Salt: the mineral that organised trade routes

Humans require sodium, and diets based heavily on plant foods do not supply enough. Populations eating a lot of meat get sufficient sodium from it; agricultural populations generally do not.

Which means that the adoption of agriculture created a requirement for a mineral that is available only in specific places.

Where it comes from

Sea salt, produced by evaporating seawater in shallow ponds. Requires a coastline, sufficient sun and wind, and a dry season. Salt pans are among the oldest continuously operated industrial installations anywhere.

Rock salt, mined from deposits left by ancient evaporated seas. Some mines have operated for many centuries.

Brine springs, where salty groundwater emerges and is evaporated, historically by boiling — which requires enormous quantities of fuel and has driven local deforestation in several regions.

Salt lakes and desert flats, where salt can be cut directly from the surface.

Plant sources, used in regions without any of the above — burning particular plants and leaching the ash, which yields a salt that is not primarily sodium chloride but serves the purpose.

Preservation is what made it critical

Beyond dietary need, salt was the primary means of preserving food before refrigeration.

Salted and dried fish provisioned ships, armies and cities, and it allowed fisheries to serve markets far inland. Salted meat did the same. Salt is essential to most cheese, to fermented vegetables, and to cured products of every kind.

Which means salt was not a condiment but an input to the food system, required in quantity, on a schedule, at the point of harvest or slaughter.

A community that could not obtain salt in autumn could not preserve its winter food.

The trade

Salt's uneven distribution and universal necessity made it an ideal trade good.

Trans-Saharan trade routes carried salt south from desert deposits and gold north, and the exchange sustained substantial commercial networks for centuries. Salt caravans crossing the Sahara continued into the modern period and a small number still operate.

Salt roads across Europe, Asia and elsewhere are traceable in place names, in the routes of old roads, and in the towns that grew at points where salt was transhipped or taxed.

In several regions salt functioned as currency, and the word for salary in English derives from a Latin term connected with salt payments, though the exact historical arrangement behind it is debated.

Taxation and revolt

A commodity everyone must buy, produced in a small number of controllable locations, is an obvious target for taxation, and states have taxed salt for as long as there have been states.

Salt monopolies and taxes have been imposed across an enormous range of societies. They are efficient for the state and regressive in effect, since consumption does not scale with income.

They have also produced resistance repeatedly. Salt taxes have been a component in several major uprisings, and the deliberate breaking of a salt monopoly has served as a potent political act precisely because everyone understands what salt is and why the monopoly is objectionable.

Smuggling of salt was correspondingly widespread wherever the tax differential made it worthwhile, with an associated apparatus of enforcement, informers and penalties.

The types and whether they matter

Culinary salt varies in mineral content, crystal structure and moisture, and these affect how it behaves.

Coarse and flaky salts dissolve differently and are used as finishing salts partly for texture. Salts with residual minerals from their source have slightly different flavour, and the differences are smaller than the marketing suggests.

What genuinely differs is density: a spoonful of fine salt contains considerably more salt than a spoonful of flakes, which matters when following a recipe written for the other kind.

Traditional salts produced by artisanal methods command high prices, and the argument for them is partly flavour and partly the survival of the production method and the communities that operate it. Both are reasonable and neither requires believing that the mineral content transforms a dish.

Iodine

Iodine deficiency causes goitre and, more seriously, developmental problems in children. It is a public health issue in regions where soils are low in iodine, which includes many mountainous and inland areas.

Adding iodine to salt is one of the more effective public health interventions of the twentieth century, precisely because salt is universally consumed in roughly predictable quantities.

The programme's success means the underlying deficiency is invisible to most people, and the shift toward specialty non-iodised salts in some markets has produced discussion about whether that matters. In populations with varied diets it generally does not; in populations relying on salt as the main source it does.

What is left of the traditional production

Industrial salt production is enormously cheaper, and most salt is now produced for industrial rather than food use.

Traditional salt pans survive where the product commands a premium, where the landscape has heritage protection, or where the salt works also function as important habitat — several historic salt pan complexes are now significant wetland reserves, which is an unexpected second life for an industrial site.