Nitrogen Needs: Assess Your Fields

June 29, 2008 07:00 PM
The heavy rainfall that has swamped much of the Midwest has potentially washed away much of the nitrogen fertilizer farmers applied earlier in the spring. If your corn is looking yellow, you may need to apply more N. Read the answers to some common FAQs about N loss, courtesy of three soil science experts.
FAQ: How do I know if my N is gone?
Jim Camberato, soil fertility professor at Purdue University, encourages farmers to consider the overall affect weather has had on corn fields this season. "Just because corn has turned yellow for a few days, doesn't mean more N is needed,” he says.
Camberato says the saturated soils and ponding that has occurred in many fields causes low oxygen content in the soil, resulting in yellow corn. He suggests waiting a few days to let the soil dry out and see if the corn is still yellow before adding more N.
Carrie Laboski, extension soil scientist at the University of Wisconsin, has been answering many farmers' questions about N loss. She tells farmers to ask themselves three questions:
  • How much N was applied?
  • When was the N applied?
  • What form of N did you apply?
She says the answers to these questions will determine if they are in a situation of major loss. For instance, if a more-water-soluble version of N was applied early in the season and a field has remained saturated for several days, major N loss has probably occurred.
Camberato says soil sampling is also a good way to analyze the amount of N left in a field, by assessing the nitrate and ammonium levels.
Farmers can also use the Nitrogen Loss Scoresheet developed by Peter Scharf at the University of Missouri.
FAQ: What type of N source to apply?
The availability and price of N sources will most likely determine which type farmers apply. Camberato says for replanted or late-planted corn, anhydrous is a good option. He says it can damage roots and retain growth on larger corn plants.
For larger corn, applying liquid N might be a better option, Camberato says. But, it is more subject to runoff if rainfall persists.
FAQ: What type of equipment should I use to apply N now?
Camberato says most farmers only have the option to use high-clearance equipment to apply N at this point due to corn height. But, he says, farmers who planted late or replanted may still have the option for side-dressing.
Richard Ferguson, professor of soil science for University of Nebraska, says many fields in Nebraska have the ability to apply fertilizer through their irrigation system. He says that is a good option for adding more N at this stage in the season.
FAQ: Is adding more N worth the price tag?
Ferguson says two or three years ago farmers would have gone ahead and added more N, without question. But, he says, with the increases in fertilizer costs, reapplying has a much higher price tag.
"Farmers should consider fertilizer costs if they haven't already purchased it,” Laboski adds. "The first amount of N applied to corn gives you your biggest yield increase.”
FAQ: When is it too late to apply N?
Ferguson says he encourages farmers to have their N applied before the silking stage. "Most corn hybrids take up half or two thirds of the N by silking,” he says. "Anything applied later is less likely to affect yield.”
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