Protecting Spring-Applied Nitrogen Fertilizer from Ammonia Volatilization
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.
| Table 1. Relative risk factors for ammonia volatilization | ||
| Factor | High risk | Low risk |
| Soil pH | >7 | <6 |
| Soil moisture | Moist | Dry |
| Soil temperature | >70 °F | <50 °F |
| Rainfall, irrigation | Little or none, heavy dew | >0.3 inch after N application |
| CEC (cmol/kg) | <10 | >25 |
| Soil surface residue | >50% residue cover (no-till, pasture, turf) | Bare |
| Application method | Surface broadcast | Incorporated, subsurface band |
| Havlin, J.L., S.L. Tisdale, W.L. Nelson, and J.D. Beaton. 2014. Soil fertility and fertilizers: An introduction to nutrient management. 8th ed. Pearson, Upper Saddle River, NJ. | ||
| Table 2. Estimated ammonia volatilization for different nitrogen sources, application methods, and rainfall scenarios on soil with pH > 7 (high risk). | |||
| Fertilizer source | Application method | Precipitation after fertilizer application | |
| > 0.5 inch rain within 2 days | Little or no rain likely within 7 days | ||
| % fertilizer nitrogen lost | |||
| Urea or UAN | Broadcast | 0-20 | 2-40 |
| Dribble | 0-15 | 2-30 | |
| Incorporated | 0-10 | 0-10 | |
| Ammonium sulfate (AMS) | Broadcast | 0-40 | 5-60 |
| Incorporated | 0-10 | 0-30 | |
| Ammonium nitrate | Broadcast | 0-20 | 5-30 |
| Incorporated | 0-10 | 0-20 | |
| Anhydrous ammonia | Injected | 0-2 | 0-5 |
| Messinger, J.J. and G.W. Randall. 1991. Estimating nitrogen budgets for soil-crop systems. In: Follett, R.F., D.R. Keeney, and R.M. Cruse, editors, Managing nitrogen for groundwater quality and farm profitability. SSSA, Madison, WI. pp. 82-214. | |||
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/