How to make soil more acidic, or raise the pH

By Joe, Founder of Dr Forest · January 2026, updated August 2026 · 17 min read

Edited 28 Sep 2026

How do you raise or lower soil pH safely in a UK garden?

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 the nutrients in your soil are actually available to your plants, 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 slowly, over months rather than days, and the size of the dose depends on your soil type. Before you change anything, test what you have, because some soils, the chalky ones in particular, will fight you the whole way and are better worked with than against.

This guide covers what pH is, the range your plants actually want, how to test it properly, and exactly how to move it up or down, along with the popular fixes that do nothing at all.

Related: Garden Calcium Carbonate · Sulphur Soil Acidifier

Why pH matters more than your next feed

You can have a nutrient in the soil and still see a shortage in the plant, because pH changes how available it is. Phosphorus is the clearest case. It is most available between about pH 6.0 and 7.0. On the acid side, iron and aluminium bind more of it; the RHS puts the point at which most phosphates become poorly available at around pH 5.1. Past roughly pH 7.5, calcium holds more of it instead.

Iron and manganese fall away as soil turns alkaline, which is why plants on chalk so often show the pale, yellow-between-the-veins look of lime-induced chlorosis even when there is iron in the ground. If you have seen that pattern on a crop and assumed it was hunger, our guide to why tomato leaves turn yellow walks through telling the two apart.

So before you spend money on feed, know your number. Fertilisers do not correct soil pH. Feeding harder when availability is the limit usually wastes the bag. Get the ground into range, then feed. We cover the feeds themselves in our plain guide to what a fertiliser is; this page is about the groundwork that lets any feed actually work.

Figure 1 · Nutrient availability across the pH scale

Why 6.0 to 7.0 is the sweet spot

A schematic view: how readily plants can take up each nutrient as soil moves from acid to alkaline. Band widths are generalised from soil-science guidance, not exact for any one soil.

What pH does your plant actually want?

The pH scale runs from 0 to 14. Seven is neutral, anything below is acid and anything above is alkaline. It is also a logarithmic scale, which is the one fact worth holding on to: each whole step is a tenfold change. Soil at pH 5.0 is ten times more acidic than pH 6.0, and a hundred times more acidic than pH 7.0. That is why nudging pH takes real material and real time, and why small overshoots are hard to walk back.

For the great majority of garden plants, the target is a slightly acidic to neutral soil, roughly pH 6.0 to 7.0. The Royal Horticultural Society calls pH 6.5 the best general-purpose figure for gardens, because that is where the widest spread of nutrients stays available and where soil life, the bacteria and earthworms that keep soil healthy, is most active. A smaller group of plants wants things distinctly more acidic.

The practical lesson is to match the plant to the soil wherever you can, and only reach for amendments when there is a real gap between what you have and what a plant needs. Chasing a precise number across a whole garden is rarely worth it. Shifting one bed, or one row of fruit, usually is.

Test before you touch anything

Guessing your pH from the weeds or the neighbour's hydrangeas is a quick way to waste a season. A test is cheap and takes minutes. A simple home kit with the colour chart gets you close enough for most decisions; a cheap probe meter is handy for repeat checks; and a laboratory test, where you post off a sample, gives you the most reliable figure along with a read on nutrients. Take several small samples from around a bed, mix them, and test the blend, since pH can vary across even a small garden.

One timing note, and it is the RHS's own: a test carried out within three months of adding lime, fertiliser or organic matter may give a misleading result. Leave it three months, then test. And there is one field test worth knowing before you commit to anything, because it tells you whether your soil will cooperate at all.

How to raise pH: making acid soil less acidic

If your test comes back too acidic for what you want to grow, the answer is lime. Garden lime is simply ground limestone, calcium carbonate, and it is the standard, well-understood way to bring an acid soil up towards neutral. Dolomitic lime does the same job and adds magnesium, useful if your soil is short of it. Calcified seaweed and ground chalk are the same carbonate chemistry in another form.

How much you need depends on two things: how far you have to travel, and what your soil is made of. Clay resists a change in pH far more stubbornly than sand, because its much larger surface area holds a bigger reserve of acidity to be neutralised before the reading moves. The RHS publishes rates indexed to a target of pH 6.5, so you read off the row that matches the number your test gave you.

Figure 2 · Ground limestone, RHS rates

How much lime to reach pH 6.5

Find your soil's current pH on the left, then read across to your soil type. Rates assume the lime is worked into the top 20cm of soil.

Your soil now	Sandy	Loam	Clay
pH 6.0	0.4 kg/m²	0.5 kg/m²	0.6 kg/m²
pH 5.5	0.7 kg/m²	0.8 kg/m²	1.0 kg/m²
pH 5.0	1.0 kg/m²	1.2 kg/m²	1.4 kg/m²
pH 4.5	1.3 kg/m²	1.5 kg/m²	1.8 kg/m²

Those numbers are larger than most gardeners expect, and that matters. Under-liming is the commonest reason people conclude that lime does not work: half a dose on a clay soil moves the reading so little that it reads as no change at all.

Timing and handling matter too. Lime is usually added in winter for annual crops, worked into the soil and given time to act. Finely ground limes start to work within weeks, but a full reaction through the soil can take a couple of years, which is another reason to apply in stages and retest rather than dumping a heavy dose at once. Take care with builders' lime, or hydrated lime: it is far stronger than ground limestone, irritating to skin and eyes, and easy to overdo.

Keep lime and manure apart, as well. Garden practice has long been to lime in winter and manure in spring, and the chemistry behind it is real: raising soil pH pushes ammonium towards ammonia gas. The oft-cited jump in ammonia loss (more than fivefold in one dataset) is from a laboratory incubation with hydrated lime on liquid manure (Mkhabela et al. 2006), not a field trial of ground limestone — useful as chemistry, not as a plot-scale rate card for garden lime.

