Acidifying garden soil will lower its pH so that ericaceous plants such as camellias, blueberries, heathers and rhododendrons can grow. It is usually only required if soil pH is neutral or alkaline. Sulphur is the most common acidifying material. Peat is no longer recommended.
Difficulty: Moderate to difficult
When to acidify soil
Soil-acidifying materials can be applied at any time of the year, but products containing sulphur take longer to work when the soil is cold so are normally best applied from spring to autumn.
Before adding any acidifying materials you need to check your soil pH to see how much (if any) you need to add.
It is especially worth testing soil pH before designing or planting a new garden that will contain ericaceous plants, or when growth of
Testing can be done at any time but, if carried out within three months after adding lime, fertiliser or organic matter, the test may give misleading results.
If your soil pH test comes back at 7.0 or lower, you already have acidic soil, but acidifying further, to between pH 5.0-6.0, may be necessary if you intend to grow ericaceous (lime-hating) plants. For other plants, if the pH is much below 6.5, you may wish to increase the pH by adding lime.
This is the common acidifying material. Soil organisms convert sulphur into sulphuric acid, so acidifying the soil. The more finely ground the sulphur the more quickly the bacteria can convert it; sulphur dust is quicker acting than sulphur chips (and more expensive). However, acidification by sulphur takes weeks to have an effect, and when the soil is cold in winter, months might be needed.
Although sulphur is the cheapest acidifier and least likely to harm plants, other materials are sometimes used:
This is used in hydrangea ‘blueing agents’ to obtain blue flowers where the soil conditions are not sufficiently acid to give blue flowers naturally. Aluminium sulphate can also be used as a soil acidifier. The effects are rapid, but large quantities can interfere with phosphorus levels in the soil and may also reduce pH excessively. In addition, repeated applications can result in a build-up of aluminium in the soil to toxic levels.
You need seven times as much aluminium sulphate as you do sulphur. The cost, while more expensive than sulphur, is similar to ferrous sulphate.
Ferrous sulphate (sulphate of iron)
This has a similar acidifying capability as aluminium sulphate and supplies iron. It acts quickly, Ferrous sulphate as it immediately dissociates in to iron ions that bind to the clay fraction of the soil, displacing hydrogen ions which remain in solution with the sulphate ions, which is in fact very dilute sulphuric acid - so washed in with plenty of water, the effect is speedy.
However, used in large quantities, it can interfere with the availability of phosphorus. This can be remedied by watering in monopotassium phosphate when any signs of phosphorus deficiency occur after the acidification process.
Most garden centres stock sulphate of iron, but as you need eight times as much compared to sulphur, it can be more expensive.
How to acidify soil
Quantities to apply
Soils rich in clay have a buffering capacity so much more sulphur is needed to change their pH than is needed to alter the pH of a sandy soil. Organic matter also acts as a buffer, so soils rich in organic matter will need more sulphur than ones with a low organic content.
Very alkaline soils will need very heavy doses of sulphur. If free lime or chalk is present, the soil cannot realistically be acidified. You can test for free lime or carbonate by adding vinegar to a soil sample. If ‘fizzing’ is seen, then free calcium carbonate is present.
To reduce the pH of the top 15cm (6in) of soil from neutral (pH 7.0), or slightly alkaline (pH 7.5), to slightly acid (pH 6.0-pH 6.5) sulphur powder may be required at 135-270g per sq m (4-8oz per sq yd), depending on whether the soil is sandy (lower figure) or clay (higher figure). Ferrous sulphate may be required at 1080-2160g per sq m (32-63 oz per sq yd), depending on whether the soil is sandy (lower figure) or clay (higher figure).
As some tree and shrubs roots penetrate deep into the soil, sulphur may have to be incorporated down to at least 30cm (1ft) which is much more laborious and costly. Because making the soil too acid can be very damaging to plants it is best to err on the cautious side and make multiple small additions over several months than to risk one large dose. Be prepared to experiment.
Method of application
Sprinkle sulphur over the soil to be treated at the rate required. Do this in still weather as the dust is very fine and drifts readily. Gloves, goggles and dust-mask are sensible precautions if treating large areas.
Sulphur is best incorporated, by cultivation, into the soil in advance of planting so it has plenty of time to take effect. Applied to the surface it can take years for the acidity to be changed at root depth. If deep-rooted trees and shrubs are to be planted it may be necessary to dig half the dose into the soil and cultivate the rest into the surface by hoeing, raking and cultivating. A rotovator is ideal, where available.
The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.