Understanding Soil pH: What It Means and How to Change It

You can feed a plant all the fertiliser it needs and still see yellow leaves and poor growth if the soil pH is wrong. pH controls how available nutrients are, so it is worth knowing yours.

What the pH scale means

pH runs from 0 to 14. Seven is neutral, below 7 is acid and above 7 is alkaline. The scale is logarithmic: soil at pH 5 is ten times more acidic than soil at pH 6. Most garden soils fall between 5 and 8.

Why it matters

• Too acid (below about 5.5): phosphorus, calcium and magnesium become scarce; aluminium and manganese can reach toxic levels.
• Too alkaline (above about 7.5): iron, manganese and zinc lock up, causing yellow leaves with green veins.
• Between 6.0 and 7.0: most nutrients are readily available, which is why most vegetables, lawns and flowers prefer this range.

What plants prefer

pH range	Plants
4.5–5.5	Blueberries, rhododendrons, azaleas, camellias, heathers
5.5–6.5	Potatoes, strawberries, raspberries, sweet potatoes
6.0–7.0	Most vegetables, lawns, roses, most flowers
6.5–7.5	Cabbage family, asparagus, lavender, clematis

How to test soil pH

• Lab soil test: the most accurate, and it also gives a lime requirement and nutrient levels. Many universities and extension services offer them cheaply.
• Home test kits: colour-change kits are good enough to tell acid from alkaline.
• Meters: cheap probe meters are often unreliable; calibrated digital meters are better.

Test several spots, 10–15 cm (4–6 in) deep, and mix them for an average.

How to raise pH (make soil less acid)

Add finely ground garden lime (calcium carbonate) or dolomitic lime, which also supplies magnesium. Roughly 3 lb per 100 sq ft on sand, 6 lb on loam and 8 lb on clay raises pH by one unit (about 150, 290 and 390 g/m²). Our soil pH calculator gives the amount for your area and splits big doses safely. If you have a lab recommendation, the lime application calculator adjusts it for your product’s strength.

How to lower pH (make soil more acid)

Use elemental sulfur, which soil bacteria turn into acid over two to three months. About 1–2 lb per 100 sq ft (50–100 g/m²) lowers pH by one unit. Very chalky soils with free lime resist acidifying; grow acid-lovers in raised beds or containers of ericaceous compost instead.

Be patient

Lime takes months to work fully, so apply it in autumn for spring crops and re-test after a season. Change pH gradually: big swings stress soil life and are hard to hold.