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Don’t Let Unused N Go to Waste

Thursday, April 15, 2004
filed under: Fertility

You’d agree that stuffing cash under a mattress (or in an old cream can for that matter) isn’t good money management. Then why leave money buried in your fields? In a sense, that’s exactly what you’re doing if you guess at crop fertility, or allow unused nitrogen deeper in the soil to go to waste.

The higher price of fertilizer is weighing heavy on growers’ crop decisions this year. Legumes are lower users of nitrogen, while corn and wheat are heavier users. Sunflower? It’s a scavenger of N, and some agronomists are advising crop producers to consider planting sunflower this spring on fields affected by drought, as well as irrigated fields, to mine for soil nutrients not utilized by other crops.

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.

A probe deeper into the soil, down to four feet, would give a better idea about N in the soil that might be there for sunflower’s taking. “You really don’t know how much residual N is down there without doing a deeper soil probe. Then you’ll know for sure how much N is needed, or if you can get by with less N than you thought,” says Duane Berglund, NDSU extension agronomist.

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 residual N, 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.

University research 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. – Tracy Sayler

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