• Sulphate of Potash — pile of off-white granular crystals
  • Sulphate of Potash — hand scattering white granules around tomato plants over dark soil in raised bed
  • Sulphate of Potash — white granules and gardening gloves on wooden bench beside terracotta pot of soil
  • Sulphate of Potash — white granules on allotment bench with terracotta pot, gloves, raised beds and greenhouse
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Dr Forest

Sulphate of Potash UK | 50% K₂O | Organic Potash Fertiliser

Sulphate of Potash UK | 50% K₂O | Organic Potash Fertiliser

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Organic sulphate of potash — 50% K₂O chloride-free potassium & 18% sulphur

50% K₂O Potash 18% Sulphur Chloride-Free Low Salt Index Certified Organic Input Recyclable Packaging

Sulphate of potash is a chloride-free potassium fertiliser — 50% K₂O plus 18% sulphur — that feeds fruiting, flowering and root crops without the chloride load of cheaper potash. Potassium drives sugar transport, fruit ripening, flower colour, drought tolerance and disease resistance. Most budget potash supplies it as potassium chloride, which builds up in soil and damages sensitive crops. Dr Forest's is naturally mined potassium sulphate (K₂SO₄), certified for use in organic production under Regulation (EC) 834/2007 and handmade in small batches in Stockport.

At 50% K₂O and 18% sulphur this isn't a diluted compound or a blend — it's pure potassium sulphate in a uniform granulate that spreads evenly and dissolves on contact with soil moisture. Two essential macronutrients, immediately available, with no chloride, no nitrogen and no phosphorus to upset a carefully balanced feeding programme.

50%K₂O (Potash)
18%Sulphur (S)
0%Chloride
0-0-50NPK Analysis

What sulphate of potash is used for in the garden

  • Fruiting and flowering crops — potassium regulates sugar transport, speeds ripening and intensifies flower colour; essential for tomatoes, strawberries, peppers, roses and all fruiting plants
  • Flavour and quality — research consistently links potassium sulphate to higher soluble sugars, vitamin C and dry matter than untreated controls
  • Chloride-sensitive crops — berries, grapes, potatoes, tomatoes, citrus and salad crops all perform better on a sulphate-based source that avoids chloride build-up
  • Drought and frost resistance — potassium controls stomatal opening and cell turgor, cutting water loss and improving survival in temperature extremes
  • Disease resistance — adequate potassium strengthens cell walls and activates plant defence enzymes, lowering susceptibility to fungal and bacterial problems
  • Sulphur supply — 18% sulphur supports amino acid synthesis, chlorophyll production and nitrogen use; particularly important for brassicas and alliums
  • Lawns and turf — potassium hardens turf for winter and improves drought tolerance and spring green-up without an excess-nitrogen flush
  • Balancing NPK feeds — zero nitrogen and zero phosphorus make it ideal for lifting the K of any feeding programme without touching the N or P

Why sulphate of potash instead of muriate of potash?

Sulphate of potash (K₂SO₄) — this product

  • 50% K₂O — high-concentration, chloride-free potassium
  • 18% sulphur — a second essential macronutrient in every application
  • Almost zero chloride — safe for sensitive fruit and veg
  • Low salt index — minimal osmotic stress on roots
  • Certified for use in organic production under Regulation (EC) 834/2007
  • Sulphate-S improves nitrogen uptake efficiency

Muriate of potash (KCl)

  • 60% K₂O — more potassium per kg, but at a cost
  • 47% chloride — accumulates in soil and the root zone over time
  • No sulphur — delivers a single nutrient
  • Higher salt index — greater risk of root burn and osmotic stress
  • Linked to lower starch, dry matter and vitamin C in potatoes (Koch et al., 2022)
  • Poorly suited to containers and tunnels where leaching is limited
Handcrafted in Stockport

Every Dr Forest product is made by hand in small batches at our workshop in Stockport, Greater Manchester. Recyclable packaging throughout, ingredients chosen for quality rather than cost, and no slaughterhouse by-products anywhere in the range.

The science of potassium and sulphur in plant nutrition

Potassium: the quality nutrient

Potassium is the most abundant cation in plant tissue and the single most important nutrient for fruit quality. It never becomes part of an organic molecule — instead it works as a free ion, regulating water pressure, activating over 60 enzymes, balancing electrical charge and moving sugars from leaves into developing fruit. Plants short of potassium produce smaller, blander fruit with poorer shelf life and weaker resistance to disease and stress.

Unlike nitrogen, which drives leafy growth and is easily over-applied, potassium can't really be made excessive in a practical garden. It is the nutrient most often under-supplied in container growing and intensive vegetable production.

Potassium decides size, flavour and shelf life — not just yield.

