Nitrogen Fixation, a process of combining atmospheric nitrogen with other elements to form useful compounds. There are only a few ways in which nitrogen, which is relatively inert, can be combined with other elements. Nitrogen is essential to living things and, because most organisms cannot use nitrogen that is not combined with other elements, nitrogen fixation is important to the continuation of life on earth. Fixed, or combined, nitrogen is also necessary for the manufacture of many substances, including explosives and commercial fertilizers.

In nature, nitrogen is fixed by some micro-organisms and by lightning. This natural fixation plays an important role in the nitrogen cycle. In the 20th century, humans learned to fix nitrogen in large quantities to supplement the amount of nitrogen fixed naturally. Synthetic processes of nitrogen fixation include the electric arc process, the cyanamide process, and the Haber process.

Natural Nitrogen Fixation
Microorganisms

In a way that is not yet completely understood, nitrogen-fixing bacteria and algae use nitrogen gas to make ammonium compounds. These compounds are absorbed by plants.

Two main groups of microorganisms carry out nitrogen fixation. The more common of the two groups is made up of organisms living in soil and water—a few species of bacteria (chiefly of the genera Azotobacter and Clostridium) and some blue-green algae.

The second group, consisting of bacteria of the genus Rhizobium, lives in plants, primarily legumes such as peas, clover, and alfalfa. The bacteria cause the roots of legumes to form root nodules (swellings) in which the organisms live. The plants supply the bacteria with food. In return, the bacteria secrete ammonium compounds that are absorbed and used by the legumes and by other plants that are grown in the same soil.

Lightning plays a minor part in the fixation of atmospheric nitrogen. The extreme heat of a lightning flash causes nitrogen to combine with oxygen of the air to form nitrogen oxides. The oxides combine with moisture in the air. The fixed nitrogen is carried by rain to the earth, where, in the form of nitrates, it is used by plants.

Synthetic Nitrogen Fixation

Haber Process, or Haber-Bosch Process. In this process, heated nitrogen (from the air) and hydrogen are mixed under very high pressure in a vessel where they combine chemically. The vessel contains a catalyst (usually iron with oxides of aluminum and potassium), which speeds up the chemical reaction. The Haber process is the most widely used process for the commercial production of ammonia. Fritz Haber, a German chemist, developed the process in the first decade of the 20th century. Karl Bosch, another German scientist, adapted the process for industrial use.

Electric Arc Process

In this process a powerful electric arc is set up in the air, causing nitrogen and oxygen to combine and form nitrogen oxides. The air containing the oxides is then sent through water, which combines with the oxides to form nitric acid. The electric arc process was the first synthetic process of nitrogen fixation, developed by Lord Rayleigh in 1895.

The Cyanamide Process

In the cyanamide process, calcium carbide—produced from lime, coke, and air—is ground into a powder and heated in an atmosphere of pure nitrogen. The process produces calcium cyanamide, which can then be used to produce ammonia.