What is polyhalite, and why does Wimbledon use it?

By Joe, Founder of Dr Forest · May 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

Polyhalite has the chemical formula K₂Ca₂Mg(SO₄)₄·2H₂O. It's a hydrated sulphate mineral, technically an evaporite. The name comes from the Greek poly (many) and halite (salt). It's one crystal carrying many salts.

The British deposit was laid down during the Permian period, around 260 million years ago. A vast, shallow inland sea called the Zechstein covered most of what is now northern Europe. The climate at the time was hot and arid, and the sea evaporated in a sequence of long episodes. As the water vanished, the dissolved salts crystallised out in dense layers. Common rock salt first, then potash salts, then polyhalite. Hundreds of metres of mineral, sealed under younger rock for the next quarter-billion years.

The polyhalite seam is reached today through two UK mines. The first, at Boulby on the North Yorkshire coast, has been operating since the 1970s, originally for potash. Polyhalite mining started there in 2010. The second, the Woodsmith Mine near Whitby, is a much larger project still being developed. Between them they account for the entire global commercial supply of the mineral.

What's interesting about polyhalite is that it needs no industrial processing. It's mined, crushed, screened to size, and bagged. There is no chemical separation, no acid treatment, no salt-extraction step. The carbon footprint of the finished product is around 0.0029 kg of CO₂ per kg, which is roughly a thousandth of the figure for synthetic nitrogen fertilisers manufactured by the Haber-Bosch process. That low number is mostly diesel for the mining equipment and the ship that takes it down the coast.

Because of that minimal handling, polyhalite is permitted under organic certification standards in the UK and the EU, and it appears on the Soil Association's list of approved inputs for organic agriculture.

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, polyhalite carries trace amounts of boron, iron, manganese, zinc and copper. Useful background micronutrients rather than feature ingredients.

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. There's no microbial conversion step, no temperature dependency, no waiting for soil biology to wake up. The crystal dissolves slowly in soil moisture and the dissolved sulphates move into the root zone.

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 published a feature on the agronomy programme behind the Wimbledon grass. The piece named polyhalite as part of the All England Club's turf nutrition. Dr Forest was named in the same piece as the only UK retailer selling it direct to gardeners.

The reason a tournament that obsesses about its grass would reach for a 260-million-year-old salt deposit comes down to two things grass courts need: dense, resilient turf, and zero chloride stress.

Tennis grass is mown at around 8mm. Players in five-set matches drag, slide and pivot on it for hours at a time. The turf has to be tough at the crown, well rooted, and capable of recovering between matches. Potassium drives that toughness. It thickens cell walls, regulates water in and out of cells, and supports recovery from physical stress. Calcium and magnesium support cell wall integrity and the chlorophyll that keeps the colour even. Sulphur supports the proteins involved in growth.

Polyhalite delivers all four in sulphate form, slowly enough not to burn the turf and without any chloride. Standard potassium fertilisers, by contrast, are mostly muriate of potash. That's potassium chloride, and on close-mown turf the chloride load can scorch leaf tips and slow recovery. The whole point of polyhalite for sports turf is the chloride-free chemistry.

The same principle applies in a domestic garden. Tomatoes resent chloride in soluble feeds. So do strawberries, soft fruit and potatoes. A lawn cut weekly is under similar stress to a competition court, just at a smaller scale. Polyhalite suits any of them.

Why I'm the only retailer doing it

It isn't usually flattering to be the only person selling something. Most of the time it just means there's no money in it. With polyhalite, the reason was simpler. Until that Telegraph piece in 2023, almost nobody in UK gardening had heard of it.

ICL produce polyhalite at industrial scale and sell it to commercial agriculture by the tonne under the brand name Polysulphate. They aren't set up for retail. The garden centres mostly stock blue tomato feeds and chicken-pellet pots. Polyhalite sat in the gap between them: a serious mineral the trade knew about and gardeners did not.

I started bagging it because it does a job nothing else on a UK garden shelf does, and because the carbon and chloride numbers stack up in a way it's hard to argue with. The granules go into our blends; the standalone product goes out in domestic-sized sacks. Made with organic ingredients, multi-input, plant-based, no slaughterhouse waste. Handcrafted in Stockport, Greater Manchester.

What the research actually shows

Polyhalite is well studied for a relatively new commercial fertiliser. The findings group around three claims: yield uplift in standard NPK programmes where sulphur or magnesium is limiting, prevention of calcium and magnesium deficiencies in greenhouse crops, and consistent performance across soil types.

The greenhouse tomato study from the International Potash Institute is the clearest one for UK gardeners to read across to. Polyhalite at 1–2 t/ha (roughly 100–200g per square metre, in garden terms) prevented calcium and magnesium deficiency outright in treated plots, while untreated controls developed visible deficiency symptoms. Marketable yield rose 5–7%. The researchers concluded that polyhalite could replace separate liquid calcium and magnesium feeds, while supplying around a third of the crop's potassium need.

A 2025 systematic review in the Journal of Plant Nutrition looked across soil types and cropping systems. The conclusion was that polyhalite is most useful on soils that are short of sulphur and magnesium, which now describes a great many UK gardens. UK soil sulphur has been falling steadily since the early 1990s when industrial SO₂ emissions dropped under the Clean Air regulations. The atmospheric deposition that used to top sulphur up in the soil is no longer there.

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.

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

Polyhalite is slow-release. The granules dissolve gradually in soil moisture rather than spiking the way a soluble feed does. That makes it a base dressing at planting, plus a top dressing every six to eight weeks during the growing season. It isn't a foliar feed, and it isn't a fertigation product.

Application rates

For a compost or potting mix, 2.5 to 5g per litre of mix, blended in once at planting. For pot top-dressings, 1 to 3g per litre of soil volume, every six to eight weeks. For outdoor beds and borders, 50 to 125g per square metre, every six to twelve weeks from spring through autumn. For lawns, 50 to 100g per square metre during the growing season.

Always water in well after application. Polyhalite needs moisture to dissolve, and watering moves the released sulphates down into the root zone where they're useful.

Which plants benefit most

Anything that's chloride-sensitive or short on sulphur, calcium or magnesium. In practice that covers most of a UK kitchen garden.

Tomatoes are the obvious one. The sulphate potassium loads sugars into the fruit; the calcium supports cell walls and helps with blossom end rot when watering is consistent; sulphur supports the volatile compounds that carry tomato flavour. Strawberries and other soft fruit respond similarly: bigger sugars, firmer flesh, better shelf life. Potatoes prefer sulphate potassium too, and resent chloride at the rates supplied by MOP.

Brassicas and alliums need sulphur for the glucosinolates and sulphur-containing compounds that produce the flavour and pungency we grow them for. Sulphur-deficient cabbages, broccoli, onions and garlic are bland. Lawns benefit from the same chloride-free, slow-release, multi-nutrient profile that suits Wimbledon. Roses suffer magnesium deficiency more often than is recognised; polyhalite covers that off alongside the potassium roses need for repeat flowering.

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 body of independent research behind it. 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.

Frequently asked questions

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