An element is a substance that is made entirely from one type of atom
TRACE ELEMENTS
Trace elements are those things required in very small quantities yet are still vital to plant growth. These are taken in by the plant in ion form. The more important trace elements are sulfur (SO4--), calcium (Ca++), phosphorus (HPO4--/H2PO4-), magnesium (Mg++) and iron (Fe++).
Sulfur, calcium and magnesium are usually found in tap water. If the water has too little general hardness (< 3 degrees dH), calcium and/or magnesium may be in short supply. This can be remedied by adding calcium and magnesium sulfate in small quantities.
Phosphorus can be measured in the water and should be present in quantities less than 0.2 ppm of phosphate. If the nitrates are OK, phosphorus levels are usually also OK.
Iron may be present in tap water in the correct ionic state (Fe++) but will quickly oxidize to a form unusable by plants. To prevent this, chelated iron mixtures can be used. The chelator prevents the iron from oxidizing and makes it easy for the plants to assimilate. The iron concentration should be less than 0.2 ppm. Other trace elements are needed in extremely small quantities and can usually be provided in fish food or specialized trace element formulations. Note that some of these elements are toxic in anything but trace amounts so the addition of trace elements should be done very carefully