Essential mineral elements

Seventeen elements are essential for plants: the macronutrients C, H, O, N, P, K, S, Ca, Mg and the micronutrients Fe, Mn, Cu, Mo, Zn, B, Cl, Ni.
Phosphorus is a macronutrient (a constituent of membranes, nucleic acids and ATP).
Iron and manganese are micronutrients (iron in cytochromes and ferredoxin; manganese in the splitting of water).
Cadmium is not in the list; it is a toxic heavy metal and has no essential role in plants.
Hence cadmium is not an essential mineral element.

Macronutrients are needed in large amounts (more than 10 mmole per kg of dry matter): C, H, O, N, P, K, S, Ca and Mg.
Micronutrients or trace elements are needed in very small amounts (less than 10 mmole per kg of dry matter): Fe, Mn, Cu, Mo, Zn, B, Cl and Ni.
Boron, molybdenum and zinc are micronutrients.
Magnesium is a macronutrient; it is a constituent of the ring structure of chlorophyll and activates enzymes of respiration and photosynthesis.
Hence magnesium is not a micronutrient.