What should you do about N: Apply more or hope you have enough?

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Getting your planting completed has been your top priority this spring, and your number one headache. The weather just has not cooperated. And that has lead to your number two headache this spring, what should you do about nitrogen for corn. Supposedly there was a lot left in the ground that was unused by the 2012 crop. You applied some last fall or this spring. But word is that the heavy rain has washed it out of the field through the tile lines. So what can be done?

Not knowing what your revenue will be this year, and fearing the worst, many farmers do not want to spend unneeded money for nitrogen if it is not needed. But there are no simple soil tests to determine how much nitrogen may be in the ground and available to the corn, once it is planted and growing. For farmers on a tight crop production budget, nitrogen could be a budget killer. Over apply it and you waste money. Under apply it and you lose money.

University of Illinois crop production specialist Emerson Nafziger can commiserate with you about the problem, since you probably had to toss a coin earlier this year on whether to apply nitrogen or plant corn because the calendar was flying by too fast and there was not time to do both, but both had to be done.

When your corn is up and growing, any application of nitrogen will be used because the corn will absorb it before it gets lost. That is the positive thing, says Nafziger, “A plan to split-apply N – right after planting and again at sidedress – might, if the first application couldn’t be made before the crop emerged, now be modified to apply less total N but in a single application.” He says it is important to provide N to the crop before it become deficient, but he also says soils with high organic matter may have enough in a mineralized form to benefit the crop. But he says the mineralization process cannot keep up with the growth and needs of the corn plant. 

What nitrogen is left from last year’s crop?

The answer to that is not friendly. There was quite a bit left from last year due to lower demand by a drought-stressed crop, but that surplus has been diminished by the warm-up of the soil and the excessive rains that have absorbed it and carried it out of fields through drainage tile lines. His research earlier this spring at 60 locations around Illinois found that 23 to 38 percent of the nitrogen content present last fall had disappeared. The range varies with the type of soil and the amount of rainfall.

Is there any nitrogen left from fall or spring applications?

Testing two fields, Nafziger says about 40 percent of what was applied to one field last fall still remains and in a second field with fall and spring applications, about 64 percent remained and 80 percent of that was in a form usable by the corn crop. Nafziger says, “Still, a considerable amount of N is present this spring, and it is not clear that the amounts “lost” are greater than normal. Applications were made in a way that minimized nitrification last fall, and soil temperatures were normal this winter. So while any nitrate-N present last fall would have been able to move with rainfall starting in late winter, it’s not likely that fertilizer N would have been mobilized (in nitrate form) earlier than normal.”

But what about my fields?

Every field is different, not only in soil type and water holding capacity, but also in how much rainfall may have fallen. Subsequently, Nafziger says there are no clear answers to your questions about how much nitrogen you may have available for your new crop. But he says there are some considerations to think about, which will help in your decision about a post-emergent application of nitrogen:

  1. Rising temperatures means biological activity is converting ammonium to nitrates.
  2. In the most saturated field, nitrates can be converting to the gaseous form of N and lost.
  3. Based on soil organic matter, as much as 120 lb/A of mineralized N may be available.
  4. The rate of mineralization will not keep up with the more rapid needs of the corn crop.
  5. Nitrogen moves into the root with water and wet soils mean shallow root systems.
  6. Heavy rains will be detrimental to N supply, but it should be better if you have had only moderate rainfall.
  7. If nitrogen needs to be applied, it should be applied as soon as possible.
  8. Do not apply UAN over emerged corn. Instead, inject it or surface-apply it with a urease inhibitor.

Is there justification for applying more nitrogen?

Nafziger says where heavy rains have obviously washed excessive amounts of nitrogen into tile systems; it may make sense to increase the side-dress rate to replace some of the lost N. He adds, “The replacement rate should be tied to how much of the previously-applied N is likely to be, or to have been, in the nitrate form before rainfall events. The earlier the application and the more N applied as nitrate, the greater the potential for loss up to now. For ammonia applications made in early April, there’s little reason to expect that a lot of N has been lost from the field.”

For areas with only moderate rainfall, Nafziger says wait and see what happens. He says there may be enough nitrogen to carry the corn, but if the weather reverts to more rain, with warm temperatures, then supplemental nitrogen may be needed.

If you have not had the opportunity to apply nitrogen, Nafziger says do that as soon as possible with normal or even lower N rates.

Summary:

While there was an initial chance that 2013 corn could use some of the leftover nitrogen from the 2012 crop, it has likely been lost into the environment, and along with some of the nitrogen you may have applied last fall or this spring. The unfriendly planting weather has also become unfriendly for retaining nitrogen in the soil, and in a form that is available to your corn. The decision of whether to apply more nitrogen depends on many factors, but primarily how much rain has fallen that might wash the nitrogen out of your field.

Source: FarmGate blog


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