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Fertilisers provide mineral ions that are needed for healthy growth in plants. As plants grow, they absorb mineral ions from the water in the soil through their root hair cells. Over time, the concentration of these ions decreases, so farmers and gardeners add fertilisers to the soil.

Meeting world food demand

The world demand for food is so high that it is difficult to supply natural fertilisers in large enough quantities. They are difficult to transport and their mineral content is variable.

Synthetic fertilisers can be manufactured and transported. The mineral content can be kept consistent.

Overuse of fertilisers

A major problem with the use of fertilisers occurs when rainwater washes them off the land into rivers and lakes. This causes an increase in the concentration of nitrate ions or phosphate ions in the water. Algae grow faster because of this, and form an algal bloom over the water surface. The bloom prevents sunlight reaching other water plants, which then die. Bacteria break down the dead plants. The bacteria respire and use up the oxygen in the water, so the lake may be left completely lifeless. This process is called eutrophication.

Nitrogen, phosphorus and potassium

Fertilisers may contain nitrogen, phosphorus and potassium compounds to promote plant growth. Fertilisers that supply all three elements are often called NPK fertilisers, after the chemical symbols for these three elements.

Fertiliser compounds must be soluble in water so they can be absorbed by the root hair cells:

• ammonium ions, NH₄⁺, and nitrate ions, NO₃^-, are sources of soluble nitrogen
• phosphate ions, PO₄^3-, are a source of soluble phosphorus
• all common potassium compounds dissolve in water to produce potassium ions, K⁺

The table shows some examples of fertilisers, their formula and the essential elements they provide.