Learning objectives
- List macronutrients and micronutrients
- Match deficiency symptoms to the missing element
- Explain soil pH and calculate simple fertiliser rates
1. The essential elements
Macronutrients are needed in large amounts: nitrogen, phosphorus, potassium, calcium, magnesium and sulphur. Micronutrients are needed in traces: iron, manganese, zinc, copper, boron, molybdenum and chlorine.
- Nitrogen (N) — leaf and stem growth, chlorophyll and protein
- Phosphorus (P) — root development, flowering, energy transfer (ATP)
- Potassium (K) — water regulation, disease resistance, fruit quality
- Magnesium (Mg) — central atom of the chlorophyll molecule
2. Reading deficiency symptoms
Mobile nutrients (N, P, K, Mg) are moved from old leaves to new growth, so their deficiency symptoms appear on the OLDER, lower leaves first. Immobile nutrients (Ca, Fe, B) show on the YOUNGER, upper leaves first. This one rule answers most deficiency questions.
- N deficiency — uniform yellowing of older leaves, stunted growth
- P deficiency — purple or reddish tinge on older leaves, poor roots
- K deficiency — scorching and browning of leaf margins on older leaves
- Fe deficiency — yellowing between the veins of young leaves, veins stay green
3. Soil pH and availability
Most crops grow best at pH 6.0–7.0. Below pH 5.5 phosphorus becomes locked up and aluminium becomes toxic; above pH 7.5 iron, zinc and manganese become unavailable, causing lime-induced chlorosis.
Acid soil is corrected with agricultural lime (calcium carbonate); alkaline soil is lowered with elemental sulphur or acidifying fertilisers such as ammonium sulphate.
4. Fertiliser calculation you will be asked
A bag labelled 15-15-15 contains 15% N, 15% P₂O₅ and 15% K₂O by weight. To supply 60 kg N per hectare from 15-15-15: 60 ÷ 0.15 = 400 kg of fertiliser per hectare. Always show the division — the working carries marks.
Key definitions
- Macronutrient
- An element required by plants in relatively large quantities, e.g. N, P, K.
- Chlorosis
- Yellowing of leaf tissue due to insufficient chlorophyll production.
- Soil pH
- A measure of the acidity or alkalinity of soil, controlling nutrient availability.
Exam tips
- Always say WHERE the symptom appears (old or young leaves) — it distinguishes the nutrient.
- State units in every fertiliser calculation: kg/ha, not just a number.
Practice exam questions & model answers
Q1.A maize crop shows uniform yellowing of the lower leaves. Identify the deficiency and justify your answer.
Nitrogen deficiency. Nitrogen is mobile in the plant, so it is translocated from older lower leaves to young growing tissue, causing the older leaves to yellow uniformly first.
Q2.How many kilograms of 20-10-10 fertiliser are needed to supply 80 kg of nitrogen per hectare?
80 ÷ 0.20 = 400 kg of fertiliser per hectare.
Q3.Explain why phosphorus becomes unavailable in strongly acidic soils.
At low pH phosphate ions react with iron and aluminium to form insoluble compounds that plant roots cannot absorb, a process called phosphorus fixation.
Continue revising
Plant Cell Structure and Function
Plant BiologyPhotosynthesis: Process, Equation and Limiting Factors
BotanyLeaf Structure, Stomata and Transpiration
Plant PathologyPlant Pathology Basics: Pathogens, Disease Triangle and Koch's Postulates
Put the theory to work — diagnose a real leaf photo or browse the disease library.