Which of the following elements is classified as a macronutrient secondary to the primary group?

Calcium is classified as a macronutrient secondary to the primary group because it is essential for plant growth and development, yet it is required in smaller quantities than the primary macronutrients, which are nitrogen, phosphorus, and potassium.

Calcium plays a vital role in plant structure by contributing to cell wall stability and signaling processes within the plant. It is also important for root and shoot development, as well as for the process of nitrogen metabolism and enzyme activity. While plants need calcium in significant amounts compared to micronutrients, it is still less than what is required for the primary macronutrients.

On the other hand, nitrogen and phosphorus are considered primary macronutrients because they are critical for plant growth and are required in larger quantities. Manganese, meanwhile, is classified as a micronutrient, as it is necessary for plant health but required in much lower concentrations than macronutrients. This classification helps farmers and agronomists in nutrient management, ensuring that plants receive balanced nutrition for optimal growth and yield.