Biological nitrogen fixation is the process through which atmospheric nitrogen (N₂) is converted into ammonia (NH₃) or related nitrogenous compounds by bacteria. BNF includes both the fixation of these compounds by bacteria that are present in the soil and by bacteria that live in the root nodules of legumes and certain other plant and tree species. The latter form of BNF is a symbiotic process: the bacteria provide the plant or tree with nitrogen compounds while the plant or tree provides the bacteria with carbohydrates. Nitrogen-fixating plant and tree species are often able to live on relatively poor soils with little need for additional manure or synthetic nitrogen fertiliser. Increased reliance on BNF as opposed to fertilisers is often considered to be an important aspect of a more sustainable food system.