If you have ever watched a rhododendron slowly turn yellow and die in an otherwise well-tended garden, you have seen soil pH at work. It is the single soil property that gardeners can least afford to ignore and, in one direction at least, the hardest to change. Testing takes ten minutes and costs a few pounds — and it should happen before you spend serious money on plants.
What pH actually measures — and why plants care
Soil pH measures acidity and alkalinity on a scale from 0 to 14, with 7 as neutral. Below 7 is acid; above 7 is alkaline. The scale is logarithmic, so soil at pH 5.5 is ten times more acid than soil at pH 6.5 — small numerical differences are bigger than they look.
Plants do not respond to pH directly. What matters is that pH controls which nutrients dissolve into forms roots can absorb. Most nutrients are freely available between about pH 6 and 7, which is why the majority of garden plants are happiest there. Outside that window, problems appear:
- In alkaline soil (above about pH 7), iron and manganese become locked up. Acid-loving plants cannot extract enough iron, and their young leaves turn yellow between green veins — a condition called lime-induced chlorosis. Phosphorus availability also falls.
- In strongly acid soil (below about pH 5), phosphorus, calcium and magnesium become scarce, while aluminium and manganese can dissolve at levels that are actually toxic to roots. Earthworms and many beneficial soil bacteria also decline.
No amount of fertiliser fixes a pH problem, because the nutrients are usually present in the soil but chemically unavailable. Matching plants to pH, or adjusting pH itself, is the only real solution.
What UK soils are typically like
Most UK garden soils fall between pH 5.5 and 7.5, and the underlying geology usually decides where yours sits. Gardens on chalk downland in the south-east, on limestone in the Cotswolds, the Yorkshire Dales and the Peak District, or on clay derived from either, are commonly alkaline, at pH 7 to 8. Gardens on sandstone, granite and in high-rainfall western and upland areas — much of Wales, the Pennines, the west of Scotland, parts of Surrey and Hampshire heathland — tend towards acid, sometimes strongly so. Decades of rainfall slowly leach calcium from soil, so old, undisturbed and high-rainfall soils drift acid over time. Urban gardens are wildcards: buried rubble, old mortar and imported topsoil often make them more alkaline than the local geology would suggest, and pH can differ from one bed to the next.
Testing: three approaches
Home test kits and meters
Chemical test kits — the sort where you shake soil with a reagent and compare the colour to a chart — cost little and are accurate enough for garden decisions, typically to within half a pH unit. Take several small samples from 10–15 cm down (not the surface), from different parts of the area, remove stones and roots, and either test them separately or mix them for an average. Avoid sampling straight after liming, mulching or feeding. Cheap dial-type probe meters pushed into the soil are, frankly, unreliable; if you prefer a meter, a calibrated digital pH meter used on a soil-and-water slurry is better.
Laboratory analysis
For a larger investment — a new garden, an orchard, a major landscaping project — a professional laboratory soil analysis is worthwhile. For a modest fee you get an accurate pH figure plus organic matter, texture and major nutrient levels, which a home kit cannot give you. The RHS offers a soil analysis service to members and non-members, and several commercial labs do the same.
Reading the local landscape
The cheapest test is observation. If mature rhododendrons, camellias, azaleas and heathers thrive in open ground in neighbouring gardens, the local soil is acid. If the district's hedgerows are full of clematis, viburnum and spindle, and gardens grow lilac and pinks well but you never see a healthy rhododendron in the ground, you are on alkaline soil. Wild indicators help too: foxgloves, gorse and bracken suggest acid conditions; old man's beard (Clematis vitalba) scrambling through hedges is a reliable sign of chalk or limestone. Local observation is a guide, not a guarantee — always confirm with a test before planting anything expensive.
