By Daniel Davidson
DTN Contributing Agronomist
Time to read the corn leaves. Yellow leaves often mean corn needs a shot of nitrogen (N). The good news is there are ways to determine if you need to apply additional N and how much you might need.
If you see corn yellowing at V1 or V2 stage, it is usually related to cold or wet spring soils, and we often get a color correction as soon as things warm up. Yellow leaves on a corn plant at V3 or bigger usually mean N applied last fall or this spring is gone. However, this year, yellowing at V3 or later probably means corn lost a good portion of N to excessive rainfall or it washed below the reach of plant roots.
Tim Smith, consultant with Crop Smith in Monticello, Ill., specializes in making nitrogen recommendations based on soil and cropping considerations. With the heavy rains this spring, he has been busy pulling soil samples to measure available nitrate levels. He told DTN that he's found measurements as low as 3 to 5 parts per million (ppm), or the equivalent of 14 to 20 pounds of N in the 12 inches of soil. At this time of year, those readings should register 25 to 30 ppm, or least 120 to 140 pounds of N.
I guess this shouldn't surprise us given the fact that some parts of the Corn Belt saw 12 to 18 inches of rainfall in April and May. John Sawyer, soil fertility specialist at Iowa State University, told DTN that last summer's drought and the dry winter allowed the soil profile to absorb much of the moisture, so nitrogen losses are probably less than they would have been. However, he also noted that nitrogen is probably deeper in the soil profile this year -- perhaps as much as the 3- or 4-foot level.
EVALUATE YOUR RISK
Before heading to the field to sidedress, let's assess the risk. Sawyer recommended that growers consider the form of N they applied and when. Fall-applied ammonia and manure and spring-applied UAN are at greater risk to loss than spring-applied ammonia or urea, he said. Nitrate will leach through the soil profile and away from the root zone, or it can denitrify and be lost as nitrogen oxide.
Most of the N applied to corn is applied in the fall or spring in the ammonium form such as anhydrous ammonia, urea or urea ammonium nitrate (UAN), which contains 25% nitrate. Sawyer told DTN that some of that ammonia is at less risk of loss because the soils were cooler for longer this spring and did not convert to nitrate as quickly. Heavy rains in April and May might not have been enough to put you at risk, but if they've continued into June, ammonia levels have likely been compromised.
Peter Scharf, soil fertilizer specialist with the University of Missouri, said, to assess risk, you have to understand nitrogen form, soil temperature and month. In April, most of the nitrogen losses are from residual nitrate in the soil from last year or from UAN applied this spring, not from ammonia applied in the fall or spring. He added that most of the leaching losses in May come as ammonia converts to nitrate when soil temperatures reach 50 degrees Fahrenheit. In June, N losses to denitrification kicks in once soil temperatures reach 60 F.
When assessing your risk of loss and crop's ability to respond, Smith reminds us that corn has yet to take up the majority of its N, which begins at V8 and continues through tassel (VT). "While there have been heavy rains, planting was also delayed and the corn crop is several weeks behind normal, so there is still an opportunity to correct these losses and not impact the crop," Smith said.
SOIL TEST NOW
Growers can test for available nitrogen in the soil by pulling soil samples from zero to 12 inches and 12 to 24 inches. Sawyer points out that you should be measuring both nitrate and ammonia levels. "If you just check for nitrate and don't see much there, how do you know it still isn't in the ammonium form?" he asked.
Scharf said to get a good average of nitrogen availability, you have to pull 15 to 20 cores and sample across different landscapes. That's a time-consuming proposition. Instead, he suggests sampling a couple fields to get an estimate of how much nitrogen to apply and use that to represent all your fields.
While Smith uses the data he gets from spring soil sampling, he says it doesn't tell the whole story. "The soil organic matter will contribute as much as half the N the crop needs with soil (fertilizer) contributing the other half," Smith said. "However, the organic contribution doesn't kick in 'til June, so much of that organic N will still be available to the crop this summer." When considering just the fertilizer N portion, Smith likes to see readings in the top 2 feet at this time of the year at 25 ppm nitrate, which is equivalent to 100 pounds of N. "If you are measuring that much N, you don't have to worry about applying more, as the soil will naturally supply the rest," he said.
Deciding what rate of N to apply is the toughest decision to make because you have already spent your budget on N and now you need to spend more. A soil test (at 2 feet) is a good indicator of what is available, but remember to account for both nitrate and ammonia forms. "Without a nitrate test, you are just guessing how much N to apply. You might put on too much or too little," Smith said.
Smith feels you need to have at least 100 pounds available in the top 2 feet at this time, and a soil test reveals what is available. If you measure 60 pounds and need 100 pounds, just make up the difference and add an additional 40 pounds. "If you are shooting for 220- to 240-bushel yields, you need to have 140 or 150 pounds available," he said.
If you are not going to measure N, Scharf suggested targeting 80 to 100 lbs. on well-drained soils and 40 to 50 pounds on poorly drained soils. In those poorly drained areas where water ponded and denitrification occurred, you may need to kick it up a notch and add 80 to 100 lbs. He recommended going to the field prepared to toggle the rate among zero, low and a high rate and switch based on the crop color and landscape position.
In Iowa, Sawyer said, based on the cool spring, 50 pounds of supplemental N will be adequate on heavier soils while lighter, sandier soils may require as much as 75 or 80 lbs.
AERIAL AND CANOPY SENSING
If you have a high-clearance sprayer and access to imagery or nitrogen sensors, you have an even longer window to watch the crop to see what happens and make a supplemental N application.
"Once corn gets knee to hip high, it can tell you if it needs nitrogen, and you can calculate how much to apply," Scharf said.
Commercial sensors such as the Minolta SPAD meter, TopCon OptRx, Holland Scientific's Crop Circle and Trimble GreenSeeker sense greenness based on chlorophyll content -- a factor that reflects N availability.
"Nitrogen sensors are quick, can calculate a rate to apply, and when connected to an application with a flow meter, you can variable-rate apply it across the field," Scharf added. "It doesn't make sense to put on too much or too little."
The form of N you apply -- ammonia gas, dry urea or liquid UAN -- is not important and depends on your preference, equipment availability or your retailers equipment availability. One thing to remember is you do not need to add a nitrogen stabilizer when sidedressing. The corn will quickly take up N and a stabilizer degrades quickly in warm summer soils.
For more information on N losses:
http://bit.ly/… (University of Missouri)
http://bit.ly/… (University of Missouri)
http://bit.ly/… (University of Illinois)
http://bit.ly/… (Iowa State University)
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