Soil pH testing is the cheapest, highest-value test in gardening. For the price of a small kit or a laboratory fee, it tells you whether your plants can actually use the nutrients in your soil — because pH controls nutrient availability more than any fertiliser does. A bed with perfect compost and plenty of feed will still grow poorly if the pH is far outside the range its plants need. Testing first and amending second is the habit that separates gardeners who fix problems from gardeners who keep buying products that do not help.
This guide explains what pH means, how to take a sample properly, the home methods and their limits, when to send a sample to a laboratory, and how to read the result and decide what to do next.
What soil pH is
pH measures how acidic or alkaline the soil is, on a scale from 0 to 14, where 7.0 is neutral, below 7.0 is acidic and above 7.0 is alkaline. The scale is logarithmic, so pH 5.0 is ten times more acidic than pH 6.0, and small differences matter more than they look. Most garden soils fall between pH 5.5 and 7.5. Most vegetables grow best at pH 6.0–7.0, where the widest range of nutrients is available. Below pH 5.5, aluminium and manganese become soluble enough to damage roots, and phosphorus, calcium and magnesium become harder to take up. Above pH 7.5, iron, manganese, zinc and phosphorus become less available, and plants can show yellow leaves with green veins even in fertile soil.
Different plants want different pH. Blueberries need pH 4.5–5.5, potatoes prefer pH 5.0–6.0, lavender prefers pH 6.5–7.5, and brassicas prefer slightly alkaline ground around pH 6.5–7.5. So there is no single "correct" pH for a garden — there is a correct pH for each bed and what it grows. That is why testing each bed separately matters, and why our acidic soil and alkaline soil guides treat the two directions differently.
How to take a soil sample
A test is only as good as its sample, and most bad results come from bad sampling rather than bad kits. Follow these steps:
1. Test each bed separately. pH varies across a garden — the vegetable bed, the lawn and the bed under the conifer can differ by a full pH unit. Take a separate sample for each area you manage differently.
2. Sample at the right depth. For vegetables and flower beds, sample the top 15 cm. For lawns, sample the top 8 cm. For trees and shrubs, sample 15–30 cm deep. Scrape away surface mulch and leaf litter first.
3. Take several subsamples. Walk a zigzag across the bed and take 8–10 small samples from different spots, using a trowel or auger. Avoid the edges of beds, compost heaps, paths, bonfire sites and anywhere fertiliser or lime was spilled — those spots are not representative.
4. Mix and dry. Mix the subsamples in a clean bucket, remove stones and roots, and let the soil air-dry on newspaper for a day. Do not dry it in an oven or on a radiator, which can change the reading. For a laboratory test, send about 500 g of the mixed, dried soil in a clean bag, labelled with the bed name and the date.
5. Sample at the right time. Test in autumn or early spring, before fertilising or liming, and at least a few weeks after any amendment. Testing a bed the week after liming tells you nothing useful. Retest every one to two years, or after any major change such as building a new bed or adding a large amount of compost.
Home testing methods
Chemical test kits. The common kits mix soil with water and a reagent capsule or solution, and you match the resulting colour to a chart. They cost little, work for a rough reading, and are accurate to roughly half a pH unit — good enough to tell acidic from neutral from alkaline, but not good enough to calculate a precise lime dose. Follow the instructions exactly, use distilled or rainwater rather than tap water, and read the colour in daylight.
Probe meters. Push-in electronic meters are convenient and reusable, but cheap ones are often inaccurate, especially in dry or very sandy soil. For a better reading, water the soil first, push the probe in fully, wait a minute, and take several readings across the bed. Calibrate the meter regularly if it has a calibration function, and do not rely on a single reading.
The vinegar and baking soda test. This is a rough field check, not a measurement. Put a spoonful of soil in a dish and add vinegar: fizzing suggests alkaline soil with free lime. Put another spoonful in a dish, wet it, and add baking soda: fizzing suggests acidic soil. No fizz in either case suggests roughly neutral soil. It tells you the direction, not the number — useful as a quick check, not as a basis for amending.
pH test strips. Strips dipped in a soil-water mixture give a rough reading, similar in accuracy to kits. They are cheap and quick, and they are fine for checking whether a container mix is roughly right, but they are not precise enough for calculating lime or sulfur doses.
