Plant Nutrition
What the domain tests
Plant Nutrition tests the essential elements, their functions, deficiency symptoms and the principles of nutrient uptake. You should be able to separate macro from micronutrients and explain why a symptom appears on old or young leaves.
Core rules
- Seventeen elements are essential: C, H, O, N, P, K, Ca, Mg, S, Fe, Mn, Zn, Cu, B, Mo, Cl and Ni.
- Primary macronutrients: N, P, K. Secondary: Ca, Mg, S. Micronutrients: Fe, Mn, Zn, Cu, B, Mo, Cl, Ni.
- N builds proteins and chlorophyll; P is involved in energy transfer and root development; K regulates water relations, enzyme activity and quality.
- Mobile nutrients (N, P, K, Mg) move from old to young tissue, so deficiency appears first on older leaves. Immobile nutrients (Ca, Fe, B) show symptoms on young tissue.
- Liebig's law of the minimum: growth is limited by the nutrient in shortest supply. Luxury consumption is uptake above need without yield gain.
- Critical concentration is the tissue level below which deficiency occurs.
Worked example
A crop shows uniform yellowing of older leaves, while young leaves remain green. Reasoning: the symptom is on older leaves, so the nutrient is mobile. Nitrogen is the most likely cause because it is mobile and involved in chlorophyll. Confirm with soil or tissue test before applying urea; check for waterlogging, which can also cause yellowing by root damage.
Traps
Do not say micronutrients are unimportant because they are needed in small amounts. Do not diagnose every yellow leaf as N deficiency. Fe deficiency is interveinal chlorosis on young leaves, not uniform old-leaf yellowing. Boron toxicity can occur from over-application.