Why the potassium source matters

Roughly 96% of the world's potassium fertiliser is sold as potassium chloride, or muriate of potash. It's cheap and concentrated. But the chloride ion it carries isn't inert — it accumulates in soil, raises salinity and damages sensitive crops directly. The choice between chloride and sulphate as the accompanying anion has measurable consequences for crop quality.

01

Potato quality — starch, sugars & vitamin C

A two-year field trial by Koch et al. (2022) compared K₂SO₄ and KCl on two potato cultivars. Potassium sulphate held higher starch and ascorbic acid (vitamin C), while KCl-treated tubers built up more reducing sugars in storage — the precursors to acrylamide during cooking. The KCl treatment also carried more lipid-derived off-flavour compounds.

02

Fruit weight & soluble solids

In pineapple, swapping KCl for K₂SO₄ at 20% less total potassium produced larger fruit and better bromatological quality, including total soluble solids and vitamin C (Arias-Vázquez et al., 2018). The sulphate-fed plants won on less total potassium — source matters as much as quantity.

03

Chloride accumulation & root-zone salinity

Potassium sulphate adsorbs to soil particles more strongly than KCl, so it leaches less (Tisdale et al., 1999). In containers, where leaching is limited, that matters. Chloride from KCl accumulates in the root zone, raising osmotic potential and reducing water uptake. Sulphate ions don't carry that risk.

04

Sulphur, the fourth macronutrient

Sulphur is essential for the amino acids cysteine and methionine — without them protein synthesis stalls. It's a structural part of coenzyme A and thiamine and is needed for chlorophyll. Sulphur deficiency caps nitrogen efficiency: adding N to a sulphur-short soil gives diminishing returns. Every application here delivers 18% S alongside the potassium.

05

Potassium & disease resistance

Adequate potassium thickens cell walls, increases cuticle wax and activates pathogenesis-related (PR) proteins. Potassium-sufficient plants show fewer fungal problems including powdery mildew, botrytis and fusarium wilt. The mechanism is mostly physical — stronger walls are harder for fungal hyphae to penetrate — backed by faster enzymatic defence.

06

Potassium & water regulation

Potassium is the main ion controlling stomatal aperture. When it's adequate, guard cells close stomata quickly under water stress to cut transpiration — which makes potassium the single most important nutrient for drought tolerance. Well-supplied plants also recover faster from frost, since potassium lowers the freezing point of cell sap and protects membranes.

Scientific references

  1. Koch, M. et al. (2022). Comparison of the effects of potassium sulphate and potassium chloride fertilisation on quality parameters of potato tubers. Frontiers in Plant Science, 13, 920212.
  2. Arias-Vázquez, E.L. et al. (2018). Effects of potassium chloride and potassium sulphate on 'MD-2' pineapple fruit yield and quality. Acta Horticulturae.
  3. Tisdale, S.L. et al. (1999). Soil Fertility and Fertilizers. 5th ed. Prentice Hall.
  4. Mengel, K. & Kirkby, E.A. (2001). Principles of Plant Nutrition. 5th ed. Kluwer Academic.
  5. Sharma, U.C. & Sud, K.C. (2001). Effect of potassium sources on potato yield and quality in acidic and alluvial soils. J. Indian Potato Assoc., 28, 70–71.
  6. Kumar, P. et al. (2004). Effect of sulphate and muriate of potash on quality of potato. Annals of Agricultural Research, 25(3).

How to use sulphate of potash: application rates & guide

Measuring & spreading

A level teaspoon of granulate is roughly 5g. The granules are free-flowing — broadcast by hand or spreader, top-dress around plants, or stir into compost and potting mixes. Always water in after a soil application so the potassium reaches the root zone.

Application rates — soil

Soil mix — potting and container preparation

Rate: 1–2g per litre of compost | When: at planting

Mix thoroughly through compost or potting soil before planting. Provides baseline potassium and sulphur for the first 4–6 weeks. Ideal for tomatoes, peppers, strawberries and other fruiting crops.

Top dressing — established containers

Rate: 2–6g per 10-litre pot | Frequency: every 4–6 weeks in the growing season

Scatter granules evenly over the soil surface and water in well. Use the higher rate for heavy-fruiting crops at peak production. Work lightly into the top centimetre of soil where you can.

Outdoor beds and borders

Rate: 20–50g per m² | Frequency: every 6–12 weeks, spring to autumn

Broadcast evenly across the soil surface and water in. Lower rate for maintenance; higher rate for heavy-fruiting crops, new plantings, or where soil potassium is known to be low.

Lawns and turf

Rate: 20–35g per m² | Frequency: twice a year — spring and autumn

Apply in spring for active growth and in autumn to harden turf for winter. Water in immediately. Potassium improves drought tolerance, wear resistance and winter colour.

