Polyhalite vs sulphate of potash: which potash should you use?
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Potash · soil amendments
Polyhalite vs sulphate of potash: which potash should you use?
By Joe, Founder of Dr Forest · May 2026 · 9 minute read
Two of the cleanest potash options a UK gardener can buy. They look similar on the label, behave very differently in the soil, and answer different questions.
The short answer: use polyhalite as a slow base feed and sulphate of potash as a fast top-up. Polyhalite is a four-nutrient mineral (potassium, calcium, magnesium, sulphur) that releases over months, which suits pre-plant beds, perennials, and containers planted for the season. Sulphate of potash is concentrated near-pure potash that works within days, which suits fruiting and flowering peaks. Both supply potassium as the sulphate ion, so both are safe for chloride-sensitive crops. If you only buy one for a mixed garden, choose polyhalite.
Both polyhalite and sulphate of potash deliver potassium in the same chemical form (the sulphate ion, K₂SO₄ chemistry), which is why both suit chloride-sensitive crops like potatoes, tomatoes, and most soft fruit. Sulphate of potash is chloride-free; polyhalite is very low in chloride. Beyond that they diverge. Polyhalite is a four-nutrient mineral that drips its potassium out slowly over months. Sulphate of potash is a concentrated single-purpose feed that supplies potassium and sulphur fast and finishes quickly.
The right choice depends on whether you want a slow background mineral that also brings calcium, magnesium, and sulphur (polyhalite), or a faster, more concentrated dose of pure potash for fruit and flower (sulphate of potash). Most established gardens benefit from having both on the shelf for different jobs.
In short, polyhalite
A slow, four-nutrient sulphate mineral mined in North Yorkshire. 14% K₂O, 17% CaO, 6% MgO, 48% SO₃. Good as a base or pre-plant amendment.
In short, sulphate of potash
A fast, concentrated single-nutrient feed. About 50% K₂O and 45% SO₃, sulphate-form. Good as a top-up during fruiting and flowering.
What is polyhalite?
Polyhalite is a naturally occurring hydrated sulphate mineral with the formula K₂Ca₂Mg(SO₄)₄·2H₂O. Mined from beneath the North Yorkshire moors, it is a 260-million-year-old evaporite that formed when ancient seas dried out. The mineral is crushed and screened (no chemical processing, no roasting, no synthetic step) and bagged as a granular fertiliser. [1] There's more on how it forms and behaves in the soil in our guide to what polyhalite is.
Because polyhalite is a single mineral salt, every granule carries the same four nutrients in fixed proportions: potassium (as K₂O), calcium (as CaO), magnesium (as MgO), and sulphur (as SO₃). All are present in sulphate form, which is the same chemistry as sulphate of potash and Epsom salt. Polyhalite is very low in chloride, low in sodium, and approved for organic use under the EU and UK regulations for organic farming. [2]
The release pattern is what separates polyhalite from concentrated salts. Independent field trials carried out under UK conditions show polyhalite releasing its potassium over several months, more slowly than sulphate of potash and more steadily than muriate of potash. [3, 4] That makes it suit pre-plant incorporation, perennial beds, and any container that's been planted for the season rather than as a quick rescue feed.
Figure 1 · nutrient composition
Polyhalite carries four nutrients in one granule; sulphate of potash carries one and a half
Headline values for each amendment, oxide convention, by weight.
What is sulphate of potash?
Sulphate of potash, often abbreviated SOP, is potassium sulphate (K₂SO₄). It contains about 50% K₂O and 45% SO₃ (18% sulphur as the element). Three production routes are used worldwide: direct mining of natural langbeinite or kainite deposits, conversion of muriate of potash via the Mannheim process (a reaction with sulphuric acid), and solar evaporation of mineral-rich brines such as the Great Salt Lake. The organic-approved grades come from the mined and solar routes. [5]
Compared with the cheaper muriate of potash (potassium chloride), sulphate of potash carries no chloride. That matters for chloride-sensitive crops where excess chloride suppresses the uptake of nitrate or causes leaf burn at higher rates. Affected crops include potatoes, tomatoes, soft fruit, most berries, tobacco, and many ornamentals. [6] It also supplies sulphur, a nutrient that's become quietly deficient in many UK soils as atmospheric sulphur deposition has fallen since the 1980s. [7]
SOP is sold as a fine to medium granule for soil application, a soluble grade for fertigation and foliar use, and as a liquid suspension. The fast release is the defining property: dissolved K⁺ is plant-available within days of watering in, which is why SOP is the workhorse of commercial tomato and fruit growing. The trade-off is that nothing slows the salt down. A heavy single dose pushes the salt index up and can scorch tender roots in close contact.
