Iron sulphate vs sulphur
Iron sulphate is quick. Sulphur goes eight times further.
Iron sulphate acts within weeks and supplies iron, so it can green yellow leaves fast, and it kills moss. For the same drop in soil pH you need about eight times as much (RHS), and it stains paving. Elemental sulphur takes a season, because soil bacteria release the acid slowly. Use iron sulphate for a quick fix; use sulphur for a big change.
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- Changing border pH for blueberries, rhododendrons or blue hydrangeas: sulphur.
- Greening a yellow plant this month, or killing lawn moss: iron sulphate.
- Both: iron for now, sulphur for next year.
Side by side
| Sulphur | Iron sulphate | |
|---|---|---|
| What it is | Elemental sulphur, 90% | Ferrous sulphate |
| Speed | A season: soil bacteria do the work | Weeks |
| Product for the same pH drop | 1 | About 8 times as much (RHS) |
| Supplies iron | No | Yes |
| Kills moss | No | Yes |
| Stains paving | No | Yes, rust-orange |
| Can interfere with phosphorus | Not directly | Yes, in large amounts (RHS) |
Amounts, speed and phosphorus: RHS, Acidifying soil. Sulphur rates as on our Sulphur Soil Acidifier page.
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.
Which to use
- New bed for acid-lovers. Sulphur, worked in at least six months before planting, ideally a year.
- Established shrub with yellow leaves. Iron sulphate or chelated iron to regreen now; sulphur at up to 25 g/m² for the long term.
- Mossy lawn. Iron sulphate kills moss. Neither fixes the shade, wet or compaction behind it.
- Pots. Neither. Use ericaceous compost and rainwater.
Common questions
Does iron sulphate lower soil pH?
Yes, quickly, but it takes about eight times as much product as sulphur for the same drop (RHS), and large amounts can interfere with phosphorus.
Can I use iron sulphate and sulphur together?
Yes. Iron sulphate greens the plant while sulphur works on the soil. Keep both within the rates on the pack.
Is iron sulphate safe for blueberries?
In small doses, as a tonic. For lowering the bed pH, sulphur is the better tool.
Why does iron sulphate stain?
It oxidises to rust. Brush it off paths and patios before watering in.
More: Sulphur Soil Acidifier · aluminium sulphate for hydrangeas · soil pH calculator · how to make soil more acidic · rhododendron leaves turning yellow · blueberry leaves turning yellow · does lime kill moss?
For a big pH change, use sulphur
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.
Buy the size for your border
Free UK delivery over £40. Buy now goes straight to checkout.
Free UK delivery over £40. Dispatched in 1 to 2 working days. Packed and posted from Stockport.