Should you apply supplemental nitrogen fertilizer this year?

By: Brad Carlson, Extension educator

Recent wet weather has led many farmers to question the fate of applied nitrogen and whether they need to apply supplemental N fertilizer. For most farmers, some N has been lost, however, probably not enough to warrant an additional application. Understanding the processes involved is key to understanding whether you need to add supplemental N.

Once applied and incorporated, the loss processes of N are water based. These processes depend on the N being in the nitrate (NO₃) form. Nitrogen in the soil converts to nitrate through a biological process that is dependent on temperature and time. In most cases the majority of applied N is now in the nitrate form (the exception being anhydrous ammonia applied later in the spring). The two loss processes of concern are:

1. Leaching: where nitrate travels with water as it drains to the bottom of the soil profile.
2. Denitrification: another biological process where the N is converted back to N₂ gas and lost to the atmosphere.

Leaching

In general, the soil profile will hold 10-12 inches of water, at which point the soil is saturated. Another term for this is “field capacity.” Despite the severely dry conditions last year, we have reached field capacity in most locations across southern Minnesota this year. A general rule of thumb is that nitrate will move down in the soil profile about six inches for every inch of water drained. This can be estimated by observing outflow from field tile. Most artificial drainage systems in southern Minnesota are designed to have a 0.5-inch drainage coefficient. This means that the system can remove 0.5 inches of water in 24 hours. For reference, this equates to three feet deep tile being spaced at about 65 feet for our typical heavy glacial soils such as Webster, Glencoe, or Le Sueur. When the spacing drops to 50 inches, the drainage coefficient goes up to 0.75 inches. Despite the recent wet weather, we have not drained enough water to significantly move applied fertilizer out of the rooting zone. It is possible that it has moved down enough that corn may appear yellow until the roots grow to reach it.

Screenshot of CoCoRaHS precipitation data from May 20, 2024 to June 3, 2024.

Denitrification

The other significant loss process of denitrification is probably more of a concern. Because it is biological, it is time and temperature dependent. Basic research shows that very little N is lost this way prior to planting due to the soil being cold. This process becomes much more significant as the soil warms and spring turns into summer. Saturated soil at 60° will lose about 6% of the N present when saturated for four days and 12% if saturated for 10 days. Loss due to denitrification increases to 12% when the soil reaches 70° and are saturated for 4 days, and up to 26% if saturated for ten days. At this time, most N applied as anhydrous ammonia is probably at the eight inch depth and soil temperature at that depth is probably at about 65°. Of more concern is N applied as urea that was incorporated to a three inch depth, as the soil temperature there is most likely above 70°. Farmers will want to monitor the length of time the soil stays saturated in order to estimate how much N they have lost. It should be noted that most locations likely to experience severe N loss are also places where the crop was drowned out, making the situation dubious.

Nitrogen carryover

We typically do not carry over N from one year to the next because of the way in which it is lost, however it is possible following a dry year. This past year was a case where there was the potential for carryover N. It is likely that N being currently lost via leaching through drain tile, or to shallow groundwater, is carryover N and not this year’s applied fertilizer.

The University of Minnesota developed a decision aid to help you decide whether you need to apply supplemental N. In general, though, most farmers will not need to apply additional N unless the saturated conditions persist over the next week or so.

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