Fertilizers containing potassium must be applied wherever soil potassium reserves are inadequate for targeted crop or pasture production. Potassium can be applied as a straight fertilizer, or as part of a blended or compound fertilizer with nitrogen and phosphorous.
There are four common kinds of straight potash fertilizers:
Muriate of Potash (MOP)
Potassium Magnesium Sulfate
Each of the major potash sources are soluble in water and will readily dissolve when there is adequate rainfall or irrigation is applied. Under high rainfall or excessive irrigation, potassium can be lost in surface runoff or from the root zone by leaching - especially in permeable soils with a low cation exchange capacity.
Potassium chloride (commonly referred to as Muriate of Potash or MOP) is the most common potassium source used in agriculture, accounting for about 95% of all potash fertilizers used worldwide.
Potash fertilizers other than MOP are used where special crop or soil needs exist.
Potassium sulfate (referred to as Sulfate of Potash or SOP) is used in crops that are sensitive to chloride or fertilizer burn like tobacco, pineapple or avocado, or where sulfur is deficient.
It is used mainly for high value crops, where all three of these nutrients are required. It is also included in many 'complete' fertilizer products since it supplies several nutrients. While it is 99.7% water soluble, the rate of solubility is too slow for application in fertigation systems.
Potassium nitrate has application as a potassium source for crops which are highly sensitive to chloride, such as tobacco. It is an oxidising agent and should be handled and stored with care.