Dr Forest
Organic Granulated Polyhalite Fertiliser. Mined in Yorkshire 14% Potassium Oxide
Organic Granulated Polyhalite Fertiliser. Mined in Yorkshire 14% Potassium Oxide
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Yorkshire polyhalite — four macronutrients in one ancient mineral
Polyhalite is a 260-million-year-old mineral mined from beneath the North Yorkshire coast. It delivers four essential macronutrients in a single granule — potassium, calcium, magnesium, and sulphur — all as plant-available sulphates, with virtually zero chloride. No chemical processing. No synthetic additives. Crushed, granulated, and packaged. It has one of the lowest carbon footprints of any fertiliser on the market.
This is the mineral that underpins every Dr Forest crop-specific blend. We use it as the primary source of potassium, calcium, magnesium, and sulphur across our entire range because nothing else delivers all four nutrients in slow-release, balanced form from a single natural input. Now available as a straight so you can apply it directly.
What polyhalite is used for in the garden
- Base mineral amendment for all crops — supplies the four secondary and tertiary macronutrients most commonly deficient in garden soils, particularly in containers and raised beds where nutrients leach rapidly
- Calcium delivery without pH change — calcium sulphate does not raise soil pH the way lime does, making polyhalite 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 remainder releasing gradually throughout the growing season
- Chloride-sensitive crops — tomatoes, strawberries, potatoes, peppers, and salad crops benefit from potassium delivered without chloride accumulation in the root zone
- Magnesium supplementation — prevents interveinal chlorosis (yellowing between leaf veins), supports chlorophyll production, and activates over 300 plant enzymes
- Lawn and turf nutrition — potassium hardens turf for winter, calcium improves soil structure, and sulphur supports colour and density
- Soil structure improvement — calcium and magnesium displace sodium on clay particles, improving drainage, aeration, and root penetration in heavy soils
- Fruit quality and flavour — a 921-trial meta-analysis across 47 crops showed polyhalite increased yields by 3.8–16.3% compared to standard NPK programmes
Why polyhalite instead of single-nutrient supplements?
Polyhalite — this product
- Four macronutrients in one natural granule — K, Ca, Mg, S
- Slow-release — nutrients available for the full growing season
- Chloride-free with a low salt index — safe for all crops
- No chemical processing — mined, crushed, granulated
- EU organic certified under EC 834/2007
- One of the lowest carbon footprints of any commercial fertiliser
- One product replaces separate applications of gypsum, Epsom salt, and SOP
Buying K, Ca, Mg, S separately
- Requires sulphate of potash + gypsum + Epsom salt — three products
- All fully soluble — higher leaching losses, more frequent applications
- Higher total salt load from multiple soluble inputs
- Harder to balance ratios correctly without overloading one nutrient
- Higher total cost per nutrient unit when buying separately
- More packaging, more shipping, more storage
Every Dr Forest product is made by hand in small batches at our workshop in Stockport, Greater Manchester. We use recyclable packaging throughout and source ingredients for quality, not cost.
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 that were sealed underground for 260 million years. The world's largest known deposit sits beneath North Yorkshire, estimated at over 2.5 billion tonnes.
The mineral is extracted from Boulby Mine on the North Yorkshire coast — sinking over 1,200 metres beneath the North Sea. It undergoes no chemical separation or industrial refining. It is mined, crushed, screened, and granulated. This minimal processing gives it one of the lowest carbon footprints of any commercially available fertiliser — just 0.034 kg CO₂ per kg of product.
Nutrient composition
| Nutrient | Content | Form |
|---|---|---|
| Potassium (K₂O) | 14% | Potassium sulphate |
| Calcium (CaO) | 17% | Calcium sulphate |
| Magnesium (MgO) | 6% | Magnesium sulphate |
| Sulphur (SO₃) | 48% | Sulphate |
All four nutrients are present as sulphates — the form plants absorb directly through their roots. The crystalline structure of polyhalite means these sulphates dissolve at different rates depending on soil moisture and temperature, creating a natural slow-release effect that soluble salts cannot replicate.
Why slow release matters
Reduced Leaching Losses
Transport and leaching of Ca, Mg, K, and S following polyhalite application is significantly lower than following application of equivalent soluble salts. The sulphate ions in polyhalite exhibit stronger adsorption to soil particles. In container growing and raised beds — where leaching is a constant problem — this extended availability is a major practical advantage over fully soluble alternatives.
Staggered Nutrient Availability
University of Nottingham research confirmed that 50–60% of the sulphur in polyhalite is immediately plant-available, with the remainder releasing gradually over the growing season. This staggered release pattern matches the way plants actually take up nutrients — demand increases as crops grow, flower, and fruit. A single application at planting can supply nutrients for months rather than days.
Superior Residual Effect
Research consistently shows a higher residual effect from polyhalite compared to equivalent soluble salts, especially for calcium, magnesium, and sulphur. This means the mineral continues to feed subsequent crops, improving the economics and efficiency of long-term soil management.
