Oliver Pemberton
Why soil pH is the quiet foundation of a good plot
Soil pH is a measure of how acidic or alkaline your soil is, on a scale from 0 to 14. Seven is neutral; below seven is acidic, above is alkaline. It matters because pH controls how available nutrients are to your plants. Even if your soil is rich in phosphorus, iron or manganese, a pH that is too high or too low can lock those nutrients up. Most vegetables grow best between pH 6.0 and 7.0, where the widest range of nutrients is soluble. Brassicas such as cabbage and sprouts prefer 6.5 to 7.5, while potatoes and rhubarb are happier at 5.0 to 6.0. Blueberries are extreme acid lovers at 4.5 to 5.5. Knowing your number saves you from wasting compost, fertiliser and hope.
How to take a sample that actually tells the truth
A pH test is only as good as the sample you take. Use a clean trowel or auger, and avoid galvanised tools that can contaminate the sample. Take 10 to 15 small cores from across the bed in a zigzag pattern, each about 15cm deep. Discard any surface mat of grass or weeds. Tip the cores into a clean plastic bucket, mix them thoroughly, then take a handful for testing. Remove stones, roots and worms. If the soil is very wet, spread it on a tray to dry for a few hours; most home tests work best with moist, crumbly soil.
- Test separate beds if you have added lime or manure to only one area.
- Wait at least three months after liming, and a few weeks after feeding, before testing.
- Spring and autumn are ideal, but any frost-free day works.
Testing with a shop-bought kit
Garden centres sell simple colour-comparison kits for a few pounds. They usually contain a test tube, a bottle of indicator solution and a colour chart. You mix a small amount of soil with distilled water or a barium sulphate solution, add a few drops of indicator, shake, and wait for the colour to develop. Then match it to the chart. Follow the instructions exactly, as ratios matter. These kits give a rough reading, often within half a pH point. For most allotment crops, that is plenty. If you want more precision, look for a digital pH meter, but cheap ones can drift, so calibrate it if you can. Always take two readings from the same sample to check consistency.
Testing with red cabbage: a pantry pH indicator
You can make a surprisingly effective pH indicator from red cabbage. Chop a quarter of a red cabbage and simmer it in about 500ml of distilled water for 15 minutes. Strain the purple liquid and let it cool. Put a tablespoon of your soil sample in a clean jar, add just enough distilled water to make a slurry, then pour in a few tablespoons of the cabbage water. Shake and leave for 30 minutes. The colour tells you the story: pink or red means acidic, purple means roughly neutral, and blue or green means alkaline. Tap water can alter the result, so use distilled or rainwater if possible.
For a quick yes-or-no test, try the vinegar and baking soda method. Put a spoonful of soil in one cup and add a splash of white vinegar. If it fizzes, your soil is alkaline. In a second cup, add a spoonful of soil and a spoonful of baking soda, then add water. If that fizzes, your soil is acidic. If neither fizzes, your soil is likely close to neutral. This method is crude but useful for a fast check before you reach for amendments.
Reading your results and what to do next
Once you have a number, you can act. A pH of 5.5 or below is acidic; 7.5 or above is alkaline. UK soils vary: heavy clay in the south and east often leans alkaline, while peaty or sandy soils in the west and north are more likely to be acidic. To raise pH, add garden lime according to the packet rate, or use wood ash and mushroom compost in moderation. To lower pH, add elemental sulphur, composted bark, leaf mould or pine needles. Sulphur is slow, taking several months to work, so apply it in autumn for spring planting. Never dump lime and sulphur on at the same time. Add organic matter either way; it buffers pH and improves soil structure. Retest after six months.
Making peace with your soil's natural lean
Rather than fighting your soil, work with it. Acidic plots are perfect for potatoes, blueberries, rhubarb, parsley and shallots. Alkaline plots suit brassicas, asparagus, beetroot, spinach and onions. If you desperately want blueberries on chalky soil, grow them in a raised bed or large pot filled with ericaceous compost. Keep a simple notebook of your pH readings and what you added, then retest each year. Soil changes slowly, and that is a good thing. A careful test now means healthier crops, fewer deficiencies and a plot that gives back more than you put in.
