Iron is the most abundant mineral ion required by the plant. It is absorbed in ferrous and ferric form from the rhizosphere of the plant.
$Fe ^{2+}$ (ferrous) form is relatively soluble but is readily oxidized to $Fe ^{3+}$ (ferric) at the junction of the protoxylem and the metaxylem, chelated by citrate, and then transported in the metaxylem to the other parts of the plant.