Iron (Fe) is essential for many plant functions including the formation of chlorophyll critical for photosynthesis. Iron is also required for plants to obtain nitrogen from the air and to produce enzymes and proteins necessary for metabolism and respiration. The most serious problem that most plants have with iron is deficiency, not toxicity. Geraniums, marigolds and impatiens of all types are especially susceptible to iron toxicity.
Iron toxicity occurs when a drop in soil pH causes an excess of iron. Too much iron can result in plants with stunted roots and tops plus dark green foliage. Rotten spots may appear on the leaves. Iron toxicity is associated with wet or flooded soil and is a particular problem with rice paddies where it causes brown to purple leaves, called the bronzing disease. If symptoms are apparent in a garden the problem is already serious.
Iron toxicity usually occurs with acidic soils, those with a pH of below 5. Acidic soils that are poorly aerated, compacted or saturated can increase the available iron to the point of toxicity. Researchers at North Carolina State University suggests that a deficiency of phosphorous, high growing temperatures and hot sun can all cause a sudden decline in soil pH that can cause iron toxicity. Iron toxicity may be avoided by adding lime to soil to increase its pH to 6 to 6.6 before planting garden vegetables or flowers.
Iron toxicity may be brought about by the use of 20-10-20, 15-15-15, 15-6-17 or other acid-forming fertilizers. Adding fertilizer with iron chelates may cause problems on plants that are sensitive to iron toxicity. Symptoms of iron toxicity often appear after excessive soluble iron salts have been applied as soil amendments or sprayed on foliage.
Iron and Other Metals
Sometimes excessive iron in the soil reduces the plant uptake of manganese. When this happens plants show symptoms of manganese deficiency. The plants may be slow to mature their leaves turn yellow between the veins. Brown, rotten spots may appear on the leaves, and they may drop prematurely. A deficiency of zinc may increase the uptake of iron to the point of toxicity.
Douglas Cox, of the Plant and Soil Sciences Department, of the University of Massachusetts, Amherst warns growers to be careful to add iron only to plants susceptible to iron deficiency. Adding them to geraniums, marigolds or impatiens, all plants susceptible to iron toxicity, could be disastrous.