The Effects of Using Anhydrous Ammonia to Supply Nitrogen to Vegetable Crops
Proper nitrogen management (N) on vegetable farms is essential for the health of the crop, soil and ecosystem. It has long been recognised that groundwater with nitrate from fertiliser origin is a serious environmental issue in areas of intensive agriculture around the world. Areas of intensive vegetable production typically produce two to three crops annually, with frequent irrigation and N applications, sometimes at rates that far exceed the rate of N removal of the harvested product1. Nitrogen fertilisers interact with soils and plants in different ways, and understanding this is key to achieving the most profitable use of nitrogen. Anhydrous ammonia is a N fertiliser that interacts with the soil clay particles and moisture in a way that increases the retention in the soil and reduces leaching into ground water. At normal room temperature and pressure ammonia is a gas, under high pressure it converts to a liquefied gas, and this is how it is transported and applied to the soil. Anhydrous ammonia has a high affinity to water and on application to the soil it combines with soil moisture to produce ammonium ions. The following reaction occurs: Ammonia gas (NH3) combines with water in the soil to produce ammonium nitrogen (NH4+), plus hydroxide ions (OH‐). Ammonia gas (NH3) is highly reactive and ionises to ammonium (NH4+) ions in the presence of water. Ammonium ions have a high positive charge and as such are attracted to negatively charged surfaces such as clay particles and organic matter. This strong attraction reduces the loss of ammonia to the atmosphere or by leaching into the soil. Ammonium is then converted into nitrate (NO3‐) by soil microbes, allowing it to move with soil moisture.2 Anhydrous ammonia is normally used as a pre‐plant fertiliser to supply nitrogen in broadacre cereals and cotton. It is applied directly under the seed or as side dressing. Anhydrous ammonia can have beneficial effects on soil microbes, nitrifying bacteria and worms. It can also increase N retention in the soil, reducing nitrate leaching, resulting in nitrogen‐useefficiency benefits. Caution needs to be used when applying anhydrous ammonia, as it can be toxic to seeds and plant roots.