Farm Progress

Subsurface imagery will monitor root activity, soil moisture and other factors related to soil conductivity.

Tyler Harris, Editor

March 7, 2017

6 Min Read
TRACKING PLANT RESPONSE: Jason Gross examines an ERT reading taken in a hay field in Lancaster County in spring 2014, after manure was applied the previous fall. "When we're collecting images, if we put manure or rainwater on, it takes three to four days for the plant to respond. I would have thought when we hayed the grass, we'd see an immediate reaction, but it took three to four days," Gross says.

For 40 years, a 1952 Plymouth Cambridge occupied a 4-by-8-foot space on the property Jason Gross now owns in Sherman County. The loess uplands nearby are dotted with smooth brome, big and little bluestem, and indiangrass. However, beneath the old Plymouth, where sunlight was obstructed for 40 years, only bare earth remains. It was the perfect place for Gross, a University of Nebraska Extension engineering technician, to test out subsurface imagery.

"We had the same soil type, same elevation, received same amount of moisture. I expected to see a homogeneous image where the car was, because there was no rooting underneath," Gross says. "We know grass roots can grow underneath a sidewalk. They can grow horizontally. So how much of those plants tried to grow underneath that car? I know it's growing into it, that's why we saw a bump about a foot and a half horizontally under the car."

Electrical resistive tomography (ERT), a kind of subsurface imagery, has been used to measure seismic activity, map the ocean floor, and locate oil and precious metals beneath the surface of the earth for years. However, in 2011, Gross, along with Bryan Woodbury and Roger Eigenberg, ag engineers with USDA's Agricultural Research Service, began using near-surface ERT imagery as a way to track root activity in pasture, row crop acres and under old cars.

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BENEATH THE SURFACE: A 32-pin array on Gross' property in Sherman County uses a custom-built resistivity meter to transmit electric current into the ground through two electrodes, with two other electrodes spaced between them. Using the Wenner array method, the resulting electric potential field is measured by the two electrodes between, and an ammeter gives a conductivity or resistivity reading.

Unlike the 1,800-foot arrays used for mining and deep-ocean measuring, Gross' 30-foot array is measuring at depths of 6 feet or less. The array, essentially a span of irrigation tubing, has 32 pins, or electrodes that are placed in the ground like a probe — each electrode is a stainless steel bolt. Using a custom-built resistivity meter, electric current is transmitted into the ground through two electrodes, with two other electrodes between them. The resulting electric potential field is measured by the two electrodes between, and an ammeter measures the reading of the conductivity or resistivity within the field – a method referred to as the Wenner array. From these readings, an image is plotted — usually in the shape of the trapezoid. This defines the conductivity and resistivity within the given electric field.

"What makes us different from other geophysicists and conventional resistivity meters is that ours is a low-powered, low-noise meter that can be permanently installed in one position that is powered with solar panels," Gross adds. "This eliminates the variables that can occur in conventional resistivity arrays, which are made to be portable."

Who's driving the bus?
Using this approach, Gross can monitor the different factors that affect conductivity and resistivity — including soil and plant root moisture, physical and chemical soil properties, microbial activity, active root tissue, and dead plant tissue. Because all of these are accounted for in the readings, it's difficult to discern one factor from another.

Woodbury, who is researching subsurface imagery along with Gross, compares these components to a bus. "I've got a whole bunch of people on the bus, but the driver determines where it's going. Unfortunately, in the environment we have, the driver changes frequently," Woodbury explains. "In some cases, we can eliminate a lot of passengers and figure out who the driver is."

However, by monitoring these changes, Gross hopes to track root growth. Recent research suggests plants have an active and inactive root mechanism, and Gross notes certain stresses will trigger active growth, meaning they use more of the entire root system to take up nutrients and water. When a part of the root system is inactive, it's not actively seeking and taking up nutrients and water. "If it's not going to need the bottom half of the root system, it's not going to use the bottom half. If there's something stressing the plant, it flips the switch," he says. "In some situations I ask, 'Why isn't it growing?' Something's telling it not to grow, or at least it’s not leaf or stem growth."

Subsurface conductivity imagery can also be used to measure response to stress. In the fall 2013, Gross and Woodbury applied manure on a cool-season hay field in Lancaster County. The following spring, they used the 30-foot array to measure root activity and response to fertilizer, rain and mowing.

"When we're collecting images, if we put manure or rainwater on, it takes three to four days for the plant to respond. I would have thought when we hayed the grass we'd see an immediate reaction, but it took three to four days," Gross says. "If we come back and reinjure it before that three or four days, how does it affect the root system? We know overtime it will start harvesting its roots to feed itself and with no rest period, it won't come back and build new roots systems. We can image how a plant’s roots react to stress."

Applications in agriculture
When it comes to agricultural applications, measuring root activity is a fairly new concept. But there are a number of potential uses in agriculture. Seven years ago, Woodbury and retired USDA Agricultural Research Service ag engineer Roger Eigenberg began using ERT to monitor leaks in holding ponds at feedlots and dairies, as well as evaluating its application for cooling tower wastewater ponds at power plants using the same method as Gross, but on a deeper level. Instead of using a straight-line array, they bend the array to create a curtain of electrical measurement around the perimeter of the pond. While the entire perimeter is measured, the image is compensated using GPS software and is displayed as a trapezoid, much like Gross' images.

"We can tell you approximately where the leak is occurring and give you a fairly good idea where the depth range is. Once we have that, or suspect there's a leak, we can bring in some other investigative tools to define the problem and try to figure out what action to take to fix it," Woodbury says. "I liken it to when a smoke alarm goes off, and you immediately come into the kitchen and realize you left something on the burner and take action. But you've got to come in and investigate before you take action."

Just like with Gross' images, he notes, the technology is intended to measure changes in conductivity, which would indicate a pond is leaking. In this case, the bus driver is the salts in water in the soil.

"As long as that pond doesn't leak, there are no changes," Eigenberg says. "The only thing that can dynamically change is a leak, and that would be a particular pathway that the liquid in the pond has found to escape through the liner and out through that subsurface soil."

If ERT sounds familiar, that's because it's similar to electromagnetic conduction (EC) mapping in farm fields. The difference is EC mapping is a no-contact, nonpermanent approach that induces a signal into the soil and measures apparent conductivity through correction factors based on the signal that comes back.

As Woodbury notes, "We can use subsurface imagery for multiple different things. I don't think our imagination has been tapped out yet as far as its applications."

About the Author(s)

Tyler Harris

Editor, Wallaces Farmer

Tyler Harris is the editor for Wallaces Farmer. He started at Farm Progress as a field editor, covering Missouri, Kansas and Iowa. Before joining Farm Progress, Tyler got his feet wet covering agriculture and rural issues while attending the University of Iowa, taking any chance he could to get outside the city limits and get on to the farm. This included working for Kalona News, south of Iowa City in the town of Kalona, followed by an internship at Wallaces Farmer in Des Moines after graduation.

Coming from a farm family in southwest Iowa, Tyler is largely interested in how issues impact people at the producer level. True to the reason he started reporting, he loves getting out of town and meeting with producers on the farm, which also gives him a firsthand look at how agriculture and urban interact.

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