Polyhalite Fertiliser

Polyhalite fertiliser — four macronutrients in one Yorkshire mineral

Polyhalite is a naturally occurring mineral fertiliser that supplies four macronutrients — potassium, calcium, magnesium and sulphur — from a single Yorkshire crystal. It's mined from beneath the North Yorkshire coast at Boulby — the world's only currently operating commercial polyhalite mine — and arrives in the bag exactly as it left the ground: crushed, screened, granulated. No chemical processing, no synthetic additives. Because it's a sulphate-based potash mineral with virtually zero chloride, it's particularly valuable for chloride-sensitive crops like tomatoes, strawberries, potatoes and salad leaves.

This is the mineral that underpins every Dr Forest crop-specific blend — the primary source of potassium, calcium, magnesium and sulphur across the range, because nothing else delivers all four in slow-release form from a single natural input. Think of it as a multi-nutrient upgrade on sulphate of potash: the same chloride-free potash, plus the calcium, magnesium and sulphur most garden soils also need. Approved for EU organic production under EC 834/2007, with one of the lowest carbon footprints of any commercial fertiliser. Now available as a straight, so you can apply the same mineral directly to your soil.

What is polyhalite used for in the garden?

• A chloride-free potash source — supplies 14% K₂O as potassium sulphate, the same chloride-free potash as sulphate of potash (SOP), but with calcium, magnesium and sulphur alongside it
• Base mineral amendment for all crops — delivers the four macronutrients most commonly deficient in containers, raised beds and intensively cropped soils, where leaching strips them out fastest
• Calcium delivery without pH change — calcium sulphate doesn't raise soil pH the way lime does, so polyhalite is safe for acid-loving plants like blueberries, rhododendrons and camellias
• Slow-release season-long feeding — University of Nottingham research confirms 50–60% of sulphur is immediately available, with the rest released over the growing season
• Chloride-sensitive crops — tomatoes, strawberries, potatoes, peppers and salad leaves benefit from potassium delivered without chloride accumulation in the root zone
• Magnesium supplementation — prevents interveinal chlorosis (yellowing between leaf veins) and activates over 300 plant enzymes, including those that build chlorophyll
• Lawn and turf nutrition — potassium hardens turf for winter, calcium improves soil structure, sulphur supports colour and density
• Soil structure on heavy clay — calcium and magnesium displace sodium on clay particles, opening up drainage and aeration without altering pH
• Yield and fruit quality — a 921-trial meta-analysis across 47 crops in 33 countries showed polyhalite raised yields by 3.8–16.3% over standard NPK programmes

Why polyhalite instead of buying K, Ca, Mg and S separately?

The science of polyhalite: a 260-million-year-old multi-nutrient mineral

What polyhalite actually is

Polyhalite (K₂Ca₂Mg(SO₄)₄·2H₂O) is a hydrated sulphate of potassium, calcium and magnesium. It formed during the Permian period when a vast shallow ocean — the Zechstein Sea — evaporated under hot, arid conditions across what is now northern Europe. As the water retreated, dissolved minerals concentrated into dense crystalline layers and were sealed underground for 260 million years. The world's largest known deposit sits beneath North Yorkshire, estimated at over two billion tonnes.

This polyhalite is extracted from Boulby Mine on the North Yorkshire coast — over 1,200 metres beneath the North Sea. It undergoes no chemical separation or industrial refining. Just mining, crushing, screening and granulation. That minimal processing gives it one of the lowest carbon footprints of any commercially available fertiliser at roughly 0.034 kg CO₂ per kg of product.

Want the full background — how polyhalite formed, how it's mined and how it compares to other potash sources? Read our companion guide, what is polyhalite, on the Dr Forest blog.

Polyhalite composition and nutrient content

All four nutrients are present as sulphates — the form plants absorb directly through the roots. The crystalline structure means these sulphates dissolve at different rates depending on soil moisture and temperature, creating a natural slow-release effect that pre-mixed soluble salts cannot replicate.

Why slow release matters

How to use polyhalite: application rates and guide

Polyhalite application rates

Step-by-step application

1. Assess your soil. Polyhalite is most useful where calcium, magnesium, potassium or sulphur are running low — typically containers, raised beds, sandy soils and intensively cropped patches.
2. Measure the rate. A level tablespoon of polyhalite is roughly 12–15g. Use the rates above as a starting point and adjust to crop demand.
3. Apply evenly. Scatter across the surface for containers. Broadcast by hand or spreader for beds. Mix thoroughly with compost for soil mixes.
4. Water in. Polyhalite needs soil moisture to begin dissolving and releasing nutrients. Water it in straight after application.
5. Pair with a nitrogen source. Polyhalite carries no N or P. Use it with a Dr Forest blend or another NPK fertiliser for complete crop nutrition.

Frequently asked questions about polyhalite