Nitrogen fixation is the process in the nitrogen cycle in which nitrogen gas from the atmosphere is converted into compounds such ammonia, nitrites and nitrates that can be used by plants. Although nitrogen, which constitutes about 78 percent of the earth's atmosphere, is essential for nearly all organisms, most cannot absorb it directly in its gaseous form and, instead, require it locked in compounds.
The main way in which nitrogen fixation occurs is by the action of specialized bacteria in the soil and in water bodies. It is also caused by lightning and ultraviolet radiation. The high temperature of lightning bolts can break the bonds of atmospheric nitrogen molecules, causing the resulting free nitrogen atoms in the air to bond with oxygen to create nitrogen oxides, which, in turn, dissolve in atmospheric moisture to form nitrates that are carried to the earth's surface by precipitation.
In addition, nitrogen fixation takes place on a massive scale artificially in the fertilizer industry, in which atmospheric nitrogen is combined with hydrogen to make anhydrous ammonia and other nitrogen fertilizer products.