What is Sulphate of Potash (SOP)?


Sulphate of Potash (SOP) is an essential mineral fertiliser for food supply, providing plants with the essential nutrients of potassium (K) and sulphur (S).  It can also be known as Potassium Sulphate and has the chemical formula K2SO4


It is predominantly used as a fertiliser for global food production and is often considered the fourth macro nutrient alongside potassium, nitrogen (N) and phosphorus (P).


What does Sulphate of Potash (SOP) do?

SOP is a mineral fertiliser and a valuable tool in crop management by positively influencing crop yield and quality.  SOP nutrients play an important role in the development of proteins, enzymes and vitamins, as well as improving plant photosynthesis and growth.


It improves nutritional value, taste and appearance (size, colour, and scent), fruit’s resistance to deterioration during transport and storage and its suitability for industrial processing.  SOP can improve the uptake of phosphorus, iron, and other micronutrients and helps the plant to be more resistant to drought, frost, insects and many diseases.  In sandy soils it can also reduce leaching of cations such as calcium and potassium.


Why is Sulphate of Potash (SOP) important?

SOP has little to no chloride and a low Salt Index (SI) when compared to alternatives, such as muriate of Potash (MOP) which contains 45% chloride, making it particularly important in semiarid regions with salinity problems and for those crops sensitive to chloride such as;


  • fruits (almonds, apricots, avocados, bananas, citrus, grapes, mangoes, peaches),

  • berries (including strawberries),

  • vegetables (lettuces, onions, sweet peppers),

  • field crops (potatoes, tobacco),

  • coffee, and

  • flowers.

In these crops, SOP cannot be substituted with MOP, making demand inelastic. 

Do we produce Potash in Australia?

No, currently there is no commercial production of either muriate of potash (MOP) or sulphate of potash (SOP) in Australia and all our SOP and MOP needs are imported. 

How is Sulphate of Potash produced?

There are three principle methods of producing SOP:

1.    Natural Brines  

About 35% of the world’s SOP production is produced from natural brines via solar evaporation.  It is a relatively low-cost primary production method, although there are a limited number of operations where geological endowment, logistics and environmental factors align.  The weighted-average cost of production for primary producers is approximately US$260/t.

Natural brines, such as that being explored for by Trigg Mining, are the only source of SOP to be certified as organic.  For organic certification SOP must come from natural sources with little or no processing, such as solar evaporation.

2.    Sulphate Salts Reaction

Potassium chloride (MOP) can be reacted with various sulphate salts, such as sodium sulphate, to produce a double salt and then decomposed to yield potassium sulphate (SOP).  Around 20% of the global SOP production comes from this process.

3.    Mannheim Process
With little over one half of the world’s SOP requirements produced from brines (35%) and reacted salts (20%) a synthetic process is required to meet total demand.  The process to synthetically manufacture SOP is known as the Mannheim process. In this method,  potassium chloride (MOP or KCl), is reacted with sulphuric acid (H
2SO4) by heating in a furnace at temperatures of approximately 800oC to produce SOP and hydrochloric acid (HCl).  A secondary process is then required to produce a soluble grade product.

The weighted average cost of production for secondary producers in 2017 was approximately US$320/t and is highly dependent on the input costs of MOP and sulphuric acid. 

Does SOP attract a price premium?

Given the inelastic nature of the demand for SOP and its exceptional qualities, produce growers are willing to pay a higher price for SOP and it effectively serves a separate market to MOP, sustaining a price premium over MOP.  Price premiums are further supported by the higher cost of manufacturing SOP in the Mannheim Process (see below). 


For example on 23 October 2019 the stated landed pricing at the Port of Kwinana in Western Australia was $1,049/t for SOP and $638/t for MOP.

Does SOP have an impact on soil Acidity?

SOP has a neutral effect on soil pH is the preferred potassium fertiliser for acidic soils. 


In Western Australia soil acidity is a major constraint to farming outputs. Extensive surveys of soil pH profiles across the south-west estimate 14.25 million hectares of wheatbelt soils to be acidic or at risk of becoming acidic to the point of restricting production. 

Does SOP have an impact on soil salinity?

SOP has a salt index of 43 vs MOP with 45% chloride has a a salt index of 120 and therefore Australian soils at risk of rising salinity would benefit from having access to a domestic supply of SOP.


In Western Australia the opportunity cost of lost agricultural production from dryland salinity has been calculated to be at least $519 million per year since 2009/10.   

Trigg Mining Limited
Level 1, Office E
1139 Hay Street
West Perth  Western Australia  6005
Office: +61 8 6114 5685
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