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Urea on Top

Tuesday, February 1, 2011
filed under: Fertility

Anyone familiar with Great Plains sunflower trends over the past couple decades understands the dramatic shift toward minimum- and no-till systems. Of all North Dakota fields evaluated during the 2009 National Sunflower Association crop survey, for instance, 33% were considered “minimum-till” and 34% were “no-till.” In South Dakota, a whopping 82% of fields surveyed in 2009 were “no-till”; in Kansas, it was 60%; in Colorado, 50%.

How do no-till producers of sunflower apply their crop’s nitrogen fertilizer needs? Quite a bit, of course, goes on at planting. “But a lot of no-tillers still put urea on the surface — and I think that’s a mistake,” says North Dakota State University soil fertility specialist Dave Franzen. Why? Product loss to volatilization. When urea comes into contact with moisture and urease (a naturally occurring soil enzyme) and crop residue, it is broken down and released into the atmosphere as ammonia.

For growers who see no other way of applying their crop needs, Franzen says at the very least they should use Agrotain®, “the only urease inhibitor that works.” Agrotain is a nitrogen stabilizer suitable for any crop where urea of 28%, 30% or 32% liquid nitrogen fertilizers are used. It will not work with other forms of N.

“There are urease enzymes in all soils,” Franzen explains. “But it’s particularly high in no-till soils because the crop residue itself contains hundreds of times the concentration of urea that bare soil does. And even in bare soil, conventional till, if you put it on top in a high-pH situation, you can get significant volatization. The higher the pH, the more problem you have. And in no-till, it’s bigger yet.”

The North Dakota soil scientist says that in a worst-case scenario, a no-till producer who surface spreads all his fertilizer could lose 30-40% of its efficacy. “Let’s say you put it on and get a little drizzle within a couple days — maybe 0.1” or less. It wets up the residue, wets the very surface of the soil, but doesn’t go any deeper. The pellets have all dissolved, there’s a crust on the surface, temps are above freezing — and the wind picks up.

“You could lose 10-20% in the first week and another 20% the second week if it continues to rain lightly but sporadically. It’s not something to be taken lightly.”

Compared to earlier-planted crops, sunflower doesn’t need the nitrogen as quickly in the spring; but losing some of it to volatilization can still be expensive. “If you figure you may need to sidedress later on because the plants are starting to look yellow when they’re a foot tall or so, that will mean putting another $20-$30 an acre into the crop,” Franzen notes.

While putting most of one’s N down at planting is a more-efficient alternative, the NDSU specialist realizes that time-wise and logistically (i.e., reconfiguring one’s planter), it’s not always realistic. So he reiterates that “if they do decide to put it on top, they should at least use the Agrotain — unless they’re absolutely sure they’ll get a half inch of rain within a couple days. And most people aren’t that sure.”

Franzen knows most no-till farmers have a lot of ground to cover at planting time. “Farmers don’t like to be slowed down in the spring; I understand that. But I also understand the agronomy of it.” - Don Lilleboe
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