Electrical conductivity maps aid soil testing Soil electrical conductivity (EC) measurements won't replace traditional soil sampling, but they can make soil sampling much more productive, say agronomists.

In 1997, Veris Technologies, Salina, KS, introduced its first commercial unit, the 3100, for measuring and mapping EC. That unit has now had practical application in the field and users seem enthused with the information the technology is generating.

Soil EC is a measurement that correlates to soil properties affecting crop productivity, explains Tom Doerge, Pioneer's precision farming agronomist. Those properties include soil texture (relative amounts of sand, silt and clay), cation exchange capacity, water-holding capacity, organic matter content and subsoil characteristics.

In the Veris system, an EC cart equipped with coulter-electrodes is pulled through the field. One pair of electrodes injects a current into the soil, while the other measures the voltage drop. Clay, with its smaller grains, conducts more current than does silt or sand.

Injected current passes through the soil to a depth of 1' for one pair of electrodes, and to 3' deep for a second pair. The voltage drops that are measured reflect the depth-averaged characteristics of the two soil-depth increments.

The soil EC mapping system, using GPS, records both field location and conductivity, and produces a geo-referenced map. A 3100 model costs about $12,000; a smaller unit runs slightly less.

Pioneer's Doerge points out that an EC cart - traveling at 10 mph, logging at one-second intervals and making a pass every 60' - generates a sampling density of 50 per acre. That's much denser than typical grid sampling of one per 2.5 acres.

The Andersons' Precision Ag Division, Delta, OH, began EC mapping in November 1998. It did 8,000 acres in the first 12 months.

"We do soil sampling by soil type, and EC mirrors soil type," points out Tim Barney, the company's precision ag specialist. "We aren't as interested in knowing whether it's a Hoytville or a Blount as in determining whether the texture is a loam, sandy loam or clay. The key soil characteristics - organic matter, drainage, exchange capacity - all relate to soil type and soil texture."

"We determine the number of soil sampling zones in a field based on soil type variability," explains Anderson agronomist Bill Meyer. "The range can be from, say, six to 12 across a field. These become management zones for the farmer.

"This system better reflects the soils in a field than does sampling in 2.5-acre grids. There can be more than one soil type in a grid," he adds.

Barney says The Andersons had been using Natural Resources Conservation Service soil survey maps to set up soil sampling zones. But those maps don't have longitude-latitude coordinates, and aren't nearly as precise as the EC maps.

The Andersons charge $4/acre for EC mapping and $4/acre for soil sampling. However, the EC mapping needs to be done only once. Often, 25-35% of a farm is done per year.

At Innis, LA, independent crop consultant Harold Lambert is using EC mapping to help establish management zones for fertilizer and lime. One of his first goals is to better determine nitrogen rates for cotton and, to a lesser extent, for corn.

"Our fields vary in soil texture and we need less N for cotton on sandy soils," Lambert explains. "It can be down to zero on sandy ground but needs to be much higher on heavier soil."

Lambert has long been sampling by soil texture, but it was mostly by visual observation. "EC mapping tells me what is underneath the surface," he notes. "It removes the guesswork."

Independent crop consultant Phil Cochran, Paris, IL, hopes to begin matching client yields to EC.

"The EC maps I have studied, especially at the 1' layer, have an amazing correlation to multiyear yield maps," Cochran reports. "They also show a close correlation to soil type."

Cochran, like The Andersons and Lambert, samples by soil type. He, too, plans to eventually set up sampling zones based on EC maps.

"I have been intrigued by EC technology since it was introduced," Cochran notes. "All the EC data I have studied makes me think it has tremendous possibilities for refining fertility management."

Although soil electrical conductivity (EC) technology may have a place in the future, Purdue University agronomist Bob Nielsen doesn't recommend it for Indiana farmers at this time.

Nielsen explains that the challenge in implementing soil EC mapping is in determining which of the many soil characteristics are best related to EC. Those include: salinity level, water content, cation exchange capacity, soil texture, topography, depth to clay pan, depth to glacial till, soil bulk density, clay content, soil temperature, surface residue cover and organic matter levels.

He points out that soil EC has long been used in the far West for mapping variability in soil salinity levels. And, more recently, Missouri researchers have used it to map variability in depth to clay pan. However, says Nielsen, salinity and depth to clay pan aren't factors that affect crop yields in most of Indiana.

"Our preliminary work with soil EC on 120 acres at our Purdue research farm in east-central Indiana and on a few farmer-cooperator fields suggests apparent, although mediocre, relationships between soil EC variability and soil types and topography," Nielsen reports. "The relationship we have seen between soil EC variability and grain yield also is less than exciting."

It's obvious that soil EC measurements vary across the landscape, says Nielsen. Purdue researchers are trying to determine whether the variability can be associated with soil properties that relate strongly to crop yields.

"At this point, we feel strongly that our farmers should not invest in custom soil EC mapping services and expect to recoup their expenses, let alone make a profit," says Nielsen. "The exception would be if a farmer knows with certainty what soil EC relates to in his specific fields."