As the spring planting season gets underway, agronomists and farmers are asking about the best ways to protect spring-applied nitrogen. How much nitrogen might I lose if I cannot incorporate it? Does vertical tillage incorporate fertilizer enough? We have compiled some resources to help answer those questions.

There are three ways to lose nitrogen fertilizer: ammonia volatilization, denitrification, and nitrate leaching. In excessively wet soils, nitrogen can be lost through nitrate leaching and denitrification. However, for spring-applied nitrogen, ammonia volatilization is the main concern with dry soil conditions and unpredictable precipitation forecasts.

When you apply ammoniacal fertilizers (e.g. anhydrous ammonia, urea, UAN, ammonium sulfate) to the soil surface without sufficient incorporation, some amount of free ammonia (NH3) can escape to the atmosphere. Sufficient incorporation with tillage or precipitation is needed to safely protect that nitrogen investment below the soil surface.

Ammonia volatilization risk depends on soil and environmental factors (Table 1) and the nitrogen fertilizer source (Table 2). Typically, we are most concerned about ammonia volatilization for surface-applied urea or UAN. It is not easy to estimate how much nitrogen might be lost, and sometimes the losses can be substantial. Although you cannot change the soil type or weather forecast, you do have control over the nitrogen source and application method (Table 2) to protect your nitrogen investment.

Practices to reduce ammonia volatilization for urea or UAN, in order of most effective practice

• Apply urea in subsurface bands at least 3 inches below the soil surface. A shallow urea band (1 or 2 inches deep) acts like a slow-release anhydrous ammonia band, and nobody should ever apply anhydrous ammonia that shallow.
• If nitrogen will be broadcast with incorporation, make sure the fertilizer is sufficiently incorporated at least 2 inches below the soil surface to ensure good soil coverage. A chisel plow or field cultivator are good incorporation tools. The popularity of high-speed disks (vertical tillage) has led some people to think that it counts as a meaningful incorporation event. In reality, it just moves soil and crop residue around on the soil surface without really incorporating the fertilizer. Take a look after you run across the field and you will see white urea granules remaining on the soil surface. Do you remember the old soil-applied herbicide incorporation videos from the 1970s? Those classic videos provide great examples of what a thorough incorporation job really requires. NDSU Extension has posted them online: https://vimeo.com/216680310/e843149fdd
• If nitrogen will be broadcast without incorporation, try to time the fertilizer application right before rain (at least 0.5 inches of precipitation). Soils with good crop residue cover (no-till) may require more rain to sufficiently move urea or UAN into the soil.
• If no rain is forecasted in the near future, consider applying a urease inhibitor on urea or UAN to provide temporary protection until rain arrives. The active ingredient NBPT has been widely researched and shown to reduce nitrogen losses; make sure the active ingredient rate is 1.3 to 1.8 lb NBPT per ton of urea to ensure effective NBPT activity and protection. NBPT begins to break down after 7 to 14 days. In addition, it is important to remember that nitrification inhibitors like nitrapyrin and DCD do not protect against ammonia volatilization.

These practices should also be considered if you will be applying in-season nitrogen fertilizer later in the summer. It is always best to apply nitrogen below the soil surface, such as injected anhydrous ammonia or coulter-injected UAN, to protect nitrogen fertilizer. For surface-applied urea or UAN, you should time the fertilizer application just before a rainfall or consider NBPT to extend the rainfall window.

The higher ammonia loss potential for ammonium sulfate (Table 2) often surprises people (and we get questions about it). On calcareous soils with high pH, the initial reaction products of ammonium sulfate [(NH₄)₂SO₄] and calcium carbonate (CaCO₃) can produce free ammonia, which may be lost if ammonium sulfate is lying on the soil surface. This is a similar reaction process to free ammonia formation with diammonium phosphate (DAP, 18-46-0) applied to calcareous soils. This is why AMS and DAP are not suggested as seed-placed fertilizers on calcareous soils because of the ammonia toxicity risk to seedlings. Please note that urease inhibitors like NBPT will not protect ammonium sulfate from ammonia volatilization.

Helpful resources

Nitrogen extenders and additives for field crops (NDSU) https://www.ag.ndsu.edu/publications/crops/nitrogen-extenders-and-additives-for-field-crops

Should you add inhibitors to your sidedress nitrogen application? (Univ. Minnesota) https://blog-crop-news.extension.umn.edu/2020/06/should-you-add-inhibitors-to-your.html

Split the risk with in-season nitrogen (AGVISE Laboratories) https://www.agvise.com/split-the-risk-with-in-season-nitrogen/

This article is shared annually to help answer frequently asked questions about nitrogen fertilizer applications and nitrogen losses in the fall.

