Agricultural input

Three elements limit plant growth almost everywhere — nitrogen, phosphorus and potassium — and the materials here are overwhelmingly the means of supplying them. Each has a different constraint: nitrogen is fixed from air at great energy cost, phosphorus is mined and has no substitute, potassium is mined and abundant.

Phosphorus is the one to worry about. It cannot be synthesised, comes from a small number of rock deposits, and is not recovered from the food system in any quantity — which makes it the clearest case of a material that is genuinely finite in a way that energy and most metals are not.

• Mineral

  Apatite

  as phosphate rock, the feedstock for phosphate fertiliser

• Rock

  Limestone

  ground limestone raises soil pH

• Material

  Peat

  as the base of most potting compost — for its water-holding and its structure rather than its nutrients, of which it has almost none

• Element

  Phosphorus

  as phosphate; irreplaceable rather than merely important

• Element

  Potassium

  The third of the three elements every fertiliser is measured by, mined as ancient evaporated seawater.

• Compound

  Quicklime

  spread to raise soil pH, which is one of the oldest deliberate soil treatments there is

• Mineral

  Sylvite

  as potash, the potassium of NPK fertiliser

Each of these uses is established and cited on its own. Whether something counts as an ore, for instance, depends on grade, price and era — and those qualifications are shown alongside it.