Testing Soil pH Before Planting a New Bed

Why Bother Testing Before You Plant?

A new bed is a bit of a mystery. Whether you have just taken on a neglected half-plot, built a raised bed from scratch or inherited a patch that has been worked for forty years, you cannot see what is going on beneath the surface. Somewhere in that soil's history there may be a heap of builder's rubble, a buried bonfire, a sack of lime applied for a brassica crop in 2014, or decades of leaf mould quietly pushing everything towards acid.

Testing pH takes about twenty minutes and costs less than a packet of seeds. Get it right before you plant and you will save yourself a season of sulking seedlings, yellowing leaves and mystery failures that look like disease but are really just chemistry. It is the single most useful tenner you will spend on a new bed.

What the Numbers Actually Mean

The pH scale runs from 0 to 14, with 7 neutral. Anything below 7 is acid, anything above is alkaline. Most of the vegetables we grow on allotments sit comfortably between 6.0 and 7.0, which is where nutrients are most freely available and soil bacteria are busiest.

Where things go wrong is at the extremes. In acid soil, phosphorus and molybdenum become locked away, and clubroot — the bane of every brassica grower — thrives. In alkaline soil, iron, manganese and zinc become unavailable, and you get that classic interveinal yellowing on raspberries and blueberries.

• Chalk and limestone areas: typically pH 7.5 to 8.0
• Typical clay or loam: pH 6.0 to 6.5
• Sandy soils and western, rain-heavy plots: pH 5.0 to 6.0
• Peat or old woodland ground: pH 4.5 to 5.5

Rainfall leaches lime downwards over time, so western and upland plots drift acid. Eastern and chalky plots rarely need liming and sometimes need the opposite.

Taking a Sample That Tells the Truth

One sample from one spot will lie to you. Take a proper composite:

• Use a trowel or soil auger to take eight to ten cores in a rough W across the bed.
• Work to spade depth, about 15cm, and stay away from the very edges.
• Drop them in a bucket, remove stones, roots and worms, then mix thoroughly.
• Test while the soil is damp but not waterlogged — crumbly, not sticky.
• Use rainwater or distilled water in the kit if you can. Hard tap water is alkaline and will skew your reading upwards.
• Run the test twice and trust the average.

A liquid indicator kit is accurate enough for our purposes, usually to within half a point. Cheap electronic meters are notoriously unreliable unless you spend proper money and calibrate them. Test in spring or autumn when the soil is workable, and ideally before you add compost or manure, which can nudge a reading slightly.

Raising pH with Garden Lime

If your bed is sitting below 6.0 and you want to grow brassicas, beans or onions, lime is the answer. Ground limestone or garden lime (calcium carbonate) is gentle and safe; dolomitic lime also adds magnesium, useful on light sandy plots.

As a rough guide, lifting the pH by half a point takes about 500g per square metre on sandy soil and up to 1kg on heavy clay. Spread it evenly over the surface in autumn or winter, rake it in lightly and let the frost and rain work it down. Do not dig it deeply — it needs to react with the topsoil.

• Leave at least six weeks between liming and adding manure or compost, or you will lose nitrogen as ammonia.
• Do not lime where you plan to grow potatoes (it encourages scab) or blueberries, rhubarb or raspberries.
• Re-test after two or three months rather than guessing.

Lowering pH with Sulphur (and Patience)

Pushing pH down is slower work and you cannot do it overnight. Elemental sulphur is converted by soil bacteria into acid, which means it only works in warm, moist, active soil. Apply in late summer or early autumn for a spring planting.

Around 70 to 100g per square metre will shift a loam by roughly one pH point, but apply it in stages and re-test between applications. Overdoing it is far easier than undoing it. Sulphate of iron is a faster option for small areas, and generous dressings of composted bark or leaf mould will gently acidify over years rather than weeks. Growing acid-lovers in a large pot of ericaceous compost is often the more sensible answer than fighting your subsoil.

Choosing Crops That Suit the Bed You've Got

Once you know your number, you can plan with confidence rather than hope.

• pH 6.5 to 7.5: brassicas, beans, peas, onions, leeks, spinach, lettuce
• pH 6.0 to 6.8: carrots, parsnips, beetroot, courgettes, most salad crops
• pH 5.0 to 6.0: potatoes, sweetcorn, tomatoes, parsley, raspberries
• pH 4.5 to 5.5: blueberries, cranberries, rhododendrons and other acid-lovers

Keep a note of your readings and the date, and re-test each bed every couple of years. Soil is a living thing and it shifts, especially after heavy cropping or a wet winter. Work with what you have rather than against it, and that new bed will reward you from its very first season.