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.
"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, 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. In the case of nitrogen, most producers applied enough to 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 be available for next year's crop.
Lack of water also may cause a drop in soil pH from previous years. 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.