Sample With Precision

Farming Foot by Foot Requires Spot-On Soils Information

Soil-mapping units in soil surveys aren't smaller than 2.5 acres and could have up to 40% other soils within a unit. Electrical-conductivity mapping (above) helps resolve these issues for precision farming by giving higher resolution of soil differences based on organic matter, clay content and water-holding capacity. (Progressive Farmer photo courtesy of Shannon Gomes)

By Lynn Betts

Progressive Farmer Contributor

Are your soil tests precise enough for precision farming? Unless you've taken extra steps to specifically identify and locate your soils for the qualities most important to precision farming, the answer is no.

"If you believe you can use your soil survey to make decisions for precision farming, you should have a heart-to-heart talk with the soil scientist who mapped your soils," says Shannon Gomes, a former soil scientist with the Natural Resources Conservation Service (NRCS) who mapped soils for six years. Gomes has a master's degree in soil fertility from Iowa State University and has been a private crop consultant in northeast Iowa for 28 years.

"Some of those soil surveys are 40 and 50 years old. Even if they were done more recently, the soils weren't mapped with precision farming in mind, nor were they georeferenced," he notes. Gomes says other shortcomings may include soil delineation lines that are off, inclusions within the soil type that have different properties and the possibility of large variations of properties in the soil.

"There was a lot of lumping together and averaging in the soil survey as it was conducted," Gomes says. "It was never intended to be used for precision farming. A soil line that was off 50 or 100 feet didn't mean a lot in the soil survey, but it does in variable-rate decision-making."

PINPOINT ACCURACY

You defeat the whole purpose of precision farming if you make decisions based on averages or ranges," adds Frank Moore, a farmer and certified crop adviser in Iowa and Minnesota. "If you look closely at soil descriptions from the soil survey, you see that organic matter of a particular soil can vary from 2 to 7%. You have to ask yourself where the 2% is and where the 7% is, and everything in between, if you're going to farm foot by foot."

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He says the Adapt-N program, a fertilizer program he uses on his farm and recommends to clients, is highly sensitive to soil organic matter levels.

"The recommendations for additional nitrogen at sidedressing can jump with just half a percent increase in organic matter," Moore explains. He would like to refine the Adapt-N program to use a variable-rate program for sidedressing nitrogen. "But we need accurate prescription zones to do that," he says.

Moore, who uses GIS (geographic information systems) and GPS systems in his own farming operation, contends advances in technology will make variable-rate planting more attractive to farmers. "It hasn't been a useful tool for them in the past. But now that planters can vary both populations and seed varieties on the go, row by row, the concern is that the soils information isn't detailed enough to make the best choices."

MISSING THE MARK

Even though using the soil survey for zone soil testing has its pitfalls, Moore says grid-sampling can lead to incorrect recommendations, too. "Grid-sampling is based on sampling a single point in every 2.5 acres for most people," he says. "Even if the soil line is in the right place, your grid-point sample could be on a corner of that 2.5 acres or miss it entirely. In precision farming, it's not a matter of getting a hit on the next sample and averaging things out; it's having the right information for every spot in the field."

Gomes and Moore say the key to soils information to guide precision farming starts with the best base map you can make. First, you need many more cores for each soil sample. "When you have a planter that can change population and variety by the foot, and you have a soil line that's off by 100 to 150 feet, that's huge," Moore says.

Typically, four to six cores, or at most, 10 to 12 cores, are taken from the center area of a 2.5-acre grid for a soil sample, Gomes says. But for precision farming, you need three to seven times as many cores for a sample—like 30 to 40 cores spaced evenly at less than 20 feet per core across the length of the grid. For 1-acre grids, take 27 cores, one every 8½ feet, across the grid for a sample.

MORE IS BETTER

Moore and Gomes also believe they have the solution to sampling too few points for soil tests. "Precision farming is a numbers and data issue, where more is better, so you need many referenced points of data for soils, too," Moore says. "It's critical to take enough cores to reflect what's in the field and where."

Moore and Gomes recently formed a partnership called Soil Investigative Services (SIS). The business puts their combined knowledge to work to benefit farmers who want to use more refined soils information to grow corn and soybeans. They use an AutoProbe soil-sample collector that's hooked to a tractor by a three-point hitch. It automatically takes a core sample every 8½ feet. Dialed into GPS, the AutoProbe can collect samples from 100 to 150 acres in an hour and automatically georeference each sample.

MEASUREMENT ACCURACY

You can take steps to improve the accuracy and detail from the soil survey. You can map relative clay content, organic matter levels, water-holding capacity and the salt levels in your soils much more precisely with electrical conductivity (EC) measurements, Gomes says. "Those functions of the soil, like organic matter content and water-holding capacity, are key to fertilizer recommendations, plant populations, seed varieties and herbicide recommendations," he says. "And the flow of electricity through the soil picked up by electromagnetic induction offers a much more detailed map than a soil survey."

Gomes adds the electrical conductivity testing can pinpoint soil characteristics such as water-holding capacity to a more precise area of the field. It also gives you information you don't get from a soil test, with more precise field locations than you get from a soil survey. "But it doesn't replace your soil map or soil-sampling," Gomes says. "You should use it as a base map to guide your management decisions you make from the soil samples you take more often."

Electrical conductivity isn't a test you need to repeat often. The levels will vary according to soil conditions at the time, but their relative readings will remain fairly constant. Gomes says since soil properties are relatively constant, a single map of a field should work for many years. "The only time you might want to do a new electrical conductivity test is after you've made major changes in management that would cause soil properties to change -- like using no-till and cover crops that build soils."

Gomes has been doing EC mapping for more than 10 years. He inserts the EC unit into a cart that he pulls with an ATV and can EC-map about 100 acres per hour. The EC unit doesn't look like much -- it reminds you of a big carpenter's level -- but it costs $18,000.

EC levels are interpreted differently in the Midwest compared to the West. "You get high EC levels on your most productive soils in the Midwest, but the highest readings in the West are often poor soils," Gomes says. "And the reading will vary depending on moisture levels in the soil. The important thing to remember is that electric conductivity maps precisely separate out your soils by organic matter and water-holding capacity, showing you where they are in the field."

SIS charges $8 an acre for EC testing. "You amortize that to one dollar an acre over eight years, and that's inexpensive for a base map that gives you much more detailed information," Moore says.

Gomes points out only a few AutoProbes are operating in Iowa. "We realize not every farm has the equipment to make use of more accurate, detailed soils information," Moore says. "I know my clients using manure aren't looking for that much soils precision, for instance. But corn and soybean farmers applying commercial fertilizer and using variable-rate technology might want to consider this."

(BAS)

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