Plants take up fertilizer as ions - when salts dissolve in water the molecules separate into ions, so KNO3 becomes K+ and NO3- ions. Those ions are taken up by the roots and the leaves of the plants and used as food. Iron is one of the nutrients which, if it is an iron oxide, needs bacteria to work on it to make it available to the plants, and it needs a chelator to protect the ions from combining with other ions in the water to make compounds that won't dissolve in water. Fortunately, the chemicals we buy as plant food work fine - there is a chelator in the trace element mix to protect the ions that need the protection, and the nutrients in the macro fertilizers are molecules that split into ions the plants use easily.