Customer highlight
I decided to try granular potash to take some of the strain from regular watering. I can’t recommend this product enough. My fruit trees and flowers are thriving with little effort from me. I’ve made some bags up for family and friends and they’re impressed with the product. A little goes a long way. It’s clean and easy to use. The potash was delivered shortly after ordering. I would definitely order from Dr Forest again.
Editorial highlight from marketplace and store reviews — separate from the full review list below.
Sulphate of potash: high potash fertiliser, 50% K₂O, chloride-free
Sulphate of potash is a chloride-free potassium fertiliser (50% K₂O plus 18% sulphur) used as a high potash feed for fruit, flowers and roots, at 15 to 20 g per m² on beds in spring or 2 to 6 g per 10-litre pot. Potassium drives sugar transport, fruit ripening, flower colour and drought tolerance. Most budget potash supplies it as potassium chloride, which builds up in soil and damages sensitive crops. Dr Forest sulphate of potash is naturally mined potassium sulphate (K₂SO₄), a mineral salt of the kind permitted in organic growing, in a uniform granulate that spreads evenly.
What sulphate of potash is used for: potassium for plants, from tomatoes to lawns
- Fruit and flowers. Potassium moves sugars into fruit, speeds ripening and deepens flower colour: tomatoes, strawberries, peppers, roses and every fruiting plant.
- Chloride-sensitive crops. Berries, grapes, potatoes, tomatoes, citrus and salad crops all do better on a sulphate source that avoids chloride build-up.
- Sulphur in every dose. 18% sulphur for protein and chlorophyll, which brassicas and alliums use most.
- Lawns and turf. Potassium hardens turf for winter and helps it through dry spells without an excess-nitrogen flush.
- Lifting K on its own. No nitrogen and no phosphorus, so it raises the potash in any feeding programme without touching N or P.
At a glance: NPK 0-0-50 · 18% sulphur · chloride-free · granulated · water-soluble · 500 g to 36 kg bags · recyclable packaging
A single mined mineral salt with no synthetic processing and no additives, in recyclable packaging. Packed in Stockport.
What potassium and sulphur do in the plant
Potassium is the nutrient that matters most for fruit quality. It works as a free ion, regulating water pressure, activating enzymes and moving sugars from leaves into developing fruit. Short of it, plants give smaller, blander fruit that keeps less well. Too much is a problem too: it makes it harder for plants to take up magnesium and calcium, and on apples it makes bitter pit more likely. That is why the rates here are kept low.
Sulphate of potash vs muriate of potash: why chloride matters
Most potash fertiliser is potassium chloride (muriate of potash): 60% K₂O, but 47% chloride, which builds up in soil and harms sensitive crops. Sulphate of potash carries sulphur instead, and the difference shows in the crop.
Better potatoes
In a two-year field trial on two potato varieties, potassium sulphate gave tubers higher starch and vitamin C than potassium chloride. The chloride-fed tubers built up more reducing sugars in storage, the sugars that form acrylamide when potatoes are cooked (Koch et al., 2022).
No salt build-up in pots
Potassium sulphate holds to soil particles more strongly than potassium chloride, so less washes out (Tisdale et al., 1999). In pots and tunnels, where little drains away, chloride builds up around the roots and makes water harder to take up. Sulphate does not.
Sulphur and water
Sulphur builds the amino acids cysteine and methionine, and a plant short of it wastes nitrogen. Potassium opens and closes the leaf pores, so well-fed plants hold water better through dry spells (Mengel & Kirkby, 2001).
It is not a complete feed: no nitrogen, no phosphorus. It does not replace a soil test on fruit, and more is not better.
Scientific references
- 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.
- Tisdale, S.L. et al. (1999). Soil Fertility and Fertilizers. 5th ed. Prentice Hall.
- Mengel, K. & Kirkby, E.A. (2001). Principles of Plant Nutrition. 5th ed. Kluwer Academic.
How to use sulphate of potash: rates for beds, pots, fruit and lawns
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
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
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
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
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.
Liquid feeding
Liquid feed: root drench
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. Then apply at the base of the plant. Lower rate for maintenance, higher rate when plants are in full production.
Foliar spray
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 through fine mesh before spraying, to protect your sprayer.
On fruit trees and soft fruit
Fruit needs potash to colour, sweeten and ripen, and to firm up its wood for winter. It needs far less per square metre than a vegetable bed, though. Feed fruit once a year, with no summer repeat.
When: once a year in late winter or early spring (February to early April), as the buds begin to swell.
