Sweet and sour

Another important soil feature is its pH.

pH is a measure of acidity or alkalinity. The pH scale ranges from 0 to 14, with 7 being neutral. Above 7 is alkaline ("sweet"), below 7 is acidic ("sour").

The majority of plants thrive best when the soil is slightly acidic, pH 5.8-6.3. Some, such as rhododendrons and azaleas, prefer it more acidic (around pH 5-5.5); others such as dianthus and some members of the cabbage family (brassicas) prefer it more alkaline (pH 7-7.5) and don't do well in acidic conditions.

Soil pH affects the availability of essential nutrients to the plants. As pH moves either above or below 5.5-7.5, many plants begin to suffer nutrient deficiencies that can severely affect growth.

Raising ph

You can raise pH by adding lime to the soil, but don't overdo it. The amount you need to apply to raise the pH depends on the existing pH, the soil type, the type of lime used and what pH you want to end up with. Unless you've had an accurate pH test done, we recommend caution. Liming every year out of habit can raise the pH too high. Don't lime unnecessarily and even then follow packet recommendations or advice based on a reliable soil test.

Results of our tests in the past suggest many pH testing kits and meters don't give accurate readings. Soil-testing labs can give much more reliable results along with some advice, usually for around $40-50. Look under Soil Testing in the Yellow Pages of larger centres.

Lime also has the added beneficial effect of encouraging clay particles to combine together into very small groups, which helps improve the structure of heavy soil. It comes in different forms but the two easiest types to use are garden lime (calcium carbonate) and the more expensive dolomite (calcium and magnesium carbonates combined). They're best applied in autumn to have an effect by the following spring. You can sprinkle lime on the surface and leave it to wash in over time, or fork in - either way works well.

Gypsum (calcium sulphate) is also useful in helping to condition a heavy soil. It has much the same effect on clay as lime but does not raise pH, and for this reason is often an ingredient in products promoted as "clay-breakers".

Lowering pH

You can lower pH by adding acidic organic matter such as peat, mixing finely ground sulphur (often sold as flowers of sulphur) into the soil, or using acidic fertilisers such as those specifically formulated for azaleas and rhododendrons.