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How low can you go?

Growing concerns of supply shortages and rising fertilizer prices have many growers rethinking their corn fertility strategies for 2001. Nitrogen fertilizer costs have increased by more than 100 percent since last spring, but availability may be even more of an issue than price, says Larry Oldham, fertility specialist at Mississippi State University in Starkville, Miss. Farmers should find and lock in a reliable supply as soon as possible.

According to Oldham, Delta farmers could purchase urea-ammonium-nitrate fertilizer solutions at the beginning of 2000 for somewhere between $170 and $185 per ton. This year, however, bulk urea ammonium nitrate fertilizer prices in the Delta are running between $250 and $260 per ton.

The question for corn growers, then, is how much can you cut back on fertilizer rates without sacrificing yields?

Not much, according to Extension corn specialists, who say that although your corn yield level won't exactly equal the amount of nitrogen you put out, at maximum efficiency, nitrogen fertilizer basically equals yield. Generally, if you cut nitrogen rates in corn, you cut yield, says Jerry Singleton, area Extension agent in Greenwood, Miss.

How much nitrogen fertilizer is absolutely required? asks Oldham. I would be hesitant to cut nitrogen fertilizer rates back very significantly on corn because it has such a dramatic effect on yield in Mississippi. We need to maintain the regular rate on heavy soils, particularly under irrigation, due to the interaction of fertilizer with the soil.

State guidelines in Mississippi call for an application of 1.3 pounds of nitrogen fertilizer per bushel of yield goal up to 100 bushels per acre. Then, for yields in excess of 100 bushels per acre, you add 1.7 pounds of nitrogen for each additional bushel of expected yield.

For example, if a farmer is expecting to produce an average yield of 200 bushels per acre, he should apply 300 pounds of nitrogen fertilizer. That's 130 pounds of fertilizer for the first 100 bushels of yield and another 170 pounds of nitrogen fertilizer for the second 100 bushels of yield.

State recommendations

All our nitrogen recommendations are based on yield expectations for corn so there's not a whole lot of flexibility when it comes to lowering your nitrogen rate, Oldham says.

In Arkansas, corn specialist William Johnson recommends farmers planting corn on sandy loam and light silt loam soils apply a minimum of 1 pound of nitrogen fertilizer per bushel of yield expectation. On silt loam soils the recommended rate increases to 1.2 pounds of nitrogen per bushel of expected yield. On cotton-type soils and silt loams we're usually running 150 to 180 pounds of nitrogen per acre, and on a little bit heavier silt loams we're running 180 to 220 pounds of nitrogen per acre, he says.

We're normally putting out 160 pounds of nitrogen preplant incorporated and then at the six-leaf stage we're putting out between 90 and 120 pounds of nitrogen. Then, if everything looks good and we can make a yield close to 200 bushels per acre, we'll apply another 30 to 40 pounds of nitrogen about a week before tassel, he says. But, if we don't have a good stand or are unable to irrigate properly, we won't make that last fertilizer application. What we're doing is hedging ourselves with these split applications.

This would probably not be the year to plant corn on clay soils because it usually takes 1.8 pounds of nitrogen to produce each bushel of yield on clay soils. And, if you're going to try to make 150-bushel corn, you'd have to put out between 250 and 350 pounds of nitrogen per acre, Johnson says.

If you are going to cut the nitrogen rates, it's probably best to go to another crop, he says. You don't cut corners on corn, you've got to do everything right to produce high yields.

There are, however, some ways to increase your nitrogen efficiency, according to corn specialist Erick Larson at Mississippi State University in Starkville, Miss.

One way, he says, is to apply nitrogen fertilizer in a split-application, which can decrease the amount of nitrogen that potentially could be lost to environmental conditions. We generally recommend applying a third to a fourth of the total nitrogen recommendation at planting time and then applying the remainder of nitrogen at the sixth- to eighth-leaf growth stage, which normally occurs about 30 days after plant emergence, Larson says.

Corn doesn't need much nitrogen until it's after knee-high, he says. And, heavy or frequent, early-season rains can cause you to lose a lot of nitrogen to environmental processes such as leaching and denitrification, he says.

Leaching is the movement of nitrogen through the soil to a level deep enough that it is no longer available for the plants to use. Denitrification is the chemical process nitrogen goes through when warm, wet soil conditions cause some of the nitrogen to convert to a gas form, which is then lost into the air. Both environmental processes result in a net loss of nitrogen available to the corn crop.

Other best management practices for corn, Oldham says, include the following:

  • Calculate rates closely, depending on state recommendations.
  • Tune-up and calibrate all fertilizer application equipment.
  • Use a balanced fertility program including testing for potassium (K), phosphorus (P) and lime. Follow state recommendations for any necessary application treatments.
  • Apply nitrogen fertilizers close to the time of actual crop need.
  • When possible, do not leave nitrogen fertilizers on the soil surface.

For more information, contact your local Extension office.

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