Q. Which among the following is NOT a primary nutrient for plants? 
Answer: Magnesium
Notes: Apart from air, 15 of the essential nutrients are supplied by the soil to plants. Out of them, Nitrogen, Phosphorus and Potassium are called primary nutrients or macronutrients. Three more elements viz. Calcium, Magnesium and Sulphur are known as secondary nutrients because the deficiency of them is less likely to be a growth limiting factor.