Depending on the requirements, plant nutrients are classified as micronutrients and macronutrients. Which one of the following is an example of a macronutrient?

Correct — C, Magnesium. Plant nutrients are grouped by the quantity a plant needs, not by how essential they are. The macronutrients are carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulphur, and plants take them up in comparatively large amounts. Manganese, copper and chlorine are all needed in traces only and belong to the micronutrient list. Magnesium's best-known role explains why so much of it is required: it is the metal atom sitting at the centre of every chlorophyll molecule, so a plant cannot make its green pigment without it.

• (a)Manganese — A micronutrient, needed in trace amounts, where it activates enzymes and takes part in the oxygen-releasing step of photosynthesis. Its name is one letter's worth away from magnesium, which is the whole point of putting it first.
• (b)Copper — A micronutrient, a component of enzymes involved in electron transport. Plants need it in very small quantities, and an excess is toxic.
• (d)Chlorine — A micronutrient taken up as the chloride ion. It has a part in photosynthesis and in maintaining the water balance of the cell, but the amount required is minute.

Seventeen elements are considered essential for plants. Carbon, hydrogen and oxygen come from air and water; the rest come from the soil. Of those from the soil, nitrogen, phosphorus, potassium, calcium, magnesium and sulphur are wanted in large amounts and are called macronutrients, while iron, manganese, boron, zinc, copper, molybdenum, chlorine and nickel are wanted in trace amounts and are called micronutrients.

The classification is about quantity and nothing else, which is the point most often missed. A micronutrient is not a less important nutrient — a plant starved of boron or zinc fails just as surely as one starved of nitrogen; it simply needs far less of it. The examiner exploits a second confusion by placing manganese first in the list. Manganese and magnesium differ by one syllable and by their symbols, Mn against Mg, and only one of them is a macronutrient. A useful hook is that magnesium sits at the heart of chlorophyll, which a plant manufactures in bulk; deficiency shows up as yellowing between the veins of the older leaves, because the plant moves its scarce magnesium to the young growth.

• The macronutrients are carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulphur.
• The micronutrients are iron, manganese, boron, zinc, copper, molybdenum, chlorine and nickel.
• The distinction is by quantity required, not by importance — a micronutrient deficiency can be fatal to a crop.
• Magnesium is the central metal atom of the chlorophyll molecule.
• Magnesium deficiency shows as yellowing between the veins of older leaves, since the element is moved to new growth.

• Reading magnesium and manganese as the same element.
• Treating 'micro' as meaning less important rather than needed in smaller quantity.
• Forgetting that carbon, hydrogen and oxygen are macronutrients too, though they come from air and water rather than soil.

Asked as a one-from-four sorting item with three micronutrients and one macronutrient, and a near-homonym planted at the top of the list.

• practice — not a real PYQ

  The metal present at the centre of the chlorophyll molecule is

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

  Answer(b) Magnesium — iron is needed for chlorophyll synthesis but is not part of the molecule.

• practice — not a real PYQ

  Which one of the following is a micronutrient for plants?

  • (a)Potassium
  • (b)Sulphur
  • (c)Zinc
  • (d)Calcium

  Answer(c) Zinc — the other three are macronutrients.