How to make soil more acidic, or raise the pH

By Joe Barrington, founder of Dr Forest · Published January 2026 · Updated 10 October 2026

Spend fifteen years in professional kitchens and you learn one thing early: season at the end and you are correcting, season at the start and you are cooking. Soil pH is the same. Get it right before you plant and everything after it goes easier.

Soil pH decides whether nutrients are actually available, which is why it matters more than the feed you reach for next. To raise pH you add lime; to lower it you add elemental sulphur. Both work over months, and the dose depends on soil type. Test first. Chalky soils will fight you the whole way and are better worked with than against.

Why pH matters more than your next feed

You can have a nutrient in the soil and still see a shortage in the plant. Phosphorus is clearest: most available between about pH 6.0 and 7.0. On the acid side, iron and aluminium bind more of it; the RHS puts most phosphates poorly available around pH 5.1.¹ Past roughly pH 7.5, calcium holds more of it instead.² Iron and manganese fall away as soil turns alkaline, which is why plants on chalk so often show pale yellow-between-the-veins chlorosis even when there is iron in the ground. Our guide to why tomato leaves turn yellow walks through telling hunger from lock-up.

Feeds aren’t a pH fix. A few nudge it, sulphate of ammonia slowly acidifies, but not by enough or predictably enough to rely on. Feeding harder when availability is the limit usually wastes the bag. Get the ground into range, then feed. The feeds themselves sit in what a fertiliser is.

What pH does your plant actually want?

The scale runs from 0 to 14. Seven is neutral, and it is logarithmic: each whole step is a tenfold change, so soil at pH 5.0 holds ten times the acidity of pH 6.0. What really sets the dose, though, is the acid held in reserve on clay and organic matter. That reserve has to be neutralised too, which is why heavy soils need more lime than sandy ones to move the same distance.

For most garden plants, aim for roughly pH 6.0 to 7.0; the RHS calls 6.5 the best general-purpose figure.¹ Blueberries want 4.5 to 5.5; rhododendron, azalea, camellia and summer heather about 5.0 to 6.0. Many Mediterranean herbs, lilac, clematis and brassicas are happy around 7 or a little above. Match the plant to the soil. Shifting one bed usually pays; chasing a precise number across a whole garden rarely does.

Test before you touch anything

A home colour-chart kit is close enough for most decisions; a laboratory test is more reliable. Sample several spots, mix, and test the blend. The RHS notes a test within three months of adding lime, fertiliser or organic matter may mislead.¹ On acid ground, retest every three or four years: rain leaches lime out, and the soil drifts back.

One field check matters before you commit. Drop ordinary vinegar onto a spoonful of dry soil. If it fizzes, free calcium carbonate is present (unless you limed that bed recently). A fizz means naturally alkaline ground, and trying to acidify it will be a long, losing battle. No fizz usually means little or no free chalk; clay and organic matter still resist change, so expect it to be slow.

How to raise pH

If the test is too acidic, use ground limestone (calcium carbonate). Dolomitic lime adds magnesium as well, which is worth having only if a test shows magnesium is low. RHS rates of ground limestone to reach pH 6.5, worked into the top 20 cm:¹

Those numbers are larger than most gardeners expect, and in my experience under-liming is the usual reason people decide lime doesn’t work. If you already sit at 6.5 or higher, skip lime. For more than 0.5 kg/m², dig half in and rake the rest on after digging. Lime in winter for annual crops; finely ground material starts within weeks, but a full reaction can take a couple of years, so apply in stages and retest.

On a vegetable plot, lime goes ahead of the brassicas, where a pH around 7 helps against clubroot, and never just before potatoes, because limed ground encourages common scab.¹ Keep lime and fresh manure apart by a few weeks too; together they drive nitrogen off as ammonia. Avoid builders’ or hydrated lime: it is stronger than limestone, caustic on skin and eyes, and easy to overdo.

From our range, Garden Lime is micronised for faster reaction. Sea Shell Meal is coarser calcium carbonate for a slower correction. Both are plain calcium carbonate, packed in Stockport.

How to lower pH

For blueberries, rhododendrons, camellias or acers on alkaline ground, use elemental sulphur. One dose round only moves soil about one pH unit, so retest after 6 months and repeat if needed. Blueberries want 4.5 to 5.5. Treat the ground at least a year before planting where you can; for a few bushes, peat-free ericaceous compost in a large pot is far easier.

