Fall Anhydrous Ammonia Applications: What to know about soil moisture

October 30, 2024 12:29 AM
Blog Post

The crops are coming out and it’s ALMOST time for farmers to start making fall nitrogen applications. Remember that soil temperatures should be below 50°F at a depth of 4 inches and continuing to drop before making fall nitrogen applications —ideally, the cooler, the better. It's important for farmers to check the soil temperatures in their specific fields, as many factors can influence them. At the very least, farmers should consult the Iowa Environmental Mesonet Soil Temperature Maps before moving forward with applications. Current soil temperatures across the state can be seen in the map below (Figure 1).

Iowa soil temperature map by county
Figure 1. Iowa 4-inch soil temperature by county as of October 28, 2024. Source: Iowa Environmental Mesonet

At this time of year, 4” soil temperatures for most of the state are expected to be around 45-50 degrees. Since soils this fall have been so dry, they are more easily cooled or warmed by the air temperature. Daytime high temperatures have been mostly above average for October, but nighttime temperatures have been near or below average. This causes the dry soils to warm during the day and cool at night. Looking ahead, the Weather Prediction Center predicts measurable rain in the next 7 days (Figure 2). The Climate Prediction Center outlooks for both 6-10 day and 8-14 day show temperatures leaning above average and increased chances for precipitation (Figure 3).

Quantitative Precipitation Forecast for October 20, 2024 to November 4, 2024.
Figure 2. 7-day Quantitative Precipitation Forecast for October 30, 2024 to November 4, 2024. Source: Weather Prediction Center

 

6-10 day and 8-14 day temperature and precipitation outlooks
Figure 3. Temperature and precipitation outlooks for November 4-8, 2024 and November 6-12, 2024. Source: Climate Prediction Center.

 

Soil moisture is equally as important as soil temperature when it comes to effective fall anhydrous ammonia applications. As of October 28th, nearly 83% of the state is facing D1 (moderate drought) or drier conditions (Figure 4), and many questions have been coming in regarding fall anhydrous ammonia applications into dry soils. 

Iowa drought monitor for October 22, 2024
Figure 4. Iowa drought monitor as of October 22, 2024. Source: U.S. Drought Monitor

What happens when anhydrous ammonia is injected into soil

When anhydrous ammonia is injected into soils it undergoes a violent reaction with water resulting in the formation of Ammonium (NH4+- plant available form of nitrogen) and a sudden increase in pH within the injection zone which can kill microbes in the immediate vicinity leading to a bit of a “protective zone” for the ammonium. After conversion to ammonium, the positively charged ion binds with organic matter and clay. As long as the nitrogen remains in the ammonium form, it can remain bound to the soil exchange sites with reduced potential for loss. However, over time a microbially driven process known as nitrification will convert the ammonium to nitrate (NO3—also plant available). Nitrate is highly susceptible to loss via leaching and denitrification. This nitrification process slows drastically under dry and cold conditions.  

So, can anhydrous ammonia be applied to dry soils? 

Well, the simple answer is yes, anhydrous ammonia can be applied to dry soils as even the “driest” of soils contains some moisture. However, it is not without its challenges. Ammonia readily dissolves in water and is retained as ammonium by clay and organic matter in the soil. However, low moisture levels in dry soils may slow the dissolution of ammonia to ammonium and reduce the rate at which it binds to the soil exchange sites. Cloddy dry soils that do not seal properly can lead to ammonia loss via volatilization either during injection or post injection as it seeps through large pores between clods. These volatilization losses can persist for days after injection until the ammonia is able to react with water and bind to the soil exchange sites. Therefore, ensuring proper injection depth and good closure of the injection slit is essential for effective anhydrous ammonia application in dry soils. 

Best practices for anhydrous ammonia applications into dry soils: 

To minimize nitrogen loss and ensure efficient application in dry conditions, consider the following best practices: 

