Why corn needs nitrogen
Nitrogen is in the news. This vital crop nutrient, essential for corn production, is being targeted as a pollutant. The Des Moines Water Works is blaming nitrogen fertilizer for high N levels in the city's drinking water supply. The utility earlier this year filed a lawsuit against drainage districts in three northwest counties, alleging the districts, and farmers indirectly, are responsible for high levels of nitrate in the Raccoon River, a source for the Water Works.
The conversation often centers on rates at which farmers apply commercial nitrogen fertilizer and manure; however, most soils in Iowa are naturally high in “organic” nitrogen, meaning large levels of nitrogen exist in soils whether fertilizer is applied to them or not. When soil conditions are right, microbes break down or mineralize organic N into ammonium and then nitrate.
It is these “inorganic” forms that can be taken up by plants or leach into groundwater. While much of this mineralization happens in summer, when corn and soybeans take up nitrates, it also occurs in spring or fall, before planting and after harvest.
Nitrogen cycling is one topic explained in the new publication “Nitrogen Use in Iowa Corn Production,” from Iowa State University Extension. John Sawyer, Extension soil fertility specialist, gathered information on nitrogen use in Iowa and analyzed results of studies. “So we have information for people when they ask: ‘Why do farmers apply nitrogen?’ ” he says. The publication also looks at economic and environmental aspects of N application.
The yield potential of corn not fertilized with N will be limited as the soil’s plant-available N becomes depleted. Long-term research in Iowa finds yields average only 60 bushels per acre for continuous corn and 115 bushels per acre for corn following soybeans without N applications. “If we do apply N at optimal rates, then we can get yields over 200 bushels per acre in both rotations,” says Sawyer. “There’s a big return from N application in terms of productivity, which of course drives profitability.”
As those optimal N rates are applied, it increases the nitrate loss in water leaving a field through tile drainage. While N application rate is critical both economically and environmentally, only so much can be done to reduce N loss strictly through rate of application. “For fields already at an optimal rate [recommended by the Corn Nitrogen Rate Calculator], those fields are where they need to be,” he says.
Reducing tile-flow nitrate to meet the government’s 10-ppm limit for drinking water by simply reducing the N application rate will result, more often than not, in a loss in net return if application is at the maximum-return-to-N rate. ISU studies show even without N applied, about 7 ppm of nitrate N is still in tile-flow drainage. A lot of organic N cycles through the soil and will produce nitrate that can be lost from the soil system, the same as nitrate from a fertilizer or manure application.
Two uncertain factors affect nitrate loss: soil and weather. “We’re working in a system that is complex regarding N,” says Sawyer. “You can’t control the weather, which can affect nitrogen loss through leaching and denitrification, and nitrogen availability to the corn plant. And you can’t control soil nitrogen cycling, also related to weather.”
Timing of application is another consideration, as more farmers are now applying N in spring instead of the fall. That can help reduce risk of N loss.
But when is the best timing for the spring application? It depends a lot on the weather. Keep in mind that just because you apply N in spring preplant, or during the growing season as a sidedress or a later application, that doesn’t mean all the N gets into the corn plant, says Sawyer. It goes through the soil cycling process. “In many studies where we’ve looked at spring preplant vs. sidedress, we get the same yields, same efficiency, same optimal rate,” he says. “Whether N is applied preplant or at planting or as a sidedress, it still goes through the soil system and has an impact on nitrate loss to water systems.”
This article published in the July, 2015 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2015.
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