Symbol: Fe. A metal element, atomic number 26, atomic weight 55.85, required by plants as a macronutrient. It is important as a constituent of the *cytochromes
, ferredoxin, and of certain enzymes, e.g. succinate dehydrogenase. Deficiency of iron leads to chlorosis since iron plays a part in chlorophyll formation. Iron is the second most abundant metal in the soil being present in such minerals as haematite, magnetite, and limonite. However in alkaline conditions iron may be precipitated as an insoluble ferric salt making it unavailable to plants. This can be overcome by adding sequestrols containing chelated iron. Ferrous compounds in the soil are also used by the filamentous iron bacteria, which derive energy by oxidizing them to ferric compounds.