You can contribute to the Aquarium Wiki by expanding this article. Dont be shy!.
Iron is the most abundant metal on Earth and it is essential to all known organisms and therefore aquatic plants require it in order to grow.
You can find out the level of iron in your aquarium water by using a iron Test Kit. This measures the amount of iron (in mg/l) in a sample of water and around 0.2 mg/l is generally required.
However iron is available in two forms - insoluble iron (ferric Fe+3) and soluble iron (ferrous Fe+2). The plants need the soluble type.
Usually Chelated ferrous iron is added to aquariums in the form of liquid fertilisers. You can buy these ready made fertilisers (for example Seachems Flourish) from any pet store or you can purchase the dry components and mix up the powder with RO water to make your own much cheaper version.
Iron is only soluble in water if the pH is less than 8. So if you are measuring the level of iron in your tank. Measure the pH too. It may be the pH is too high.