Potassium is found throughout the plant, being needed for all activities that have to do with (water) transport. These include the opening and closing of the stomata (air holes in the leaf). Potassium also helps determine the sturdiness and quality of the plant and controls countless other processes, such as carbohydrate metabolism.
The Romans and the Etruscans enriched their soils by burning the local vegetation. This form of robbery has come to be used all over the world in subsequent centuries, with enormous levels of soil erosion as a result. In the Netherlands in the 1930s they began to use wood ash mixed with cattle manure, and this practice is still frequent today.
Potassium is a soft, silver-white metal that in its pure form reacts violently with air and water. 300 million years ago minerals such as potassium, sodium, and magnesium were eroded from the land and dissolved in the sea. The seawater in huge sea basins evaporated and the salts crystallised. This is how salt deposits such as those in Alsace in southwest Germany came into existence.
Around the turn of the century only kitchen salt was being reclaimed from the mined salt deposits. The excess potassium salt produced was dumped in the Rhine. Thanks to the introduction of inorganic fertilizers, these days in addition to sodium and potassium salts a whole range of other mineral salts are won from the mines, such as magnesium, sulphur (in the form of Epsom salts), phosphorus and boron.
When there’s a potassium deficiency the plant suffers from reduced transpiration (evaporation via the leaves). Thanks to this the temperature in the leaf rises too high, so that cells burn out. This happens primarily at the edge of the leaf, where normally the evaporation is the highest (edge disease).
Watch out: dead leaf edges can also be symptomatic of other causes of burning, such as the air being too hot, or salt burn!
It is effectively impossible to diagnose a potassium deficiency by visual symptoms alone. In general you can talk of there being a deficiency of something if around 10% of the element is missing from the level that is necessary in the plant tissues. The symptoms are visible above ground from changes in colour and dying off (necrosis).
Progression of deficiency in chronological order:
A well-constituted fertilizer contains sufficient potassium.