Fertilisers

Fertilisers are substances added to soil to give plants essential elements they need to grow well. Plants make their own sugar by photosynthesis, but they still need minerals from the soil. Farming and gardening remove minerals from soil, so fertilisers replace them.

The big three: N, P, K

• Nitrogen (N): needed to make proteins and chlorophyll. Encourages green, leafy growth.
• Phosphorus (P): helps strong roots, healthy seeds, and energy transfer in cells (ATP). Supports early growth.
• Potassium (K): helps enzymes work, improves flowers and fruit, and supports disease resistance and water balance.

Common fertiliser compounds

Cambridge IGCSE expects you to know that ammonium salts and nitrates are widely used as fertilisers. They dissolve in water to give ions that plant roots can absorb.

• Ammonium nitrate: $\text{NH}_4\text{NO}_3$ provides nitrogen as $\text{NH}_4^+$ and $\text{NO}_3^-$.
• Ammonium sulfate: $(\text{NH}_4)_2\text{SO}_4$ provides nitrogen as $\text{NH}_4^+$.
• Potassium nitrate: $\text{KNO}_3$ provides potassium $\text{K}^+$ and nitrate $\text{NO}_3^-$.
• Phosphate fertilisers supply phosphorus (e.g. compounds giving $\text{PO}_4^{3-}$ ions).

NPK fertilisers (mixed fertilisers)

NPK fertilisers are mixtures designed to supply all three key elements: nitrogen (N), phosphorus (P), and potassium (K). A bag might be labelled “NPK 15–15–15” to show a balanced mix. Different plants or seasons may need different ratios (for example, more N for leafy lawns, more P for root growth).

Environmental points and wise use

• If too much fertiliser is used, nitrates and phosphates can wash into rivers and lakes. This can lead to extra plant growth in water and deoxygenation, which harms aquatic life.
• Apply the right amount, at the right time, and avoid spreading before heavy rain.

Common misconceptions

• Fertilisers are not “food” for plants; they supply minerals. Plants make sugar using light, water, and carbon dioxide.
• More fertiliser is not always better; excess can damage plants and pollute water.
• Compost and manure add nutrients and improve soil, but NPK fertilisers give precise amounts of N, P, and K.