Potassium is slightly less common than sodium in igneous rocks, but potassium is more abundant in all of the sedimentary rocks. This points up the very different behavior of these two alkali metals in natural systems.
Sodium tends to remain in solution rather persistently once it has been liberated from silicate mineral structures. Potassium is liberated with greater difficulty from silicate minerals and exhibits a strong tendency to be reincorporated into solid weathering products, especially certain clay minerals. In most natural waters, the concentration of potassium is much lower than the concentration of sodium.
The principle potassium minerals of silicate rocks are the feldspars, and the micas. In sediments the potassium commonly is present in unaltered feldspar or mica particles or in illite or other clay minerals. Evaporite rocks locally include beds of potassium salts and constitute a source of high potassium concentration in brines.
Although geologic areas where the potassium content of rocks is greatly in excess of the sodium content are not unusual, there are a few fresh waters in which the potassium concentration nearly equals or even exceeds the sodium concentration. In the more dilute waters where sodium contents are below 10 mg/L, the potassium concentration may commonly be a half or a tenth that of sodium. Potassium concentrations of more than a few tens of mg/L, however, are decidedly unusual, except in water with veryhigh dissolved solids concentrations or in water from hot springs.
The rather narrow range of concentration of potassium observed in natural water suggests a significant chemical control mechanism may be involved. Two general principles which have been referred to are the resistance to solution exhibited by potassium feldspar, and the apparent preferential incorporation of potassium into clay or mica minerals. The potassium ion is substantially larger than the sodium ion and it normally would be absorbed less strongly than sodium in ion exchange reactions.
In addition to trace amounts of a long list of other elements necessary for plant growth, three important nutrients are required; potassium, nitrate and phosphate. Potassium is always present in considerable amounts in the most unmineralized waters. This leaves nitrogen and phosphorus as the two plant nutrient elements which are most likely to be important.
Potassium seems to be rarely, if ever, a factor in limiting algal growth even in still water. However, phosphate and also sometimes nitrate, silica and even sulfate are in short supply in lakes.
Hynes (1971) states that we know nothing of any possible influence of the
relative shortage of potassium or sulfate on algal growth in running water,
and it seems unlikely that a shortage would occur except under very
Potassium acts as a cathartic. A dose of 1 to 2 grams of potassium is cathartic. The extreme limit of potassium permissible in drinking water is regarded as 1000 to 2000 mg/L. Potassium in low concentrations in irrigation waters is essential for plant nutrition, but for good plant development it must be maintained in proper balance with other mineral requirements such as phosphorus. In irrigation water, potassium and sodium act alike upon the soil although potassium is considered less harmful than sodium.
The toxicity of potassium to fish is reduced by calcium and, to a lesser degree; by sodium. Potassium is more toxic to fish and shellfish than calcium, magnesium or sodium. It has been observed that potassium stimulates plankton growth in some lakes. The content of potassium in lakes is reported to vary from 0.4 to 1.5 mg/L in oligotrophio and mesotrophic lakes, and it runs as high as 5 to 6 mg/L in very eutrophicn lakes.