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Material World
Application

Fertiliser

Supplying the elements crops remove from soil, chiefly nitrogen, phosphorus and potassium.

Harvesting a crop removes elements from the soil, and continuous cultivation depletes them. Fertiliser replaces the three that limit growth soonest: nitrogen, phosphorus and potassium, the N-P-K on every bag.

The three are obtained in completely different ways — nitrogen fixed from the air, phosphorus mined from rock, potassium mined from ancient evaporite beds — which means their supply risks have nothing in common with each other.

Medium confidence Weak evidence checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

uses

  • Hydrogen — element · via ammonia synthesis, which consumes about half of world production
  • Potassium — element · the K in N-P-K, mined as potash
  • Nitrogen — element · the N in N-P-K, fixed from the air by Haber-Bosch
  • Phosphorus — element · the P in N-P-K, and the one with no chemical substitute
  • Sulfur — element · as sulfuric acid, used to convert phosphate rock into soluble fertiliser
  • Sulfuric acid — compound · treating phosphate rock to make phosphoric acid and superphosphate
  • Pyrite — mineral · roasted for the sulfur that becomes sulfuric acid, most of which makes phosphate fertiliser
  • Peat — material · as the base of most potting compost, which is misleading — it is used for its water-holding and structure and supplies almost no nutrients at all

Sources

  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Fertiliser back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Fertiliser → uses (via ammonia synthesis, which consumes about half of world production) → Hydrogen → is produced by (the third product, often burned for process heat rather than sold) → Chlor-alkali electrolysis → takes as input (the largest single use of salt, and the process the whole chlorine and caustic soda industry rests on) → Salt → is produced by (in the solar route — the same process, read from the other end) → Brine evaporation → takes as input (the feedstock for solar salt and, in the salars, for the far stronger brines that lithium comes from) → Seawater
  • Fertiliser → uses (the K in N-P-K, mined as potash) → Potassium → is produced by (from potash brines, where the potassium salts are among the last to crystallise and so the last ponds in a series) → Brine evaporation → takes as input (and the pond sequence is run so that the less soluble salts crystallise out first, leaving the sodium chloride to be harvested on its own) → Salt → is sourced from (by solar evaporation, which needs a dry sunny coast and is the cheapest route there is) → Seawater → is composed of (about 96.5 per cent by mass — seawater is 3.5 per cent dissolved solids and the rest of it is this) → Water
  • Fertiliser → uses (the P in N-P-K, and the one with no chemical substitute) → Phosphorus → is produced by (white phosphorus, reduced from phosphate rock with coke and silica in an electric furnace and condensed under water) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
  • Fertiliser → uses (treating phosphate rock to make phosphoric acid and superphosphate) → Sulfuric acid → is produced by (absorbed into existing acid as oleum, then diluted — never added to water directly) → Contact process → takes as input (burned to sulfur dioxide, increasingly recovered from oil and gas rather than mined — and including the sulfur dioxide captured from ore roasting, which turns a smelter's principal pollutant into a feedstock) → Sulfur → is extracted from (historically the principal source of sulfur for sulfuric acid) → Pyrite
  • Fertiliser → uses (the N in N-P-K, fixed from the air by Haber-Bosch) → Nitrogen → is produced by (the most abundant component of air and the first to boil off) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
  • Fertiliser → uses (as the base of most potting compost, which is misleading — it is used for its water-holding and structure and supplies almost no nutrients at all) → Peat → is sourced from (waterlogging is the whole mechanism: a bog is anoxic below a few centimetres, so the organisms that would decompose the plant matter cannot work, and it accumulates instead) → Water → is sourced from (by desalination, where energy is cheap and coastline available — the only route that adds fresh water rather than moving it) → Seawater

These are the most distinct paths back. Fertiliser can be traced through others besides.