Ammonia is produced by reacting nitrogen from the air with hydrogen at high pressure and temperature in the presence of a catalyst. The hydrogen is most often produced by reacting natural gas with water at high temperature and pressure in the presence of a catalyst. Natural gas is also used as a process gas (i.e., an energy source) to generate the heat required in the ammonia production process, but this use is minor compared to its use as a raw material in ammonia production.
Because there are natural gas deposits in many locations, ammonia and nitrogen fertilizers are produced in many countries. British Sulphur, a specialist publisher and independent consultant to the fertilizer industry, has reported that ammonia is produced in approximately 68 countries; urea, the most common of nitrogen fertilizer products, is produced in about 56 countries.
Although the number of chemical processes used in the production of nitrogen fertilizer products is small, there is a wide variety of finished products. The diversity of products facilitates site-specific applications, which take into account factors such as soil type and the requirements of the crop, thus making it possible to achieve optimal plant nutrition.
Ammonia – used as a fertilizer and as a building block for other nitrogen products, including intermediate products for industrial applications and finished fertilizer products. Ammonia, consisting of 82 percent nitrogen, is stored as a liquid under pressure or refrigerated. It is gaseous at ambient temperatures and is injected into the soil as a gas. The direct application of ammonia requires a considerable investment by farmers in pressurized storage tanks and injection machinery.
Urea – formed by reacting ammonia with carbon dioxide (CO2) at high pressure. From the warm urea liquid produced in the first, wet stage of the process, the finished product is mostly produced as a solid product (containing 46 percent nitrogen) typically applied in solid form. Urea can be combined with ammonium nitrate solution to make liquid nitrogen fertilizer (urea ammonium nitrate or UAN).
Ammonium nitrate (AN) – produced by reacting nitric acid, an intermediate chemical feedstock produced from ammonia, with ammonia to form a concentrated, watery solution that is subsequently solidified in a prilling or granulation process. Ammonium nitrate is a solid fertilizer (containing approximately 34 percent nitrogen) typically applied in solid form. Ammonium nitrate is water soluble and used in various fertilizer solutions.
Calcium ammonium nitrate (CAN) – a mixture of AN and calcium or magnesium carbonate (containing 25-28 percent nitrogen), produced by mixing calcium and/or magnesium carbonate into an ammonium nitrate solution before the solidification process. The lime content of CAN also helps to neutralize soil acidity.
Ammonium sulfate (AS) – has a relatively low nitrogen content (21 percent). In addition to nitrogen, it contains sulfur (24 percent). It is used where the lack of sulfur in the soil is a limiting factor in plant growth.
Calcium nitrate (CN) – produced by dissolving a calcium salt such as limestone or the calcium phosphate of phosphate rock in nitric acid. In the latter case, it is a co-product with nitrophosphate products. CN is used to remedy plant deficiencies in calcium and ameliorate soil acidification. It contains 15.5 percent nitrogen in nitrate form and 19 percent water-soluble calcium. The product is water soluble and particularly suited for water-born fertilizer application systems.
Potassium nitrate (PN) – produced by reacting sodium nitrate with potassium chloride. Potassium nitrate is used as a potassium and nitrogen fertilizer. Potassium nitrate contains 13.5 percent nitrogen and 45 percent water-soluble potassium as K2O. The water suitability makes it particularly suited for liquid-based applications.
Because of their chemical similarity, AN, CAN, CN and PN are often collectively referred to as “nitrates.”