Plant nutrients

Essential nutrients

A nutrient is called essential if (Arnon's criteria): the plant cannot complete its life cycle without it, its role cannot be replaced by another element, and it is directly involved in plant metabolism. Seventeen elements are accepted as essential.

• From air and water: carbon (C), hydrogen (H), oxygen (O).
• Primary macronutrients: nitrogen (N), phosphorus (P), potassium (K).
• Secondary macronutrients: calcium (Ca), magnesium (Mg), sulphur (S).
• Micronutrients: iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), chlorine (Cl), nickel (Ni).

Functions and deficiency symptoms

• N: part of proteins, chlorophyll and nucleic acids. Deficiency: yellowing (chlorosis) of older leaves first, stunted growth.
• P: energy transfer (ATP), root growth, flowering. Deficiency: purple or dark green leaves, poor roots.
• K: enzyme activation, water regulation through stomata, disease resistance. Deficiency: scorching of leaf margins of older leaves.
• Mg: central atom of chlorophyll. Deficiency: interveinal chlorosis of older leaves.
• S: part of some amino acids; important for oilseeds.
• Fe: chlorophyll formation. Deficiency: interveinal chlorosis of young leaves.
• Zn: khaira disease of rice. B: hollow heart and browning in cauliflower. Mo: whiptail of cauliflower; needed for nitrogen fixation.

Mobility rule: nutrients mobile in the plant (N, P, K, Mg) show symptoms on older leaves first; immobile ones (Ca, Fe, B, S) show them on younger leaves.

Soil analysis

Soil testing measures pH, electrical conductivity, organic carbon and available nutrients so fertiliser doses can be matched to need. Samples are taken from several spots in a zig-zag pattern, usually from 0–15 cm depth, mixed, and a composite sample is sent to the laboratory. Soil Health Cards give farmers these results with recommendations.

Key points

• 17 essential nutrients; N, P, K are primary.
• Mg is in chlorophyll; Mo is needed for nitrogen fixation.
• Khaira of rice = zinc deficiency.
• Mobile nutrients: symptoms on old leaves; immobile: on young leaves.
• Soil samples are composite samples from many spots.

Exam tips

• Learn "old leaf vs young leaf" symptoms; they appear in statement questions.
• Do not call C, H, O "minerals"; they come from air and water.