The acidity of your soil

Pay attention to plants that require either an acidic or alkaline soil!
The acidity of soil along with its texture plays an important role for certain plants that require specific factors in this domain. An alkaline soil is often charachterised by its high chalk content. An acidic soil is likewise characterised by its total lack of chalk or limestone. A plant that demands an acidic soil will suffer in a chalky soil. Conversely an alkaline loving plant will perish in an acidic soil. It is therefore very important to know the acidity of your soil.

To find this out there are several methods available. First of all use your sense of observation; certain plants that establish themselves on their own can indicate an acidic soil, such as gorse, foxgloves, broom, wild sorrel... and of course heather, which always grows in an acidic soil. In contrast plants that like an alkaline soil such as white oaks, laburnum, field maple, iris, rosemary etc. all suggest a surrounding alkaline soil.

This information is useful because where you find wild growing plants it is normally easy to grow the cultivated variety. These indications are always relative as the soil can change within quite a short distance. To be on the safe side try and take a more accurate measure.

Heather and the Maritime pine are good indicators of an acidic soil

A scale for measuring
Acidity is measured in terms of pH ("potential Hydrogen"). It is always between 1(extreme acidity) and 14 (extreme alkalinity), a value of 7 is considered to be neutral (neither acidic nor alkaline). In the garden the pH value is usually between 5 (very acid soil) and 9(very alkaline soil). The exact pH cannot be guessed and you need to use a special kit, which allows you to estimate it. Their use is very simple and informative.

Values between 6 and 7 indicate that the soil is slightly acidic. It is favourable for acid loving plants but can be used for growing a large number of plants. A lower value would indicate a really acidic soil: vegetables would grow badly, certain shrubs would be unlikely to do well such as the Judas tree, lavender and rock roses… however camellias, Japanese maple and hortensia would thrive...

The Aleppo pine is usually found on chalky soils

Alkaline soils
A value of more than 7 shows that the soil is alkaline (this soil is sometimes called basic meaning that it is rich in the alkaline based materials). It will contain a high level of chalk especially if the pH is between 8 and 9. To grow rhododendrons or pieris will be very difficult: you would need to fill the planting holes with a peat based soil. The plants that are most sensitive to alkaline conditions like roses and hortensia would probably turn yellow due to the iron chlorate, an unfortunate element often found in alkaline soils. The only solution is to spray with a sequestered compound as soon as the symptoms are noticed. A regular dose of well rotted manure around their base will help avoid this problem. Many plants will thrive in alkaline soil including pines, fig trees, box cotoneasters etc.

Nature however is not always so fickle and many plants are indifferent to the soil’s acidity and are equally at home in either acid or alkaline soils. Good examples would be clematis, agapanthus, cultivated ivy, apple trees, lilacs etc. And if as an adventure you try to change the acidity of your soil, well it is possible! By adding sulphur powder or iron sulphate it becomes more acidic and in the opposite sense the incorporation of slaked lime or dolomite to the soil will turn it more alkaline.