Electrical conductivity measurements might be a shortcut to identifying management zones for fields in precision agriculture programs.

The march of technology won’t stop. A few years ago came yield monitors. They were married to GPS receivers, and precision farming was born in earnest. There’s been tweaking along the way—better signals, better software, more precise data, but now, another technology has emerged to prove its worth in the effort to better match agriculture inputs with the land’s ability to produce. Measuring soil electrical conductivity (EC) is an increasingly popular way to help designate management zones in fields—a way to narrowly define soil productivity and farm it accordingly.

Bruce Wilson and Ralph Windmann, both farmers in Audrain County, Mo., have been working toward building management zones on their farms. They’ve gone the traditional route of getting grid-soil-sample analysis and comparing those results with multiple years of yield-monitor data. Both farmers agree that the goal for the time, expense and work they’ve put into precision farming is to maximize yield, to manage fertilizer inputs specific to location to get the best return on cost. So recently, in order to better identify soil productivity, Wilson and Windmann jointly purchased an EC sensor and cart.

For farmers like Wilson and Windmann, who have existing soil grid maps and yield data, maps generated from the EC sensor serve as a kind of truthing device. On the claypan soils of Audrain County, EC maps are effectively a measure of topsoil, a good correlative to productivity.

Wilson said that with the kind of data they’ve amassed through yield monitoring and soil sampling, they’ve gotten a good picture of soil productivity. But with an EC map they may find better detail.

“I know the soils on my place, but this is confirmation. And it’s a double check. If you get an EC reading that suggests part of a field should be getting good yields and isn’t, you can go back and look at why it isn’t getting the yield the EC map would predict.”

Windmann agreed, adding that the data from yield maps and soil sampling might not detect yield drag related to long-term problems external to those measurements—something like a high soybean cyst nematode problem.

How it works
The Veris 3100 cart (Veris Technologies, Salina Kan.) that Windmann and Wilson use measures the electrical conductivity of soil by sending a charge into the soil and measuring how much it drops. Charge is sent out through one set of coulters and measured by a second set. Coulters are smooth and straight and need only go a few inches in the ground, so the process isn’t invasive. The result is basically a measure of clay content and soil texture. Clays have higher moisture holding capacity and are made up of particles that cling more closely together. Thus, they are highly conductive. Meanwhile, coarser soils such as sand have more space between particles and don’t hold moisture well. They are poor conductors.

Readings from the EC sensor are geo-referenced via GPS receiver. Using a light bar, the EC sensor can be used at various swath widths. Windmann said that they’ve been using a 40-foot width. Any more narrow, he said, and the amount of data pulled in would be overkill, too much to compare with yield and existing soil data. Any wider and the data may be too loose, not reflecting the changes in the soil. MFA precision systems manager David Hughes said that 30- to 40-foot passes are probably ideal. Because the EC map is a measure of soil characteristics, changes should be gradual in most cases.

Since soil texture typically dictates critical productivity measures such as water holding capacity and cation exchange capacity, an EC map is a useful layer along with soil samples and yield data.

MFA’s Hughes said that building management zones is basically trying to discover how productivity potential changes spatially across the field.

“One way to do that is with yield monitoring over time. And soil testing is part of the equation. The EC map allows us to come up with a similar picture. It is a good map of the spatial variability of a soil’s conductivity, which is related to the physical properties of the soil that drive yield—texture, structure, plant-available water capacity.”

Hughes is believer enough in the value of EC data that he has hired Windmann and Wilson (Site Specific Services, LLC) to provide EC mapping for MFA Agricultural Systems Information Lab customers.

“The way we see it as a fit in our program,” said Hughes, “is as another tool to understand soil productivity.”

Hughes said that an initial round of soil testing is critical because it offers insight about the soil’s ability to provide adequate crop nutrition that can be acted upon immediately.

“We have to soil test,” he said. “That gives us a picture of pH and nutrient issues that are affecting yield. We can work on correcting or improving crop nutrition right away. Plus, soil testing gives us the base knowledge that we need to make nutrient recommendations as we collect long-term yield data and other information such as an EC map. Without soil test data, yield and EC measures provided limited basis to act on.”

And understanding soil productivity has been a constant driver toward fine-tuned fertility.

Hughes continued: “Maybe instead of looking at a field and saying we’ve got a 150-bushel expected yield for corn, we can look at the field and say, based on patterns we have from yield history and EC maps, that we know the expected yield will change across the field. That may significantly affect the nitrogen recommendation. Now we can make an improved pre-plant nitrogen recommendation that accounts for the soil’s spatial ability to yield.

Electrical conductivity maps correlate with productivity maps. They can help quickly identify management zones. The top map is the Electical conductivity soil depth, and the bottom is the Normalized Yield.

Good first step
Wilson and Windmann believe that an EC survey is a good first step for a field that is being added to a precision program.

“You can approach soil sampling more knowledgeably,” said Wilson. “If you have a good handle on the types of soil, you may not take as many samples in one area and more in another.”

Hughes said a producer entering MFA’s program might choose to include an EC survey to get a quick picture of soil productivity and more options in choosing how to invest in soil sampling.

“We like to go out and find the nutrient availability and pH picture via soil sampling right away. Those are things we do well and can begin to adjust right away, following up the other aspects in discovering in-field management zones.

“Right now we only go as intensive as a 2.5 acre grid for soil sampling,” said Hughes. “That’s based on getting an accurate look at an affordable price. With an EC survey, we might be able to move from a 2.5-acre grid up to a 6-acre grid on some soil. And we might be able to better position our sample points relative to where we can capitalize on productivity.”

Different by soil type
EC maps will mean different things to different soil profiles. On claypan, where Hughes has worked most with EC (and where the University of Missouri has done extensive research), EC maps are largely a way to look at top soil depth.

In deeper soils, the device will provide measurements that show not so much the depth to clay but the amount of clay in the soil. In alluvial soils the EC map will be driven by soil texture. Maps of riverbottom land will show soils changing from sand to rich silt to tough clay gumbo.

EC surveys are best done following harvest. Soil moisture will affect the readings. Hughes said that the ideal time is after wheat or soybean harvest when soil moisture conditions are likely to be more uniform across the field. Wilson said from a logistic standpoint, post soybean harvest is a good time.

MFA offers EC surveys in combination with soil sampling services or along with multiple-year yield analysis.

For more information, call MFA’s Precision Agronomy Systems at (573) 876-5383.