Drought conditions change the way farmers should fall-apply nitrogen. There could be an opportunity for farmers to spread out or delay input costs, and they also want to avoid sending excess N downstream that the soil is currently holding.

Experts from Iowa State University and Agribusiness Association share research and recommendations for applying N during a drought year, such as the one Iowa has been experiencing in 2021.

Soil Retains More N During a Drought

Dr. Michael Castellano, professor of agronomy at Iowa State University, and William T. Frankenberger, professor of soil science at Iowa State University, cite research the university’s lab has done over the last 30 years concerning N following a drought. Based on prior years’ soil nitrate levels and precipitation, research shows there are higher-than-normal N levels in Iowa soils in 2021 due to the ongoing drought.

“We know that during a drought year, the soil retains more N for a number of reasons,” Castellano says. “Not only does the crop utilize less N, but there is also a major reduction in the amount of N lost to leaching and denitrification. As a result, we encourage farmers to have their soil tested before making fall application decisions that not only impact the environment, but also farmers’ profitability.”

During an average year, denitrification occurs at a rate of about 30 pounds per acre.

“But during a drought, we have less N losses to the atmosphere, to the waterways and in the root zone to the plant and eventually the crop,” he explains.

A little more than half a pound of N is removed in each bushel of corn harvested.

“For a farmer that applies 200 pounds of N an acre and has 200 bushels per acre corn yield, in this example, there could be nearly 100 pounds of N still residing in the soil,” Castellano says.

Management Begins with a Soil Test

Shawn Richmond, environmental services director at the Agribusiness Assn. of Iowa, encourages farmers to work with their ag retailer or certified crop adviser to test soil this fall to verify N levels, which is the first step in putting together a plan.

“Fertility decisions are always important, but especially following drought conditions,” Richmond says. “We encourage farmers to take advantage of residual N and consider split applications of N in the fall, spring or even during the growing season to provide the crop what it needs when it needs it. Split application is becoming more common because it gives farmers a better opportunity to fine-tune the N needs for our crops based on the weather pattern for any given year.”

Avoiding a Repeat of 2013

Agronomists and water quality experts are sounding the alarm and sending urgent messages to the agriculture community in an effort to avoid a repeat of 2013, which followed a drought year.

“2013 was a relatively wet year, and the nitrate load to the Gulf of Mexico increased by nearly 300% from 2012,” Castellano says. “During the same period, flow-weighted nitrate concentrations in Iowa watersheds increased by more than 80%.”

The U.S. EPA nitrate standard for primary drinking water from public water supplies is 10 parts per million (ppm). In 2013, the nitrate levels in the Raccoon River and Des Moines River were over 10 ppm for roughly 3 months.

“There are major consequences of drought on environmental outcomes and with improved management we can do our best to avoid the worst,” adds Castellano.

Cover Crops Mitigate Nitrate Risk

Cover crops also provide a means of scavenging and holding excess N in the soil so it’s available for the following crop instead of moving downstream. Castellano says a well-managed cover crop can easily retain more than 25 pounds of N an acre and has additional soil health and economic benefits.

CCAs can learn more about the importance of the 4Rs of nutrient management and the “Plus Side” of conservation practices and earn a total of 2.5 credits in the Soil and Water Management or Nutrient Management categories by successfully completing the 4R Plus CCA course.

Learn more about the predictive tools ISU has created to determine and evaluate N conditions on a regional basis.