Short answer: Take readings from several spots in moist, settled soil (at least three months after liming or feeding), clean the probe between each one, and average the results. Most garden plants do best at around pH 6.5.
What does a soil pH reading tell you?
The pH scale runs from 0 to 14. A value of 7 is neutral, lower numbers are acidic and higher numbers are alkaline. Soil pH matters because it controls how easily plants can take up nutrients. The RHS describes pH 6.5 as the best general-purpose level for gardens. At that level major nutrients are most available and soil bacteria and earthworms are most active. Lime-hating (ericaceous) plants such as rhododendrons, camellias, heathers and blueberries are the exception, because they prefer more acidic ground.
A meter gives you a quick indication on the spot. It does not replace a laboratory analysis, which can also report nutrients and detect calcium carbonate (chalk or limestone).
When should you test?
Test before you design a new bed, start a vegetable plot or plant fruit. Test again if growth is disappointing or leaves turn yellow. You can test at any time of year, but avoid the three months after adding lime, fertiliser or organic matter. The RHS warns that readings taken in that window can be misleading.
For the most consistent picture, test at the same time each year and under similar soil conditions.
What kind of meter are you using?
There are two broad types on the market:
- Analogue probe meters. These have two metal tips and a needle display, and they need no battery. They are quick and cheap, but they are sensitive to dry soil and dirty probes.
- Digital meters with a glass or spear electrode. These are more precise and often show temperature too. Many can be calibrated against buffer solutions.
The steps below work for both. Where they differ, follow the instructions that came with your instrument.
How do you prepare the probe?
- Wipe the probe with a clean cloth. Any film of oxidation, fertiliser residue or old soil will skew the reading. Many analogue meters come with a small abrasive pad for polishing the tips.
- If you have a digital meter that can be calibrated, check it against a pH 7 buffer first, then pH 4 (or pH 10 for alkaline soils). Do this before a season of testing, or whenever readings start to look wrong.
- Rinse an electrode-type probe with distilled water and blot it dry.
How do you take a reading?
- Pick representative spots. Choose three to five locations across the area, avoiding the edges of paths, compost heaps and bonfire sites.
- Clear the surface. Scrape away mulch, stones and the top centimetre or so of soil.
- Check the moisture. The probe needs moist soil to work. If the ground is dry, dig a small hole about 10 to 15 cm deep, add a little distilled water (not tap water, which can change the reading), stir it into a slurry and wait a minute.
- Insert the probe. Push it in to the depth the manufacturer recommends, usually the full length of the tips. Avoid forcing it past stones.
- Wait for it to settle. Analogue needles typically settle in about a minute. Digital displays show a stable icon.
- Record and clean. Note the value, wipe the probe and move to the next spot.
How do you make sense of the numbers?
Average your readings. A single outlier often means a buried lump of mortar, lime or ash.
- Below 5.0: strongly acidic. Most vegetables will struggle unless lime is added.
- 5.1 to 6.0: acidic. Suits ericaceous plants, but other crops may need lime.
- 6.1 to 7.0: moderately acidic to neutral. Suits most plants, and usually needs no correction.
- Above 7.0: alkaline. Lime is not needed, and an acidifying material is only worth using if you want to grow acid-loving plants.
If you do plan to lime, base the rate on soil texture as well as pH. The RHS publishes rates for clay, loam and sandy soils, derived from DEFRA guidance, for raising soil to pH 6.5.
Why are your readings inconsistent?
- The soil is too dry. This is the most common cause. Moisten the soil with distilled water and test again.
- The probe is dirty or oxidised. Clean or polish the probe.
- You tested soon after liming or feeding. Wait the full three months.
- The electrode is old. Glass electrodes age. If calibration no longer holds, the probe needs replacing.
- The soil is genuinely variable. That is real information. Map the zones and treat them separately.
When should you use a laboratory instead?
Use a lab for new orchards, large plots, commercial growing, or any time a meter result surprises you. A lab test is more accurate, gives nutrient levels as well, and can detect calcium carbonate. You can do a quick home check for chalk by adding vinegar to a soil sample: fizzing means calcium carbonate is present.
FAQ
Can I leave the meter in the ground? No. Long contact with moist soil corrodes the probe tips. Take the reading, then remove and clean the probe.
Does a soil pH meter need batteries? Analogue probe meters generally do not. Digital meters do.
Should I use tap water to wet the soil? No. Use distilled or deionised water so you are not adding minerals that shift the result.
How many readings do I need for a vegetable bed? Take three to five per bed, then average them.
Can the same meter test compost? Yes, if the instructions allow it, but expect readings from compost to vary more than from soil.
Sources
- RHS, "Soil: understanding pH and testing": https://www.rhs.org.uk/soil-composts-mulches/ph-and-testing-soil
- RHS, "Lime and liming" (DEFRA-based liming rates): https://www.rhs.org.uk/soil-composts-mulches/lime-liming
Related products: Soil & Garden Testing
Cover photo: PhysicsScotland, CC BY-SA 4.0 via Wikimedia Commons
Photo in article: CSIRO, CC BY 3.0 via Wikimedia Commons
