Iron (Fe) is one of the micronutrients or trace elements, needed by plants in relatively small or trace, but essential, amounts. It is important in photosynthesis (the process plants use to harness the sun’s energy to produce sugars and oxygen) and the production of chlorophyll (the light-absorbing plant pigment), so is important for deep green leaf colour.
Iron deficiency is often confused with other problems. Leaf yellowing between the veins can be confused with magnesium or manganese deficiencies, while overall leaf yellowing can be caused by numerous factors – including nitrogen or potassium deficiencies, waterlogging, drought, cold weather, plant virus diseases and even natural leaf drop – including on evergreen plants.
Iron deficiency affects the youngest leaves first, whereas manganese and magnesium deficiencies start with the older leaves. Nitrogen deficiency produces a general yellowing of the whole leaf and in potassium deficiency the yellowing is usually more pronounced at the leaf edges.
Iron deficiency may be caused when there are insufficient suitable iron-containing materials in the growing medium (soil or compost). However, it is more likely when iron is present, but is unavailable for plant roots to absorb it – “locked” in the soil or compost. This is common in alkaline growing media, where the pH is above 7 – which is why acid-loving/lime-hating ericaceous plants, which have a relatively high iron need, are prone when grown in alkaline growing media. It can also be caused when plants are growing in dry soil or compost, which prevents the roots taking up the iron.
Use a simple soil pH testing kit to make sure your soil is acidic – especially when growing lime-hating and ericaceous plants.