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N is for new, nitrogen
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By Vicky Boyd |
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Despite the University of Arkansas’ plant area board being a simple and helpful tool to gauge rice nitrogen requirements, it never caught on with growers. As a result, the university has discontinued promoting it and instead has issued new rice nitrogen guidelines that take into account growers’ cultural practices. “In uniform plots, the plant board works, but fields are not uniform,” says Dr. Charles Wilson, University of Arkansas rice Extension specialist in Stuttgart, Ark. “And growers didn’t have the confidence in it. “We know that with a single pre-flood [application of nitrogen], which is what we were trying to push, the farmers were having problems. They can’t get flooded in time to effectively use urea without losing some of it. So we made the decision that without the plant area board, we can’t effectively recommend a single preflood treatment.” New guidelines are strictly guides “You have to keep in mind we have a limited number of [test] locations, and because we don’t have a soil test, these are guidelines,” Wilson says. “If your experience tells you we don’t recommend enough nitrogen, then you need to add some. “If your experience tells you you’re always laying [the crop] on the ground, then you better back off.” If anything, Wilson says, the guidelines tend to be on the conservative side of fertilization. Too much nitrogen can cause lodging as well as increase the severity of diseases, such as sheath blight, blast and kernel smut. With many crops, you can use a preplant soil test to gauge nitrogen levels and calculate nitrogen fertilizer rates based on the results. But as soon as you flood a rice field where nitrogen has been applied, Wilson says the nitrates in the fertilizer convert to nitrogen gas. And within three to four days, the nitrogen gas begins to volatilize. Currently, two University of Arkansas graduate students are trying to take a University of Illinois nitrogen soil test for corn and refine it for rice. More than 10 years’ of research and verification trials showed that the single or optimum preflood nitrogen method was equal to or better than the split application method and typically required less total nitrogen. The optimum preflood method is based on the theory that the larger preflood application sets yield potential in many of the newer varieties that were bred with the system in mind. To take advantage of the optimum preflood system, growers needed to use the plant area board to check plants at green ring to see if they were running short on N and make minor adjustments, if they were. The plant board, manufactured of corrugated plastic, measured plant height and plant width. The numbers, compiled during surveys at midtillering and the beginning of green ring, were then multiplied to determine plant area. By comparing the plant area to critical levels developed by the University of Arkansas, growers could determine whether they needed to apply midseason fertilizer. But without the plant area board, Wilson says Extension didn’t feel comfortable recommending the optimum preflood method. The second application is made midseason between green ring (beginning of internode elongation) and 1/2-inch internode. The new guidelines still call for applying more nitrogen preflood and less midseason than what has historically been recommended. Altogether, the new guidelines call for the same total amount of nitrogen applied during the season as with the system based on the plant area board or with a three-way nitrogen split application. The two-way split application also saves a plane trip compared to the three-way program, Wilson says. Using the plant area board, growers would have applied 120 units preflood, then returned to check the crop at green ring. Based on their readings, they would have applied up to 30 units additional N. Growers who relied on the three-way split would have applied 90 units N preflood, followed by 30 pounds at 1/2-inch internode and 30 pounds a week later. Some things to consider... If you’re growing rice on clay soils, you may have to increase the early N rate by 30 pounds per acre. If rice follows a previous rice or cotton crop in rotation or if you have a stand density of less than 10 plants per square foot, you may need to increase the early N rate by 20 pounds per acre. If rice follows grain sorghum, wheat or corn in rotation, increase the early N rate by 10 pounds per acre. On the other hand, if rice follows setaside or fallow that is not continually tilled in a rotation, decrease the early N rate by 10 pounds per acre. And if rice follows fish, long-term pasture or the first year after clearing in a rotation, omit the early N rate. Contact Vicky Boyd at (209) 571-0414 or vlboyd@att.net. Internet Hotlink University of Arkansas’ 2004 rice fertilizer recommendations:
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