Conservation Now

March 27, 2009 07:00 PM
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.”

Managing Fertility in No-Till

If you cut back on tillage, you may need to modify your fertility program, say Montana State University soil fertility specialist Clain Jones and cropping system specialist Kent McVay.

 With a colleague, they have authored Nutrient Management in No-Till and Minimum-Till Systems to help northern Great Plains dryland growers make the transition. About half the state's crops are planted with reduced-tillage systems.

"Soil nitrogen and fertilizer nitrogen are consumed when stubble residue decomposes, especially when the fertilizer is broadcast on the stubble,” Jones says. "Nitrogen release from organic matter is generally slower in reduced-till systems due to less oxygen in the soil. Compensate for this by applying more nitrogen fertilizer for the first few years after converting to no-till, especially if you broadcast it on large volumes of stubble.”

No-till and minimum-till systems often result in a greater concentration of nutrients near the soil surface, Jones adds. Although the recommended rate of phosphorus (P) application remains the same among tillage systems, Jones and McVay recommend subsurface banding P with the seed, or up to 2" below the seed if you are using a reduced-tillage system.

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