Understanding Soil pH for Healthier Plants

Why Soil pH Matters More Than You Might Think

Soil pH measures how acidic or alkaline your soil is, on a scale running from 0 to 14. Seven is neutral, below seven is acidic, above is alkaline. Most UK garden soils sit somewhere between 4.5 and 8.5, and that number quietly governs almost everything that happens underground.

The reason pH matters so much is nutrient availability. It is not that your soil lacks iron, phosphorus or manganese — it is that at the wrong pH, plants simply cannot reach them. In alkaline soils above pH 7.5, iron and manganese lock up, and you see it as yellowing between the veins of rhododendrons, camellias and blueberries. In strongly acidic soils below pH 5.5, phosphorus binds tightly to aluminium and iron, and aluminium itself becomes soluble enough to damage root tips. Soil life suffers too: earthworms, bacteria and fungi that break down organic matter all work most efficiently around neutral.

Get the pH roughly right and you will spend far less time puzzling over pale leaves and stunted growth.

Testing Your Soil Before You Plant

Always test before you dig, feed or plant anything new. It takes twenty minutes and saves years of guesswork.

• Take samples from ten to fifteen spots across the bed, using a clean trowel or auger.
• For beds and borders, sample to about 15cm deep; for lawns, 10cm is plenty.
• Mix the samples in a bucket, remove stones and roots, and let the soil dry slightly.
• Test with a colour comparison kit — a few pounds from any garden centre and accurate enough for gardening purposes.
• Cheap probe meters are tempting but unreliable, especially in dry or stony ground. For a definitive reading, send a sample to a laboratory.

Autumn and early spring are ideal testing times, when soil is moist but not waterlogged. Repeat every two to three years, sampling the same spots, so you can see which way your soil is drifting.

What Your Plants Actually Want

Rather than chasing a single perfect number, aim for the range your plants prefer:

• Strongly acid (pH 4.5–5.5): rhododendrons, camellias, pieris, heathers, blueberries, Japanese maples.
• Slightly acid (pH 5.5–6.5): potatoes, strawberries, raspberries, most conifers, hydrangeas (though pH decides whether the flowers turn blue or pink).
• Neutral to slightly alkaline (pH 6.5–7.5): the majority of vegetables, brassicas, onions, beans, roses, clematis, most lawns.
• Alkaline (pH 7.5+): lavender, buddleja, wallflowers, carnations, sea holly.

Raising pH with Lime

If your soil is too acidic for what you want to grow, lime is the answer. Use garden lime (calcium carbonate) or dolomitic lime (magnesium carbonate), the latter being useful on light soils where magnesium is low.

Apply in autumn or early winter, scattering it evenly over the surface and letting rain and worms carry it down. On a loam soil, roughly 500g per square metre raises pH by about 0.5. Heavy clay may need double that; sandy soils noticeably less. Never exceed 500g per square metre in a single application — split it over two seasons instead.

Hydrated lime acts faster but is caustic, so wear gloves and a mask and keep it away from skin and eyes. Do not lime at the same time as spreading manure or fertiliser, as lime drives off nitrogen. Wood ash is a gentler alternative, but never use ash from coal or treated timber. Re-test the following spring before adding more, and keep lime well away from acid-loving plants.

Lowering pH with Sulphur

Alkaline soil is harder to shift, but it can be done. Elemental sulphur (often sold as flowers of sulphur) is the standard choice. It is slow-acting: soil bacteria convert it into sulphuric acid over several months, so apply in autumn for the following spring.

Around 100g per square metre lowers pH by roughly 0.5 on loam, and less on sandy ground. Do not exceed 100g per square metre at once. Sulphate of iron works more quickly but is a short-term fix. For small beds and containers, ericaceous compost, leaf mould, composted bark and pine needles are easier and kinder — dig them in generously.

If you garden on chalk or limestone, be realistic. You can acidify a planting hole, but the surrounding soil will pull it back. Choose lime-tolerant plants and work with your soil rather than against it.

Keeping It Right Through the Seasons

Soil pH is not fixed. UK rainfall is slightly acidic and steadily leaches calcium downwards, so most gardens drift towards acidity over the years — particularly in the wetter west and north. Chalky subsoil, hard water, concrete paths and liming push things the other way.

A practical routine: test every two to three years in the same spots, add organic matter annually, and mulch regularly. Organic matter buffers pH swings and improves structure, which matters just as much as the number itself. Rotate crops sensibly, note what you add and when, and make small adjustments rather than dramatic ones — they are far easier to correct.

Aim for the range your plants enjoy rather than a perfect figure, and remember that drainage, light and soil structure all play their part. Get the pH into the right zone, though, and your plants will reward you with stronger roots, greener leaves and better harvests, season after season.