NPK fertilisers contain compounds of nitrogen, phosphorus and potassium. Explain why farmers use NPK fertilisers and describe how the nitrogen-containing compound in an NPK fertiliser is produced industrially.

NPK fertilisers are used because they supply the three essential nutrients – nitrogen, phosphorus and potassium – that plants need for growth, so they improve agricultural productivity and increase crop yield.
The nitrogen in an NPK fertiliser is produced industrially as ammonia (NH₃). Ammonia is made by the Haber process, in which nitrogen gas (N₂) and hydrogen gas (H₂) react:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The reaction is reversible, so unreacted N₂ and H₂ are recycled back into the reactor. Ammonia is then separated from the gas mixture and removed for use in the fertiliser.