Soil testing

Acid remarks over soil pH

Knowing the pH of your soil is vital so you know what you can – and can’t – grow.

Soil testing kit
Soil testing kit and my results

You wouldn’t buy a house without knowing whether its foundations were solid, yet most of us never bother to check whether our soil is suitable for the plants we want to cultivate.

I failed to grow a Rhododendron years ago. This genus (plus azaleas, hydrangeas, and heathers) are acid-loving plants, which hate lime, so I guessed my soil must be neutral or slightly alkaline.

Variegated Rhododendron
Variegated Rhododendron – no good in my soil. Picture; Bill Docherty’s Plants

I was right. Using a small pH test from the garden centre, it’s easy to take a sample, mix it with the chemicals and compare the resulting colour with the chart.

There’s a comprehensive list of what does best on acid, neutral or alkaline soils and suggestions on how you can improve what you’ve got.

My soil ranges from pH 7 (neutral) to 7.5 (alkaline), which is a decent value for most things. If you want to grow lime-haters, use ericaceous compost in large tubs or raised beds – that’s how I’m growing a Camellia at the moment.


Plants for alkaline or chalky/limestone soils

Chalky (and often limestone) soils are alkaline and the opposite of acid soils – on or above 7.5 if you do a pH test. They are free draining, so are drought-prone and lacking in nutrients. Your best bet is to think Mediterranean – plants that thrive in dry summers, need good drainage and moderate to low fertility.

Lavender will tolerate alkaline soil

If you have some clay in the mix, your soil’s nutrient levels will be higher, and its water retention, too. However, some chalky soils are incredibly shallow, making the normal planting of shrubs impossible without adding raised beds or adding huge quantities of topsoil.

The RHS has a comprehensive list of suitable plants – but establish how alkaline your soil is with a pH test and dig test holes to find out how deep it is before you hit bedrock.