Why soil pH matters on a British plot

Every allotment and kitchen garden has its own character, and one of the quietest influences on what grows well is soil pH. The scale runs from 1 to 14, with 7 neutral. Below 7 is acidic; above 7 is alkaline. Most familiar vegetables sit comfortably between 6.0 and 7.0, but a few have strong preferences. pH also affects how easily roots take up nutrients. In very acid soil, phosphorus and molybdenum can be less available; in very alkaline soil, iron, manganese and zinc can become locked up. Before you order seeds or plan a rotation, it is worth knowing which side of neutral your plot sits on. UK soils vary hugely: many western and upland gardens are acidic, especially over peat or granite, while chalk and limestone areas give alkaline soil. Allotment sites can be a patchwork because of years of ash, lime, manure and builders' rubble.

What you need for the cupboard test

This is a rough field test, not a laboratory analysis, but it is surprisingly useful. You need:

• A clean glass jar or mug for each test
• Clear white vinegar — not malt vinegar
• Bicarbonate of soda, also called baking soda
• A trowel and a clean bucket
• Rainwater or distilled water
• An old spoon and a pair of gloves

Take samples from five to ten spots across the plot. Remove surface weeds and debris, then dig a small amount from about 10–15cm down. Mix them together in the bucket. Spread the mixture on a tray and let it dry somewhere warm, or use a little gentle warmth, until it crumbles easily. Do not test soil you have just limed or fed: wait at least a few weeks, and ideally test in autumn or early spring before the growing season begins.

The vinegar test: checking for alkaline soil

Put two tablespoons of dry, crumbled soil into a clean jar. Add about 50ml of clear white vinegar. Watch closely for a few minutes. If the mixture fizzes, bubbles or foams like lemonade, your soil is alkaline — likely containing chalk or limestone. The stronger the fizz, the more alkaline it is. If you see nothing at all, your soil is probably not alkaline. A few tiny bubbles are not conclusive; look for a proper reaction. Do this test in its own jar, and keep the sample separate from the next one.

The baking soda test: checking for acidic soil

Put two tablespoons of the same dry soil into a second clean jar. Add rainwater or distilled water a spoonful at a time until you have a thick paste. Then add one tablespoon of bicarbonate of soda and stir. If it fizzes or bubbles, your soil is acidic. If there is no reaction after two or three minutes, it is probably neutral or alkaline. Tap water can be slightly alkaline in hard water areas, which may confuse the result, so rainwater is better. Never mix vinegar and bicarbonate of soda in the same sample: they simply neutralise each other.

Reading the results and choosing crops

Once you have your results, the practical decisions become easier.

• Vinegar fizzes, bicarbonate does not: alkaline soil. Brassicas — cabbage, kale, sprouts, broccoli and cauliflower — usually thrive here, as do onions, garlic, leeks, asparagus and spinach. Clubroot, a serious brassica disease, is less active in alkaline conditions.
• Bicarbonate fizzes, vinegar does not: acidic soil. Potatoes, rhubarb, strawberries, raspberries and blueberries are happy. Acid soil also helps suppress potato scab. Brassicas may struggle and are more likely to suffer clubroot.
• Neither fizzes: roughly neutral. Most vegetables, including carrots, parsnips, beans, peas, courgettes, lettuce and tomatoes, will do well.
• Both fizz: unusual. It usually means contamination, a very variable sample, or residues in the jar. Wash everything and retest with fresh samples from several spots.

If you garden on chalk or limestone, blueberries are best grown in pots of ericaceous compost rather than open ground. On acid soil, a traditional allotment rotation often limed one bed before brassicas and keeps another lime-free for potatoes.

What to do next: gentle adjustments

Do not try to chase a perfect number. Soil pH changes slowly, and the best long-term strategy is to add organic matter: garden compost, leaf mould and well-rotted manure. For acidic soil, garden lime raises pH. Use a proper garden lime product, not builders' lime, and follow the packet rate. Apply it in autumn or winter, spread it evenly, and avoid liming ground that will grow potatoes the following season. For alkaline soil, adding compost and mulching helps buffer the pH, while elemental sulphur can lower it slowly, though results vary. Raised beds and containers give you more control, especially for acid-loving crops. Test again next year, keep notes, and let your soil — rather than the seed catalogue — guide what you grow. A plot that suits its crops is a far more productive and pleasant place to spend a Saturday morning.