SOP fertilizer is sulphate of potash, or potassium sulphate (K2SO4). It contains about 50% potassium, shown as K2O, plus around 17 to 18% sulphur, and almost no chloride. That last point is why growers pay more for it than for ordinary potash.
Most of the world’s potash is sold as MOP, muriate of potash, which is potassium chloride. It is cheap and effective. But some crops dislike chloride. Tobacco burns badly, potatoes lose quality, and many fruits and vegetables suffer. For those crops, SOP is the potassium source of choice.
SOP at a glance
| Item | Detail |
|---|---|
| Chemical name | Potassium sulphate, K2SO4 |
| Potassium | About 50% K2O |
| Sulphur | About 17 to 18% S |
| Chloride | Very low |
| Grade | 0-0-50 (often written 0-0-50-18S) |
| Forms | Granular for soil use; soluble fine grades for fertigation and foliar feeding |
SOP vs MOP
| SOP (sulphate of potash) | MOP (muriate of potash) | |
|---|---|---|
| Potassium | About 50% K2O | About 60 to 62% K2O |
| Second nutrient | About 17 to 18% sulphur | Chloride |
| Best for | Chloride-sensitive and high-value crops | Most field crops |
| Price | Higher | The lowest-cost potassium source |
| World trade | A small, specialised market | One of the largest fertilizer markets |
MOP delivers more potassium per tonne for less money, so most field crops use it. SOP earns its premium where chloride would hurt quality or yield, or where the crop also needs sulphur. Read more in MOP fertilizer and potash vs potassium.
Which crops need SOP?
- Tobacco: chloride in the leaf hurts curing and burn quality. North Carolina extension guidance limits chlorine from fertilizer to about 20 to 25 lb per acre, which is why SOP or potassium nitrate is the usual potassium source. See best fertilizer for tobacco.
- Potatoes: chloride can reduce dry matter and processing quality.
- Fruit, vegetables and berries: many are chloride-sensitive and high in value, so the extra cost pays back.
- Tea, coffee and some tree crops: often grown with SOP or other low-chloride potassium sources.
- Saline soils and fertigation: SOP adds potassium without adding more chloride, and soluble grades suit drip irrigation.
How SOP is made
The U.S. Geological Survey describes potash as a group of potassium salts that includes MOP, SOP and potassium magnesium sulphate. SOP is produced in several ways:
- From natural brines: salt lakes are evaporated in solar ponds and the potassium sulphate is recovered. In the United States, one company processes brine from the Great Salt Lake by solar evaporation to produce SOP, according to the USGS.
- By reacting MOP with sulphuric acid (the Mannheim process), which gives SOP and hydrochloric acid.
- From potassium and magnesium sulphate minerals, such as langbeinite or kainite, by chemical conversion.
Who trades SOP?
SOP trade is small compared with MOP. In 2024, countries reported only around 0.6 million tonnes of SOP exports to UN Comtrade (HS 310430), against more than 30 million tonnes of MOP. Much SOP is used in the country where it is produced.
| Largest reported importers, 2024 | Thousand tonnes | Average import value (US$/t) |
|---|---|---|
| United States | 122 | 643 |
| South Africa | 121 | – |
| Australia | 92 | – |
| Peru | 90 | – |
| Mexico | 80 | – |
| India | 73 | 637 |
Those average import values, around US$640 per tonne, compare with roughly US$250 to 300 per tonne for MOP in the same year. That gap is the SOP premium in practice. For potash prices, see K2O fertilizer price, and for producers and country data, the potash manufacturers database.
Buying SOP in bulk
When you request SOP, specify:
- Minimum K2O (usually 50%) and sulphur content
- Maximum chloride, if your crop is sensitive
- Form: granular, standard or soluble grade
- Packaging: bulk, 50 kg bags or big bags
- Destination and delivery terms: FOB, CFR or CIF
Agromer supplies potash to importers, distributors and large growers. Tell us the grade, quantity, packaging and destination, and we will check what we can offer. See also our potash fertilizer page.
Frequently asked questions
What is SOP fertilizer?
SOP is sulphate of potash, or potassium sulphate (K2SO4). It contains about 50% K2O and 17 to 18% sulphur, with very little chloride.
What is the difference between SOP and MOP?
MOP is potassium chloride with about 60% K2O. SOP is potassium sulphate with about 50% K2O plus sulphur. SOP costs more and is used on crops that are sensitive to chloride.
Which crops need SOP?
Chloride-sensitive and high-value crops such as tobacco, potatoes, many fruits and vegetables, tea and coffee.
Why is SOP more expensive than MOP?
It is made in smaller volumes, often by more costly processes, and it serves a specialised market. In 2024, average SOP import values were around US$640 per tonne, about twice the level of MOP.
Is SOP an organic fertilizer?
SOP from natural sources, such as brines, can be approved for organic farming in some markets. Check your certification body’s rules for the specific product.
How is SOP made?
From salt lake brines by solar evaporation, by reacting MOP with sulphuric acid (the Mannheim process), or by converting potassium magnesium sulphate minerals.