Dissolving for liquid feed

Granulate dissolves more slowly

The granulate is fully water-soluble, but it dissolves more slowly than a fine grade. Stir well, or dissolve in warm water and leave to stand for a few minutes before topping up. For a foliar spray, dissolve completely and strain through fine mesh first to protect your sprayer.

Liquid feed / fertigation — root drench

Rate: 1–2g per litre of water | Frequency: weekly to fortnightly during fruiting/flowering

Dissolve, then apply at the base of the plant. Ideal for topping up potassium when demand peaks. Lower rate for maintenance, higher rate when plants are in full production.

Foliar spray

Rate: 5–10g per litre of water | Frequency: every 2 weeks during fruiting

Apply to both leaf surfaces in early morning or late evening. Foliar potassium is absorbed quickly and can ease deficiency within days. Dissolve fully and strain before spraying.

Step-by-step

  1. Identify your potassium need. Fruiting and flowering crops have the highest demand. Scorched leaf edges, poor fruit set and weaker disease resistance are classic deficiency signs.
  2. Measure the rate. Use the rates above as a starting guide — a level teaspoon of granulate is about 5g.
  3. Apply and water in. For soil, scatter evenly and water thoroughly. For liquid feeding, dissolve fully before applying. Watering in moves potassium into the root zone.
  4. Time it to demand. Potassium demand peaks through flowering and fruit development. Start when the first flowers appear and carry on to harvest.
Works well combined with…

Pair with Dr Forest crop feeds — Tomato, Rose & Flower and Fruit & Vegetable — where extra potassium is wanted during peak fruiting. Combine with Seaweed for biostimulant activity and Cal-Mag where calcium is also needed. Zero nitrogen means it won't disturb bloom-phase ratios.

Further reading

New to potassium minerals? See our guide to polyhalite, a multi-nutrient potassium mineral, and why tomato leaves turn yellow — often a potassium or magnesium signal.

Frequently asked questions about sulphate of potash

Both deliver potassium, but the accompanying ion differs. Muriate of potash (potassium chloride) carries 47% chloride, which can accumulate in soil and damage sensitive crops. Sulphate of potash is almost chloride-free and adds 18% sulphur. For container growing, tunnel crops and chloride-sensitive plants like tomatoes, strawberries and potatoes, sulphate of potash is the safer, better-quality choice.
Yes. This sulphate of potash is certified for use in organic production under Regulation (EC) 834/2007 as a naturally mined crude mineral salt — no synthetic processing and no chemical additives. It's approved for use in organic growing systems. (In the EU and Northern Ireland the equivalent regulation is now (EU) 2018/848.)
Potassium demand peaks through flowering and fruiting — begin when the first flowers appear and continue to harvest. For lawns, apply in spring and autumn. For general soil maintenance, once or twice in the growing season. Leaf-edge scorching, poor fruit set or rising disease are deficiency signs that warrant an immediate application.
As a guide: 1–2g per litre when mixing into compost, 2–6g per 10-litre pot as a top dressing every 4–6 weeks, 20–50g per m² on outdoor beds, and 20–35g per m² on lawns. A level teaspoon of granulate is roughly 5g. Start at the lower end and increase for heavy-fruiting crops.
Yes. Potassium sulphate is water-soluble, though the granulate dissolves more slowly than a fine grade. Stir well, or dissolve in warm water and let it stand for a few minutes. For foliar spraying, dissolve completely and strain through fine mesh first to protect your sprayer. For most gardeners, applying to the soil and watering in is the simplest route.
Sulphate of potash has a low salt index compared with muriate of potash, so it's much less likely to scorch roots. Apply at the recommended rates, water in afterwards and keep granules off stems. At normal garden rates the risk is very low.
Most liquid tomato feeds are balanced NPK fertilisers with nitrogen, phosphorus and potassium together. Sulphate of potash is a single-nutrient supplement — only potassium and sulphur. That makes it ideal for lifting potassium on its own, without adding nitrogen (which pushes leafy growth at the expense of fruit) or phosphorus.
All fruiting and flowering crops: tomatoes, peppers, chillies, strawberries, raspberries, grapes, courgettes, cucumbers, roses, dahlias and sweet peas. Root crops like potatoes and carrots benefit too, since potassium improves starch and storage quality. Brassicas and alliums make particular use of the sulphur.
Yes. It pairs naturally with any Dr Forest blend where extra potassium is wanted — usually at peak fruiting or flowering. With zero nitrogen and zero phosphorus, it won't disturb the NPK ratio of your base feed. Many growers run a crop-specific fertiliser as the base and add sulphate of potash as a targeted booster during heavy production.
Keep it cool and dry in the sealed bag. Potassium sulphate isn't hygroscopic under normal conditions, so it won't draw in moisture and clump if kept sealed. Stored properly, shelf life is effectively indefinite — it's a stable mineral salt.
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