Did You Leave Soil N Behind in 2003?
Some agronomists are advising crop producers to consider planting sunflower next year on fields affected by drought, as well as irrigated fields, to mine for soil nutrients not utilized by other crops.
“On fields that have been on irrigated corn over multiple years, it’s not at all uncommon to find 200 to 400 lbs/ac of nitrate below the corn root zone, typically three or more feet deep in the soil. Sunflower will root down and extract that, provided there’s not a compaction zone to inhibit root growth, and if subsoil moisture is adequate to encourage root growth,” says Roger Stockton, Kansas State University extension crops and soils specialist.
On-farm anecdotal evidence supports sunflower’s ability to dig down for soil N. As an example, Stockton points to sunflower producers Mark and Marci Peden, whose sunflower acreage under center-pivot irrigation last year averaged 4,500 lbs/ac in yield near Goodland, Kan.
An article on Peden’s incredible two-and-a-half ton yield was detailed in the January, ’03 issue of The Sunflower. (Article can be found online at www.sunflowernsa.com: click on “Sunflower Magazine” link, then “View Archives,” and finally “Optimizing Plant Development/Yields”)
In the article, Mark Peden said that he takes soil samples to three feet, and accounts for the residual nitrogen. He fertilized with a yield goal of 4,000 lbs, putting down 190 lbs of N and 60 lbs of phosphorus before planting.
He pointed out sunflower’s ability to use residual nutrients as a benefit to a corn-sunflower rotation. “Corn sends roots down about three to four feet, and sunflower seven to eight feet. Whatever N leaches down beyond corn the sunflower will get the following year,” he said.
University research also validates the sunflower plant’s ability to nab soil N. A three-year Kansas State University study conducted near Tribune in the late 1980s demonstrated that sunflower roots extended into the soil about 9.9 ft deep, while grain sorghum rooted to about 8.3 ft deep—nearly a 2 ft difference.
A study at the USDA-ARS Central Great Plains Research Station in Akron, Colo., in 1997 and 1999 analyzed recovery of N fertilizer placed deep in the soil profile with different placement methods.
The Akron researchers found that sunflower recovered half the fertilizer N placed two feet deep. They measured 23% recovery from fertilizer N placed four feet deep, and 12% recovery at five and a half feet deep.
Following a drought, phosphorus and potassium levels are likely to be similar to what they were the previous year, according to David Franzen, North Dakota State University extension soils specialist. However, the amount of residual nitrogen would be expected to be quite high. The only way to really know for sure is soil testing to a two-foot depth in the fall or early spring.
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.
“With sunflower, a farmer can extract soil N he already paid for, and clean up nitrates that can potentially be a problem because of leaching. It’s a win-win situation,” says Stockton. – Tracy Sayler
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