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).
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.
Most garden centres stock sulphate of iron.