How do I test my soil pH and change it?

Taking a sample that means something, reading the result, and the realistic limits of what you can change in an open garden.

Difficulty: beginner
Time: 40 min
Read: 3 min

Short answer

Take several small samples from across the area, mix them, and test with a cheap chemical kit — they are accurate enough for garden decisions. Lime raises pH and takes months; lowering pH in open ground barely works, so grow lime-haters in pots of ericaceous compost instead.

pH matters because it controls whether nutrients already in your soil are available to plants. An iron-deficient rhododendron in chalky soil is not short of iron; it is unable to take up the iron that is there.

What you'll need

Soil test kit or pH meter
Trowel
Clean container
Garden lime (optional)
Sulphur chips (optional)

Step by step

Sample properly.Take five or six small samples from different points across the bed, at about 100mm depth, avoiding recently fed or limed spots. Mix them together and test the mixture.
Let the soil dry a little.Very wet soil dilutes chemical test kits and gives a misleading result. Crumble it and let it air-dry before testing.
Test more than one part of the garden.pH varies considerably across a single plot, especially where builders' rubble or old mortar is buried. A single reading tells you about one square metre.
Interpret the number.Most UK garden plants are happy between 6.0 and 7.0. Below 5.5 is acid; above 7.5 is alkaline and lime-haters will suffer. Vegetables generally prefer 6.5, brassicas nearer 7.0.
Confirm with the local evidence.Thriving rhododendrons, camellias and heathers in neighbouring gardens mean acid soil. Wild clematis, scabious and abundant chalk in the subsoil mean alkaline.
Raise pH with lime, slowly.Garden lime applied in autumn or winter at the rate on the pack. It takes months to act, and the effect is stronger on light soils than on clay. Never lime and manure in the same season — they react and waste the nitrogen.
Accept that lowering pH is hard.Sulphur chips lower pH slowly, but on chalk or limestone the underlying rock keeps buffering it back up. In genuinely chalky ground it is not a fight worth having.
Work with the soil instead.Grow acid-lovers in containers of ericaceous compost, watered with rainwater. Choose plants suited to the pH you have for the open ground.
Retest after a season.Changes take months. Test again the following year rather than assuming a treatment worked.

Tips

Test several spots — one reading describes one square metre, not a garden.
Look at what thrives in neighbouring gardens; it is free and it is accurate.
Rainwater is naturally slightly acidic, which is why lime-haters in pots need it rather than hard tap water.

Common mistakes

Liming every year as routine — Over-liming locks up iron, manganese and phosphorus and creates deficiencies that look like disease. Lime only in response to a test result.
Trying to acidify chalky ground for a rhododendron — The chalk beneath keeps neutralising whatever you add. The plant will limp along for years. A large container of ericaceous compost gives a far better plant.

If it doesn't work

Yellow leaves with sharply green veins on new growth

Cause: Iron unavailable because the pH is too high — Fix: Apply chelated iron as a short-term fix, and move the plant to a pot of ericaceous compost as the real one.

Clubroot in brassicas

Cause: Acid soil favours the disease — Fix: Lime to bring the pH towards 7.0 in the brassica bed and rotate crops.

Test readings vary wildly across the garden

Cause: Buried rubble, old mortar or made-up ground — Fix: Treat areas separately and choose plants per area rather than averaging.

No change after liming

Cause: Applied too recently, or a heavy clay soil buffering it — Fix: Wait a full season and retest. Clay needs more lime for the same shift than sand does.

Questions people ask

Are cheap soil pH kits accurate enough?

For garden decisions, yes. The chemical colour-change kits are more reliable than the cheap probe meters, which often read whatever moisture is present rather than pH.

How do I make my soil more acidic?

Realistically you do not, in open ground over chalk. Sulphur lowers pH slowly on neutral soils, but the practical answer for lime-hating plants is a container of ericaceous compost and rainwater.

When should I apply lime?

Autumn or winter, so it has months to act before the growing season, and never in the same season as manure or high-nitrogen feed.