Which nutrients are classified as "secondary nutrients" in fertilizing fields?

Secondary nutrients are essential nutrients that are necessary for plant growth, but are required in smaller amounts compared to primary nutrients. The primary nutrients are nitrogen, phosphorus, and potassium, often referred to as N-P-K. Secondary nutrients include calcium, sulfur, and magnesium.

Calcium plays a vital role in cell wall structure and helps in regulating various plant physiological processes. Sulfur is important for the formation of amino acids and proteins, as well as for photosynthesis. Magnesium is a central component of chlorophyll, making it crucial for photosynthesis and overall plant health.

The other options include elements that either fall into the category of primary nutrients, micronutrients, or are not classified as secondary nutrients. For instance, zinc, iron, and copper are considered micronutrients, which, while critical for plant health, are needed in even smaller quantities than secondary nutrients. Chlorine, sodium, and boron, similarly, do not belong to the secondary nutrient category but rather include elements that serve more specialized roles in plant nutrition.