Nitrogen fixation by free-living bacteria in the soil results in the formation of ammonia (NH₃), and most of it is rapidly ionized to form ammonium (NH₄⁺). The latter can be taken up by plant roots and is then used as a source of amino groups (—NH₂) in the formation of such molecules are amino acids and nucleotides. Soil bacteria called nitrifiers oxidize ammonia to nitrate ions (NO₃^–)—another form that plants can take up—by the process of nitrification. Soil pH affects which form of nitrogen is taken up by plants: nitrate ions are taken up preferentially under more basic conditions, and ammonium ions under more acidic ones. To use nitrate that has entered the roots, the plant must reduce it to ammonium in a process called nitrate reduction, which occurs in two enzyme-catalyzed steps. First, nitrate is converted to nitrite (NO₂^–) in the cytoplasm, and then nitrite is converted to ammonium in the plastids.