  1. Wait for Rain- You’re probably reading this and thinking “gee, thanks captain obvious” and I get it. If only I could control the weather, then I’d never have to work another day in my life. I totally understand that this may not be feasible, especially if there are no foreseeable chances of precipitation in the near future. However, if there are chances for precipitation and you are comfortable doing so, then consider delaying your anhydrous ammonia application until after the rainfall. Even a small amount of precipitation can improve soil conditions by increasing moisture levels and help the soil seal better behind the applicator. 
  2. Application and Equipment- Inject the ammonia at least 6 to 8 inches to help minimize volatilization occurs within the soil rather than at the surface. Deeper applications also place the ammonia in contact with more moisture, improving the conversion to ammonium. Additionally, using wing sealers immediately above the outlet port on the knife can help close the knife track, limit the retention zone's size, and reduce vertical movement of ammonia. Fields with poor soil structure may benefit from tillage or another soil management practice before application to ensure better sealing of the ammonia bands. Closing disks can also reduce ammonia loss by covering the injection track with soil, trapping the ammonia as it moves to the soil surface. Furthermore, reducing the application rate or narrowing knife spacing can minimize ammonia concentration in each injection band, further mitigating losses. 
  3. Use a Nitrification Inhibitor- Yes, nitrification rates are drastically reduced in dry soils. Yes, the risk of immediate nitrogen loss is lower in dry soils. No, the nitrification inhibitor will not speed up the conversion of ammonia to the stable ammonium form. No, including a nitrification inhibitor does not replace proper nitrogen management practices such as waiting for soil temperatures to drop below 50°F or ensuring proper depth of application. So, do I still recommend the use of nitrification inhibitors under dry soil conditions. Yes, Absolutely I do. Nitrification inhibitors are a relatively inexpensive risk mitigation tool to protect your nitrogen fertilizer, which is typically the second highest input expense per acre in corn production. It should be noted that nitrification inhibitors may not always result in increased yields, but they can result in increased nitrogen use efficiency and reduced leaching and denitrification losses even when increased yields are not realized. Yes, the conversion of ammonium to nitrate is slower in dry soils, but once moisture becomes available this process can rapidly accelerate. As noted above, none of us can control the weather and what are currently dry conditions could change in an instant. So, if for instance, we experience an extremely wet spring again like we did this year, then having the nitrification inhibitor applied could help us maintain higher levels of available nitrogen in the soil for crop production.  
  4. Avoid Tillage Following Application- Ammonia applied into dry soils may take longer to interact with soil moisture and convert to the stable ammonium form which can be held on the soil exchange sites. Tillage shortly after ammonia application, especially in dry soils, could lead to greater volatilization losses of nitrogen by disturbing the injection site, mixing the soil, and exposing unconverted ammonia to the atmosphere. Tillage could also lead to greater rates of nitrification and greater susceptibility to loss due to disturbance of the “protective zone” and exposing the ammonium to new microbial populations. Additionally, tillage could also reduce the efficacy of applied nitrification inhibitors, again due to the exposure to new microbial populations.  
  5. Be Vigilant During Application- Producers should be alert to signs of ammonia escaping during application. If ammonia can be smelled, it may indicate that the injection was not deep enough or that the soil is not sealing properly (this can also happen if soils are too wet). In such cases, adjusting equipment setup or waiting for better soil moisture conditions is crucial. 

Anhydrous Ammonia Safety Reminder 

Anhydrous ammonia is a highly hazardous substance, and safety should always be a top priority during application. See Safety First with Anhydrous Ammonia Applications for a full list of precautions for working with anhydrous ammonia. Always wear protective gear when handling anhydrous ammonia and be cognizant of wind direction when filling or changing tanks and during application. See Play it Safe with Anhydrous Ammonia for information on regulations and transport requirements.

References

Sawyer, J., 2011. “Anhydrous Ammonia Application and Dry Soils.” Iowa State University Extension and Outreach Integrated Crop Management News, October 28, 2011. https://crops.extension.iastate.edu/cropnews/2011/10/anhydrous-ammonia-application-and-dry-soils

Sawyer, J., 2019. “Understanding Anhydrous Ammonia Application in Soil.” Iowa State University Extension and Outreach Integrated Crop Management News, March 15, 2019. https://crops.extension.iastate.edu/cropnews/2019/03/understanding-anhydrous-ammonia-application-soil

Ruiz Dian, R. and P. Tomlinson, 2022. "Can dry soils affect anhydrous ammonia applications?" Kansas State University Agronomy eUpdates, September 1, 2022. https://eupdate.agronomy.ksu.edu/article_new/can-dry-soils-affect-anhydrous-ammonia-applications-510-4

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Authors: 

Richard Roth Assistant Professor

Dr. Richard Roth is an Assistant Professor and Extension Soil fertility Specialist at Iowa State University.  He joined the Department of Agronomy and at Iowa State in September 2023. His primary responsibilities are nitrogen and sulfur management. Dr. Roth's research and extension work focu...