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:
- Rising temperatures means biological activity is converting ammonium to nitrates.
- In the most saturated field, nitrates can be converting to the gaseous form of N and lost.
- Based on soil organic matter, as much as 120 lb/A of mineralized N may be available.
- The rate of mineralization will not keep up with the more rapid needs of the corn crop.
- Nitrogen moves into the root with water and wet soils mean shallow root systems.
- Heavy rains will be detrimental to N supply, but it should be better if you have had only moderate rainfall.
- If nitrogen needs to be applied, it should be applied as soon as possible.
- 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.”