Optimal Conditions for Anhydrous Ammonia Application

Anhydrous ammonia (NH3) is a widely used nitrogen fertilizer in Iowa. It is applied as a liquid under pressure and quickly turns into a gas after it is injected into the soil. The effectiveness of anhydrous ammonia application is heavily dependent on the factors that affect the reaction rate such as temperature and moisture content of the soil. These factors have a large impact on the chemical reaction that takes place (NH₃ + H₂O → NH₄+ + OH- ) but also affect the effectiveness of the fertilizer itself as a nitrogen supplement for crops, as well as environmental impacts.

Soil moisture content must contain enough moisture for the ammonia to react, so it doesn't leak as a gas into the air, but not too much moisture where it will wash out of the soil as runoff. Anhydrous ammonia should be applied in soil temperatures less than 50^oC to control the slow rate of reaction, so there is still nitrogen left in the soil when crops are planted.

Improper application can lead to leaching of nitrates into the water, and ammonia gas leaving the soil leading to ineffective nitrogen replacement in the soil. Understanding factors that affect chemical reaction rates is essential to proper anhydrous ammonia application.

The photos were taken February 2025 and the video was March 2025.

Image

Indianola

Driving Question

How do the conditions determine the effectiveness of anhydrous ammonia application?

Probing Questions

What time of year have you seen anhydrous ammonia being applied to crop fields?
Why might this be? Why is there machinery to inject anhydrous ammonia into the ground?
Why might NH₃ be commonly used as a fertilizer?
What might be some issues if anhydrous ammonia is applied in the wrong conditions?
What weather conditions do you think are best for the application of anhydrous ammonia?

Classroom Suggestions

Students could:

Research other real world chemical reaction rates and how temperature and concentration can have an effect.
Conduct in-class labs with other chemicals to gain understanding of the effects temperature and concentration have on the rate of chemical reactions, then apply that knowledge to the reaction for anhydrous ammonia in the soil to develop an explanation for best conditions for anhydrous ammonia application.
Test various soils (school test plots, local farmers, etc) for nitrogen, review actual or mock scenarios for conditions when anhydrous ammonia would be applied, make observations about the application of anhydrous ammonia, refine solutions to apply, control conditions, or optimize the application of anhydrous ammonia.
Brainstorm ways in which anhydrous ammonia application can have environmental effects, calculating the conservation of mass based on amounts of anhydrous ammonia applied per acre.

Resources

Libre Texts | Effects of Temperature, Concentration, and Catalysts on Chemical Reactions: General background knowledge on the effect of temperature and concentration on chemical reactions.
Iowa State University Extension | Understanding Anhydrous Ammonia Application in Soil: General information about anhydrous ammonia application and the chemical reactions that happen in the soil with it.
MooMoo Math and Science | Factors that affect the Rate of Reaction: Video describing the factors that affect chemical reaction rates. Scientific knowledge that can be applied to the desirable conditions for the application of anhydrous ammonia.

Contributors