Aquatic Pollution and Coastal Management

Unit 3 : Agricultural wastes and eutrophication

3.13.2.4.Iron
Iron crosses over the border as a macro/micro-nutrient. Ferrous matter is necessary in only small concentration, but is often a nutrient deficiency cause of 'yellowing'. Your tap water may well not contain enough iron material to meet your plants needs or be too alkaline, precipitating it out of solution.
A sound approach to iron supplementation is inclusion in soil additive(s), or periodic addition in a chelated commercial preparation Unfortunately, you can have "too much of a good thing" with iron. About half a part per million is what you want. You will have to invest in and use an iron test kit, rely on water changes and uptake of a chelated formula, and/or add iron judiciously and not worry.