Which of the following is NOT classified as a plant micro-nutrient (trace element) for growth and metabolism?

Introduction / Context:
Plant nutrients are grouped into macronutrients (required in large amounts) and micronutrients (required in trace quantities). Clear classification aids fertiliser formulation and diagnosis of deficiency symptoms in crops.

Given Data / Assumptions:

• Typical micronutrients: B, Cl, Cu, Fe, Mn, Mo, Zn, Ni.
• Macronutrients: N, P, K, Ca, Mg, S, plus C, H, O from air/water.

Concept / Approach:
Carbon is a fundamental macronutrient obtained primarily from atmospheric CO2 through photosynthesis, not a trace element supplied as a micro-nutrient in fertilisers. In contrast, chlorine, iron, boron, and molybdenum are classic micro-nutrients supplied in small amounts through soil amendments or foliar feeds.

Step-by-Step Solution:

Verification / Alternative check:
Agronomic texts and extension guides consistently classify C as a macronutrient, while B, Cl, Fe, and Mo are trace elements with defined deficiency/toxicity ranges in soils and plant tissues.

Why Other Options Are Wrong:

• Chlorine, Iron, Boron, Molybdenum: all are required in small amounts and classed as micro-nutrients.

Common Pitfalls:
Confusing the term “micro-nutrient” with “minor importance”; micro-nutrients are critical despite low required quantities.

Final Answer:
Carbon