Plant nutrients — Which of the following is NOT classified as a secondary nutrient for plant growth?

Introduction:
Essential plant nutrients are grouped by the quantities required: primary (N, P, K), secondary (Ca, Mg, S), and micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl, Ni). Differentiating among these categories is fundamental to fertilizer formulation and soil testing.

Given Data / Assumptions:

• Primary macronutrients: nitrogen, phosphorus, potassium.
• Secondary macronutrients: calcium, magnesium, sulphur.
• Non-mineral elements like carbon, hydrogen, oxygen are obtained largely from air and water.

Concept / Approach:
Secondary nutrients are supplied via soil amendments and fertilizers when deficient. Oxygen is essential for life but is not classified as a mineral plant nutrient in fertilizer practice; plants obtain O (and H) primarily from water and CO2, not as a separate fertilizer nutrient category.

Step-by-Step Solution:
List secondary nutrients: Ca, Mg, S.Compare with options; identify oxygen as not on the secondary list.Select oxygen as the correct “NOT” answer.

Verification / Alternative check:
Agronomy guides and fertilizer labels enumerate secondary nutrients explicitly as Ca, Mg, and S.

Why Other Options Are Wrong:

• Calcium, magnesium, sulphur are standard secondary nutrients.
• “None” is incorrect because oxygen is not in that category.

Common Pitfalls:
Confusing essentiality with classification; all 17 essential elements are necessary, but categories depend on required amounts and fertilizer practice.

Final Answer:
Oxygen (O)