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Into thin air
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By Vicky Boyd |
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If you’re not able to flood fields within three days of applying urea, you could see some of your profits essentially vaporize as your fertilizer is converted to ammonia and volatilizes. But university tests have shown that Agrotain, when added to urea, can greatly slow nitrogen loss and do so economically. “If you look at this tracer work, we are losing more nitrogen than what we thought we were losing, even in the five-day flood-up time,” says Rick Norman, a University of Arkansas agronomy professor at Fayetteville. Research conducted by Norman and agronomists Chuck Wilson and Nathan Slaton involved using the stable-isotope tracer N-15 to monitor ammonia volatilization. Ammonia volatilization occurs when the urease enzyme present in soils and organic residue acts on urea and converts it to an unstable form. The unstable form can quickly change to ammonia and carbon dioxide. When incorporated into the soil, the ammonia is converted to ammonium, a salt, for plant uptake. When urea is left on the soil surface—such as delayed flooding or wet soils—ammonia can quickly be lost to the atmosphere, a process known as volatilization. N losses can be as high as 35 percent His research also showed that more nitrogen was lost from fields using fall stale seedbeds than spring-prepared fields. If it takes you seven to 10 days to flood a field, you typically lose nitrogen in the high 20-percent range on conventionally tilled fields and in the low 30-percent range with fall stale seedbeds. Ammonia volatilization occurs more rapidly when urea is applied to silt loam soils compared to clay soils. Regardless of the soil, the longer you wait to flood, the more nitrogen you may lose. Not only does this cost you in the pocketbook, both with inefficient fertilizer use and a reduction in yield potential, but it’s not the most environmentally sound practice, either. The U.S. Enviromental Protection Agency is closely looking at agricultural activities and their contributions to air pollution. If you try to compensate for the losses by applying an additional 30 percent urea, you could be setting yourself up for more problems, Norman says. That could be the year that it rains right after you apply urea, so your nitrogen losses are greatly reduced. Too much nitrogen can promote disease, lodging and other problems. Product slows N loss Agrotain is an urease inhibitor applied to urea at the fertilizer blender. Basically, it blocks the urease enzyme for up to 15 days. The company recommends 1 gallon of product per ton of urea, says Jimmy Johnson, a salesman for Agrotain covering the South and mid-South rice belt. Depending on the dealership from which you buy your fertilizer, adding Agrotain costs about 2 to 2.25 cents per pound of urea. Because Agrotain slows volatilization, it keeps more nitrogen in the soil and available for the rice plants when they need it most—in the early vegetative growth stages. Most of the newer varieties have been developed to have the greatest nitrogen uptake in the early growth stages. That’s why University of Arkansas Extension recommends putting the bulk of your nitrogen—105 to 135 units, depending on variety—preflood and only about 45 pounds midseason. But if you use Agrotain, Norman says you probably shouldn’t adjust your urea rates unless you’ve been applying more than Extension recommends. Real-world situations If they flew on urea without Agrotain onto the wet soils, it would speed volatilization. If they waited for fields to dry, it would promote more weed growth and they’d miss the opportunity to apply urea at the crucial pre-flood period. “It would have been more expensive to do it any other way,” Taylor says, adding he heard no complaints from growers. “We didn’t have any negatives show up, and we put it out under some situations where nobody was really sure—even the researchers—whether it would work. But it didn’t hurt anything.” He also had one large grower tell him he used the product on his entire acreage as a type of inexpensive insurance. If the grower had a well go out or a levee break, he was still assured that most of the nitrogen would still be available to the rice once he was able to flood back up. Based on what he saw last year, Taylor says he’s a proponent of Agrotain because of the importance of preflood nitrogen with the newer, shorter-maturing rice varieties. “Preflood nitrogen sets the yields on these rice varieties, so if you mess it up, you can never recoup your [yield] potential,” Taylor says. “It’s very important not to short your preflood nitrogen.” Put to the test The Agrotain-treated lanes averaged 184.7 bushels per acre dry compared to the untreated lanes, which averaged 176.5 bushels per acre. “It was economically feasible at $4 per acre to put the stuff on,” Gipson says. “Even at $3 rice, you are talking about $24 gross, so the grower had a $20 bottom-line return. In 2003, Gipson conducted a trial on a 70-acre field of Drew on sandy-loam soil. The field was split, with half receiving 250 pounds of urea and half receiving 250 pounds of Agrotain-treated urea. Each half was harvested separately. The Agrotain half yielded 172.2 bushels per acre compared to the untreated half yielding 164 bushels per acre. A trial involving 90 acres of Wells on silt-loam yielded similar results with the Agrotain-treated half yielding 193.2 bushels per acre and the untreated half yielding 183.4 bushels per acre. But another trial showed less than 1 bushel different between Agrotain-treated urea and untreated urea. The disparity in results points out how different soils and cultural practices may influence the outcome. “That is why we do multiple locations,” Gipson says. “I am going to try to do more side-by-side comparisons this season. I’m encouraged by what I’ve seen, and I’m going to try it on corn, too.” Trying it on your field “If a grower said, ‘I’m thinking of treating every acre I’ve got,’ I wouldn’t have a problem with it,” Gipson says. “It’s not like he’s putting out $30 bucks an acre. If he wants to do some side-by-sides, that would be good—I’d suggest that.” Contact Vicky Boyd at (209) 571-0414 or vlboyd@att.net. Internet Hotlinks “Research shows nitrogen makes the difference,” Arkansas
Rice 2004
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