Plant growth needs nitrogen. In the atmosphere, nitrogen exists as a molecule made up of two atoms bonded together. For plants to use the nitrogen, the atoms must separate, or be "fixed." There are many ways for this to happen–some naturally and some by human action. Plants can easily take up and use two forms of nitrogen, ammonia (NH4) and nitrate (NO3). Other forms of nitrogen must convert to one of these compounds by natural or artificial means before plants can utilise them directly as a source of nitrogen for plant growth.
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Plants and other organisms make nitrogen fertiliser naturally. As plants decay, the nitrogen locked in their tissues breaks down and releases into the soil as nitrogen compounds. Bacteria and fungi then break down these compounds to fix the nitrogen. Some types of plants, such as soybeans, alfalfa and clover, can also fix nitrogen. Bacteria, called Rhizobium, live on the plant roots and form nodules. Within the nodules, the bacteria use plant sugars as a form of energy, and in return, they fix nitrogen in the soil. When a farmer ploughs plants into the soil, the fixed nitrogen in the nodules returns to the soil for use by other types of plants that cannot fix nitrogen themselves. Manure also contains fixed nitrogen, and has been in use as an organic fertiliser since time immemorial.
Because organic fertilisers are too inefficient to use with modern agricultural methods, farmers have found new ways to fix nitrogen artificially and add it to the soil. The Haber-Bosch process places nitrogen and hydrogen or methane gas under high pressure and temperature in the presence of iron oxide, which acts as a catalyst to produce a molecule of anhydrous ammonia gas (NH3). This then goes under pressure to make a liquid fertiliser. This is the most common form of inorganic fertiliser because it is very high in nitrogen and is relatively cheap to produce. However, it is dangerous to transport, and farmers must inject it into the ground or lose large amounts to evaporation.
Manufacturers make all of the other types of inorganic fertilisers from anhydrous ammonia. They are more expensive, but also more stable and safer because of no need to place them under pressure for use as a liquid.
Production of urea fertilisers uses the Bosch-Meiser process. In this process, anhydrous ammonia combines with carbon dioxide to produce ammonium carbonate, which, when heated, gives off urea and water. The urea molecule contains fixed nitrogen and is a stable solid. It often sells as a pellet.
Urea-ammonium nitrate fertiliser
Urea-ammonium nitrate fertiliser results from dissolving urea and ammonium nitrate in water. This creates a stable solution containing fixed nitrogen. Although it has less nitrogen than urea or ammonia fertilisers, it is very easy to use.
Ammonium nitrate fertiliser
Another dry fertiliser is ammonium nitrate, made by combining anhydrous ammonia with nitric acid. When dissolved in water, the ammonium and nitrate separate--each contains fixed nitrogen. The nitrate remains dissolved in the soil water while the ammonium attaches to the soil particles. Plants can then take up and use both.
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