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Every gardener eventually meets the term pH, usually on the back of a seed packet or a bag of compost. It is simply a measure of how acid or alkaline your soil is, running on a scale from 0 to 14. Seven is neutral. Below seven is acid, above seven is alkaline, and each whole number represents a tenfold change in acidity, so the difference between pH 5 and pH 6 is far greater than it sounds.
What matters for plants is not the number itself but what it does to the soil. pH controls how easily roots can take up nutrients. Iron, manganese and zinc become locked away in alkaline soils, whilst calcium and magnesium are harder for plants to reach in very acid ground. Even a soil stuffed with nutrients can starve your plants if the pH is wrong. Get it roughly right and many other problems quietly disappear.
The great majority of vegetables, flowering annuals, perennials, shrubs and lawn grasses grow best between pH 6.0 and 7.0. In that band, the widest range of nutrients sits in a form roots can absorb, and the bacteria and worms that break down organic matter are at their busiest. If you do nothing else, aiming for around 6.5 is a sensible target for a mixed British garden.
British soils tend to fall into two camps. In the wetter west and on heathland, sandy and peaty soils are often acid, sometimes as low as pH 4.5. In the east, on chalk and limestone, soils are frequently alkaline, sitting at 7.5 or even 8.0. Neither is a problem in itself, but it does shape what will thrive in your borders.
A soil test kit costs only a few pounds from a garden centre, and a simple probe meter costs a little more. Either is accurate enough for garden decisions. Testing takes twenty minutes and is worth repeating every two or three years.
If you need a precise figure, or you are managing a large plot, a professional laboratory analysis will also report nutrient levels alongside pH. For most gardens, though, a cheap kit tells you everything you need to know.
Once you know your number, plant selection becomes much easier. Working with your soil saves endless effort and disappointment.
If you desperately want acid lovers on chalky soil, grow them in containers filled with ericaceous compost and water them with rainwater. Planting them in a pocket of the wrong soil never works for long.
To make acid soil less acid, apply ground limestone or dolomitic lime. As a rough guide, one handful per square metre, about 100 to 150g, will lift a sandy soil by roughly half a pH point. Clay soils need more, sometimes double, because they resist change. Apply in autumn or early winter, fork it lightly into the surface, and retest the following spring before adding more.
Do not be tempted to overdo it. Raising pH above 7.5 locks up iron and manganese and encourages clubroot in brassicas, so liming to that level is counterproductive. Never lime at the same time as adding manure or general fertiliser, as the reaction releases ammonia and wastes nitrogen. Leave at least a month between the two.
Lowering pH is harder. Sulphur, applied at around 50 to 100g per square metre, acidifies slowly over several months, and repeated dressings of composted bark, pine needles or ericaceous compost help over the long term. For most gardeners on alkaline soil, choosing the right plants is far more rewarding than fighting the ground.
Think of soil pH as the foundation of everything else you do. Before buying a dozen new shrubs, before blaming a fertiliser for poor growth, spend a few pounds and half an hour finding out what your soil really is. Keep a note of the result, retest every few years, and let the number guide your planting rather than your hopes.
Gardeners who understand their soil spend less time nursing sickly plants and more time enjoying healthy ones. It is one of the quietest, cheapest and most useful habits you can build.
Jade Gorse shares practical, down-to-earth guidance on british gardening and plant care for readers across the UK.
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