Editor’s note: The following was written by Fabian Fernandez, University of Minnesota Extension nitrogen management specialist, for the university’s Minnesota Crop News website May 12.
Spring has finally arrived in Minnesota and the field season is in full swing.
The wet and cool start to spring delayed farmers’ ability to get into the field, so deciding which field activities to prioritize now is key. Here are a few things to consider related to corn planting and nitrogen management.
1. Prioritize planting over nitrogen fertilizer application.
The No. 1 priority should be planting crops, even if that means applying nitrogen (N) fertilizer later.
The soil, even sandy soil, has enough N to get the crop started. Corn does not need very much N until later in the growing season. Available soil N and corn’s low early season N requirements usually give us enough time to apply N during the season without compromising crop development or yield.
That said, if your original plan was to fertilize before planting, switching things around requires a little extra caution (see below).
2. Leaving urea on the soil surface is not a good idea.
If your original plan was to broadcast urea and till it in before planting, but now you are planting and applying urea later, it is important to realize that leaving urea on the soil surface is not a good idea.
Soil, especially if it has a lot of crop residue on the surface, has high levels of an enzyme called urease. When urea is left on top of the soil, the urease enzyme increases the rate of urea hydrolysis. This temporarily causes the pH in the soil underneath the urea granule to increase, transforming the nitrogen to ammonia gas. You can end up losing quite a bit of your nitrogen fertilizer investment to the atmosphere.
Soil moisture conditions this spring are ideal for urea hydrolysis. While soil temperatures have been low, they are increasing, and with warmer temperatures the hydrolysis process increases.
It is also important to keep in mind that nitrogen loss from volatilization tends to be higher in soils with low cation exchange capacity and when the nitrogen rate is high. Related to high rates, banding the fertilizer on the soil surface creates a high-rate effect, which can result in more volatilization than broadcasting the same amount of fertilizer.
3. Watch the forecast before you apply urea or use a urease inhibitor.
The best two things to do when you cannot use tillage to incorporate broadcast urea into the soil are: Apply it within 24 hours of rain (when you expect to get at least a quarter to a half inch of rain), or use products that have an adequate amount of urease inhibitor to keep urea from hydrolyzing.
There are a handful of active ingredients that have been shown to reduce ammonia volatilization loss. By far the most common and well-known is NBPT. Research has shown that, to be effective, you should use one pound (16 ounces) of NBPT per ton of urea at the very least.
However, I would strongly encourage you to use products that have around double that amount (2 pounds; 32 ounces) to make sure you get the protection you need from ammonia volatilization loss.
You should be wary of products that claim to have NBPT but do not provide a concentration amount on the label. Chances are, the amount of NBPT is too low to make a difference.
4. Using a urease inhibitor is not an insurance policy against all risks of nitrogen loss.
NBPT degrades over time, so make sure you are using fertilizers that were treated with NBPT for this growing season. Studies have shown that after 20 weeks of storage at room temperature, about half of the active ingredient of pure NBPT was degraded.
In addition, once applied in the soil, the inhibitor undergoes microbial degradation and is effective for about 7 to 14 days depending on soil conditions. Degradation is faster under acidic soil pH conditions and warmer temperatures that increase microbial activity.
So, when using NBPT, what you are doing is increasing the window of opportunity for urea to be incorporated into the soil and converted to ammonium by several days, but the protection does run out.
5. Consider nitrogen sources other than urea.
Finally, if you are applying nitrogen on the soil surface after planting, there are other N sources that are not subject to ammonia volatilization under most Minnesota soil conditions. These include:
- Ammonium sulfate (AMS)
- Ammonium nitrate (though it is very difficult to find a commercial supplier)
- Polymer-coated urea (Ex. ESN)
Urea ammonium nitrate (UAN) solutions are another alternative, but half of the nitrogen in UAN is urea and is subject to ammonia volatilization. So, injection of UAN rather than dribbling on the surface is a good idea.
While it is becoming increasingly more difficult to find, anhydrous ammonia is an excellent nitrogen source. The important thing to keep in mind with anhydrous ammonia is to get a good seal on the soil surface and to apply as far as possible from the crop rows to avoid seedling/root injury.