Although trace amounts of iron are required for plants to grow properly, exposure to too much of the element can cause just as many problems as its absence. Excessive iron not only proves toxic to plant tissues, but it also displaces other nutrients that plants need.
Plants that absorb too much iron have inefficient roots and difficulty regulating photosynthesis, the process by which light is converted into usable energy. Unbalanced levels of many other important nutrients like zinc and manganese often arise with iron poisoning, compounding problems in unpredictable ways (Reference 1).
Areas with wet, poorly-aerated soil, such as swamps and fens, as well as lands subjected to acid rain promote dangerously fast absorption of iron in plants (Reference 1).
Leaves stained red, orange or dark green that develop spots of rot are among the first visible symptoms of excessive iron uptake in plants. This reaction begins in the tips of leaves, spreading inward to cover the plant if high-iron conditions persist (Reference 3).
Excessively iron-rich soil causes many plants to develop sparse, ragged root systems. Large sections of the roots will die, and the weakened portions that survive will often be stained brown or black (Reference 3).
High levels of iron are known to reduce the yield of fruit, vegetable and cereal crops by promoting stunted growth (Reference 2).
Rice farmers in many portions of western and central Africa experience poor harvests due to iron poisoning. Large amounts of iron from the highlands wash down into the wet valleys where rice would otherwise flourish (Reference 2).