Side-by-side: nutrient content and release
The cleanest way to read the difference is on a per-bag basis. The same kilogram of granules carries quite different totals depending on which mineral you're holding.
| Property | Polyhalite | Sulphate of potash |
|---|---|---|
| Mineral form | Natural hydrated quadruple-salt, K₂Ca₂Mg(SO₄)₄·2H₂O | Potassium sulphate, K₂SO₄ |
| Potassium (K₂O) | 14% | 50% |
| Sulphur (SO₃) | 48% | ~45% |
| Calcium (CaO) | 17% | n/a |
| Magnesium (MgO) | 6% | n/a |
| Chloride | Very low | None |
| Release pattern | Gradual, weeks to months | Rapid, days to weeks |
| Salt index | Low | Moderate |
| Best application | Pre-plant, base feed, perennials | Top-up during fruiting and flowering |
| Organic-approved | Yes (mined natural mineral) | Yes (mined and solar-evaporation grades) |
A 1 kg bag of polyhalite carries roughly 140 g of K₂O, plus calcium, magnesium, and sulphur on the side. A 1 kg bag of sulphate of potash carries about 500 g of K₂O, three and a half times more concentrated, but with nothing else useful in the bag. To match the potassium delivery of a single 1 kg bag of SOP, you would need about 3.5 kg of polyhalite. That is why polyhalite makes sense as a broad-spectrum amendment, and SOP makes sense when potassium is the only nutrient you want to push.
Sulphate form: why both products earn that label
"Sulphate-form potash" is the cleanest type of potassium fertiliser available, and it's worth understanding why. The alternative, muriate of potash (potassium chloride), accounts for the bulk of world potash production but carries 47% chloride by weight. Chloride is an essential micronutrient at trace levels but a problem at fertiliser-application rates. [6]
For potatoes, the salt suppresses dry matter and starch content. For tomatoes and soft fruit, chloride competes with nitrate uptake and can reduce yield quality. For container-grown ornamentals, the salt accumulates because pot drainage doesn't flush it the way an open field does. [8]
Both polyhalite and sulphate of potash deliver their potassium as the sulphate ion. The K⁺ released into the soil solution is chemically identical from either source. The plant cannot tell the difference at the root surface. What differs is the rate at which K⁺ enters the soil solution, and what else comes along for the ride.
Which one for which crop
Tomatoes
The textbook potash-hungry crop. A base of polyhalite mixed into the compost at potting (about 30 g per 30 L pot) supplies a slow background of K, Ca, Mg, and S. The calcium in particular helps with blossom end rot prevention by keeping uptake steady during fruit set. From first flower onwards, switch to a weekly liquid feed containing sulphate of potash to push the fruiting phase. Most commercial tomato feeds, including ours, use SOP as the potassium source for exactly this reason.
Roses and established perennials
Polyhalite is the workhorse here. A spring top-dress of around 40 g per square metre worked lightly into the surface gives the rose a season-long supply of potassium and magnesium (the latter often the limiting nutrient in older rose beds). A second, lighter dose of sulphate of potash in early-to-mid summer pushes the second flush of flowers. Avoid heavy SOP doses on roses, as the concentrated salt can scorch surface feeder roots in dry years.
Soft fruit (strawberries, raspberries, blackcurrants)
Soft fruit are chloride-sensitive, which is why both sulphate-form options work and muriate of potash should be avoided altogether. A polyhalite top-dress at planting and again each spring covers the base. From fruit set onwards, a low-rate sulphate of potash feed (about 10 g per square metre, watered in) supports berry sizing and sugar accumulation.
