Structure: Nitrogen-fixing vegetation

Leguminous plants (Fabaceae) are used in agriculture in order to transform atmospheric nitrogen (N₂) into compounds such as ammonia, nitrates and nitrogen oxides that are suitable for organisms.

Red clover (Trifolium pratense) is the most common plant mixed with grasses in crop rotation fields. The yield is used for animal fodder. It has been studied that the share of red clover should exceed 40% in order to increase the nitrogen level of the soil and to benefit the subsequent crops such as cereals. The nitrogen uptake in organic clover-grass fields has been estimated to be around 125 kg/ha.

Pea (Pisum sativum) is another important nitrogen fixing crop in Finland. It is cultivated in rotation fields in every 4–5 years. The area under cultivation of pea varies annually but on average the area has remained quite stable at about  4 500 ha during the past twenty years. The yields of pea has varied as well. The average annual yield has been about  10 000 tonnes. The amount of fixed nitrogen of pea is 50–150 kg/ha.

Other nitrogen fixing crops are for example alfalfa (Medicago sativa), broad bean (Vicia faba), goat’s rue (Galega), vetches (Vicia) and clovers (Trifolium). Associative nitrogen fixation of cereals and grasses occurs as well but compared to the function of the Fabaceae it is fairly insignificant, about 4 kg/ha.

In other habitats alders and cyanobacteria are important in nitrogen fixing. Cyanobacteria live in the soil, in the Baltic Sea or in symbiosis with fungi forming lichens.

Antikainen, R., Lemola, R., Nousiainen, J. I., Sokka, L., Esala, M, Huhtanen, P., Rekolainen, S. 2005. Stocks and flows of nitrogen and phosphorus in the Finnish food production and consumption system. Agriculture, Ecosystems & Environment. Volume 107, Issues 2–3. pp. 287–305.

Rajala J. (ed.) 2006. Luonnonmukainen maatalous. Helsingin yliopiston Maaseudun tutkimus- ja koulutuskeskus, julkaisu no 80. 494 p. [In Finnish]