A low pH reading (below 7) indicates an acid soil that is sandy or peaty. A high pH reading (above 7) indicates an alkaline soil that is limestone or chalk based.
How acid or alkaline a soil is depends on how much calcium it contains. This has a significant effect on plant growth.
Too little calcium means the soil is acid and many plants will not thrive. The answer is to add lime. Another important nutrient, magnesium (Mg), may be low in acid soils. Remedy magnesium deficiency with magnesium sulphate (Epsom salts).
Too much calcium can restrict the uptake of trace elements ('locking up' nutrients), depending on plant species. This happens on alkaline soils.
The leaves of plants which prefer acid conditions turn yellow in alkaline soils, as they cannot absorb iron and manganese.
The pH scale
Calcium levels are measured by the pH scale, 0-14. You can measure garden soil with a pH testing kit bought from a garden centre, so you know whether to increase the level of lime or select appropriate, chalk-loving plants.
The kit indicates the approximate pH value, which is precise enough in the garden because all plants possess some tolerance. Many plants thrive in most soils, and even those which prefer lime are usually tolerant of slightly acid conditions.
You can use a pH meter to test soil, but these can be influenced by variations in the level of soil moisture.
Soil must not be too acid for cabbages and other brassicas, so add lime in the winter if your test shows a pH below 6.
BUYING TIP
With plants that require acid soil, be sure to buy compost described on the label as ericaceous (suitable for lime-hating plants which require soils of pH 6.5 or less). This compost also allows bulbs and other plants, which tolerate a higher pH, to thrive.
Correcting acidity with lime
Any application of lime takes time to be distributed through the soil. Suitable materials are hydrated lime, ground chalk, ground limestone, dolomitic (magnesian) limestone and mushroom compost.
• The best are ground chalk or limestone, and the finer the grade the more rapid the effect.
• Hydrated lime is also fast acting but it is highly soluble and rainfall can quickly wash it into the subsoil. Garden lime is often a mixture of hydrated lime and ground limestone.
• Dolomitic (magnesian) limestone supplies useful amounts of magnesium as well as lime.
• A cheaper and easily applied alternative is discarded mushroom compost, but do not apply it to soils planted with acidloving plants.
Clay soils are extremely difficult to cultivate when either very wet or very dry because they compress into putty-like lumps. Adding bulky organic matter is the best way to improve the structure, but you can also apply ground chalk or limestone.
As well as making it easier to dig, adding these materials helps plants to become established and grow more quickly.
Rhododendrons demand acid soil. Many grow moderately well on slightly acid soil but none tolerate an alkaline pH.
IMPORTANT
Lime is significant in at least two soil-borne diseases.
• Clubroot, which is highly damaging to brassicas (the cabbage family), can be weakened by adding lime to acid soil.
• Common scab, which is a disfiguring disease of potatoes, is aggravated by highly alkaline soils. Do not add lime.