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Nimble Nabbers of N

Thursday, February 1, 2001
filed under: Fertility

Nimble Nabbers of N

Colorado research indicates that sunflower is effective at extracting nitrogen deep in the soil profile—noteworthy findings, given high fertilizer costs

Sunflower has the ability to extract nitrogen deep in the soil profile, therefore requiring less N fertilizer than other crops, according to research at the USDA-ARS Central Great Plains Research Station in Akron, CO. The findings are noteworthy, given the jump in fertilizer costs expected to continue into the next planting season.

Research in 1995-97 indicated no yield response in dryland sunflower planted in the same plot three years in a row, with no additional N applied to the soil. The more limiting factor on the dryland sunflower appeared to be available soil moisture at grain fill, not available soil N. The Akron crop scientists suspected sunflower was digging deep for available N in the soil, so they conducted a study in 1997 and 1999 to analyze recovery of N fertilizer placed deep in the profile, with different placement methods.

They applied a urea solution (23% N) on sunflower at the four-leaf stage, using a subsurface band (4”x 4” placement) surface dribble (4”) and surface broadcast with N placed and measured at soil depths of 2, 4, and 5½ feet.

Broadcasting was half as effective as the band-placed N application, and the effectiveness of the dribble-application was about the same as the surface broadcast N.

“The sub-surface band is the most effective method of placing fertilizer N in this study. Concentrated fertilizer placement zones are better than broadcast treatments,” says Merle Vigil, soil fertility specialist at the Akron research station. Although less effective than banding, dribbling liquid N over the rows is an option to consider for no-till producers who don’t want to disturb the soil.

In analyzing soil samples for N recovery, the Akron researchers found that sunflower recovered half the fertilizer N placed 2 feet deep. “That surprised us. You wouldn’t get that kind of recovery with other crops,” says Vigil. He and Akron researchers Joe Benjamin and Jim Schepers measured 23% recovery from fertilizer N placed 4 feet deep, and 12% recovery at 5½ feet. They measured 60% recovery from banded N placed 4” deep.

“It proves that sunflower roots are effective at extracting nitrogen down to two to three feet,” says Vigil. “It pays to fertilize wheat, because typically you’ll get a yield response. But not with sunflower, because they’re so deep-rooted and efficient at getting the N that’s there. That’s why you might not see the same response to N applied to sunflower as you would with other crops.”

Soil scientists generally recommend that you’ll need 50 pounds of soil N plus fertilizer N in the top 2 feet of soil for every 1,000 pounds of expected sunflower yield. – Tracy Sayler

Effect of micronutrients, row spacings

Micronutrients and row spacings were analyzed in related research on dryland sunflower at the Central Great Plains Research Station in Akron. Sunflower yields were significantly greater (200-600 lbs) in 20-row spacings compared to 30-row spacings. Micronutrients (zinc, copper, manganese, and boron) increased seed oil contents, but not seed yield in this study.

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