Acidity and alkalinity are measured by what is known as the pH scale, which runs from 0 to 14; neutral is 7. Readings higher than 7 mean that the soil is progressively more alkaline; readings below 7 show that it is progressively more acidic. Although most vegetables do best in a slightly acid soil with a pH level of about 6.5, some – including brassicas, beans, onions and asparagus – prefer nearly neutral soil. Strawberries and potatoes require acid soil of pH 5.0–6.0. Most fruit trees and bushes require a neutral or slightly acid soil, although figs and grapes will tolerate conditions that are slightly alkaline. Most soils can be treated and corrected if they are too acid or too alkaline. Use a soil-testing kit to find out the pH of your soil.

Adding lime
If your vegetable plot is overly acid but you are not sure to what degree, it’s best to adopt a cautious approach so as to avoid the possibility of overliming. Apply a dressing of garden lime or dolomite at the rate of 180 g per square metre, or ground limestone at 250 g per square metre. Spread the lime on the surface and leave it to be washed in by rain or irrigation. Liming benefi ts clay soils and other heavy soils, as well as loams and acid sandy soils. Routine liming as part of a crop rotation program will probably be needed on these soils. Add it before growing cabbages, cauliflowers and other members of the Brassica family in your crop rotation.

Neutralising lime
Although soils that are acid can be corrected relatively easily, it is more difficult to make a limey soil neutral. A method worth trying, when digging in autumn, is to mix 250 g of flowers of sulphur with a bucket of partially rotted pine needles, which are acidic. Fork in a bucketful per square metre. In addition, build up the humus content of the soil with manure and compost, particularly leaf mould.