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 car

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