Potassium (K)

An adult human body emits somewhere in the region of four to five thousand radioactive decays every second, and roughly ninety per cent of them come from potassium. Nothing about this is avoidable or unusual. Potassium is the most abundant positive ion inside your cells, a fixed fraction of it has been unstable since before the Earth formed, and no diet or environment changes that fraction.

One isotope in ten thousand, decaying three different ways

Potassium-40 makes up about 0.012% of natural potassium — roughly one atom in eight thousand — and has a half-life of about 1.25 billion years. That is short enough for most of the original supply to have gone, and long enough for a useful remainder to survive from the nucleosynthesis that preceded the solar system.

Its decay is unusually indecisive. Around 89% of the time it emits a beta particle and becomes calcium-40. Around 11% of the time it captures one of its own inner electrons and becomes argon-40 instead, releasing a characteristic gamma ray. A very small branch emits a positron. A single nuclide that decays to two different elements by two different mechanisms in comparable proportions is rare, and both daughters turn out to matter: calcium-40 is the reason that isotope dominates natural calcium, and argon-40 is the reason Earth has an argon-rich atmosphere.

Potassium-40 is also, alongside uranium and thorium, one of the significant contributors to the radiogenic heat that keeps Earth's interior molten and its plates moving.

The banana comparison is worse than it sounds

Bananas are frequently used to make radiation doses intuitive — the "banana equivalent dose" — and the arithmetic is real: a banana contains enough potassium to deliver a small, calculable exposure.

The comparison is still misleading, and the reason is physiological rather than nuclear. Total body potassium is homeostatically regulated within a narrow range. Eating a banana does not raise your potassium content for long; the kidneys excrete the surplus and the body returns to the same level it held before. The dose from a banana is therefore not additive in the way the dose from an X-ray is. Your potassium-40 burden is set by your body mass and composition, and it is essentially constant from one day to the next.

The effect is measurable in the opposite direction, though. Whole-body counters at nuclear facilities are sensitive enough that the gamma signature of a person's own potassium has to be subtracted as background, and shipments of bananas or of low-sodium salt substitutes have occasionally triggered radiation portal monitors at ports.

Davy danced

Potassium was the first metal ever obtained by electrolysis, in October 1807, a few days ahead of sodium. Humphry Davy passed a current through slightly moist caustic potash and small globules of bright metal appeared at the negative electrode, burst through the crust, and took fire.

His cousin and assistant Edmund Davy left a description of the reaction in the room. Davy, he wrote, danced about in ecstatic delight and needed some time to compose himself sufficiently to continue the experiment. It is one of the very few first-hand accounts of what a major chemical discovery felt like at the moment it happened.

Two words for ashes

The English name comes from potash, which is exactly what it describes: plant ashes leached with water in an iron pot and the solution boiled dry. This was a genuine frontier industry — clearing forest in colonial North America produced ash as a by-product, and potash was among the earliest chemical exports from the continent.

The symbol comes from kalium, via Latin from the Arabic al-qalyah, meaning calcined ashes. That same Arabic root gives English the word alkali. So the element's two names describe the identical substance in two languages, and one of them has independently become the general term for the entire chemical class that potassium belongs to.

The nutrient that never becomes part of the plant

Nitrogen ends up in proteins. Phosphorus ends up in DNA and ATP. Potassium, alone among the three macronutrients on a fertiliser bag, is not built into any biological molecule at all. It stays a free ion in solution, and everything it does is done in that state.

Its principal job is osmotic. Plants open and close the stomatal pores on their leaves by pumping potassium into and out of the two guard cells flanking each pore; the water that follows osmotically swells or slackens them, and the pore opens or shuts. A potassium-starved plant loses control of its own water balance, which is why deficiency shows up first as wilting and scorched leaf margins under drought stress rather than as stunted growth. Potassium also activates dozens of enzymes and balances the negative charge of the nitrate the plant takes up.

The cartel that broke in a single afternoon

Around 95% of mined potash goes into fertiliser, and production is geographically concentrated: Canada, chiefly the vast Devonian evaporite beds under Saskatchewan, together with Russia, Belarus and Germany.

For years two trading groups — Canpotex in North America and Belarusian Potash Company, a joint venture between Russia's Uralkali and Belaruskali — coordinated most seaborne supply, holding volumes back to hold prices up. On 30 July 2013 Uralkali announced it was leaving the venture and would sell at maximum volume instead. The potash price fell around 25% within days and the share prices of every listed producer worldwide fell with it. Belarus responded by arresting Uralkali's chief executive on a business trip to Minsk. It is one of the clearest natural experiments in what a commodity cartel is actually worth.

Sanctions imposed on Belarus after 2021, and then on Russia, removed a large fraction of exports again and sent prices to record highs in 2022.

The ocean chose sodium instead

Potassium and sodium are present in the continental crust in broadly comparable quantities. Seawater is not remotely balanced between them: it holds around thirty times as much sodium.

The difference is a matter of what happens to each ion once weathering releases it. Sodium is poorly retained by soil minerals and washes to the sea, where it stays. Potassium's ionic radius happens to fit neatly into the interlayer sites of clay minerals such as illite, which grip it and take it out of circulation, and plants take up much of the rest. Most potassium liberated from rock therefore never completes the journey. The ocean is salty with sodium chloride specifically because of a size mismatch in a clay lattice.

A narrow window in the blood

Potassium's biological indispensability comes with an unusually tight tolerance. The gradient across every cell membrane — high potassium inside, high sodium outside — is maintained by the sodium-potassium pump, which exports three sodium ions and imports two potassium ions for each molecule of ATP it consumes. Running that pump accounts for a large share of a resting body's energy expenditure, and in neurons a very large share indeed.

Because the resting electrical potential of a cell depends directly on the potassium ratio across its membrane, serum potassium outside a narrow range disturbs the electrical behaviour of cardiac muscle, and sufficiently severe deviation in either direction stops the heart. This is a plain property of the ion's role in membrane potential, and it is the reason potassium levels are among the first things measured in acute medicine.