For it turns out saltpetre is not the only rock we have used to help us grow our food. While nitrogen is by far the most important fertiliser, there are also two others: phosphorus and potassium. Together these three—NPK as they are often called after their chemical symbols—constitute the holy trinity of fertilisers, sprinkled and sprayed on fields around the world in their millions of tonnes to help grow the crops that feed the world’s 8 billion mouths.
