Potassium (K)

Potash plumps up fruit, vegetables and grains, and the potassium it contains is an essential nutrient. Yet India is completely dependent on imports of this critical fertiliser to feed its population. Presenter Justin Rowlatt visits a farm on the Ganges plains to see how this mineral is used, and speaks to the head of the national importer Indian Potash Ltd about their efforts to promote its use by farmers. We also hear from Paul Burnside, analyst at CRU Group, how a bust-up in Belarus has helped turn potash into a global buyers' market. Meanwhile Prof Andrea Sella of University College London recreates everyone's favourite school chemistry experiment, with some unexpected consequences...

(Picture: Indian labourer carries bananas in Chennai; Credit: Arun Sankar/AFP/Getty Images)