Potassium chloride (0-0-60) is the most commonly used source of potassium. Potassium chloride is mined as sylvinite from underground mines or by solution mining in New Mexico, Utah, California, and Michigan. Granular forms of this material can be soil-applied, alone, or in dry bulk blends. Finely ground material is used in the production of 7-21-7 from a 10-34-0 base. Potassium chloride is the lowest cost potassium fertilizer available.
Potassium sulfate (0-0-50 +18S) is used as a potassium fertilizer source for some crops, like tobacco, potatoes, grapes and citrus, where the chloride content of potassium chloride is undesirable. Also, potassium sulfate contains 18 percent sulfur. Thus, it is a good fertilizer material to use when both nutrients are needed.
Potassium-magnesium sulfate (Sul Po Mag) (0-0-22 + 22S) is a double salt of potassium and magnesium sulfate. It is commonly used as a dry granular source of potassium, magnesium, and sulfur. The material is an effective source of sulfur and contains 22 percent sulfur in the sulfate form.