Potassium, also called potash, is mined from the soil like phosphate fertilizers. Being soluble in water, it is sometimes dissolved from the soil and handled as brine. Processing potassium ore, unlike phosphorus, is a matter of purification, not increasing solubility.
Muriate of potash (potassium chloride) (0–0–60) is by far the most common and, hence, least expensive form. Two other sources of potassium worth noting are potassium sulfate (0–0–50) and potassium nitrate (13–0–44). Potassium chloride is the most salty of the above. Also some plants, citrus for instance, are sensitive to excess chloride. Potassium sulfate is used primarily in crops that don’t tolerate the chloride.
Potassium nitrate is relatively expensive to make so is not used much. It is a good non-chloride potassium fertilizer and supplies nitrate nitrogen as well.
All potassium fertilizers easily dissolve in water and are immediately available to the plant. In use, they may be applied directly mixed into the soil, dissolved and applied as a liquid, or used in the manufacture of mixed fertilizers.