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Soil testing still important in dry conditions

Farmers shouldn’t stop analyzing soil samples to determine lime and fertilizer needs despite the effects drought and low soil moisture might have on the results, a Purdue University agronomist says.

“Accurate analysis of representative soil samples to determine lime and fertilizer needs is fundamental to crop production,” said Jim Camberato. “Unfortunately, persistent, dry weather resulting in prolonged periods of low soil moisture can affect potassium and pH, resulting in somewhat misleading results.”

Soil tests can be useful in dry weather if farmers understand the way low moisture can affect potassium and pH test results.

In a dry fall, soil test potassium levels often are lower than expected because most of the potassium the crop had taken up during the growing season remained in the crop residue. There wasn’t enough rainfall to return it to the soil.

“This is a larger issue with corn than soybeans because corn stover contains much more potassium than soybean straw,” Camberato said.

Stover from a corn crop of 200 bushels per acre contains 220 pounds of potassium oxide (K2O) per acre, compared with 35 pounds an acre in a field of 55 bushels of soybeans per acre.

For every 100 pounds of potassium oxide per acre retained in the stover, potassium in a typical sample of soil 8 inches deep would range 100-150 parts per million (ppm) depending on soil type and texture. In an abnormally dry year potassium sample levels could be lowered about 8-30 parts per million (ppm), again depending on soil type, Camberato said.

Low rainfall totals during the growing season also can result in lower soil pH measurements because a high level of fertilizer salts remain in the soil at the end of the growing season. Those salts affect the electrode used to measure pH and can result in an inaccurate, lower pH measurement.

While the end of the season was dry in Indiana, early-season rainfall was enough to ensure normal crop uptake of fertilizer. That means, Camberato said, in most cases there was little fertilizer salt left in the soil to affect pH measurements.

“Bottom line, farmers should take advantage of the early harvest and dry soil conditions and continue soil sampling and fertilizing and liming where needed,” he said. “Most of the potassium taken up by the crop remains in the residues, so farmers need not be alarmed if soil test potassium levels are lower than expected.”

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