Why Soil pH Matters More Than Most Growers Realise

Soil pH measures how acid or alkaline your plot is, running from around 4 on heathland to 8.5 on chalk downland. Most vegetables are happiest between 6.0 and 7.0 — slightly acid to neutral. Hit that range and the nutrients already in your soil become available to roots. Miss it badly and you can pile on compost and fertiliser and still watch crops sulk.

At pH 5.5, phosphorus locks up and stays largely out of reach however much you add. Above pH 7.5, iron, manganese and zinc start to drop out of solution, which is why brassicas on chalky soil so often look yellow and hungry. Soil life has its preferences too. Worms, bacteria and fungi that break down organic matter mostly work best near neutral, so an extreme pH slows the whole recycling system your plot depends on.

Testing Your Soil Properly

Test before you do anything else. A colour-comparison kit from a garden centre reads to within about half a pH point, which is plenty for a vegetable plot. Digital meters are quicker but drift, so calibrate them and keep the probe damp. On a new allotment, or where you suspect heavy liming or contamination in the past, a laboratory test earns its cost — it also reports phosphorus, potassium and magnesium.

Sample correctly, or the result means nothing:

• Take 10 to 15 small cores across the plot in a zigzag or W pattern.
• Scrape off surface debris and sample the top 15cm (6in), where roots actually feed.
• Avoid sampling within a fortnight of liming, manuring or feeding.
• Mix the cores in a clean bucket, air-dry a handful, then test that.
• Test distinct areas separately — raised beds, the old bonfire patch and a shaded corner can differ wildly.

Repeat every three years, or after any serious application of lime, manure or mushroom compost.

What Your Result Means in Practice

• Below 5.5: distinctly acid. Fine for blueberries, rhubarb and potatoes, but brassicas and legumes will struggle.
• 5.5–6.0: workable and ideal for potatoes, since acidity suppresses scab.
• 6.0–7.0: the sweet spot. Peas, beans, carrots, onions, lettuce, courgettes and brassicas all thrive.
• 7.0–7.5: mildly alkaline. Brassicas love it — lime is a real ally against clubroot — but potatoes will scab and blueberries will fail.
• Above 8.0: true chalk. Expect trace-element deficiencies, so lean on brassicas, beetroot, spinach and asparagus.

Raising pH: Liming Done Well

If your soil sits below 6.5, lime is the tool. Ground limestone is the standard, gentle choice; dolomitic lime also supplies magnesium and suits light, hungry sandy soils. As a rough guide, raising pH by half a point takes about 250g per square metre on sand, 350g on loam and 500g on clay — but always follow the rate on the bag, because over-liming is far harder to undo than under-liming.

Apply in autumn or early winter, scatter evenly by hand or with a spreader, and fork it into the top 15cm. Never exceed 500g per square metre in one go; split heavy applications across two seasons. Do not lime and manure the same ground at the same time — lime drives nitrogen off as ammonia and you lose the value of both. Leave six to eight weeks between them. Wood ash is alkaline and rich in potash, so it is useful on acid plots, but use it sparingly and never on potato ground.

Lowering pH: The Slower Job

Making soil more acid is genuinely difficult, and on chalky ground it is a losing battle. Elemental sulphur works, but soil bacteria must convert it first, so allow several months and expect a modest shift. Work it in at the recommended rate and retest the following spring rather than repeating blindly.

Organic matter helps more gradually. Leafmould, composted bark and pine needles all nudge pH downwards as they break down. For blueberries, cranberries and lingonberries, the honest answer on most UK plots is to grow them in containers filled with ericaceous compost, topped up with rainwater rather than tap water, which is hard and alkaline across much of the country. Note that well-rotted manure and garden compost usually push pH towards neutral rather than acid, and mushroom compost is distinctly alkaline — keep it away from acid-lovers.

Working With What You Have

The best growers stop fighting their soil and start planning around it. If your plot runs acid, make potatoes, rhubarb, parsley, carrots and swedes the backbone of your rotation, and lime only the brassica bed each winter. If it runs alkaline, grow magnificent brassicas, beetroot, spinach, onions and asparagus, and grow your potatoes in bags of ericaceous or multipurpose compost instead.

Keep pH in mind when you rotate. Group crops by their preferences, lime the brassica quarter in autumn, and leave the potato quarter alone. Test again after three years, adjust slowly, and add organic matter every year whatever the reading says — it buffers pH, feeds soil life and makes any plot more forgiving. Get the basics right and your soil will do most of the work for you.