Which of the following is a macro nutrient?

Concept: The mineral elements essential for plant growth are classified as macronutrients or micronutrients based on the QUANTITY the plant requires, not their importance. Macronutrients (for example nitrogen, phosphorus, potassium, calcium, magnesium, sulphur) are needed in relatively large amounts, usually measured in milligrams per gram of plant dry matter. Micronutrients, or trace elements (for example iron, manganese, zinc, copper, boron, molybdenum, chlorine, nickel), are needed only in very small amounts, usually measured in micrograms per gram (parts per million) of plant dry matter.

Application: Checking each of the four options against this classification, sulphur is one of the essential macronutrients required by plants in bulk quantity as a building block of proteins and other cell components. Manganese, chlorine, and iron, by contrast, are all classified as micronutrients — plants need only trace amounts of each.

• Sulphur is a macronutrient: it is required in bulk quantity because it is a structural component of the amino acids cysteine and methionine, and therefore of most plant proteins.

• Manganese is a micronutrient: it is required only in trace amounts, mainly as a cofactor for the water-splitting enzyme complex of photosynthesis.

• Chlorine is a micronutrient: it is required only in trace amounts to help regulate osmotic and ionic balance inside plant cells.

• Iron is a micronutrient: it is required only in trace amounts for chlorophyll synthesis and for electron-transport proteins.

Since sulphur is the only option among the four that a plant requires in bulk (macro) quantity, sulphur is the macronutrient — the correct answer.