Better Nitrogen Management
Jun 13, 2019
Nitrogen is absorbed in one of two forms
Part of the reason nitrogen management can be so complex is because it can appear in different forms in the soil. Nitrogen can only be taken up by the plant in either the nitrate or ammonium forms. Most soils in the United States are naturally negatively charged and therefore can only hold positively charged nutrients. Nitrogen in the ammonium form is positively charged, meaning it can be held by the soil.
Over time, through a process called nitrification, ammonium in the soil will be converted to nitrate, which is negatively charged in the soil. As you can see from figure 1, the nitrification process occurs quickly in warm soils, and you can convert almost all of the ammonium nitrogen to nitrate within a matter of three weeks with 60 degree soil temperatures. Nitrate, being negatively charged, can be a problem because it is not held by the soil, and therefore can easily leach with water through the soil profile and out the drainage tile.
Timing is critical for conversion to the nitrate form
Because of this leaching possibility, nitrate is generally talked about in negative terms. However, as figure 2 demonstrates, almost 98% of nitrogen is taken up by mass flow in the nitrate form. The key to nitrogen management is to get it in the right form when the plant actually needs it. Figure 3 shows the overall nitrogen uptake of corn. Nearly 75% of a corn plant’s nitrogen uptake takes place before tassel, and almost 80% of that is taken up during the grand growth stage of corn (V10-VT). This means that if we are truly trying to manage nitrogen, we need to convert as much nitrogen as possible to the nitrate form during the grand growth stage of corn.
Methods for converting nitrogen at the optimal stage
Making enough nitrogen available for the corn plant during the grand growth stage can be a challenge, but there are several different methods that can increase the amount of nitrogen available during this time.
1. Side dress or top dress nitrogen. The goal with this method is to apply nitrogen closer to when it is actually needed by the plant. The key with side dressing or top dressing, is that we need to provide enough time for the nitrogen to convert in order for it to be efficiently taken up by the plant. I recommend that these operations be done between V4 and V6 in corn so that the nitrogen has time to move with rain into the ground, and then convert to nitrate to be taken up.
2. Stabilize the nitrogen. We often talk about stabilizing fall nitrogen to limit the amount that is lost over the winter, but we can lose just as much with spring applications if the weather does not cooperate with us. If you look at figure 3 you will notice that the majority of the nitrogen that is being placed on the field at planting is not going to be taken up until about 60 days later, but in figure 1 we see that the conversion process can take place within a matter of weeks. If the nitrogen converts to the nitrate before the crop is able to use it, it can very easily be lost, and it often becomes a race between the nitrate leaching and the growth rate of the plant’s roots. In a wet year, the nitrate generally wins, and ends up reaching the tile lines before the plant has a chance to take it up. Stabilizing nitrogen can help keep it in the ammonium form longer, which greatly reduces leaching. Instead of the roots chasing the nitrogen down to the tile lines, stabilizers can help keep the nitrogen in the root zone longer and increase the chances of seeing a return on the nitrogen that was placed in the field.