What is not a secondary nutrient?

In the context of soil nutrients, secondary nutrients are those that are essential for plant growth but are required in smaller quantities compared to primary nutrients. The primary nutrients are nitrogen (N), phosphorus (P), and potassium (K).

Calcium (Ca), magnesium (Mg), and sulfur (S) are classified as secondary nutrients because they play important roles in plant development and are required in various processes, such as photosynthesis and the formation of cell walls.

Nitrogen (N) is considered a primary nutrient because it is crucial for the synthesis of amino acids and proteins and is involved in chlorophyll production. This high demand for nitrogen in plants distinguishes it from the secondary nutrients, which are still important but are needed in smaller amounts. Thus, nitrogen is not a secondary nutrient, making it the correct answer to the question.