Potassium in Fertilisers

Fertilisers containing potassium must be applied wherever soil potassium reserves are inadequate for targeted crop or pasture production.  Potassium can be applied as a straight fertiliser, or as part of a blended or compound fertiliser with nitrogen and phosphorous.

There are four common kinds of straight potash fertilisers:

Muriate of Potash (MOP) 
Sulfate of Potash (SOP)
Potassium Magnesium Sulfate 
Potassium Nitrate

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 is not lost to the atmosphere, as can occur with some nitrogen fertilisers under certain soil and environmental conditions.

Potassium can become unavailable if it is fixed by expanding clay minerals, but will be slowly re-released if soil exchangeable potassium levels fall.

Muriate of Potash (MOP)

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 fertilisers used worldwide.

Its nutrient composition is approximately:

Potassium: 50%
Chloride: 46%

MOP has a high nutrient concentration and is therefore relatively price competitive with other forms of potassium.  The chloride content of MOP can also be beneficial where soil chloride is low. Recent research has shown that chloride improves yield by increasing disease resistance in crops.  In circumstances where soil or irrigation water chloride levels are very high, the addition of extra chloride with MOP can cause toxicity. However, this is unlikely to be a problem, except in very dry environments, since chloride is readily removed from the soil by leaching.

Sulfate of Potash (SOP)

Potash fertilisers 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 fertiliser burn like tobacco, pineapple or avocado, or where sulfur is deficient.

Its nutrient composition is approximately: 

 

          Potassium: 41% 

          Sulfur: 18%

Potassium Magnesium Sulfate

Potassium magnesium sulfate is an excellent source of three of the major nutrients - potassium, magnesium and sulfur.

Its nutrient composition is approximately:

Potassium: 18%
Magnesium: 11% o
Sulfur: 22%

It is used mainly for high value crops, where all three of these nutrients are required. It is also included in many 'complete' fertiliser 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

Potassium nitrate, sometimes also known as saltpetre, is often used in foliar sprays or fertigation because it is highly and quickly water soluble. 

Its nutrient composition is approximately:

Potassium: 38%
Nitrogen: 13%

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.

Move on for more information about:

Production of Potash Fertilisers >>

Methods of Fertiliser Application >>

Potash in Organic Agriculture >>