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Why this soil consultant prefers sampling at one depth

Greg Kneubuhler
SINGLE CORE DEPTH: Greg Kneubuhler believes pulling soil cores at one consistent depth — about 7 inches — gives the most consistent results.
Corn Illustrated: His goal is doing whatever it takes to get consistent soil test results.

The speaker at a field day doesn’t usually open the session with this question. “What do you want to get out of this topic?” Yet that’s how Greg Knuebuhler began his talk to soil conservation personnel from Indiana, Ohio and Michigan.

“I want to know how I should advise people pulling samples at various depths in no-till and reduced tillage,” someone answered.

Kneubuhler, a crop consultant and owner of G&K Concepts, Harlan, Ind., answered, but it likely wasn’t the answer the person expected. He doesn’t routinely pull split-depth samples when sampling for customers.

“Some people talk about taking one core at zero to 3 inches and another at 3 to 6 inches in the same location, or perhaps one at zero to 2 inches and another at 2 to 6 inches,” he began. “What they’re assuming is that fertilizer nutrients are stratified if there is little or no tillage going on. They’re attempting to determine how that stratification stacks up.

“The problem is that the soil testing system is set up on the assumption that the top 6 inches of soil weighs 2 million pounds. Labs calibrate their tests based on that assumption.”

That’s why if the soil testing lab reports findings in parts per million and you want to talk in terms of pounds per acre, you multiply the parts per million number by 2, he explained.

For example, if your lab reports that Sample 17 in Field B contained 20 parts per million of phosphorus, it’s the same as saying the phosphorus level is 40 pounds per acre. That’s typically considered as an adequate phosphorus level for corn.

Skew results
Knuebuhler’s point is that if you pull a sample at zero to 3 or zero to 3 inches and the testing standards are calibrated for 6 inches, the results could be skewed and not as accurate. People pulling soil samples often pull 7-inch cores and mix multiple cores together to get one sample that is sent to the lab.

“The goal is to be consistent and sample at the same depth each time,” Kneubuhler said. “There are different amounts of nutrients at different levels in the soil. If you mix some cores pulled at 5 inches with some pulled at 8 inches or deeper, you’re not mixing apples with apples. If you want repeatable results, you need to be consistent when pulling samples.”

Kneubuhler also doesn’t see the need to pull at two depths, even in no-till. “When you pull at one consistent, single depth, you’re getting a representative sample of what plants will see,” he said. “Fertilizer applied on the surface works down over time. If you’re pulling 7-inch cores, you’re sampling the soil which plants will pull nutrients from no matter the tillage system.”

Automatic probes are available today to speed up sampling. Kneubuhler noted that while he has one, he typically doesn’t use it.

“The automatic sampler can’t tell if it gets a piece of residue or a stone in the sample,” he said. “We strive to get quality samples every time. We’re more confident pulling them by hand.”

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