Which of the following plant nutrients is classified as a secondary nutrient?

Sulfur is classified as a secondary nutrient because it plays a crucial role in plant growth and development, although it is needed in smaller quantities compared to primary nutrients like nitrogen, phosphorus, and potassium. Secondary nutrients, which also include calcium and magnesium, are essential for various physiological functions in plants, such as protein synthesis, enzyme function, and photosynthesis. Sulfur specifically aids in the formation of essential amino acids and contributes to the health of plants by enhancing their resistance to stress.

In contrast, potassium is considered a primary nutrient due to its significant role in water regulation and photosynthesis. Iron and boron are classified as micronutrients because they are required by plants in very small amounts but are still vital for various cellular processes. Understanding these classifications is important for effective nutrient management in crop production.