Sample With Precision
Mon Mar 23, 2015 02:07 PM CDT
(Page 1 of 3)
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."


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."

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."


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."

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