Sulphur does not acidify by itself. Soil bacteria turn it into sulphuric acid, so the process needs warmth and moisture. To lower the top 15 cm by one pH unit: 55 g/m² on sandy soil in one dose, 110 g/m² on loam in two doses and 175 g/m² on clay in three doses, 6 months apart. Never more than 80 g/m² in one dose, 125 g/m² a year on planted ground or 160 g/m² a year on bare soil before planting (University of California). The RHS figure of 150 to 300 g/m² is an upper limit, not a target.¹ Work it in and retest after 6 months. Do not use sulphur in pots; use ericaceous compost instead (RHS). A level tablespoon of pastilles weighs about 13 to 18 g, so weigh a spoonful on kitchen scales first.

To go further, towards blueberry level, repeat rather than pile on. Apply the rate, wait a growing season, retest, and apply again if you still need to. Around established rhododendrons, azaleas, camellias and heathers, use up to 25 g/m² a dose over the root area, at most twice a year (University of Georgia), and no more than 94 g per blueberry plant a year (Oregon State University). Raspberries only need it if pH is above 7.0. Sulphur is not recommended for potatoes. For brassicas and onions, use a sulphate instead. Expect one to two years for a big shift. Do not double the rate to hurry; overshoot is far harder to undo.⁴

Our Sulphur Soil Acidifier is 90% elemental sulphur with 10% bentonite in pastilles: dust-free to spread; soil moisture swells the clay and bursts each pastille into fine particles. That cuts both ways. Pastilles work more slowly than fine sulphur powder, because the sulphur released from each pastille stays in a small cluster where it landed. What you gain is a dust-free dose that goes where you aim it. Allow a full season. Apply spring to autumn, when the soil is warm enough for the bacteria to work.

On an established lawn I'd go gentler. Only use sulphur on a lawn if a test shows pH above 7.5. Use 15 g/m² (about one level tablespoon), at most twice a year, and water it in straight away. Do not spread it above 27 °C or on close-mown turf. Sulphur does not control moss (RHS). Starting a new lawn? Work the bed rate into the top 15 cm before you seed or turf.

Apply from spring to autumn, into warm, moist ground. Acidification by sulphur takes weeks to months because it depends on soil bacteria; cold ground slows it sharply. A big shift to blueberry level (around pH 4.5 to 5.5) can take one to two years of staged applications and retests, which is why a few bushes in ericaceous compost is often the kinder route.

Iron sulphate acts faster but needs about eight times as much for the same result.¹ Aluminium sulphate is faster still at about seven times the sulphur rate, but repeated use builds aluminium toward root-toxic levels. Those two work because the metal releases acid in the soil, not because of the sulphate. Plain sulphate feeds (gypsum, sulphate of potash, polyhalite) are neutral salts and do not lower pH. For a garden, elemental sulphur is the one to reach for.

What will not work, and the chalk problem

Coffee grounds, pine needles and vinegar do not lower pH in any lasting way. Oregon State puts used grounds near neutral (about pH 6.5 to 6.8); a Washington State review found published values from 4.6 to 8.4.³ Grounds belong in the compost heap. Vinegar drops pH briefly, soil microbes break it down within days, and a strong dose can scorch roots. Useful as a fizz test; useless as a treatment.

Gypsum will not correct an acid soil. It is calcium sulphate: pH stays put.⁵ That is why it is useful for calcium, sulphur and dispersive clay without raising pH; our guide to gypsum on clay soil covers that job. Granulated Gypsum and Liquid Gypsum sit there; polyhalite is another pH-neutral sulphate. Rock dust is sold for minerals, not pH; its liming effect is small and varies with the rock (more in the rock dust guide).

Then the hard case: soil that fizzed. Free calcium carbonate keeps neutralising acid as you add it. The RHS puts it flatly: if free lime or chalk is present, the soil cannot realistically be acidified.¹ Choose lime-tolerant plants, or grow acid-lovers in peat-free ericaceous compost and water with rainwater.

So, raise, lower or leave it?

Test first. Too acidic: lime over winter from the RHS table, ahead of brassicas, not potatoes. Too alkaline and no fizz: elemental sulphur through the warmer months. Fizz: plant for alkaline ground or build a controlled space. Near 6.5 already: leave it and put effort into feeding and structure. Split applications, retest before adding more. Slow and tested beats fast and overshot every time.

Common questions

Sources