Take soil tests before applying fall fertilizer
Experts are encouraging farmers in drought-stressed areas to have their soil tested and consider delaying or spreading out fertilizer applications.
Based on research, there are clearly areas of Iowa that have large amounts of fall residual nitrogen in the soil due to this summer’s drought, said Iowa State University (ISU) professor of agronomy Michael Castellano.
“We know that during a drought year, the soil retains more nitrogen for a number of reasons,” he said. “Not only does the crop utilize less nitrogen, but there is also a major reduction in the amount of nitrogen lost to leaching and denitrification.
“As a result, we encourage farmers to have their soil tested before making fall fertilizer 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 nitrogen losses to the atmosphere, to the waterways and in the root zone to the plant and eventually the crop,” Castellano explained.
A little more than one-half pound of nitrogen is removed in each bushel of corn harvested.
“For a farmer that applies 200 pounds of nitrogen an acre and has 200 bushels per acre corn yield, in this example, there could be nearly 100 pounds of nitrogen still residing in the soil,” Castellano said.
Start with a soil test
Shawn Richmond, environmental services director at the Agribusiness Association 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 said. “We encourage farmers to take advantage of residual nitrogen and consider split applications of nitrogen 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 nitrogen needs for our crops based on the weather pattern for any given year,” he added.
Avoiding a repeat of 2013
Agronomists and water quality experts are sending 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,” said Castellano. “During the same period, flow-weighted nitrate concentrations in Iowa watersheds increased by more than 80%.
“There are major consequences of drought on environmental outcomes, and with improved management, we can do our best to avoid the worst,” added Castellano.
Cover crops mitigate risk
Cover crops also provide a means of scavenging and holding excess nitrogen in the soil so it’s available for the following crop instead of moving downstream. Castellano said a well-managed cover crop can easily retain more than 25 pounds of nitrogen an acre and has additional soil health and economic benefits.
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