Soil Test Results May Not Reveal Field Nutrient Availability

Drought-stressed corn fields can trap potassium so growers conducting soil tests this fall could find unexpectedly low potassium levels.

Published on: Nov 16, 2012

The drought has been long and tough in states like Texas.

Corn growers conducting soil tests this fall may be surprised at unexpectedly lower potassium (K) levels. It's likely, however, that fields tested in drought-stricken areas actually have plenty of nutrients waiting to move back into the soil.

"The K is actually quite safe in the plant residues, so growers shouldn't be alarmed if soil test levels of this nutrient are lower than expected," says Andy Heggenstaller, DuPont Pioneer agronomy research manager. "Just be aware that more nutrients will be released into the soil with precipitation."

With little rain on most fields this year, the K, absorbed by corn plants during the 2012 growing season, has not yet been released back into the soil from deteriorating corn stalks. In a drought year, K also can become fixed between clay layers until water moves through the soil again. Expect that K test levels will increase to more normal values if you can wait to sample following a significant fall rain event."

NOT SURPRISING. Its not surprising if soil tests show low potassium (K) as drought-stressed corn fields trap potassium. But K can reside in plant residues and be released back to the soil when rain comes.
NOT SURPRISING. It's not surprising if soil tests show low potassium (K) as drought-stressed corn fields trap potassium. But K can reside in plant residues and be released back to the soil when rain comes.

"Some growers may think it's a better idea to wait and sample soil in the spring because it will give them a more reliable nutrient reading," adds Heggenstaller. "But I would caution against spring sampling unless this is your normal practices, because you would end up comparing apples and oranges and couldn't rely on previous soil tests as a basis."

Even though K levels from this year's soil test likely will be lower than actual amounts, farmers can rely on crop removal rates and previous year's soil test results as a guide to estimate next year's K needs. To determine crop removal this year, multiply the field's harvested bushels by an estimated 0.3 pounds of K removed per bushel of corn, or 1.5 pounds of K removed per bushel of soybean. The calculated amount is a good estimate of how much K was consumed by the crop during the growing season, and thus the minimum amount that should be replaced for the next crop if historical levels were in the optimum range.

Other key nutrients, including phosphorus (P), should not show as much variability in soil tests as a result of drought. Phosphorus does not get fixed in clay soils to the extent that K does, and is not nearly as abundant in crop residues as K.

If producers applied nitrogen (N) in amounts aimed at achieving a high-yielding crop in 2012, but received little rainfall and had low yields, chances are that extra N also is still in the soil.

You can get more information on soil testing and other topics online at www.pioneer.com.