Why plants need fertilisers
Plants make food from air and water, but they also need mineral nutrients from the soil. When crops are harvested, these minerals are taken away from the field. Fertilisers are substances added to the soil to put them back.
The three main nutrients are:
- Nitrogen (N): makes leaves, proteins and chlorophyll. Lack: yellow, pale leaves, stunted growth. Fertilisers: urea CO(NH₂)₂, ammonium nitrate NH₄NO₃, ammonium sulfate (NH₄)₂SO₄.
- Phosphorus (P): helps roots, flowers, seeds and energy transfer (ATP). Lack: purple tint, poor roots. Fertilisers: superphosphate, DAP.
- Potassium (K): helps water balance, enzymes, strong stems, fruit quality. Lack: brown leaf edges, weak stem. Fertilisers: potassium chloride KCl (muriate of potash), potassium sulfate.
A fertiliser label such as NPK 20-10-10 gives the percentage of N, phosphorus (as P₂O₅) and potassium (as K₂O).
The law of the minimum
A plant is limited by the nutrient that is in shortest supply, like a barrel that leaks from its shortest plank. Adding more of a nutrient that is already plenty does nothing. So farmers test the soil first and add only what is missing.
Soil acidity and liming
Soil pH shows how acidic or alkaline the soil is (pH 7 is neutral). Most crops grow best at pH 6 to 7.5. Rain, some fertilisers (like ammonium sulfate) and rotting plant matter can make soil acidic. In acidic soil some nutrients get locked, and metals like aluminium become harmful to roots.
Liming: adding lime (CaO), slaked lime Ca(OH)₂ or ground limestone CaCO₃ neutralises the acid and raises the pH. Very alkaline soil can be treated with gypsum or sulfur and organic matter. Test the soil pH with indicator paper or a pH meter first. See the pH scale and soil.
Good and bad effects of fertilisers
Good: higher yield, healthy crops, feeding more people from the same land.
Bad when overused: nitrates and phosphates wash off the field into ponds, rivers and wells. They make algae grow wildly (eutrophication); when the algae die, bacteria use up oxygen and fish die (see eutrophication). Nitrates in drinking water are harmful, especially for babies. Too much fertiliser burns roots. Soil life can also suffer.
Wise use: soil tests, right dose and time, mix with organic manure and compost, crop rotation with legumes (which fix nitrogen).
Biological role of elements
- C, H, O: the body of all organic matter (carbohydrates, fats, proteins).
- N: proteins, DNA, chlorophyll.
- P: DNA, ATP, bones and teeth (calcium phosphate).
- S: some amino acids (proteins).
- Ca: bones, teeth, cell walls, muscle and nerve action.
- K and Na: nerve signals and water balance.
- Mg: centre of chlorophyll.
- Fe: haemoglobin carries oxygen.
- Zn, Cu, I and others: tiny amounts for enzymes and hormones (iodine for thyroid).
Qualitative tests for ions
To know what a fertiliser or a salt contains, we test for ions. A few quick ones:
- Ammonium (NH₄⁺): warm with NaOH; a pungent gas that turns damp red litmus blue.
- Nitrate (NO₃⁻): brown ring test.
- Chloride (Cl⁻): add AgNO₃ and dilute HNO₃; white precipitate.
- Sulfate (SO₄²⁻): add BaCl₂; white precipitate insoluble in acid.
- Carbonate (CO₃²⁻): add acid; CO₂ fizz turns lime water milky.
- Phosphate (PO₄³⁻): yellow precipitate with ammonium molybdate.
More in Tests for ions.
Genetic links between classes of inorganic substances
Inorganic substances are related like a family tree:
- Metal → basic oxide → base → salt. Example: Ca → CaO → Ca(OH)₂ → CaCO₃ or CaCl₂.
- Non-metal → acidic oxide → acid → salt. Example: P → P₂O₅ → H₃PO₄ → Ca₃(PO₄)₂ or phosphate fertilisers.
- An acid and a base make a salt and water (neutralisation), which is exactly how fertiliser salts like ammonium nitrate are made: NH₃ + HNO₃ → NH₄NO₃.
Silicate materials
Glass, cement and ceramics are silicate materials made from sand and clay. They are explained in the carbon family lesson, glass, cement and ceramics.
Key formulas and definitions
- NPK label = % N : % P₂O₅ : % K₂O
- Urea: CO(NH₂)₂ (46 % N)
- NH₃ + HNO₃ → NH₄NO₃
- Liming: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂
- Metal → basic oxide → base → salt; non-metal → acidic oxide → acid → salt
Worked examples
1. A bag is labelled 10-26-26. What mass of nitrogen is in a 50 kg bag?
10 % of 50 kg = 5 kg of N.
2. Leaves of a crop are yellow and growth is slow. Which nutrient is most likely missing?
Nitrogen. It builds chlorophyll and leaves.
3. Why does lime help acidic soil?
Lime is basic. It reacts with the acid (H⁺) in the soil and raises the pH, so nutrients are released and aluminium becomes less harmful.
4. Find the % of nitrogen in urea CO(NH2)2. (C = 12, O = 16, N = 14, H = 1)
Molar mass = 12 + 16 + 2 × (14 + 2) = 60. N mass = 28. % N = 28 / 60 × 100 = 46.7 %.
Common mistakes
- Thinking more fertiliser is always better. Too much burns roots and pollutes water.
- Mixing up which nutrient does what: N leaves, P roots and flowers, K stem and fruit.
- Adding lime to soil that is already alkaline.
- Forgetting that nutrients can be in the soil but locked by wrong pH.