921-Trial Meta-Analysis
A landmark meta-analysis published in the Agronomy Journal (2025) combined data from 921 replicated field trials across 47 crops in 33 countries, conducted between 2014 and 2023. Polyhalite increased yields by 3.8–16.3% over NP controls. Crops with the strongest responses included potato, peanut, onion, and oilseeds — all crops with high calcium and sulphur demands.
Calcium Without pH Change
Most calcium amendments (lime, dolomite) raise soil pH significantly, making them unsuitable for acid-loving plants or already-alkaline soils. Polyhalite delivers calcium as calcium sulphate (gypsum form), which is pH-neutral. This means you can correct calcium deficiency — preventing blossom end rot in tomatoes, bitter pit in apples, and tip burn in lettuce — without disturbing your soil pH.
Soil Microbial Activity
Research shows polyhalite application positively influences soil microbial biomass carbon and FDA hydrolase activity — indicators of a biologically active, healthy soil. The calcium and sulphur content improve cation exchange capacity and aggregate stability, creating better habitat for beneficial soil organisms. Field trials on wheat demonstrated that 100% K from polyhalite produced the highest soil organic carbon and microbial activity compared to conventional fertiliser treatments.
Scientific References
- Meta-analysis of polyhalite's yield performance across diverse soil, crop, and environmental conditions (2025). Agronomy Journal, 117, e70259. (921 trials, 47 crops, 33 countries.)
- Gopinath, K.A. et al. (2024). Exploring the use of POLY4 for the improvement of productivity, peanut quality, and soil properties in Southern India. Frontiers in Plant Science, 15, 1448909.
- Singh, S.P. et al. (2025). Polyhalite as an alternate nutrient source for improving growth, yield, and nutrient use efficiency in onion and garlic. Scientific Reports, 15.
- Kumar, R. et al. (2025). Polyhalite nutrients driving balanced crop nutrition and sustainable agricultural productivity. Discover Soil.
- Barbarick, K.A. (1991). Polyhalite applications to sorghum-sudangrass and leaching in soil columns. Soil Science, 151, 159–166.
- Tiwari, D.D. et al. (2015). Effects of polyhalite as a fertilizer on yield and quality of oilseed crops mustard and sesame. e-ifc, 42, 10–17.
- Mello, S.D.C. et al. (2018). Potato response to polyhalite as a potassium source fertilizer in Brazil: yield and quality. HortScience, 53(3), 373–379.
- Mello, S.D.C. et al. (2018). Response of tomato to polyhalite as a multi-nutrient fertilizer in southeast Brazil. J. Plant Nutr., 41(16), 2126–2140.
How to use polyhalite: application rates & guide
Polyhalite contains no nitrogen and no phosphorus. It supplies potassium, calcium, magnesium, and sulphur. For a complete feeding programme, use alongside a nitrogen and phosphorus source — such as a Dr Forest crop-specific fertiliser blend (Tomato, Rose & Flower, Fruit & Veg, etc.) or an all-purpose NPK fertiliser.
Application rates
Soil mix — potting and container preparation
Mix thoroughly into compost or potting soil before planting. Provides baseline calcium, magnesium, potassium, and sulphur for the first 8–12 weeks thanks to the slow-release profile. Ideal for all container crops.
Top dressing — established containers
Scatter granules evenly across the soil surface and water in thoroughly. The slow-release profile means less frequent applications compared to soluble alternatives. Particularly valuable for tomatoes, peppers, and other heavy-feeding fruiting crops.
Outdoor beds and borders
Broadcast evenly across the soil surface and water in well. Use the lower rate for general maintenance; the higher rate for heavy-feeding crops, new plantings, or clay soil improvement. Can be applied at planting and as a mid-season top-up.
Lawns and turf
Apply and water in. Potassium hardens turf for winter, calcium improves soil structure, and sulphur supports colour and density. Polyhalite is the mineral used in professional turf nutrition programmes at the highest level.
Clay soil improvement
The calcium in polyhalite displaces sodium on clay particles, improving aggregate structure, drainage, and workability. Unlike lime, it does not alter soil pH — making it safe for all soil types. Work into the top 10–15cm if possible.
Step-by-step application
- Assess your soil. Polyhalite is most beneficial where calcium, magnesium, potassium, or sulphur are low — particularly in containers, raised beds, sandy soils, and intensively cropped areas.
- Measure the correct rate. A level tablespoon of polyhalite granulate is approximately 12–15g. Use the rates above as a starting guide and adjust based on crop demand.
- Apply evenly. For containers, scatter across the surface. For beds, broadcast by hand or spreader. For soil mixing, combine thoroughly with compost.
- Water in. Polyhalite needs soil moisture to begin dissolving and releasing nutrients. Water immediately after top-dressing or broadcasting.
- Combine with nitrogen and phosphorus. Polyhalite contains no N or P. Use alongside a Dr Forest blend or other nitrogen/phosphorus source for complete crop nutrition.
Polyhalite is the ideal companion to any nitrogen-focused feed. Use with Dr Forest All-Purpose 6-6-6 for general gardens, crop-specific blends (Tomato, Rose & Flower, Potato) for targeted feeding, or Seaweed Powder for biostimulant activity. The calcium content also pairs well with Humic Acid Granules for long-term soil structure building.
Frequently asked questions about polyhalite
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