How to increase or decrease your soil pH

Soil science

How to increase or decrease soil pH

By Joe, Founder of Dr Forest · June 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 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 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.

Why pH matters more than your next feed

Here is the part most people miss. You can have plenty of a nutrient in your soil and still see a deficiency in the plant, because at the wrong pH that nutrient is chemically locked up and the roots cannot take it in. Phosphorus is the classic case: below about pH 5.1 it binds tightly and becomes hard to reach, and at high pH it locks up again. Iron and manganese drop away as soil turns alkaline, which is why plants on chalky ground so often show the pale, yellow-between-the-veins look of lime-induced chlorosis even when there is iron in the soil.

So before you spend money on feed, it is worth knowing your number. A balanced fertiliser cannot fix a pH problem, and feeding harder to compensate usually just wastes nutrients and your money. 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 points to about 6.5 as the best all-round figure, 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. Do not test in the weeks straight after adding lime, sulphur, fresh compost or any feed, because the reading will be unsettled and misleading. Leave it a couple of months for things to settle, 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 almost entirely on your soil texture, because clay resists a change in pH far more stubbornly than sand. Clay's large surface area buffers against change, so a clay soil can need several times the lime a sandy one does to shift the same amount. The figures below are a general starting point for nudging a soil up by roughly half a pH point; treat them as a guide and let a soil test, ideally a lab one, set the real rate.

Figure 2 · A starting point for liming

Roughly how much garden lime, by soil type

Representative quantities to raise pH by about half a point, worked into the top 20cm. General guidance only; a soil test should set your real rate.

Soil type	How it behaves	Garden lime, guide rate
Sandy / light	Shifts easily, needs the least	~50–100 g/m²
Loam / medium	Middle of the road	~100–200 g/m²
Clay / heavy	Buffers hard, needs the most	~150–300 g/m²

Timing and handling matter. The traditional moment to lime is over winter or before planting, working it into the soil and giving it 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. Avoid hydrated or builders' lime in the garden unless you know exactly what you are doing; it is caustic, far stronger than garden lime, and easy to overdo. Do not apply lime at the same time as manure, either, as the two react and waste nitrogen.

Dr Forest does not sell a bag labelled simply "lime", but two products in the range do the same job in a cleaner, more controlled form.

How to lower pH: making alkaline soil more acidic

If you need to bring pH down, for blueberries, rhododendrons or any of the ericaceous group, the standard material is elemental sulphur. It does not acidify the soil on its own; instead, soil bacteria slowly oxidise it into sulphuric acid, and that is what lowers the pH. Because it relies on living bacteria, the process is gradual and all but stops in cold soil, so autumn or spring applications into warm, moist ground work best.

The RHS gives a clear working rate: roughly 135 to 270 grams of sulphur powder per square metre to bring the top 15cm of soil down from around pH 7.0 or 7.5 to 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. Finer sulphur acts faster than coarse, and the amendment needs to be worked into the soil rather than left on the surface, where it would take years to reach root depth.

There are two alternatives worth knowing, with caveats. Iron sulphate (sulphate of iron) acts faster than elemental sulphur, but you need a great deal more of it for the same drop, roughly eight times as much, which makes it expensive for anything beyond a small area. Aluminium sulphate works fast too and is the classic trick for turning hydrangeas blue, but in quantity it interferes with phosphorus uptake and aluminium is toxic to most plants that are not natural acid-lovers, so it is best kept to that one specialist job. Peat, once the go-to 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. The acidity washes out in brewing, so used grounds come out close to neutral, around pH 6.5 to 6.8, and any small effect is short-lived. They are a fine addition to the compost heap, but they will not move your soil pH in any reliable 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 does not change pH. This one trips people up because gypsum is calcium sulphate and sulphur is an acidifier. But gypsum is pH-neutral: it supplies calcium and sulphur and improves the structure of heavy clay without nudging pH in either direction. That 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. You can pour on sulphur, but as fast as it acidifies, the chalk buffers it straight back, and the amount needed to win outright is impractical and quickly cost-prohibitive. 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, leaning on soil type for the rate and applying over winter. If it is too alkaline and the vinegar test does not fizz, sulphur will bring it down slowly through the warmer months. 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.

Common questions