Growers conducting soil tests this fall may be surprised at unexpectedly low potassium levels. It's likely, however, that fields tested in drought-stricken areas have plenty of nutrients waiting to move back into the soil, according to DuPont Pioneer agronomy experts.
"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. Just be aware that more nutrients will be released into the soil with precipitation," says Andy Heggenstaller, DuPont Pioneer agronomy research manager.
With little rain on most fields in 2012, K, absorbed by corn plants during the 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.
"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," says 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 will likely be lower than actual amounts, farmers can rely on crop removal rates and previous years' 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 soil test levels were in the optimum range.