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Dig into science behind dry soil sampling

The soils in much of Indiana were very dry this fall. How does that affect soil sampling results?

Dig into it, and you’ll find a variety of opinions. However, certain principles are consistent. “Persistent dry weather resulting in prolonged periods of low soil moisture can affect soil test potassium and pH, resulting in somewhat misleading results,” explains Jim Camberato, Purdue University Extension specialist.

Key Points

• Potassium soil test results may be lower in a dry fall.

• Phosphorus test results won’t be affected.

• Soil pH could be lower, but the question is how much lower.


“Even so, dry weather soil tests can still be useful if one understands the potential impact of low soil moisture on soil test potassium and pH, and uses this knowledge to adjust interpretation of the results.”

Potassium effect

Soil test potassium levels will typically be lower in dry soil, Camberato says. Without rainfall, potassium doesn’t return from the residue to the soil quickly after harvest.

It’s especially an issue after corn because corn stover from a 200-bushel corn crop can contain 220 pounds of potash per acre, Camberato says. Residue behind a 55-bushel-per-acre soybean crop contains only 35 pounds of potash per acre.

For every 100 pounds of potash per acre held in residue, Camberato says a typical 8-inch soil sample would be lowered 8 to 30 parts per million for potassium, depending upon soil factors. You would double the ppm to get pounds per acre. So it’s possible to lower the K soil test 16 to 60 pounds per acre per 100 pounds of potash still in residue.

Another factor

There’s a theory that in very dry soils all potassium may not be measured. “We believe that the potassium can get in between clay sheets as soils dry and then not get measured,” says Betsy Bower, an Indiana Certified Crop Adviser with Ceres Solutions, Terre Haute.

There’s no evidence phosphorus test results are affected, she says. The value for P this fall should be comparable to a normal fall. Some farmers ask why sampling dry soil is a problem when labs dry soil anyway.

“The difference is what happens to the soil when we get very dry conditions during the crop season,” Bower says. “Soils can accumulate soluble salts near the soil surface.”

The pH issue

“In dry years a lower pH soil test reading is often a result of a high level of fertilizer salts remaining in the soil at the end of the growing season,” Camberato says. Salts affect electrodes used to measure pH. Too many salts result in a slightly lower pH measurement.

However, Camberato points to good rainfall early in most of Indiana and notes that normal uptake of fertilizer should have been sufficient to leave little fertilizer salt in the soil at sampling time.

Bower isn’t so sure. She points to a University of Kentucky study where UK agronomists pulled samples from a drought-stricken tobacco field and leached out salts from half the sample. Leaching out the salts caused the pH to increase more than 0.5 pH unit, returning to the reading of the other sample.

However, lime index wasn’t affected in the Kentucky sample. “We’ve been concerned about soil pH readings from dry soil in the past,” Bower says.

This article published in the December, 2010 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.