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March 28, 2009
By: Sara Schafer, Farm Journal Media Business and Crops Editor

Onion and Corn Rotations May Require Less Nitrogen

Many growers in Colorado's Arkansas River Valley who rotate onions with corn could increase profits and also protect groundwater by applying less nitrogen (N) fertilizer on corn. That's the conclusion of a study conducted by Ardell Halvorson, Mike Bartolo and others. Halvorson works for the USDA–Agricultural Research Service's Soil Plant Nutrient Research Unit in Fort Collins, Colo., and Bartolo is with Colorado State University.

The scientists launched their study in 1998 because high nitrate levels were showing up in the region's groundwater. "We found that onions used only 12% to 15% of the 200 lb. of nitrogen applied to the crop,” Halvorson says. "Much of the rest stayed in the soil.”

The following year, the researchers grew 239-bu.-per-acre corn without applying any N fertilizer. The corn crop utilized about 24% of the N applied to the onions the previous year. "It required four consecutive corn crops to get the soil nitrogen level back to what would be considered normal,” Halvorson says.

Because the area receives only 10" or so of annual rainfall, excess N is not leached out of the soil as rapidly as it would be in higher-rainfall areas, Halvorson notes. But eventually irrigation water carries it into groundwater.

Continuing the study through 2008 confirmed that shallow-rooted onion crops consistently leave large amounts of N in the soil, which can be "scavenged” by corn.

"Guidelines from universities suggest you need to apply 100 lb. to 235 lb. of nitrogen per acre to grow 240-bu. corn,” Halvorson says. "Following vegetables, you may not need as much. The secret is to take soil nitrate tests before planting corn and adjust the rate of nitrogen fertilizer based on nitrate in the upper 3' of the soil.”

The study also revealed farmers could apply much less N fertilizer on onions if they switched from furrow irrigation (the most common method) to drip irrigation.

"Using drip irrigation, we maximized onion yield with 80 lb. per acre of applied nitrogen,” Halvorson says.

"Under furrow irrigation, it required 200 lb. per acre.”

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FEATURED IN: Farm Journal - Early Spring 2009

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