Potassium Facts:

Potassium is chemical element 19 on the periodic table and makes up to 3.5% of the earth crust. It is a balanced element and is second only to nitrogen in the quantities required by the plants. K plays a role in photosynthesis, enzyme activation, stomata control and transport of plant sugars.

Muriate of Potash (MOP)
Potassium chloride (commonly referred to as Muriate of Potash or MOP is the common potassium source used in agriculture, accounting for about 95% of all potash fertilizers used worldwide.
Keep in mind chloride reduces microbiological activity in the soil and at high levels can become toxic.
Its nutrient composition is approximately:
Potassium: 50%
Chloride: 46%

Sulfate of Potash (SOP)
Potassium sulfate is used on crops that are sensitive to chloride or fertilizer, or where sulfur is deficient.
Its nutrient composition is approximately:
Potassium: 41%
Sulfur: 18%

Potassium Magnesium Sulfate K2Mg(SO4)2
Potassium magnesium sulfate is an excellent source of three of the major nutrients - potassium, magnesium and sulfur. It is used mainly for high value crops, where all three of these nutrients are requied.
While it is 99.7% water soluble, the rate of solubility is slow and not recommended for application in crop irrigation systems.
Its nutrient composition is approximately:
Potassium:18%
Magnesium: 11%
Sulfur: 22%

Potassium Nitrate (KNO3)
Potassium nitrate, sometimes known as saltpeter, is often used in foliar sprays or fertilization because it is highly and quickly water soluble.
Potassium nitrate has application as a potassium source for crops which are highly sensitive to chloride. It is an oxidizing agent though which leads to rusting of metal elements .
Its nutrient composition is approximately:
Potassium: 38%
Nitrogen: 13%