Plant require Fe and Mg for

Plants require Fe and Mg for the synthesis of chlorophyll.
The presence of magnesium in chlorophyll was discovered in 1906, and was that element's first detection in living tissue.
Chlorophyll share a common biosynthetic pathway with porphyrins, including the precursor uroporphyrinogen III. Unlike hemes, which contain iron bound to the N₄ center, most chlorophylls bind magnesium. The axial ligands attached to the Mg²⁺ center are often omitted for clarity.
Mg²⁺ is essential for the function of many cellular enzymes and the aggregation of ribosomes.
Without iron, a plant can't produce chlorophyll and, therefore, can't get oxygen.