The primary means by which nitrogen is added to soil are biological nitrogen fixation and atmospheric deposition. Fixation is the conversion of dinitrogen gas, which is chemically quite unreactive, to nitrogen combined with other elements, such as oxygen or hydrogen, which can readily undergo chemical reactions.

The atmosphere contributes about 11.4 pounds of nitrogen per acre to soils annually (Stevenson, 1982). Biological nitrogen fixation accounts for 8.2 pounds or about 72 percent of the nitrogen contributed per acre per year. Biological nitrogen fixation occurs symbiotically (dinitrogen-fixing bacteria, such as Rhizobium bacteria in conjunction with legumes) and non-symbiotically (free living organisms such as photosynthetic bacteria, blue-green algae, and free-living Azotobacter species). The balance, 3.2 pounds of nitrogen per acre per year, consists of various sources of ammonium nitrogen (NH₄+), nitrate nitrogen (NO₃-), and nitrite nitrogen (NO₂), deposited in precipitation. The amount of nitrogen added each year from atmospheric deposition varies considerably with climate and nearness to industrial sources of atmospheric nitrogen, but generally this deposition is too small to significantly affect crop production.