Which of the following is NOT considered a macro element?

Iron is classified as a micro element or trace element, which means it is required by plants in smaller quantities compared to macro elements. Macro elements are nutrients that plants need in larger amounts for their growth and development. The primary macro elements include nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. These elements play crucial roles in various physiological processes such as photosynthesis, cell division, and nutrient transport within the plant.

On the other hand, iron is important for plants as it is necessary for chlorophyll synthesis and enzyme functions, but it is only required in trace amounts. This distinction is why iron is not considered a macro element, making it the correct answer to the question. Understanding the difference between macro and micro elements is essential for effective plant nutrition management.