Plant Nutrition
What this domain tests
The essential elements, criteria of essentiality, functions, deficiency and toxicity symptoms, nutrient mobility in the plant, antagonism and the choice between soil and foliar application.
Core rules
- Seventeen elements are essential: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulphur as macronutrients; iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel as micronutrients.
- Criteria of essentiality (Arnon and Stout): the element is required for completion of the life cycle, its function is specific and cannot be replaced, and it acts directly in plant metabolism.
- Mobile nutrients (N, P, K, Mg) show deficiency first on older leaves because the plant reallocates them; immobile nutrients (Fe, Mn, Zn, Cu, B, Ca) show it on young leaves.
- Signature symptoms: N - uniform pale yellowing of older leaves; K - scorching at leaf margins; Mg - interveinal chlorosis of older leaves; Zn - interveinal whitish bands and stunted young leaves in maize; B - brown heart, hollow stem, poor seed set; Fe - interveinal chlorosis of young leaves with green veins, common on alkaline black soils.
- Antagonism: excess potassium depresses magnesium uptake, excess phosphorus locks zinc, and high calcium or lime reduces iron and manganese availability.
- Hidden hunger means a nutrient is deficient without visible symptoms - only soil or tissue analysis detects it.
- Foliar sprays correct acute micronutrient deficiency; soil application or fertigation suits bulk macronutrients.
Worked example
Young maize leaves show interveinal chlorosis with green veins, older leaves stay normal, and the field is alkaline. Reasoning: symptoms occur on young leaves, so the nutrient is immobile; interveinal chlorosis with veins green narrows the cause to iron or manganese; alkalinity reduces iron availability sharply, so soil pH is the trigger. Conclusion: iron deficiency induced by high pH - spray ferrous sulphate at 0.5% with a little citric acid, and correct the alkalinity with organic matter and gypsum where sodicity is the cause.
Traps
Assuming any yellowing is nitrogen deficiency; ignoring which leaf age shows symptoms first; and treating a toxicity (marginal burn from excess boron or salt) as a deficiency.