AMES, Iowa – Despite last year’s severe drought that parched a huge swath of the Midwest’s cropland, farmers can take steps right now to boost soil fertility this spring planting season.

“They should evaluate the yields produced in 2020 and see if they can adjust P and K applications,” said John Sawyer, Iowa State University (ISU) professor of agronomy, and extension soil fertility specialist. “That is, there might be considerably less P and K removed due to low yields as a result of drought or wind-damaged corn.”

Sawyer said, however, farmers may have already addressed many of these questions last fall. “Similar for tillage, if soils are dry and precipitation limited in the winter/spring, then tillage could be reduced (or no-till) in order to conserve soil moisture for planting and seedling establishment,” he said. “There might be an opportunity to adjust yield expectation in the areas most impacted.”

Antonio Mallarino, ISU professor of soil fertility, said it is important to carefully interpret soil test results of samples taken in the fall with drought conditions.

“The phosphorus soil tests are not much affected by drought, but the potassium and pH soil tests are affected,” he said. “With drought from early fall until the sampling time, potassium test results usually are lower than they should be because the potassium in crop residues has not been leached out to the soil by rainfall.

“It is not possible to predict how low exactly, but I have seen from 10 to 50 ppm lower values than by sampling after rain have wettened the top six inches of soil (the recommended sampling depth),” he added.

He said if farmers were not planning to apply P and K for the 2021 crop (for example, when they applied the two-year rate for the two-year corn-soybean rotation for the 2020 crop), “drought may have reduced yield and both P and K removal, so they should not be nervous, and do not have to apply additional P or K for the 2021 crop.”

With drought, Mallarino said, pH results tend to be higher, from 0.1 to 0.4 pH units, for example, 5.5 instead of 5.6 or 5.9.

“Since pH is used to decide if lime is needed or not, farmers or consultants should consider this to avoid lime when it is not needed,” he said. “The buffer pH measurement – which is used to determine the lime application rate when the pH suggests lime is needed – is not much affected by drought.”

Concerning nitrogen for corn, he said last year’s drought may have reduced nitrate leaching, so by the end of last fall, there may have been higher-than-normal nitrate levels in the soil profile.

However, this year’s spring rainfall is the main factor determining the nitrogen rate needed, he said.

“But soil sampling for nitrate before planting corn is not a good practice, except when drought continues until around April, when expensive soil profile sampling (say, down to three feet) could tell how much carryover nitrate still is in the root zone,” he said.

“With drought in spring until planting time, tillage only aggravates the moisture loss. Farmers should be careful that soil properly close nitrogen injection tracks (especially with anhydrous ammonia) to minimize N volatilization losses,” he added.

He said urease inhibitors can help reduce N volatilization losses when applying urea or UAN – especially with surface applications.

“With drought in the spring before planting, subsurface banding or injection of P or K fertilizers will help,” he said, “but only if there is moisture at the banding or injection depth, which only sometimes is the case.”

In the meantime, spring soil moisture – or lack thereof – is high on the list of concerns among farmers, Dennis Todey, director of the USDA Midwest Climate Hub in Ames, told DTN.

“If you’re in a very dry soil area right now, I would be in some level of increased concern,” he said. “If your soils are in pretty good shape, I wouldn’t sweat things too much at this point.”

Regarding the irrigated and non-irrigated areas in the drier-outlook Midwest states, he added, “One thing is, you have to consider where you are. If you have irrigation, drier conditions in spring are maybe not a big consideration. If you’re dryland (non-irrigated), it’s more of a concern.”