Mean content of potassium at 50 cm depth in mineral soil under forest, (expressed as mass percentage of potassium oxide)

Factfile

Metallic element (alkali metal) belonging to sub-group 1A in the periodic system. It is a soft, silver-white metal that reacts with air and violently with water. Potassium is the seventh commonest element in the Earth's crust (counting oxygen as the commonest). The metal was discovered and isolated in 1807 by Humphrey Davy in London.

It occurs in the Earth's crust mainly as a silicate. The mean concentration by mass of potassium in rocks in the Earth's crust is 2.10-2.59% K, in the oceans 380 ppm. Average abundance in igneous rocks is 2.5% K. To facilitate review of the main element in total geochemical analysis, in MarkInfo potassium is expressed as its oxide. The figure above thus corresponds to 3.1% K2O. With a composition corresponding to the Swedish average bedrock (mafic rocks making up 7-8% of the crystalline basement), the content by mass is 3.55% K₂O. The arithmetic mean of forest soils lies around 3.0-3.15%, showing that the potassium content of the regolith is somewhat underrepresented in relation to the composition of the underlying bedrock. A possible explanation for this is the relative effect that an increased quartz content in the regolith has on other elements in the calculation. A median value for 24000 analyses of regolith (mat <0.063 mm) is 2.88% K₂O (SGU).

In Swedish rocks, regoliths and soils, potassium occurs mainly as compounds with silicon (silicates). The dominant source of potassium is feldspars such as microcline and orthoclase. In Sweden, potassium occurs almost exclusively in gneisses and granites as microcline and potassium-bearing micas (muscovite and biotite) while orthoclase dominates Dala porphyries and alum shales.

When minerals weather, much less potassium than sodium is carried away by water, since the potassium ions are adsorbed and retained in the loose soil layer, particularly in clay minerals, much more strongly than sodium ions. However, under favourable crystallisation conditions potassium salts are accumulated in large amounts in some salt lakes and likewise in previously formed solid sedimentary deposits. When the water evaporates from the salt solution, this leads to the formation of evaporites, of which sylvite (KCl) and the double salts carnallite (KMgCl₃.6H₂O) and kainite (KMgClSO₄.3H₂O) are the most important. Sylvite is the principal ore mineral of potassium compounds. However, Sweden does not have potassium salt deposits.

Potassium compounds have been known in components of terrestrial plants for centuries. Combustion of wood and leaching of the solution produces potash, potassium carbonate (K₂CO₃). The difference between soda (Na₂CO₃) and potash was demonstrated in the 1700s, primarily by Andreas Sigismund Margraf in Berlin.