Nitrogen fixation is the process through which atmospheric nitrogen (N₂) is converted into ammonia (NH₃) or related nitrogenous compounds that, when present in the soil, can be utilised by plants. Plants are unable to utilise atmospheric nitrogen (N­₂) for plant growth because it is a relatively unreactive gas. There are two main processes through which nitrogen fixation occurs in the food system: artificial nitrogen fixation through the Haber-Bosch process – the process underlying the production of synthetic fertiliser – and biological nitrogen fixation (BNF) through bacteria in the soil and roots of particular plant and tree species such as legumes.