Calcium, magnesium and sulphur are defined as

Calcium, Magnesium and Sulphur are known as secondary nutrients.
There are 17 essential nutrient elements required by the plants for proper functioning and growth, which are further classified as micro and macro-nutrient or as primary and secondary nutrients.
The primary nutrients without which the plants may not grow are NPK. (Nitrogen, Phosphorus and Potassium) whereas the secondary nutrients are required for specific functions in plants and these are calcium, magnesium and Sulphur.