Potatoes
Potatoes need a lot of potassium but punish chloride harder than most crops. High chloride applications reduce starch content and storage quality. [6] Polyhalite pre-plant is the cleanest option. Sulphate of potash works too, but at the rates needed (potatoes are heavy feeders), the cost difference becomes meaningful. Most allotment growers use polyhalite as the base and skip the SOP top-up.
Brassicas and leafy crops
Light potash-feeders compared with fruiting crops. A modest polyhalite base at planting is usually all that's needed; the calcium it supplies also nudges a slightly acid soil towards the near-neutral pH brassicas prefer. Sulphate of potash top-ups are rarely justified here, since leafy growth doesn't draw down potassium fast enough to need them.
Where polyhalite shines is in containers and perennial beds, anywhere you want one mineral to do four nutritional jobs across a long stretch of weeks.
Combining the two
Polyhalite as the base, sulphate of potash as the top-up. That is the simplest mental model and it covers most outdoor gardens. The combination plays to the strengths of each: polyhalite handles the long, steady release of calcium, magnesium, and a moderate amount of potassium, while sulphate of potash answers any short-term peak demand during heavy fruiting or flowering without adding more calcium and magnesium than the soil can use.
A worked example. For a row of cordon tomatoes in 30-litre pots: mix 30 g of polyhalite into the compost at potting. From first truss onwards, water in 2-3 g of sulphate of potash (or use a tomato-specific liquid feed at the label rate) once a week through the picking season. The polyhalite holds the background; the SOP responds to the demand peak.
The two are not antagonistic and can be used in the same season without worry about salt buildup, provided the rates are sensible. Where the combination becomes excessive is on very alkaline soils with naturally high calcium. Adding polyhalite there pushes calcium and magnesium up further, which can compete with potassium uptake. [9] On chalky beds, sulphate of potash on its own is usually the better choice.
Cost and practicality
Per kilogram of granules, polyhalite tends to be the cheaper option in the UK market. Per kilogram of K₂O delivered, sulphate of potash is the cheaper option, because three and a half times more of the polyhalite bag is matrix water and other minerals. Whether that matters depends on what else you want from the bag. If your soil already has enough calcium, magnesium, and sulphur, SOP is the more efficient buy. If you're amending a tired bed and want to drip-feed several nutrients at once, polyhalite is doing more work per pound spent.
Both store well. They are chemically stable, granular, and don't deliquesce in normal UK storage humidity provided the bag is closed. Both spread easily by hand or with a basic spreader. Neither needs gloves or a mask in routine garden use, though sensible practice for any granular fertiliser includes washing hands and avoiding inhalation of dust during pouring.
Which should you choose?
Reach for polyhalite when
You're setting up a new bed, repotting perennials, or want a single mineral that supplies potassium, calcium, magnesium, and sulphur. Soft fruit, roses, ornamentals, and any container you've planted for the season are textbook fits.
Reach for sulphate of potash when
You need a fast top-up of potassium during fruit set or flower formation. Tomatoes in their picking phase, chilli plants pushing fruit, cut flower beds, and any container showing a clear potash deficiency mid-season.
Use both when
The garden is mixed and you want to set a steady background with polyhalite while keeping SOP on hand for short bursts during heavy demand. This is the default for most home gardens: base in spring, top-up through summer.
Skip both when
A soil test shows potassium already in the high range, or the bed is being used for leafy crops only. Adding more potash on already-rich soil suppresses magnesium uptake without lifting yield.
If you only buy one, get polyhalite
For a typical UK home garden with mixed crops, polyhalite is the more versatile single purchase. It covers four nutrients, suits more situations than SOP alone, and the slow release reduces the risk of over-application. Add sulphate of potash separately once you have a clear use case for it (heavy tomato or fruit growing, container-only setup).
Frequently asked questions
Is polyhalite a type of potash?
