Nitrogen is a dietary requirement for all organisms, because it is a constituent of all proteins and nucleic acids. Plants consists of approximately 7.5% nitrogen (dry mass). Nitrogen is essential for plants, and can be found in air in large amounts. This elementary nitrogen cannot be taken up directly. Nitrogen must first be bound and converted, for instance to nitrate. This so-called nitrification process is carried out by bacteria, which convert ammonia and ammonium to nitrate and nitrite.