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Electrical conductivity of soil a key to precision farmingElectrical conductivity of soil a key to precision farming

Roy Roberson 2

April 28, 2006

8 Min Read

By knowing the electrical conductivity of their soils, farmers can make more precise management decisions about fertilizer applications, irrigation, use of nematicides, and other pesticide applications, according to Clemson University researcher Ahmed Khalilian.

Though it sounds high tech, and the science behind it has been, the use of electrical conductivity, or EC, is really an extension of the good old common sense farmers have been using for thousands of years.

They’re keenly aware of the strong and weak places in fields and have adjusted inputs accordingly. Precision agriculture techniques, such as using EC to develop zone maps, allows them to be even more selective in using the right input at the right time in the right place.

Khalilian explains that the same hardpan that hampers crop production in Coastal Plain soils from Virginia to Texas is particularly troublesome for farmers in South Carolina. These soils can be dramatically different from the top to the bottom of the root zone, even at the 16-inch depth needed to break up the hard pan.

Clearly, soils with higher sand than clay content will need different nutrients, provide a different environment for pests, and require different amounts of moisture to produce a crop. Knowing these differences is critical to the efficient use of zone sampling, variable rate application, and other time, labor, and money-saving benefits of precision farming.

Will Henderson, an Extension precision agriculture specialist and colleague of Khalilian at Clemson’s Edisto Research and Education Center at Blackville, S.C., has taken the EC findings from test plots to grower fields — with some astonishing results.

In one 50-acre cotton field, Henderson used EC ratings to develop three zones, and composite soil samples were used to create a soil profile for each zone. Fertilizer use was based on the zone soil samples.

Using grid sampling, or more typically, just taking random samples based on observations, most cotton farmers in the area would have applied nitrogen and other nutrients based on an average of the 50-acre field. In that case, the farmer would have wasted a lot of money on both ends of the productivity spectrum.

In the most productive areas, the farmer would tend to use more fertilizer than is needed. In the less productive areas, an average amount of fertilizer wouldn’t be enough for the plant to reach maximum productivity. In today’s economic environment, with spiraling fuel and fertilizer costs, losing at either end of the productivity spectrum can cost big dollars.

Using yield monitors to harvest the 50-acre cotton crop, Henderson found the EC ratings for the field, which were combined with aerial maps to produce zone maps, were right on target in predicting yield in each zone of the field. In one zone with high EC ratings, 1,200 pounds per acre was harvested, in another zone, 600 pounds, and in the third zone, less than 300 pounds.

Khalilian points out that each of these zones will require different amounts of fertilizer to reach optimum growth. In the least productive areas, the grower may even consider other land management options than growing row crops.

“We will go back and further evaluate the data from this 50-acre field, using aerial maps, and we may add a fourth zone for sampling,” Henderson says. By further refining the map, the grower will add another level of precision in making decisions on applying fertilizer.

“Using EC to develop zone sample maps tells me that grid sampling techniques currently being used are a thing of the past in South Carolina,” Henderson says. “Our soil type doesn’t change in a square. In a 2.5-acre grid, in a composite sample, we may have three different soil types that are being combined in one.

“We know, for example, that sandier soils leach out nutrients faster than heavier soils, so for the next crop, you’d have to apply more nutrients to one area. Unless you know where the soil characteristics change, you can’t be precise in applying fertilizers.”

In the 2006 season, Henderson and Khalilian will work with South Carolina cotton growers to zone-map over 400 acres of land using EC correlation as a basis for inputs.

Obtaining EC ratings and using these to develop zones for nutrient sampling is where the high-tech science comes into play.

The Clemson scientists use a Veris 3100 machine that measures electricity movement in the soil. The higher the clay content, the better the soil conducts electricity, and thus the higher the EC number. Sandier soils have less conductivity. “Unless soil is in a noticeably wet area or erosion area, the higher the EC number, the more productive the soil will be,” Khalilian says.

Simple in design, the Veris system uses electrically-charged disks or coulters that send electric impulses to a box mounted on the machine’s frame. One set of sensors is set at a 12-inch depth and a second set at 36 inches. The composite data provides an accurate understanding of soil textures up and down the root zone of a plant.

By using global positioning satellite imagery maps and overlaying the Veris data, the researchers can build a field map that is laid out in zones based on the electrical conductivity recorded in the soil by the Veris.

“Soil texture is important in making all sorts of management decisions, whether for applying irrigation, fertilizer, or pesticides, or determining whether to deep-plow to break up the hardpan,” Khalilian says.

For making tillage decisions alone, the information is valuable. In South Carolina, some growers have to plow twice a year to break up the hardpan, and with the cost of diesel fuel soaring and farm labor expensive and scarce, knowing precisely where to deep plow can save a lot of money in a large acreage operation.

For the past six years, Khalilian has documented the correlations between EC and different crop inputs, documenting these at multiple sites over multiple years. “The correlations are unmistakable,” he says. “Yield data have consistently supported the EC correlations with water, fertilizer, and pesticide use.”

Despite the popularity of glyphosate-tolerant cotton varieties, he says 60 percent of South Carolina’s cotton growers still use soil-applied herbicides to combat weeds, such as Florida pusley, that glyphosate doesn’t control.

To test his EC theories, Khalilian separated a field at the Edisto Research Center into three zones, as determined using the Veris 3100. With a variable rate applicator, he applied different rates of Cotoran.

On sandier soils with low EC ratings, it took only a quarter-pound of active ingredient in the herbicide to get 80 percent control of morningglory. On heavier soils with higher EC ratings, it took up to five times that amount to achieve the same level of control.

He found similar correlations for nematodes. Using the zones developed by EC ratings from the Veris 3100 to manage Columbia lance nematodes, cotton production was increased by 7 percent, while Temik use was reduced 34 percent.

In zones where EC ratings indicated heavier nematode pressure, he used Telone II to manage the microscopic soil-borne pests. Using zone management, he increased cotton yields by 5 percent, while reducing Telone use by 78 percent. Earlier tests indicated similar correlations between irrigation water and EC-driven zone management. Using variable rate irrigation technology to bypass poorly-drained areas of a field, roadways, even farm ponds, can save farmers a lot of money.

At the end of the day, farmers are more interested in bottom line economic results than in small plot findings under controlled conditions. Using data from the Veris 3100 to develop zones, in six on-farm tests, Khalilian says the proof comes from overlaying a yield map, developed long after the crop is harvested, and comparing that to an EC map developed before the crop is planted. The two maps match perfectly.

Currently, systems for measuring electrical conductivity of soil are available commercially. The Veris 3100 system retails for approximately $12,000. Most crop consultants in North and South Carolina can help farmers in mapping their fields using this equipment.

Khalilian points out that the Veris 3100 can be used in the off season and is ideal for growers who want to share its cost with other farmers in the area. He says farmers in the state who want to borrow the Edisto Research Center machine can do so, based on availability, by contacting their county Extension office. Already, more than 20 farmers have used the system to map fields on their farms.

The possibilities for precision agriculture are virtually limitless, says Henderson. Using the EC ratings provides a relatively low cost, research-proven system for determining exactly what is going on in the soil. By using this system and currently available computer-driven variable rate application methods, farmers can be very precise with all their inputs, he says.

The obvious benefits of such precision agriculture techniques are reaped by the farmer, but the environment also benefits from reduced chemical use and less tillage. And in the long run, the profitability, and even survivability, of many farming operations may hinge on how well this type of technology is applied.

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