Why soil pH matters more than fertiliser
Soil pH runs from 0 (strongly acid) to 14 (strongly alkaline), with 7 as neutral. Most vegetables grow best between 6 and 7. Each point on the scale represents a tenfold change in acidity, so a pH of 5 is ten times more acidic than pH 6.
UK soils vary significantly by region. Upland areas of Scotland, Wales, and the Pennines often have acidic soils (pH 4.5 to 6) suited to heathers and blueberries. Chalk downlands across southern England run alkaline (pH 7.5 to 8), making it difficult to grow acid-loving plants without serious amendment. Heavy clay soils common across the Midlands and parts of East Anglia tend towards neutral to slightly alkaline.
Check your plant requirements against your soil with the soil pH guide and calculate fertiliser inputs with the NPK calculator.
How to test and adjust your soil
Inexpensive soil testing kits are available from garden centres, but a laboratory test through the RHS or a soil testing service gives far more accurate results, typically including pH, organic matter, phosphorus, potassium, and magnesium levels for £15 to £40.
To lower pH (make more acidic): add sulphur chips or acidic mulch such as pine bark or bracken. Ericaceous compost helps for container growing. To raise pH: apply ground limestone (calcium carbonate) at around 100 to 200 g per square metre for light soils. Allow several months before rechecking.
What pH testing does not tell you
Correct pH does not guarantee plant health. Soil structure -- compaction, drainage, aeration, and organic matter content -- matters equally. A well-structured soil with good drainage at pH 6.5 will outperform a waterlogged, compacted soil at the same pH every time.
Use the companion planting tool to identify plant combinations that reduce pest pressure and support soil health without additional amendment.