Calcium, magnesium and sulphur are defined as

Calcium, magnesium and sulphur are defined as Secondary nutrients.
Secondary nutrients are those that plants require in smaller quantities.
Calcium regulates the transport of other nutrients into the plant and is also involved in the activation of certain plant enzymes.
The outstanding role of magnesium in plant nutrition is as a constituent of the chlorophyll molecule.
Sulphur is a structural component of some amino acids and vitamins, and is essential for chloroplast growth and function.