Laboratory testing
A laboratory test is the most accurate option and the one to choose before any major amendment. A standard test reports pH, and usually also nutrient levels (phosphorus, potassium, magnesium, calcium), organic matter content and soil texture. Many laboratories and extension services add a lime recommendation in grams per square metre — the single most useful number you can get. Your local extension service can usually test soil for a small fee, and its recommendations are written for your area and soil type, which a generic kit cannot match. Check your local extension service for sampling instructions, fees and turnaround times before you send anything.
Laboratory results are worth the wait when pH is below 5.5 or above 7.5, when plants are failing despite good care, when you are starting a new garden or raised beds, or when you are deciding between lime types — calcitic or dolomitic lime depends on your magnesium level, which only a laboratory test reports. For routine monitoring of a healthy garden, a home kit once a year is usually enough.
Reading your result and deciding what to do
Use this table as a starting point, then read the detailed guides for your situation:
| pH range | What it means | First step |
|---|---|---|
| Below 5.0 | Strongly acidic; aluminium toxicity likely | Laboratory test, lime in stages, grow acid-lovers meanwhile |
| 5.0–5.5 | Acidic; most vegetables struggle | Lime to raise pH, add compost, consider potatoes and blueberries here |
| 5.5–6.0 | Mildly acidic; many crops fine | Light liming if growing brassicas or lavender, compost annually |
| 6.0–7.0 | Ideal for most vegetables | Maintain with compost, retest yearly |
| 7.0–7.5 | Mildly alkaline; most crops fine | Compost annually, watch for iron chlorosis on sensitive plants |
| 7.5–8.0 | Alkaline; iron and phosphorus less available | Sulfur in stages, heavy compost, grow alkaline-lovers |
| Above 8.0 | Strongly alkaline | Laboratory test, sulfur and compost, containers for acid-lovers |
Do not try to fix pH in one dose. Lime and sulfur both work slowly over months, and over-correcting is harder to undo than under-correcting. Change pH by half a unit at a time, retest after three to six months, and then decide whether to repeat. And remember that pH is one number among several: a bed at pH 6.5 with no organic matter will still grow poorly, while a bed at pH 5.8 with rich compost and good structure can grow well. Test pH, but also keep up the compost, mulch and rotation that build healthy soil generally. Our loamy soil, composting basics and compost ratio guides cover those habits.
Common mistakes in soil pH testing
The most common mistake is testing one spot and treating the result as the whole garden. pH varies bed to bed, so sample each bed separately. The second is testing with tap water, which is often alkaline and skews the reading — use distilled or rainwater. The third is testing straight after liming, fertilising or adding fresh manure, which gives a temporary reading that does not reflect the settled soil. The fourth is trusting a single cheap probe reading; take several readings and cross-check with a kit or a laboratory test before spending money on amendments. The fifth is amending on the basis of a rough home-kit reading alone: a kit that reads pH 5.5 when the true value is 6.0 can lead to over-liming. Use home kits for direction and laboratory tests for doses.
Safety notes
The chemicals in pH test kits are mild, but keep them away from children and pets, wear gloves when handling soil samples, and wash your hands afterwards. If you are sending soil from a former industrial or urban site, ask the laboratory about heavy metal testing as well — pH alone does not tell you whether soil is safe for food growing. If results are hard to interpret, check your local extension service for help rather than guessing.
Frequently asked questions
How often should I test my soil pH?
Test every one to two years for established beds, and before starting a new garden, building raised beds, or liming. Retest three to six months after any lime or sulfur application to see whether it worked before applying more.
Are home pH test kits accurate?
They are accurate to roughly half a pH unit, which is enough to tell acidic from neutral from alkaline, but not enough to calculate a precise lime or sulfur dose. Use a kit for routine checks and a laboratory test before major amendments.
What is the best pH for vegetables?
Most vegetables grow best at pH 6.0–7.0. Potatoes prefer pH 5.0–6.0, blueberries need pH 4.5–5.5, and brassicas and lavender prefer pH 6.5–7.5. Match the bed to the crop rather than chasing one number for the whole garden.
Can I test pH with vinegar and baking soda?
You can get a rough direction: vinegar fizzing suggests alkaline soil, baking soda fizzing suggests acidic soil. But it gives no number, so it cannot tell you how much lime or sulfur to use. Treat it as a field check, not a measurement.
Why is my pH different in different parts of the garden?
pH varies with parent rock, past liming, fertiliser use, tree cover, drainage and building rubble. Beds under conifers, near concrete, or on old paths often differ from the main vegetable beds by a full unit or more. That is normal, and it is why each bed should be sampled separately.