How: scatter it evenly over the ground under the branches, out to the branch tips, or along the row for cane fruit and strawberries. Keep it 30 cm from tree trunks, and off the stems of bushes, canes and strawberries. Water it in, or put it down just before rain. For trees growing in grass, clear a circle of turf first, or the grass takes the potash.
Used on its own (no other fruit feed), per m² of ground under the branches or along the row:
- Apples: none without a soil test. Only use it if a test shows low potash: 44 g at Index 0 (low), 30 g at Index 1. None at Index 2, or at Index 3 or above.
- Pears: 30 g with no soil test. With a test: 58 g at Index 0 (low), 44 g at Index 1, 30 g at Index 2, 14 g at Index 3 or above.
- Plums, cherries: 20 g with no soil test. With a test: 44 g at Index 0 (low), 30 g at Index 1, 16 g at Index 2, none at Index 3 or above.
- Raspberries, blackcurrants, redcurrants, gooseberries: 30 g with no soil test. With a test: 50 g at Index 0 (low), 36 g at Index 1, 24 g at Index 2, 12 g at Index 3 or above.
- Blackberries and hybrid berries, strawberries: 20 g with no soil test. With a test: 44 g at Index 0 (low), 30 g at Index 1, 16 g at Index 2, none at Index 3 or above.
Topping up a general fruit feed (such as All Purpose 4-4-4 or Fruit & Vegetable 4-5-6): raspberries, currants and gooseberries need only 10 g per m² on average soil, or up to 25 g per m² where a soil test shows low potash.
A level teaspoon holds about 5 g and a level tablespoon about 15 g. A plum tree 2 m across covers about 3 m², so it needs about 60 g, or 4 level tablespoons.
- Apples: more isn't better. Too much potash makes bitter pit (brown, corky spots in the flesh) more likely, shortens storage and crowds out magnesium. That's why apples only get it on soil tested low in potash. If older leaves yellow between the veins, use Kieserite rather than more potash.
- On soil already at Index 2 or above, you can give a double dose every other year instead of a single dose every year.
- Planting new fruit: only add potash before planting if a soil test shows it's low. Use 40 g/m² at Index 0 or 20 g/m² at Index 1, forked through the whole bed in autumn. Don't put it in the planting hole; concentrated potash can scorch new roots.
- Fruit in pots: mix 1–2 g per litre into the compost at potting. Then give 2–4 g per 10 litres of compost every 4–6 weeks from flowering, watered in. Stop by early July for trees; strawberries can carry on to the end of picking. Figs in pots get just two of these doses, in April and late May, and nothing after midsummer.
- Figs in the ground fruit best on lean soil. A fig with its roots restricted (in a lined planting pit) can have one light dressing of 5–10 g per tree (1–2 level teaspoons) in March–April. A fig growing unrestricted in mulched soil needs nothing unless growth is weak (less than about 30 cm of new shoot a year). Give a fig only one spring feed, this or a general fruit feed, and never feed after midsummer.
- Strawberries, raspberries, redcurrants and gooseberries are sensitive to chloride. Sulphate of potash is the potash form fruit growers use for them.
- Blueberries are sensitive to salts. Don't add it at planting. On an established bush, only use it if a soil test shows low potash, and then no more than 15 g per m².
Pair with Dr Forest crop feeds, Tomato, Rose & Flower and Fruit & Vegetable, where extra potassium is wanted during peak fruiting. Add Cal-Mag where calcium is also needed. No nitrogen means it won't disturb bloom-phase ratios.
Which potash? Slow-release rock potash: Potassium Rich Fertiliser 11% K₂O. Potassium plus calcium, magnesium and sulphur: Polyhalite. A straight, chloride-free dose: this one.
See why tomato leaves turn yellow, often a potassium or magnesium signal.
Sulphate of potash FAQs
Very few plants are harmed by potash at sensible rates, but not every plant needs extra. Leafy crops such as lettuce, spinach and cabbage, and lawns in spring, want nitrogen more than potassium. Acid-loving shrubs are fine with sulphate of potash, which doesn't raise pH, unlike wood ash. The real risk is overdoing it: too much potassium makes it harder for plants to take up magnesium and calcium, especially in pots and sandy soils. Keep it for flowering and fruiting crops, at 15 to 20 g per m² on beds in spring, and don't stack it on top of a high-potash feed.
It's water-soluble, so it's available to roots within days of watering in. How long it lasts depends on the soil. Clay and soils rich in organic matter hold potassium well; light sandy soils let it wash down in a wet winter. That's why the rate here is 15 to 20 g per m² in spring, with one repeat in summer for heavy-fruiting crops or light, sandy soil, ideally after a soil test. In pots, top-dress 2 to 6 g per 10-litre pot every 4 to 6 weeks. There's little point applying it in late autumn for next year.