How to make soil more acidic
To make soil more acidic, mix in sulphur and give it a season.
The RHS rates elemental sulphur the cheapest way to acidify soil and the least likely to harm plants. Soil bacteria turn sulphur into acid a little at a time, and once the pH is down it stays down for years. To lower the pH by one unit you need about 55 g/m² on sandy soil, 110 g/m² on loam and 175 g/m² on clay. Test first, and go gently.
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- Test first. Most veg and lawns are happiest at pH 6.0 to 7.0 and need no sulphur.
- Apply from spring to early autumn, when the soil is warm, and retest after three to six months.
- Soil that fizzes with vinegar is chalky and cannot realistically be acidified (RHS).
How much sulphur per square metre
| Where | To lower pH one unit | How to apply |
|---|---|---|
| Sandy soil | 55 g/m² | One dose |
| Loam | 110 g/m² | Two doses of 55 g/m², six months apart |
| Clay | 175 g/m² | Three doses of about 58 g/m², six months apart |
| Established planting | Up to 25 g/m² a dose | At most twice a year, early spring and late autumn |
| Lawns, only above pH 7.5 | 15 g/m² | At most twice a year; water in straight away |
| Pots | Do not use | Use ericaceous compost |
Beds: Ohio State, Purdue and University of California. Established planting: University of Georgia and Cornell. Lawns: Purdue (pH range) and Kansas State (timing). Pots: RHS. For half a unit, use half. Never more than 73 g/m² in one dose on beds, 146 g/m² a year on bare soil, or 112 g/m² a year on planted ground.
Work out your exact amount. Enter your area, your test reading and your soil type, and the calculator gives the dose, the timing and the bags to buy.
How to do it
- Know your target. Blueberries want pH 4.5 to 5.5, rhododendrons and azaleas 4.5 to 6.0, camellias 5.0 to 6.5. Not sure of your reading? See how to test soil pH.
- Scatter, water, then fork in. Scatter the pastilles and water them. After a few days they swell and break apart; then fork them into the top 15 cm. Left on the surface, sulphur can take years to work (RHS). Wear gloves, and on large areas goggles and a dust mask.
- Wait, then retest. At least three months, ideally six, before any further dose.
- Keep it there. Water with rainwater and avoid mushroom compost, which contains lime. Composted bark makes a good lime-free mulch, though it will not lower the pH itself.
What does not work
Vinegar is buffered back within a day or two. Used coffee grounds are close to neutral, about pH 6.5 to 6.8 (Oregon State University Extension). Peat works but is no longer recommended (RHS). More on the home remedies in how to make soil acidic naturally.
Faster options, and the catch
Iron sulphate and aluminium sulphate act within weeks, but you need several times as much for the same drop: about eight times as much with iron sulphate (RHS). Large doses can interfere with phosphorus, and aluminium can build up in the soil (RHS). Use them for a quick nudge; use sulphur to change the soil. See iron sulphate vs sulphur and aluminium sulphate for hydrangeas.
Common questions
How long does sulphur take to make soil acidic?
It starts within weeks in warm, moist soil and slows right down in cold soil: the bacteria that do the work slow two- to four-fold for every 10 °C drop (Chapman, 1996). Allow a full season, and start a year ahead for a new blueberry bed.
Can I make chalky soil acidic?
Not realistically. Chalk holds free calcium carbonate, which neutralises the acid as fast as it forms (RHS). If a dry spoonful of soil fizzes with vinegar, grow acid-lovers in pots or raised beds of ericaceous compost.
Can I lower the pH of clay soil?
Yes, but clay resists change, so it needs about three times as much sulphur as sand, in three doses six months apart.
Do I need to make the soil acidic for veg or lawns?
Rarely. Most veg and lawns do best at pH 6.0 to 7.0. Only treat soil that tests above the pH your plants need, and never the brassica bed.
Can I use sulphur in pots?
No. The RHS advises against sulphur in potting media. Use ericaceous compost and rainwater instead.
Is iron sulphate better than sulphur?
Faster, not better. It regreens plants within weeks but needs about eight times as much product for the same pH drop (RHS) and stains paving. See iron sulphate vs sulphur.
More: Sulphur Soil Acidifier · soil pH calculator · how to test soil pH · make soil acidic naturally · soil for blueberries · how to make hydrangeas blue · what is ericaceous compost?
One pH unit, one season, years of acid soil
90% elemental sulphur in pastilles that spread without dust and break apart when wet. 55 to 175 g/m² per pH unit, by soil type.
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Free UK delivery over £40. Buy now goes straight to checkout.
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