Plant Nutrition
Plant nutrition questions test the list of essential elements, the criteria that make an element essential, and your ability to read a deficiency symptom from the leaf position and pattern.
Core rules
- Seventeen essential elements: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, sulphur, iron, manganese, zinc, copper, boron, molybdenum, chlorine, nickel.
- Criteria of essentiality: the element is indispensable for a complete life cycle, has a direct function not replaceable by another element, and its effect must be direct rather than a side effect on soil or microbes.
- Macronutrients: N, P, K, Ca, Mg, S. Micronutrients: Fe, Mn, Zn, Cu, B, Mo, Cl, Ni.
- Mobile elements (N, P, K, Mg) move to young tissue, so deficiency shows first on older leaves. Immobile elements (Ca, S, Fe, B) cannot move, so deficiency shows on young leaves.
- Key symptoms: N - general yellowing of older leaves; P - purple/dark green leaves; K - scorching at leaf margins; Zn - khaira (white/yellow bands) in wheat and rice; B - brown heart or hollow stem in cauliflower; Fe - interveinal chlorosis of young leaves with green veins; Ca - blossom end rot in tomato.
- Law of the minimum (Liebig): yield is limited by the scarcest nutrient. Law of diminishing returns explains why extra fertilizer gives smaller and smaller gains.
Worked example
Sugarcane shows interveinal chlorosis on the youngest leaves while veins stay green; older leaves are normal. Diagnose and correct. Reasoning: because young leaves are affected, the deficient element is immobile; the pattern is interveinal chlorosis typical of iron. Iron is unavailable in high-pH or waterlogged soils, so first improve drainage and then spray 0.5 per cent ferrous sulphate with a little citric acid. Nitrogen would have yellowed older leaves first.
Traps that cost marks
- Reading every yellowing as nitrogen deficiency.
- Forgetting nickel and chlorine in the list of 17.
- Confusing the law of the minimum with the law of diminishing returns.