Farmers who are conducting soil tests this fall may be surprised at unexpectedly low potassium (K) levels. It's likely, however, that fields which are 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 crop 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 not the soil with precipitation," says Andy Heggenstaller, DuPont Pioneer agronomy research manager.
With little rain in 2012, the K absorbed by corn plants during the growing season has not yet been released back into the soil by decomposing cornstalks. In a drought year, K also can become fixed between clay layers of the soil until water moves through the soil again. Heggenstaller tells producers to expect that K test levels will increase to more normal values if they can wait to sample after a significant fall rain event.
How to figure out the right amount of potassium fertilizer to apply
"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 of soil, unless this is your normal practice, because you would end up comparing apples and oranges and couldn't rely on previous soil tests as a basis."
Even though K levels show in their year's soil test will likely be lower than actual amounts that are present in the field, farmers can rely on crop removal rates and previous years' soil test results as a guide to estimate next year's K needs.~~~PAGE_BREAK_HERE~~~
To determine crop removal this year, Heggenstaller recommends multiplying the field's harvested bushels by an estimated 0.3 pounds of K removed per bushel of corn and 1.5 pounds of K removed per bushel of soybeans. He says 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.
Soil testing is still best way to get handle on potassium needs; although calculating K needs often works best in drought year
"Depending on management style, this method of calculating K needs might not be the best long-term approach for managing K fertility, but in drought years it is often the best option for growers because of the variability in soil sampling," says Heggenstaller. "I still recommend that producers test soils this year because it will prove useful when determining future fertilizer needs."
Other key nutrients, including phosphorous, or P, should not show as much variability in soil tests as a result of drought. Phosphorous does not get fixed in clay soils to the extent that K does and is not nearly as abundant in crop residues.
For nitrogen, or N, most crop producers applied enough o achieve a high-yielding crop in 2012, but ended up receiving little precipitation and lower than anticipated yields. In these fields, it is very likely that extra N is present in the soil. Depending on rainfall between now and next spring, some of this N may carryover and still be available for next year's crop, he says.
What about testing soil for pH, to get a handle on lime needs in a drought year? Lack of water also may cause a drop in soil pH from previous years, says Heggenstaller. Without precipitation, lime applied to help balance pH in spring 2012 cannot infiltrate the soil and take full effect. Additionally, dry soil conditions are often associated with increased salt concentration at the soil surface, which can also result in lower-than-expected pH test values. Find more information on soil testing and other topics online.