Iron

Iron is required for the synthesis of chlorophyll and is a component of cytochromes and other proteins involved in the electron-transport chains of photosynthesis and respiration. It also acts as a cofactor for several other enzymes involved in plant metabolism.

Because iron underpins chlorophyll formation directly, its deficiency shows up quickly and visibly as impaired leaf greening, even though the total amount of iron required by the plant is small.

Although iron is abundant in most soils, its solubility — and therefore its availability to plants — is governed largely by soil pH and oxygen status rather than by total quantity. Plants and their associated microorganisms release organic acids and iron-binding compounds (siderophores) that help make soil iron more available.

Iron deficiency appears as interveinal chlorosis on young leaves — the veins stay green while the tissue between them turns pale yellow to white — sometimes called iron chlorosis or lime-induced chlorosis when it occurs on high-pH soils. It can occur even where total soil iron is abundant, because the underlying issue is availability rather than total supply.

Iron toxicity occurs mainly in flooded, low-oxygen soils, notably in lowland rice, where the reduced Fe²⁺ form accumulates to levels that cause a characteristic "bronzing" of leaves and reduced yield. It is much less common in well-drained upland soils.

Iron competes with manganese, zinc, and copper for uptake and for binding sites in chelating compounds, so an excess of one of these micronutrients can induce a deficiency of another. High soil or fertilizer phosphorus can also reduce iron availability, sometimes described as phosphorus-induced iron chlorosis.