October is here, and many people are preparing for fall nitrogen fertilizer applications. Before you hit the field, we want to share these important reminders about fall nitrogen application timing and placement to help you reduce potential soil nitrogen losses through fall and winter.

It is important to wait until soil temperatures reach 50 °F (10 °C) before applying fall nitrogen to reduce the risk of soil nitrogen loss. Once nitrogen fertilizer is applied, soil microbes begin converting ammonium-nitrogen (NH4+) to nitrate-nitrogen (NO3-), a process called nitrification. In the nitrate form, nitrogen is vulnerable to loss through nitrate leaching or denitrification. Soil temperatures cooler than 50 °F help slow microbial activity and keep nitrogen in the safer ammonium-nitrogen form longer. This applies to any ammoniacal nitrogen fertilizer source, which includes anhydrous ammonia, urea, UAN, and ammonium sulfate.

Quick rules for fall-applied nitrogen timing

1. Wait until after October 1 because cool soil temperatures are more consistent and reliable after this date. After October 1, measure soil temperature in the early morning (6 a.m. to 8 a.m.) at the 4-inch soil depth.
2. When the 4-inch soil temperature has reached 50 °F (10 °C), it is relatively safe to start applying anhydrous ammonia.
3. Wait one week after the anhydrous ammonia-safe date to apply banded urea.
4. Wait two weeks after the anhydrous ammonia-safe date to apply broadcast urea.

Soil temperature map from the North Dakota Agricultural Weather Network (NDAWN) from 21 September 2025. You can find an updated daily soil temperature map at the NDAWN website.

It is a good idea to keep a soil thermometer with you to measure the current soil temperature in the field. In addition to NDAWN, a number of regional climate mesonets have online tools to search for local and regional soil temperatures.

• Alberta Climate Information Service
• Manitoba Agriculture – Soil Temperature Trends
• Minnesota Department of Agriculture – Soil Temperature Map
• Montana Climate Office Mesonet
• NWS North Central Region – Soil Temperature Map
• North Dakota Agricultural Weather Network (NDAWN) – Soil Temperature Map
• South Dakota State University Mesonet

The 50 °F soil temperature rule of thumb is particularly important for soils prone to nitrogen loss: well-drained, coarse-textured soils are prone to nitrate leaching and poorly-drained, fine-textured soils are prone to denitrification. If such soils receive excess precipitation or become saturated (waterlogged) through fall or spring, soil nitrate can be lost through leaching or denitrification. In general, it might be better to apply nitrogen fertilizer on such soils in spring. But, if you must apply nitrogen fertilizer in the fall, make sure you wait until soil temperatures are cold enough to keep it in the ammonium-nitrogen form for a longer period of time to reduce potential soil nitrogen losses.

For fall-applied nitrogen, subsurface banding or incorporation is also important to reduce ammonia volatilization, another potential nitrogen loss mechanism. Fall precipitation (rain or snow) is often too sporadic and unreliable to be considered an effective incorporation “strategy” for broadcast applications. Fall-applied urea should be banded below the soil surface (3 inches or deeper) or incorporated with tillage (at least 3-4 inches) to ensure complete coverage.

Shallow fertilizer bands or shallow incorporation with vertical tillage does not provide adequate soil coverage to prevent ammonia volatilization. If soils are very dry, successful incorporation may not be possible because tillage can produce large, uneven clods that leave nitrogen fertilizer exposed to the atmosphere and vulnerable to ammonia volatilization. Although dry soil poses a lower risk of ammonia volatilization than moist soil, soil moisture is not the only factor that contributes to ammonia volatilization risk (Table 1).

Fall-applied anhydrous ammonia should be banded 5 to 6 inches deep. Ensure that anhydrous ammonia trenches are sealing properly to prevent gaseous ammonia losses from the trench. In addition, the nitrification inhibitor nitrapyrin (brand name N-Serve) can be added to anhydrous ammonia to slow nitrification, offering additional insurance to keep nitrogen in the safer ammonium-nitrogen form for longer. Nitrapyrin is also available in formulations for dry and liquid nitrogen products. Please note that nitrapyrin degrades faster and loses its effectiveness at warmer soil temperatures, so it is no substitute for cooler soil temperatures (<50 °F).

Fall-applied nitrogen is a convenient way to allocate time and labor resources, leaving one less thing to do in the spring. But, you must be smart and consider fertilizer source, timing, and placement options to make sure that the nitrogen applied in the fall will still be there next spring. With fertilizer prices still remaining high, now is not the time to risk soil and fertilizer nitrogen loss.