Iron is the fourth most abundant mineral in the earth's crust after oxygen, silicon and aluminum. Plants derive 2.5 percent concentrations of iron from all the elements they derive from the soil. Iron is classified as a trace element, or micro-nutrient, because it is only needed in small amounts. Too much iron can be toxic to plants.
Excess iron can produce stunted growth of roots and tops, dark green foliage, or dark brown to purple leaves on some plants. Iron toxicity is a special problem in rice paddies that show the symptom of brown leaves, called "bronzing."
Iron and Photosynthesis
Plant scientists have yet to identify the exact role of iron in plant health. Although iron is not part of a chlorophyll molecule, the green pigment of plants, plants need iron to make chlorophyll, which is critical to photosynthesis. That is why iron affects the color of foliage.
It is difficult for soil scientists to make conclusions about iron by analyzing plant tissue unless they are meticulous. Iron is applied as a liquid on the foliage of golf course grasses, so unless the grass is carefully washed before being analyzed the results can be skewed. Also iron collects on the surface of plant roots; if a technician does not remove the roots, the plant may appear to have more iron than it actually does.
In acidic soils--those with a pH lower than 5--anaerobic bacteria reduce iron to the soluble Fe2, providing plants with too much iron. Iron toxicity commonly occurs in water-logged soils.
Acid soils that are poorly aerated or compacted can increase iron to the point of toxicity.
Plants use iron in the form of ferrous iron Fe2. In alkaline soil--those with a high pH--Fe2 is changed into iron oxides or hydroxides that are less soluble or insoluble.
Soluble iron in excess of 1,000 parts per million is found at a soil pH of 6.3. At pH 6.5, that number drops to 352 ppm, at pH 7 it is 35 ppm and at pH 7 the number is 3.5 ppm. The higher the soil pH the likely problem is iron deficiency; at lower pH levels the problem more likely is iron toxicity.
If your soil has a low pH, make sure to keep it well drained.
Iron and Manganese
There have been cases where excessive iron has reduced the uptake of manganese, causing plants to show symptoms of manganese deficiency. A chelated mineral is one that plants can absorb. When growers add chelated manganese or iron, it increases iron without solving the problem of manganese deficiency.
Fertilizers and Amendments
Plants can suffer iron toxicity when excess soluble iron salts are applied as soil amendments or foliar sprays.