How to lower pH: making alkaline soil more acidic

If you need to bring pH down, for blueberries, rhododendrons, camellias, acers or any of the ericaceous group, the material to use is elemental sulphur. It does not acidify the soil by itself. Soil bacteria of the genus Acidithiobacillus, which older texts call Thiobacillus, oxidise it into sulphuric acid, and that is what lowers the pH. Because the work is done by living bacteria, the process is gradual and it depends on warmth and moisture.

The RHS gives a clear working rate: 135 to 270 g of sulphur per square metre to bring the top 15cm of soil down from neutral (pH 7.0), or slightly alkaline (pH 7.5), to slightly acid (pH 6.0 to 6.5). Use the lower end on light sandy soils and the higher end on clay, which, just as with lime, buffers against change. The sulphur has to be worked into the soil rather than left on the surface, where it would take years to reach root depth.

Our own sulphur, and how to use it

Dr Forest makes a Sulphur Soil Acidifier: 90% elemental sulphur bound with 10% bentonite clay and formed into pastilles. The bentonite is the useful part of the design. Loose sulphur powder is fine enough to blow about and unpleasant to spread, so the pastilles let you spread it like any other granule, dust-free. Once they take up soil moisture the clay swells and bursts the pastille apart into fine sulphur particles. Fineness is what governs the speed of the reaction, because the bacteria work on the surface of the particle, so a fine particle is oxidised faster than a coarse chip.

Figure 3 · Sulphur Soil Acidifier, application rates

How much 90% sulphur pastille to deliver the RHS acidifying dose

These are product weights of our 90% sulphur bentonite pastilles for moving the top 15 cm from about pH 7.0–7.5 towards pH 6.0–6.5 — the band that delivers the RHS elemental-sulphur dose. The loam figure is ours; the RHS publishes sand and clay only. Work the pastilles in rather than leaving them on the surface.

Soil type	How it behaves	Rate per square metre
Sandy / light	Shifts easily, needs the least	~150 g/m²
Loam / medium	Middle of the road	~220 g/m²
Clay or high organic matter	Buffers hard, needs the most	~300 g/m²

Apply from spring to autumn, into warm, moist ground. The RHS is plain about the pace: acidification by sulphur takes weeks to show, and in cold winter soil months might be needed. Getting a neutral soil down by a full point can take one to two years. The upside of a slow mechanism is a durable result, since once the soil sits at the pH you want, it tends to hold there for years rather than bouncing straight back.

Two practical cautions. Wear gloves and spread in still air. And do not be tempted to double the rate to hurry things along, because over-acidified soil has to be limed back, which wastes both bags and a season.

The alternatives, and why sulphur usually wins

Iron sulphate, sold as sulphate of iron, acts faster than elemental sulphur, but the RHS puts the requirement at eight times as much for the same result, which makes it expensive beyond a small patch. Aluminium sulphate is faster still and is the classic trick for turning hydrangeas blue, and the RHS puts that one at seven times the sulphur rate. It carries a real cost, though: repeated applications build aluminium in the soil towards levels toxic to roots, and aluminium ties up phosphorus into the bargain, so it is best kept to that one specialist job. Peat, once the standard acidifier, is no longer recommended on environmental grounds.

What will not work, and the chalk problem

A few popular fixes get repeated endlessly and simply do not deliver. Worth clearing up before you waste time or money.

Coffee grounds do not acidify soil. Most of the acidity washes out in brewing, and Oregon State puts used grounds close to neutral at around pH 6.5 to 6.8. A wider literature review by Washington State found published values ranging from 4.6 all the way to 8.4, which is the more useful point: grounds are too variable to be a reliable acidifier even when a given batch happens to be acidic. They are a fine addition to the compost heap, but they will not move your soil pH in any dependable way.

Vinegar is not a soil acidifier. It drops pH for a few hours then is gone, and on the way it can harm roots and the soil life you want to protect. Useful as a fizz test, as above; useless and slightly risky as a treatment.

Gypsum will not correct an acid soil. This one trips people up because gypsum is calcium sulphate and sulphur is an acidifier. But gypsum is the neutral salt of a strong acid and a strong base: dissolving it adds no hydrogen ions, and its neutralising value is zero, so it is not a liming material. It adds calcium without raising pH, and where heavy clay is dispersive it improves the structure, which is exactly why it is useful, and why we are clear about it.

Then there is the hard case: naturally chalky or lime-rich soil, the kind that fizzed in the vinegar test. Here the soil holds a reserve of free calcium carbonate that keeps neutralising any acid you add. The RHS puts it flatly: if free lime or chalk is present, the soil cannot realistically be acidified, because the quantity of sulphur needed to counter it would be huge and would have to be repeated over many years. The honest advice is to stop fighting it.

Work with the soil you have, not the one you wish you had.

So, raise, lower or leave it?

Pulling it together: test first, then decide. If your soil is too acidic for what you want to grow, lime it over winter, reading your rate off the RHS table by your starting pH and soil type. If it is too alkaline and the vinegar test does not fizz, elemental sulphur will bring it down through the warmer months, at roughly 150 to 300 g per square metre for a full point depending on your soil. If the test fizzes, accept that you have alkaline soil and either plant for it or build a controlled space. And if your number already sits near 6.5, the best move is usually to leave it alone and put your effort into feeding and soil structure instead.

Whatever you do, go gently. pH change is measured in months, not days, so split your applications, retest before adding more, and check again each year, since soils drift back towards their natural state over time. Slow and tested beats fast and overshot every time.

Related: Sulphur Soil Acidifier · Garden Calcium Carbonate · Sea Shell Meal · Granulated Gypsum · Start with our guide to what a fertiliser is

Common questions