Understanding Soil pH: Recommendations for Soil Types

By Libby Reeves

One of the first questions I always ask clients that come to Nicholsons for advice is, ‘What is your soil like?’ The soil is essential to anchor the roots and hold the plant upright. It is vital as a water source and is the supply tank for nutrients that will keep the plant healthy and enable it to grow. After discussions regarding drainage and levels of clay, the conversation turns to pH. But what is pH, and why do we worry about it so much?

Understanding Soil pH: Recommendations for Soil Types

The pH is the level of acidity or alkalinity within the soil. This is brought about by chemical reactions within the soil, and results in a pH value between 1 and 14: 1 being highly acidic, 14 being very alkaline. The middle point (7), is referred to as neutral. The chemical reactions when the soil is acidic are very different to those when it is alkaline.

An acidic soil is regularly high in aluminium, which is poisonous to plants in high enough doses. An alkaline soil is often deficient in iron and phosphorus, and both can be low in magnesium, which is one of the building blocks for chlorophyll, where photosynthesis takes place and creates food for the plant to survive. The pH of our soil is, therefore, significant for us as gardeners, as the success of the plants we grow is actively dependent upon this.

As you travel around the country, each area has its own soil type which is often either acidic or alkaline. The South Downs and the Chilterns are both known for having a chalk base rock which makes the soil alkaline, whereas the soil around Bracknell Forest (on the way to Heathrow Airport) is nearly all acidic. However, plants are well known for living in these challenging circumstances and many have adapted to thrive in these less-than-ideal areas.

So how do you determine the pH of your soil? The most reliable method is via a pH testing kit, which can be sourced reasonably cheaply from garden centres or on the internet. The ideal pH is around 6.5 (as most plants will live happily here and most nutrients required for growth are available), but if your soil falls in the range of 5.5 to 7.5, this can usually be easily remedied. In some areas of chalk soils, there may be loose lumps of chalk that have not broken down within the soil which will ensure a raised pH. You can test if this is the case in your garden by adding vinegar to a soil sample. If fizzing is witnessed, then free chalk is present.

Trees that are particularly suited to alkaline conditions include:

Understanding Soil pH: Recommendations for Soil Types

Whitebeam (Sorbus aria ‘Lutescens’)

Whitebeam is a member of the Sorbus or mountain ash/rowan family, but looks very different. A medium-sized tree that can reach a height and spread of around 10 metres, makes this a lovely statement for the larger garden. It has clumps of white flowers in spring, a lovely powder-grey foliage with large red berries in autumn, which are very popular with birds!

Understanding Soil pH: Recommendations for Soil Types

Beech (Fagus Sylvatica)

Beech is a wonderful statement tree for large gardens and parkland. Easily reaching 20 metres in height and spread, this has lovely classic bright green foliage which is particularly vibrant when resting beneath it on a hot summer’s day. The smooth grey bark is attractive, as are the bronze leaves that hold onto the tree well into winter.

Understanding Soil pH: Recommendations for Soil Types

Ornamental Pear Tree (Pyrus calleryana ‘Chanticleer’)

This is an upright tree of around 6 metres, making it suitable for small gardens. It has lovely white blossom in spring, followed by a glossy green leaf, typical of its edible counterparts. The autumn colour of this tree is lovely, and varies from yellow to a gentle orange. The foliage of this tree also holds well into November in a good year, it is often the last tree to lose its leaves on the nursery.

Trees that are particularly suited to acidic conditions include:

Understanding Soil pH: Recommendations for Soil Types

Liquidambar styraciflua

Liquidamber is a medium-to-large tree, often reaching at least 10-15 metres tall when mature. It has an upright habit and multiple lobed leaves that can sometimes cause this tree to be confused with a maple. Its greatest season is autumn when the leaves turn the most beautiful red before dropping over winter to start again in spring.

Understanding Soil pH: Recommendations for Soil Types

Magnolia sayonara

This is a small tree, ideal as a statement in any garden. It has white flowers, blushed purple, that appear in later spring, and therefore is less likely to be damaged by bad weather earlier on in the year. The foliage soon follows, and the tree creates a rounded habit. The RHS have given this tree an award of garden merit.

Understanding Soil pH: Recommendations for Soil Types

Rhododendrons

These are shrubs or trees that are well known to thrive in an acidic soil. There are a wide variety that have large showy flowers, some in bright colours from late winter to early summer, and there are plenty of evergreen varieties too.

Understanding Soil pH: Recommendations for Soil Types

The Fragrant Snowbelle (Styrax obassia)

This is a small-to-medium deciduous tree, ideally suited to a woodland garden with shelter and some dappled shade. It has long chains of small white fragrant flowers in late spring and the leaves turn butter yellow in autumn.

If you find your soil is not a suitable pH for a plant you really want to grow in your garden, do not despair. Container growing allows you to manipulate the soil pH easily. Choose ericaceous compost for acid-loving plants or add horticultural lime for alkaline-loving plants. Do keep in mind the extra watering and feeding vigilance that comes with this type of growing, and consider that many mulching materials will also affect the soil pH. The addition of well-rotted organic material is always helpful as, even if only for a short time, the addition of a range of nutrients will help feed the plant in a situation where they may not get as much as they need.

As well as helping with nutrient availability, some plants become less susceptible to diseases if grown in the right pH soil: ensure any brassicas in a vegetable garden have an alkaline soil to prevent the development of clubroot.

You can also add sulphur to an alkaline soil to decrease the pH, and it’s worth feeding acid-loving plants with sequestered iron can help these plants gain the nutrients they require to thrive. Any huge change to the pH of the soil in borders is often short-lived, so it’s better to consider your plant selection to suit your soil for the best results.

Our Plant Centre staff are on hand to help you with choosing the right plants for your soil type – pop in to see us with any questions!