Which of the following is a macronutrient?

Calcium is classified as a macronutrient because it is required by plants in relatively large quantities compared to micronutrients, which are needed in smaller amounts. Macronutrients are essential elements that play critical roles in various physiological processes within plants, including structural functions, metabolic activities, and nutrient uptake and assimilation. Calcium, in particular, is vital for maintaining cell wall integrity, promoting root development, and facilitating the overall growth of the plant.

In contrast, the other options—iron, boron, and manganese—are classified as micronutrients because they are needed in much smaller amounts. Although these micronutrients are also essential for plant health, their roles are more focused on specific functions such as enzyme activity and chlorophyll synthesis, which can occur in much smaller concentrations compared to macronutrients like calcium. Understanding the distinctions between macronutrients and micronutrients is important for effective plant nutrition management.