How to test soil pH at home, and what to do with the reading

In short. Take soil from five or six spots, 10 to 15 cm down, mix it, and test it with a chemical colour-chart kit and distilled water. Read the colour in daylight. Skip the cheap two-prong probe meters: in a university test they showed no meaningful link to the true pH.[2] Then drop vinegar on a dry spoonful: if it fizzes, the soil holds chalk.

• Most garden plants want pH 6.0 to 7.0, and 6.5 is the best all-round figure.[1]
• Below your target, lime raises the pH. Above it, sulphur lowers it.
• Do not test within three months of adding lime, fertiliser or compost.[1]
• Once a bed is in range, retest every three to five years.[3]

I test before I add anything. Fifteen years in professional kitchens taught me to season at the start rather than correct at the end, and with soil the start is a pH reading. Lime or sulphur on a guess is as likely to make things worse as better.

Why it is worth testing

Acid soil is common. More than 40% of UK arable soils and 56% of grassland sit below their target pH.[4] And pH decides what a plant can reach: phosphorus is most available between pH 6.0 and 7.0, and is locked up by iron and aluminium below that and by calcium at very high pH.[5] Feeding harder will not fix a pH problem.

Which test to use

A chemical colour-chart kit from a garden centre gives a reading good enough to decide what to do, and a lab test tells you more.[1] What I would not trust is the cheap two-prong probe that needs no battery. When researchers tested 21 field sensors against a lab-grade glass electrode, the metal-probe meters showed no meaningful correlation with the true pH, while glass-electrode meters, at over £100, tracked it closely.[2] The same study found pH sensors read best in a mix of soil and deionised water. The full comparison is in which soil pH tester to trust.

How to test soil pH, step by step

1. Pick your areas. Test each bed, border and the lawn separately. A border against the house wall can read a whole unit higher than the veg patch, because mortar leaches lime.
2. Dig five or six small holes. Take a slice of soil from 10 to 15 cm down in each, across the whole area. For a lawn, take it from the top 7 to 10 cm, below the thatch.
3. Mix and dry. Crumble the samples together in a clean bucket, pick out stones and roots, and let a handful air-dry on paper.
4. Follow the kit. Add the soil and reagent to the tube, top up with distilled water, not tap water, and shake.
5. Let it settle, then read it in daylight. Match the colour by a window, not under a kitchen light.
6. Do the fizz test. Drop a little vinegar on a dry spoonful of the same soil. Fizzing means free lime or chalk.[1]
7. Write it down. Note the bed, the date and the reading, so you can see the trend next time.

What your reading means

Reading	What it means	What to do
Below 5.5	Acid. Fine for blueberries and rhododendrons; too low for most else	Lime, unless you grow acid-lovers there
5.5 to 6.0	Slightly acid. Fine for camellias and most lawns; low for veg	Lime veg beds; leave lawns above 5.5
6.0 to 7.0	The sweet spot for most plants	Nothing
7.0 to 7.5	Slightly alkaline. Too high for acid-lovers	Sulphur, only where acid-lovers grow
Above 7.5, fizzes with vinegar	Chalky, with free lime	Grow acid-lovers in pots of ericaceous compost

How much lime or sulphur depends on how far you need to move, your soil type and the area. Clay needs about three times as much sulphur as sand for the same drop. The soil pH calculator does the sum: put in your reading, your target, your soil and the area, and it gives the dose, the timing and the bags.

The mistakes I see most

• Testing straight after liming or feeding. Wait three months.[1]
• One sample from one spot. A single hole can sit on an old bonfire site or a lump of mortar. Mix several.
• Tap water in the tube. Use distilled water, as the kit says.
• Reading the colour under artificial light. Use daylight.
• Treating moss as proof of acid soil. It is a hint, not a reading.

Common questions

Can I test soil pH without a kit?

Only roughly. Drop vinegar on a dry spoonful of soil: fizzing means free lime, so the soil is alkaline.[1] The bicarbonate of soda test, a paste of soil and distilled water, fizzes only on quite acid soil, so no fizz does not rule acidity out. For a number, use a kit.

Are cheap soil pH meters accurate?

The two-prong metal-probe meters are not. In a 2017 test of 21 sensors, the metal-probe types showed no meaningful correlation with the true pH, while glass-electrode meters tracked it closely.[2] See which soil pH tester to trust.

When is the best time to test soil pH?

Autumn or winter, before you lime or plan the next season, and never within three months of adding lime, fertiliser or compost, because fresh amendments skew the reading.[1]

How often should I test soil pH?

Before you add anything, then six months after sulphur and at least three months after lime. Once a bed is in range, every three to five years is enough, the interval the UK farm guide RB209 uses.[3]

Does moss in the lawn mean my soil is acidic?

Not on its own. Moss grows in shade and on wet or compacted lawns at any pH. Moss with sheep's sorrel is a stronger hint, but test before you lime.

What pH should my soil be?

Most garden plants are happy between 6.0 and 7.0, and the RHS gives 6.5 as the best all-round figure.[1] Blueberries want 4.5 to 5.5, and rhododendrons and azaleas 4.5 to 6.0.

How deep should I take the soil sample?

About 10 to 15 cm down for beds, where most roots feed. For a lawn, take it from the top 7 to 10 cm, under the thatch. Always mix several samples from across the area.

Sources

1. Royal Horticultural Society. Soil: understanding pH and testing soil; Lime and liming, https://www.rhs.org.uk/soil-composts-mulches/lime-liming; Acidifying soil, https://www.rhs.org.uk/soil-composts-mulches/acidifying-soil.
2. Scheberl, L., Scharenbroch, B. C. and Fite, K. (2017). Comparing field measurement methods of soil pH and moisture for use in an urban site assessment. ASA, CSSA and SSSA Annual Meeting, Tampa. University of Wisconsin–Stevens Point and The Morton Arboretum. Poster (PDF)
3. AHDB (2023). Nutrient Management Guide (RB209), Section 1: soil pH and liming recommendations for arable and grass systems. https://ahdb.org.uk/knowledge-library/soil-ph-and-liming-recommendations-for-arable-and-grass-systems
4. Holland, J. E. et al. (2018). Liming impacts on soils, crops and biodiversity in the UK: a review. Science of the Total Environment, 610 to 611: 316 to 332.
5. Penn State Extension. Managing phosphorus for crop production.