Where’s My Nitrogen? How To Make A “Nitrogen Map”

Many growers apply nitrogen in the fall to take advantage of the wide application window. But there are risks with fall application. Applying seven months prior to planting leaves nitrogen vulnerable to loss from rain events and warm winters.

Because nitrates easily move in the soil profile, it is important to map that movement to get a picture of your nitrate availability.

Pulling soil samples before planting, after emergence and prior to the rapid uptake period — v10 — gives you a timeline of N availability. In season sampling is different than the full-spectrum sampling done semi-annually. This is a specific sampling technique designed to find the ammonium and nitrates in the soil profile. Use row profile sampling, taking 10 cores every 3 inches across a 30 inch row to give an average of what that profile contains. This process allows you to find the bands of N from the injector knife or coulter and to see how this band is dispersing through the soil profile.

When you apply NH3, it’s important that your analysis look at nitrate — N03 — and ammonium — NH4 — levels. Ammonium is relatively stable, but converts to nitrate. Nitrate is mobile. Knowing how much of these two forms of nitrogen are in your field will tell you the amount of total N that could be available to the plant and how much of the total N is at risk of movement and loss. As the crop gets closer to silking, N03 levels are all that are necessary since most of your ammonium will be converted to nitrate.

Manage the root zone. Pull one-foot and two-foot cores in each of your management zones. The one-foot sample shows what’s available in the root zone. The two-foot samples show if the nitrogen has moved deep into the profile.

Here is an example. This field had 200 pounds of anhydrous ammonia applied in the fall. Samples were pulled prior to planting in two management zones.


  12” 24”
  Lbs % Lbs %
Nitrate 139.6 71 20.1 70
Ammonium 56.6 29 8.5 30
Total 196.2   28.6  

This zone is doing fairly well. This area is well drained, not holding water. All of the nitrogen is accounted for, and we are getting some mineralization from the soil microbials, as you can see from the higher levels of nitrogen in the soil than was initially applied. Keep in mind though, a good amount of conversion has taken place and a majority of N is in the N03 form, which is more susceptible to loss than NH4.

Now, let’s compare zone 1 to zone 2:

Zone 2

  12” 24”
  Lbs % Lbs %
Nitrate 108.3 80 36.3 85
Ammonium 26.6 20 6.2 15
Total 134.9   42.5  

This zone appears to be a more low lying, poorly drained portion of the field, because it is saturated compared to the surrounding area. You can see there is more N03 in the second foot than in zone one, indicating that N03 is moving down through the profile. Nitrate moving down in the soil profile before planting is not good – it means that it will likely be gone by the time the plant needs access to it around V8-V10. The young corn plant can’t reach nitrogen at two feet deep. In this zone, we’re also about 23 pounds shy of our 200 pound application.

If your sampling and nitrogen map show you are at risk of running low of nitrate in the root zone by V10, you have a couple of options. Supplement nitrogen with a weed and feed or planter application. But, again, you run the risk of this early N moving out of the root zone after heavy spring rains. To decrease these risks and match application rates to the plant’s need, delay the supplemental application until the plant is ready to use it — V6 or later — using 360 Y-DROP Sidedress  on an applicator bar or 360 Y-DROP on a self propelled sprayer.

Nitrogen can be a mysterious ingredient in a successful cropping plan. Frequent and focused soil tests can eliminate the mystery and ensure you have the right amount of N in the right place at the right time.