CDS (II) 2024 General Knowledge · Question 1 of 120

Correct — C, Magnesium. The other three appear on every standard list of plant micronutrients and magnesium does not. NCERT's table of nutrients supplied by the soil splits the thirteen soil-borne elements into six macronutrients — nitrogen, phosphorus, potassium, calcium, magnesium and sulphur — and seven micronutrients — iron, manganese, boron, zinc, copper, molybdenum and chlorine. Iron, manganese and chlorine, offered in options (a), (b) and (d), all sit in the second list; magnesium sits in the first. The division is about quantity, not importance: a plant needs magnesium in bulk because a magnesium atom occupies the centre of every chlorophyll molecule, while iron, manganese and chlorine do their work in trace amounts inside enzymes and electron-transfer complexes.

• (a)Iron — Iron heads NCERT's list of soil micronutrients. Plants need it only in traces, for chlorophyll synthesis and for the iron-sulphur proteins of the electron transport chains, and its shortage shows first as yellowing of the youngest leaves.
• (b)Manganese — Manganese is a micronutrient, and easy to confuse with magnesium because the two symbols look alike. Its best-known job is inside the water-splitting complex of photosystem II, where a manganese cluster takes electrons from water.
• (d)Chlorine — Chlorine is the one candidates most often doubt, but it is on NCERT's micronutrient list. Plants absorb it as the chloride ion and use it for osmotic and charge balance and in the same photosystem II reaction that releases oxygen.

Plants need about sixteen elements to complete their life cycle. Carbon and oxygen come from the air, hydrogen and oxygen from water, and the remaining thirteen come from the soil. Those thirteen are graded by how much of each the plant needs: macronutrients are required in large quantity and micronutrients in trace quantity. A micronutrient is not less essential — a plant starved of molybdenum dies as surely as one starved of nitrogen — it is simply needed in a far smaller dose.

The item is a list-recall test dressed as reasoning, and the fastest route is to hold NCERT's two rows in memory. The trap is that magnesium sounds like a minor metal and is one letter away from manganese, which really is a micronutrient. The way to break the tie is to remember what magnesium does: it is the metal atom at the heart of chlorophyll, so a green leaf needs it by the gram, not by the microgram. That single fact moves magnesium into the macronutrient row and settles the question.

• NCERT Class 9 Science lists the soil macronutrients as nitrogen, phosphorus, potassium, calcium, magnesium and sulphur.
• The same table lists the soil micronutrients as iron, manganese, boron, zinc, copper, molybdenum and chlorine.
• Air supplies carbon and oxygen; water supplies hydrogen and oxygen; the other thirteen essential elements come from the soil.
• Magnesium is the central metal atom of the chlorophyll molecule, which is why a plant needs it in macronutrient quantities.
• Ordinary N-P-K fertilisers supply only nitrogen, phosphorus and potassium, so micronutrients such as iron and zinc usually have to be added separately.

• Reading magnesium as manganese; the symbols Mg and Mn are one letter apart and only one of them is a micronutrient.
• Assuming micronutrient means less important — the grading is by quantity required, not by necessity.
• Forgetting that chlorine is on the micronutrient list, and rejecting it because chlorine is thought of as a disinfectant.

Usually as a which-one-is-not item on the macro/micro split, or as a deficiency-symptom match; the same list also drives questions on what fertilisers do and do not supply.

• practice — not a real PYQ

  Which one among the following elements is present at the centre of the chlorophyll molecule?

  • (a)Iron
  • (b)Magnesium
  • (c)Manganese
  • (d)Zinc

  Answer(b) Magnesium — the chlorophyll molecule is built around a central magnesium atom, which is why plants need magnesium in macronutrient quantities.

• practice — not a real PYQ

  Ordinary N-P-K fertilisers supply which one of the following sets of nutrients?

  • (a)Nitrogen, phosphorus and potassium
  • (b)Nitrogen, phosphorus and calcium
  • (c)Nitrogen, potassium and iron
  • (d)Phosphorus, potassium and sulphur

  Answer(a) Nitrogen, phosphorus and potassium — NCERT states plainly that fertilisers supply nitrogen, phosphorus and potassium, which is why other soil nutrients often have to be added separately.