Polyhalite vs sulphate of potash: which potash should you use?
By Joe Barrington, founder of Dr Forest · Published May 2026 · Updated 10 October 2026
Dr Forest Polyhalite Fertiliser is the slow four-nutrient base, at 50 to 125 g/m² on beds; Dr Forest Sulphate of Potash is the fast potash top-up, at 20 to 50 g/m² on beds.
TLDR
Polyhalite as the slow base; sulphate of potash as the fast top-up. Polyhalite is a Yorkshire four-nutrient mineral (K, Ca, Mg, S) that drips out over months — pre-plant beds, perennials, season-long pots. Sulphate of potash is near-pure potash (~50% K₂O) that works in days — fruiting and flowering peaks. Both are sulphate-form and suit chloride-sensitive crops. Mixed UK garden, one bag only: buy Dr Forest Polyhalite Fertiliser, at 50 to 125 g/m² on beds every 6 to 12 weeks. Keep Dr Forest Sulphate of Potash for tomatoes and soft fruit in full swing, at 2 to 3 g per pot watered in weekly, or about 10 g/m² on soft fruit from fruit set.
What is polyhalite?
A natural hydrated sulphate mineral, mined at Boulby, between Staithes and Loftus, about ten miles north-west of Whitby. Rough analysis: ~14% K₂O, 17% CaO, 6% MgO, 48% SO₃. What polyhalite is.
What is sulphate of potash?
Potassium sulphate (K₂SO₄), ~50% K₂O and ~45% SO₃. Chloride-free (unlike muriate). Mined and sun-dried grades can be approved for organic growing. Factory-made (Mannheim) sulphate of potash is not. Ours is mined. Dissolved K⁺ available within days; heavy doses raise salt risk on tender roots.
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 mineral, a sulphate of potassium, calcium and magnesium (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
Both deliver potassium as the sulphate ion — chloride-free or very low chloride — which suits potatoes, tomatoes, soft fruit and many ornamentals better than muriate (KCl). The K⁺ at the root is the same; what differs is release rate and what else rides along (Ca and Mg with polyhalite; near-pure K₂O with SOP).
Not feeding until spring? Leave your email and we will send a note when it is time to start again, with 10% off your first order.
Which one for which crop
Same rule everywhere: polyhalite in before or at planting for the long drip; sulphate of potash when fruit or flower is actually demanding it.
Tomatoes. Mix about 30 g polyhalite into a 30 L pot at potting. From first flower, use a weekly SOP-based liquid (or 2–3 g SOP watered in) through picking. The calcium in polyhalite supports uptake when watering is steady — it is not a blossom-end-rot cure on its own. Irregular watering still wins that argument.
Roses and established perennials. Spring top-dress around 40 g polyhalite per square metre, worked into the surface. A lighter SOP dose in early-to-mid summer can push a second flush; heavy SOP on dry feeder roots scorches.
Soft fruit. Chloride-sensitive — skip muriate. Polyhalite at planting and each spring; from fruit set, about 10 g SOP per square metre, watered in, for sizing.
Potatoes. Heavy on potassium, harsh on chloride. Polyhalite pre-plant is the clean allotment default. SOP works, but at potato rates the bag cost climbs; most growers skip the top-up.
Brassicas and leafy crops. Cabbages, sprouts and other brassicas take up a lot of potash, so use a full polyhalite base at planting, especially on light soil. The calcium arrives as sulphate (gypsum chemistry) — pH-neutral, not a lime substitute — so do not count on it to "nudge" acid soil toward neutral.
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 the first truss, water in 2–3 g of sulphate of potash once a week through the picking season. If you use a slow-release tomato feed instead, top-dress it every three weeks and skip the weekly potash. 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. On chalky beds, sulphate of potash on its own is usually the better choice.
Cost and practicality
On our own price list, polyhalite is slightly cheaper per kilo of granules. Per kilo of potash, sulphate of potash works out about a third of the price. Per kilogram of K₂O delivered, sulphate of potash is the cheaper option, because only 14% of a polyhalite bag counts as potash, against 50% for sulphate of potash. 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. Adding more potash on already-rich soil suppresses magnesium uptake without lifting yield. Do not skip potash just because the bed is leafy — brassicas take a lot.
If you only buy one, get polyhalite
For a typical UK home garden with mixed crops, Dr Forest Polyhalite Fertiliser is the more versatile single purchase, at 50 to 125 g/m² on beds every 6 to 12 weeks. 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).
Related: Yorkshire polyhalite · organic fertilisers page · Yorkshire Polyhalite · Sulphate of Potash · Tomato 3-4-6 (SOP-based) · Bloom 2-8-4 (bone meal — not plant-based) (multi-input) · what polyhalite actually is
Frequently asked questions
Is sulphate of potash organic?
Mined and solar-evaporation grades can be; Mannheim synthetic usually not. Check the organic certification mark or certifier language on the bag.
How much polyhalite per m²?
Typical garden top-dress around 40 g/m² for roses/perennials in spring; tomatoes ~30 g per 30 L pot at potting. Adjust for soil test and crop.
Does polyhalite raise soil pH?
No. Calcium is present as sulphate (gypsum chemistry) — pH-neutral, not a lime substitute.
Founder of Dr Forest. Joe started the business and formulates the blended fertilisers and mixes and packs them in small batches in Stockport.
The Dr Forest Newsletter
Grow with us, from our plot to yours
Feeding guides, seasonal growing advice and research-backed articles like this one, sent from our Stockport workshop. New subscribers get 10% off their first order.
★★★★★ 4.95/5 from 513 reviews · No spam, unsubscribe any time