Plants for acid soil
A large and glamorous group of plants, mostly from the heath and woodland families, demands acid conditions — ideally pH 4.5 to 6. These are the "ericaceous" plants, and the label is a hard requirement, not a preference:
- Rhododendrons and azaleas (Rhododendron)
- Camellias (Camellia japonica, C. × williamsii)
- Pieris (Pieris japonica)
- Most heathers (Calluna vulgaris, Erica cinerea; though winter-flowering Erica carnea tolerates near-neutral soil)
- Blueberries (Vaccinium corymbosum) and cranberries
- Skimmia (Skimmia japonica), Enkianthus and witch hazel (Hamamelis)
Blue-flowered mophead hydrangeas (Hydrangea macrophylla) are a special case: they grow perfectly well at any reasonable pH, but flower colour depends on aluminium uptake, which needs acid soil. On alkaline soil the same plant flowers pink.
Plants that shine on alkaline soil
Chalk and limestone gardeners are not hard done by. Many first-rate garden plants prefer or happily tolerate pH 7 to 8:
- Lilac (Syringa vulgaris), philadelphus and viburnums
- Clematis, honeysuckle (Lonicera periclymenum) and most roses
- Lavender (Lavandula angustifolia), rosemary (Salvia rosmarinus) and most Mediterranean herbs
- Pinks and carnations (Dianthus), scabious (Scabiosa) and Verbascum
- Peonies (Paeonia lactiflora), bearded iris and hellebores (Helleborus × hybridus)
- Fruit such as plums and cherries (Prunus), which prefer neutral to slightly alkaline ground
Most vegetables do best around pH 6.5 to 7, and brassicas positively benefit from lime, which also suppresses clubroot disease.
Raising pH: liming acid soil
Making acid soil more alkaline is the easy direction. Garden lime (ground limestone, calcium carbonate) is cheap, safe to handle and effective. Apply it in autumn or winter — traditionally to the vegetable plot before growing brassicas — raking or digging it into the top 15 cm and letting winter rain wash it down. How much you need depends on your starting pH and soil type: clay and humus-rich soils are strongly "buffered" and need considerably more lime than sandy soils to shift the same amount, so follow the rates on the packaging for your soil type and retest the following season rather than guessing. A few practical rules: never lime at the same time as applying manure (the two react, wasting nitrogen as ammonia — leave a couple of months between them), make changes gradually over more than one season rather than in one heavy dose, and do not lime at all unless a test shows you need to. Wood ash also raises pH and adds potassium, but is much weaker and more variable than lime.
Lowering pH: why it rarely sticks
Going the other way — acidifying alkaline soil — is where gardeners waste the most money. Elemental sulphur is the standard material: soil bacteria slowly convert it to sulphuric acid over months, nudging pH down. It works measurably on slightly alkaline sandy soils. But on chalk and limestone soils it is a losing battle, because the soil contains an effectively unlimited reserve of calcium carbonate that neutralises acid as fast as you add it. The pH creeps straight back up, and you are committed to repeated applications forever, for marginal results. Sequestered (chelated) iron feeds can keep a mildly chlorotic plant going, and acidic mulches such as composted bracken or pine needles help a little at the surface, but none of these turn chalk into rhododendron country. The honest advice on genuinely alkaline soil is simple: do not fight it.
Containers: the honest workaround
If you garden on alkaline soil and want blueberries or camellias, grow them in containers of ericaceous (lime-free) compost. This works properly because you control the entire root zone. Use the largest pot you can manage — 30 litres or more for a blueberry — with a peat-free ericaceous compost, and water with rainwater from a butt rather than tap water, which in hard-water areas is alkaline enough to undo your good work within a season or two. Feed with an ericaceous fertiliser in spring and summer, refresh the top few centimetres of compost each spring, and repot or root-prune every few years. Two blueberry bushes of different cultivars in pots will cross-pollinate and crop better than one. A raised bed filled with ericaceous compost over alkaline ground, by contrast, is only a temporary fix: lime-rich water moves up and roots grow down, and chlorosis usually appears within a few years.
Sources and further reading
- Royal Horticultural Society — soil pH advice, plant lists for acid and alkaline soils, and a soil analysis service.
- Garden Organic — organic approaches to soil management and liming the vegetable garden.
- Woodland Trust — how underlying soils shape native woodland and hedgerow species.