precision ag a lot farther than 2_1/2-acre grids. We're able to look at the mean productivity on each soil type and identify areas that aren't reaching their potential."

Rob Stouffer, Precision Insights, Lee's Summit, MO, works with farmers in west-central Missouri as a precision ag consultant. He has looked at soil EC data from more than 3,000 acres. "I have seen some really strong correlation between yield and soil EC. In our part of the country we tend to find an inverse relationship. As EC goes up, our yields go down," Stouffer says. "I haven't seen any other precision farming data layer as strongly correlated to yield as this."

He uses the soil EC data in conjunction with multiple years of yield data to define productivity or management zones in the field and to create variable rate seed prescriptions for corn and soybeans. He also uses it with other information to implement smart soil sampling programs. Some clients use the information to determine tiling projects, and one client is using it to direct a no-till ripper in the tight, clay soils.

Finding the meaning "Like a lot of precision farming information, the problem with soil conductivity is knowing how to use the information," says Gary Malzer, soil scientist at the University of Newell Kitchen, soil scientist with USDA-ARS, and his colleague Ken Sudduth, an ag engineer, have conducted soil EC surveys throughout much of the country. "There is a strong relationship between yield and soil EC data, particularly on Missouri claypan soils. We hope in a year or two that we will be able to provide a tool to manage nitrogen based on a soil EC yield potential map," says Kitchen. "The art of using electrical conductivity is knowing what physical or chemical soil property causes changes in soil EC. Our current research program is working to find the best procedures to put meaning to that variation."