Polyhalite: why Wimbledon feeds its lawns a Yorkshire-mined mineral

Why does Wimbledon feed its lawns a Yorkshire-mined mineral?

By Joe, Founder of Dr Forest · May 2026

Edited 24 Sep 2026

In summer 2023, the Daily Telegraph ran a piece on the fertiliser used to keep the Wimbledon centre court alive through a fortnight of professional tennis. It comes from a mine 1,200 metres beneath the North Sea, and at the time, almost nobody in the UK was selling it to gardeners.

Polyhalite is a naturally occurring mineral fertiliser containing potassium, calcium, magnesium and sulphur, all four locked into a single crystal. It formed roughly 260 million years ago when a shallow sea evaporated and left a vast bed of mineral salt under what is now the North Yorkshire coast. The world's only commercial polyhalite mine sits at Boulby, just south of Whitby, and shafts the rock from over a kilometre beneath the seabed.

The mineral is used by professional turf growers, including the All England Lawn Tennis Club. ICL, the company that owns the Boulby mine, sells most of its production to commercial agriculture by the tonne, branded as Polysulphate. We pack it into garden-sized bags. As far as I can tell, Dr Forest is the only UK retailer doing that.

This guide covers what polyhalite is, why a 260-million-year-old salt deposit ended up on the Wimbledon centre court, what the four nutrients do for plants, and how to use it in a domestic garden.

First, what polyhalite actually is

Formula K₂Ca₂Mg(SO₄)₄·2H₂O — one crystal, four nutrients as sulphates. Permian evaporite; commercial supply from Boulby under the North Sea (polyhalite mining from 2010), with Woodsmith near Whitby still under development. Mined, crushed, screened, bagged — no acid separation step.

ICL publishes a very low cradle-to-gate CO₂e figure for Polysulphate (~0.0029 kg/kg product) because it is mined and crushed. That is an industry LCA, far below typical synthetic nitrogen fertiliser footprints on a per-kg-product basis — not a like-for-like nutrient comparison, and not a journal trial. Permitted as an organic input under UK/EU rules when the batch is on your certifier’s list (incl. Soil Association schedules for the Boulby product) — check the certificate.

Four nutrients in one crystal

Every granule of polyhalite carries the same guaranteed composition. The numbers below are the manufacturer's spec sheet, expressed both as oxide (the convention used on UK fertiliser labels) and as the elemental mass percentage.

Alongside those four headline elements, Boulby polyhalite can contain trace boron and iron. Those traces are not a substitute for a micronutrient programme — the fertiliser story is the four macros (K, Ca, Mg, S).

The form matters as much as the figures. Every nutrient in polyhalite is delivered as a sulphate, which is a form plants can take up directly — no microbial conversion step of the kind elemental sulphur needs. The crystal dissolves gradually in moist soil and the dissolved sulphates move into the root zone; moisture, particle size and temperature still affect how quickly that happens.

Worth knowing: the potassium in polyhalite is sulphate-form (the same chemistry as sulphate of potash). It is not muriate of potash, which is potassium chloride. That distinction matters for tomatoes, strawberries, potatoes and other chloride-sensitive crops, where chloride at higher rates causes leaf scorch and reduced quality. There's more on that below.

What polyhalite does not contain is nitrogen, or phosphorus. Nothing on a polyhalite-only programme will get a leafy plant established. It works as part of a feeding regime, not as a sole input. Compost, an organic nitrogen source, or a multi-input blend covers that gap. Several of our blends, including the Premium Tomato Fertiliser and the Premium Fruit and Vegetable Fertiliser, use Yorkshire polyhalite as the potassium and sulphur backbone alongside other organic ingredients.

How polyhalite ended up on the centre court

In summer 2023 the Daily Telegraph named polyhalite in Wimbledon’s turf programme, and noted Dr Forest packing garden-sized bags when most UK retail ignored it. Close-mown tennis turf wants tough crowns, recovery, and no chloride scorch. Polyhalite’s sulphate K + Ca + Mg + S, slow and nearly chloride-free, fits that brief. The same chemistry suits tomatoes, strawberries, potatoes and a weekly-cut lawn — chloride-sensitive crops resent MOP.

ICL sells Polysulphate by the tonne to agriculture. Garden centres mostly stock blue tomato feeds. We bag the mineral for domestic use because the analysis and chloride profile do a job little else on a UK shelf does.

Mineral fertiliser — no animal by-products, no synthetic N. Granules also go into our multi-input blends.

What polyhalite actually offers

Polyhalite is a four-nutrient sulphate mineral: potassium, calcium, magnesium and sulphur in one crystal, with only minor chloride as impurity. That chemistry is the honest sell — not a yield percentage.

UK soils often need sulphur more than they did when atmospheric deposition was higher. Whether your bed is short is a soil-test question, not a brand claim. Polyhalite can supply that sulphur (and K, Ca, Mg) without the chloride load of muriate of potash. It does not supply nitrogen or phosphorus.

Until a verified bibliography sits on the page, lead with the bag analysis and the chloride distinction rather than yield-percentage panels.

Polyhalite or muriate of potash?

The most widely used potassium fertiliser in the world is muriate of potash, sometimes shortened to MOP. It's potassium chloride. The figures look attractive on the bag (60% K₂O against polyhalite's 14%) but the whole-product comparison is closer than it looks, especially for chloride-sensitive crops.

If you are choosing between polyhalite and sulphate of potash specifically, see our comparison: polyhalite vs sulphate of potash — which to use.

For a chloride-tolerant crop in a low-input commercial setting, MOP is hard to beat on cost per unit of potassium. For anything fruiting, anything in pots where salt builds up, anything organic, or anything close-mown, polyhalite is the better pick. The lower headline percentage is offset by the three secondary nutrients that come with it, the absence of chloride, and the slow release pattern.

How to apply polyhalite

Slow-release base dressing, not foliar, not fertigation. Compost mix 2.5–5 g/L once at planting; pot top-dress 1–3 g/L every 6–8 weeks; beds 50–125 g/m² every 6–12 weeks; lawns 50–100 g/m² in the growing season. Water in — needs moisture to dissolve.

So, briefly

Polyhalite isn't a marketing invention. It's a 260-million-year-old salt deposit, mined in North Yorkshire, with a clear analytical profile and a growing research literature behind it (much of the high-profile yield evidence is producer-linked, so treat company trial headlines cautiously). It addresses the sulphur and magnesium gaps that UK soils have developed since the 1990s, and it does so in a chloride-free form that suits exactly the crops most home gardeners care about. If your tomatoes have ever cracked, your alliums have ever been bland, or your lawn has gone yellow at the tips, this is most of the answer.

Browse our potash and potassium fertilisers if you want the Yorkshire mineral alongside other chloride-sensitive potash options.

Frequently asked questions