Nitrogen, phosphorus, potassium, calcium, magnesium and sulphur are classified as the soil's:

Nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S) are called macronutrients, because plants need them in relatively large quantities for healthy growth.

Essential plant nutrients:

• Plants need 17 essential elements for growth.
• Three of them, carbon, hydrogen and oxygen, are obtained from air and water.
• The remaining 14 are obtained mainly from the soil and are divided into macronutrients and micronutrients.

Macronutrients:

Primary macronutrients: nitrogen, phosphorus and potassium (NPK). They are needed in the largest amounts and are most often deficient in soils, so they are the main components of fertilisers.

Secondary macronutrients: calcium, magnesium and sulphur. They are needed in moderate amounts.

Roles of the main macronutrients:

Nitrogen: essential for proteins, chlorophyll and leafy growth. Its deficiency causes yellowing of older leaves.

Phosphorus: important for root development, flowering, seed formation and energy transfer (ATP).

Potassium: helps in water regulation, disease resistance, and the quality of fruits and grains.

Calcium: needed for cell walls and root and shoot tips.

Magnesium: the central atom of the chlorophyll molecule.

Sulphur: a part of some amino acids and proteins; important for oilseeds.

Micronutrients:

They are needed in very small amounts, but are equally essential. They include zinc, boron, manganese, copper, molybdenum, chlorine and nickel, among others.

Zinc deficiency is the most widespread micronutrient deficiency in Indian soils, causing diseases such as "khaira" in rice.

Fertilisers:

• Urea supplies nitrogen; DAP supplies nitrogen and phosphorus; MOP supplies potassium; gypsum supplies calcium and sulphur.
• The Soil Health Card scheme tests soils for these nutrients and recommends balanced fertiliser use.

Liebig's law of the minimum:

Plant growth is limited by the nutrient that is in shortest supply, even if all others are abundant.

Why not the other options?

• ✗Micronutrients are needed only in trace amounts.
• ✗Heavy metals, such as lead, cadmium and mercury, are pollutants that can be toxic to plants and animals.
• ✗Rare earth elements are not plant nutrients; they are used in magnets and electronics.