Simply put, soil pH is the measure of how acid or alkaline your soil is.
Soil pH is measured on a simple scale that ranges from 0 to 14. The midpoint, pH 7, denotes a neutral soil, which is neither acid or alkaline. Anything below pH 7 is acidic (or ericaceous), and the lower the pH number, the more acidic the soil is. Alkaline soils are those with a pH above pH 7. Similarly, the higher the pH number, the more alkaline the soil is. Most soils in the UK range from pH 4.5 to pH 8.
Through many years of evolution, some plant species have specialised at growing in certain soil types. While this restricts them to growing in specific locations, it also gives them a competitive advantage. Fewer plants are able to grow in very acidic or very alkaline soils, meaning those that can have fewer plants to compete with for growing space.
Ericaceous or calcifuge plants are adapted to growing on acidic soils, and will grow poorly on alkaline soils. Calcioles are plants that are adapted to growing in alkaline or lime-rich soils, and will grow poorly on acidic soils.
The signs that a plant is not suited to the pH of the soil it’s growing in include chlorosis (yellowing of the leaves), stunted growth and curling leaves. This is because the availability of nutrients varies in soils according to their pH. For example, iron levels are relatively high in acidic soils, but lower in alkaline soils, whereas alkaline soils contain a relatively high amount of calcium compared with acid soils. Plants suited to growing in soils of a high or low pH are therefore suited to the nutrient components associated with them, and struggle to thrive in different soils.
More advice on understanding your garden soil:
Find out more about soil pH, below.
How to find out your soil pH
Soil pH test
Finding out your soil’s pH is simple. You just need to use a soil pH test kit – there are many available on the market and they’re cheap to buy.
Video guide: How to test your soil pH
If you don’t yet have a pH kit to hand, you might be able to get an idea of the soil pH by looking at the plants growing in the garden and surrounding landscape. The presence of plants like rhododendrons, camellias, heathers, pieris, magnolias and conifers like Scot’s pine and larch can all be indicative of an acid soil. Conversely, if you spot honeysuckle, lilac, clematis and lavender growing, the soil is more likely to be on the alkaline side.
Lilac ‘Lois Amee Utley’
In the UK, alkaline soils are typically those that sit on top of a bedrock of chalk or limestone. You might even come across chunks of these rocks in the soil. Areas of chalk soil in the UK include the North and South Downs, Yorkshire Wolds, Salisbury Plain and Chiltern Hills.
Discover some of the plants you can grow on alkaline soils and find out how to get the best from alkaline soil.
Honesty, Lunaria annua
Most soils in the UK are neutral, with a pH of 7, or slightly lower. They can vary in texture, from heavy clays to sandy loams and with this pH, you’ll find you’re able to grow the broadest range of plant, with no issues.
Judas tree, Cercis siliquastrum
In the UK, acid soils are more widespread than alkaline soils, and can be found making up large areas of heathland and coniferous woodland, such as Dartmoor National Park, York Moors, Yorkshire Dales, Brecon Beacons and the Cairngorms.
Discover some of the plants you can grow on acid soils and how to get the best from acid soil.
Changing your soil pH
Magnolia ‘Leonard Messel’
On a large scale, there’s little point in attempting to change your soil pH. The process would be expensive, time-consuming and there’s no guarantee it will work. You’d be better off moving house if you’re gardening on chalky alkaline soil and want acid soil, and vice versa.
However, you can grow acid-loving and alkaline-loving plants, even if you don’t have the right soil. Lots of plants can be grown in containers, including shrubs and trees, where you can more easily control the soil conditions. Raised beds and containers can be filled with ericaceous compost, and kept acidic with applications of sulphur or ferrous sulphate. As for acid soils, you can raise the pH by adding lime. Generally, it’s much easier to raise soil pH (make it more alkaline and less acidic) than it is to lower it.
However it’s also worth working with what you’ve got. There are many beautiful plants that thrive in alkaline and acid soils, and growing the plants that suit your soil pH will make your life easier than trying to grow the wrong type.