Ammonia (NH₃) is the foundation for the nitrogen (N) fertilizer industry. It can be directly applied to soil as a plant nutrient or converted into a variety of common N fertilizers, but this requires special safety and management precautions.
Almost 80 percent of Earth’s atmosphere is composed of nitrogen gas (N₂), but in a chemically and biologically unusable form. In the early 1900s, the processfor combining N₂ and hydrogen (H₂) under conditions of high temperature and pressure was developed.
This reaction is known as the Haber-Bosch process: [3H₂ + N₂ 2NH₃].
A variety of fossil-fuel materials function as a source of H₂, but natural gas (methane) is most common. Therefore, most NH₃ production occurs in locations with a ready supply of natural gas.
Ammonia is a gas in the atmosphere, but is transported in a liquid state by compressing or refrigerating it below its boiling point (-33°C). It’s shipped globally in refrigerated ocean vessels, pressurized rail cars and long-distance pipelines.
Ammonia has the highest N content of any commercial fertilizer, making it a popular source of N despite the potential hazard it poses and the safety practices required to use it. For example, when NH₃ fertilizer is applied directly to soil, it’s in a pressurized liquid that will immediately become vapor if exposed to air after leaving the tank. To prevent such releases into the atmosphere, growers use various tractor-drawn knives and shanks to place it at least 10 to 20 cm (4 to 8 inches) below the soil surface. Ammonia will then rapidly react with soil water to form ammonium (NH₄⁺), which is retained on the soil cation exchange sites.
Ammonia is sometimes dissolved in water to produce aqua ammonia, a popular liquid N fertilizer. Aqua ammonia doesn’t need to be injected as deeply as NH₃, which provides benefits during field application and has fewer safety considerations. Aqua ammonia is frequently added to irrigation water and used in flooded soil conditions.
Handling NH₃ requires careful attention to safety. At storage facilities and during field application, appropriate personal protection equipment must be used. Since it’s highly water soluble, free NH₃ will rapidly react with body moisture, such as lungs and eyes, to cause severe damage. It should not be transferred or applied without adequate safety training.
Immediately after application, the high NH₃ concentration surrounding the injection site will cause a temporary inhibition of soilmicrobes. However, the microbial population recovers as NH₃ converts to NH₄⁺, diffuses from the point of application, and then converts to nitrate. Ammonia’s inhibiting effect on microbes can also damage seeds during germination, which farmers can prevent by keeping seeds out of close proximity to recent zone of NH₃ applications.
Aside from its personal and crop safety concerns, escape of NH₃ to the atmosphere causes other problems and should be avoided as much as possible. Emissions of NH₃ are linked to atmospheric haze and changes in rainwater chemistry. And elevated NH₃ concentrations in surface water can harm aquatic organisms.
Though more than 80 percent of NH₃ production ends up as fertilizer, many industrial applications exist as well. Household cleaners are made from a 5 to10 percent solution of NH3 dissolved in water (to form ammonium hydroxide). Because of its vaporization properties, NH₃ is used widely as a refrigerant as well.
Source: Nutrient Source Specifics, No. 10, International Plant Nutrition Institute.