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The Downside of Too Much N: Two Stories

Tuesday, February 1, 2011
filed under: Fertility

— Colorado —

Joel Schneekloth, Akron-based regional extension specialist with Colorado State University working primarily with irrigated cropping systems, has recorded the effects of “too much” N as having dramatic impact on yield and oil.

“The maximum N on irrigated sunflower should be in the 150- to 200-lb range between residual and applied as the upper limit,” Schneekloth says. He’s encountered growers who planted sunflower after a failed corn crop, and they can’t understand why they didn’t get the results they expected. The answer can be found in the residual N from the failed corn combined with the applied N that created the incorrect growing conditions for the sunflower crop.

Schneekloth has found that excessive water coupled with large amounts of N can create an unfavorable situation. With good moisture and maximum application of N, you have a sunflower plant that is building a large factory – tall, with big leaves. You get a large plant population doing that, and you end up with stand that’s dense, thick, lush and prone to disease. So when soil fertility tests suggest there’s enough residual soil N to grow corn without applying much additional N, that’s too much N for sunflower.

North Dakota has also been in a wet cycle for the past few years. Excessive rainfall has allowed for good soil moisture reserves. The same theory would hold true for dryland sunflower when it comes to too much nitrogen coupled with large amounts of moisture. — Sonia Mullally

— North Dakota —

Adequate supplies of soil + applied nitrogen are essential to producing top yields of any crop, including sunflower. But adding more than what the crop can use is an unnecessary — and expensive — proposition.

That long-understood reality was reaffirmed in a 2008 study at North Dakota State University’s Carrington Research Extension Center. The study was established under conventional tillage practices, with wheat being the previous crop.

A prior fall 0-2’ soil test had indicated 53 lbs of nitrogen per acre at the central North Dakota dryland site. Urea nitrogen was preplant incorporated on April 10. Spring soil samples taken May 9 from the untreated check plots indicated 71 lbs N in the 0-2’ zone and 168 lbs of N in the 2-4’ sample. An oil sunflower variety was seeded in 30” rows on May 14. Trial N rates were 0, 30, 60, 90 and 120 lbs/ac.

The trial was harvested on October 27. Seed yields ranged from 1,458 lbs/ac for the “0” N rate plots on up to 1,687 lbs for the “90 + NutriSphere -N” plots. While that difference was more than 200 lbs, the top-end yield was not considered significantly different, statistically, from the low-end one.

“With 239 lbs of total N available in the top four feet, it is not surprising that additional N did not increase seed yield when compared to the untreated check,” noted Paul Hendrickson, an NDSU-Carrington research specialist who conducted the 2008 study. — Don Lilleboe

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