Polyhalite contains potash (K₂O) but is not pure potash. It is a quadruple-salt mineral that supplies potassium, calcium, magnesium, and sulphur together in a fixed natural ratio. Pure potash fertilisers like sulphate of potash and muriate of potash contain only potassium and a counter-ion (sulphate or chloride). Polyhalite carries about 14% K₂O by weight; sulphate of potash carries about 50%.
Is sulphate of potash organic?
The mined-mineral and solar-evaporation grades of sulphate of potash are approved for organic farming under EU and UK organic regulations. The synthetic Mannheim-process grade (made by reacting muriate of potash with sulphuric acid) is not. When buying for an organic garden, check the supplier states the SOP is mined or solar-derived and approved by a recognised certifier such as the Soil Association.
What crops benefit most from polyhalite?
Crops with sustained potassium and sulphur demand and sensitivity to chloride: tomatoes, potatoes, soft fruit, brassicas, roses, ornamentals, and grass leys. Polyhalite also suits long-cycle container plantings because the slow release matches a season-long demand curve. Brewing barley and oilseed rape are large-scale field-crop uses outside the home garden.
Can I use polyhalite and sulphate of potash together?
Yes. The two are not antagonistic and combining them is the standard approach for many UK gardens: polyhalite as a pre-plant or spring base, sulphate of potash as a faster top-up during fruiting and flowering. Avoid combining them at heavy rates on naturally alkaline or chalky soils where calcium is already abundant.
Is polyhalite better than sulphate of potash?
Neither is universally better. They answer different questions. Polyhalite is better when you want a slow, multi-nutrient mineral for pre-plant or perennial use. Sulphate of potash is better when you want a fast, concentrated single-nutrient top-up during peak demand. Most established gardens use both for different jobs across the season.
How much polyhalite should I apply per square metre?
For a general garden base, 30–50 g per square metre worked into the top 5 cm of soil at planting or in early spring. Perennial beds can take 40–60 g per square metre once a year. Heavy-feeding crops (potatoes, tomatoes in beds) up to 80 g per square metre at planting. Always lighter on chalky soils.
Does polyhalite raise soil pH?
Polyhalite is approximately pH-neutral and does not significantly raise soil pH. The calcium it supplies is present as sulphate (CaSO₄, gypsum chemistry), which dissolves into Ca²⁺ and SO₄²⁻ ions without releasing hydroxide or carbonate. Lime, by contrast, releases carbonate and does raise pH. Polyhalite can be used on acid soils without changing their acidity.
Sources
- Kemp, S. J., Smith, F. W., Wagner, D., et al. (2016). An improved approach to characterise potash-bearing evaporite deposits, evidenced in North Yorkshire, United Kingdom. Economic Geology, 111(3): 719–742.
- Yermiyahu, U., Zipori, I., Faingold, I., et al. (2017). Polyhalite as a multi-nutrient fertiliser: potassium, magnesium, calcium and sulphate. Israel Journal of Plant Sciences, 64(3–4): 145–157.
- Pavuluri, K., Malley, Z., Mzimbiri, M. K., et al. (2017). Evaluation of polyhalite in comparison to muriate of potash for corn grain yield. African Journal of Agronomy, 5(3): 325–332.
- Potash Development Association (PDA). Leaflet 23: Potash for Organic Growers (originally produced 1999, reviewed 2005).
- International Plant Nutrition Institute (IPNI). Nutrient Source Specifics No. 5: Potassium Sulfate.
- Xu, G., Magen, H., Tarchitzky, J., Kafkafi, U. (2000). Advances in chloride nutrition of plants. Advances in Agronomy, 68: 97–150.
- McGrath, S. P., Zhao, F. J. (1995). A risk assessment of sulphur deficiency in cereals using soil and atmospheric deposition data. Soil Use and Management, 11(3): 110–114.
- Geilfus, C. M. (2018). Chloride: from nutrient to toxicant. Plant and Cell Physiology, 59(5): 877–886.
- Zhang, F., Niu, J., Zhang, W., et al. (2010). Potassium nutrition of crops under varied regimes of nitrogen supply. Plant and Soil, 335: 21–34.