Soil pH: why it matters and how to adjust it

Soil pH measures whether the soil is acidic, neutral or alkaline, on a scale of 0 to 14 in which 7 is neutral. It matters much more than it seems because it controls which nutrients the plant can absorb. You can have soil full of iron and manganese and, if the pH is too high (chalky), those nutrients are chemically blocked even though they're there, and the young leaves turn yellow with green veins (iron chlorosis). The other way round, in very acidic soil phosphorus, calcium and magnesium are blocked, and aluminium becomes soluble and toxic to roots.

The pH scale is logarithmic: a soil of pH 5 is ten times more acidic than one of pH 6, and a hundred times more than one of pH 7. That's why changes take so much work and have to be made gradually, and why you can't "fix" a very chalky soil from one year to the next. In this guide you'll see the ideal range by plant type; how to measure pH three ways and how reliable each is; how to lower it (acidify) and how to raise it (alkalise) safely and with rough doses; the most common mistakes; and when it's better to give up and choose plants adapted to what you already have. At the end, twelve frequently asked questions.

The pH range you want

Most garden plants are comfortable between pH 6.0 and 7.5, which is the band where the most nutrients are available at once. Within that, each group has its preference:

How to measure soil pH

1. Probe meter (€10-15): push it into moist soil (not dry or waterlogged) at several points in the garden and wait a minute. It's a guide — it doesn't give reliable decimals and you have to clean the probe between readings — but enough to know whether your soil is clearly acidic, neutral or alkaline.
2. Strip or reagent kit (€10-20): you mix a sieved soil sample with distilled water, let it stand and compare the colour of the liquid with a scale. More accurate than the probe; the reagent one (capsules + tubes) is the most reliable of the home methods.
3. Laboratory analysis (€30-60): essential before planting fruit trees, an expensive hedge, a large veg garden or a blueberry patch. It gives you pH, active lime (important: not the same as pH), texture, organic matter and nutrients all at once.
4. Free visual clue: if hydrangeas grow blue naturally in your area, along with camellias, chestnuts and ferns, your soil is acidic. If olives, rosemary, wild almonds and rock roses dominate, it's chalky (alkaline). Look at the neighbours' gardens: if everyone has pink or chlorotic hydrangeas, the soil in the area is alkaline.

Take several samples from different points and at 15-20 cm deep, not just from the surface (where mulch and rain can distort the reading). And bear in mind that pH varies within the same garden: next to a wall or the foundations it's usually more alkaline because of the cement; under a conifer or an oak, more acidic because of the leaf litter.

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How to lower pH (acidify a chalky soil)

This is the most common case in Spain, where many soils are chalky and the irrigation water is hard.

• Elemental sulphur: the most lasting and economical method. Soil bacteria turn it into sulphuric acid over months. Apply in autumn, work it in lightly and measure again in spring. Rough dose to lower one point: 50-80 g/m² in sandy soil, 100-150 g/m² in clay; split it into two applications a few months apart and measure before the second.
• Iron sulphate: faster (weeks) and it also visibly corrects iron chlorosis. Ideal for a one-off boost under a particular plant that is yellowing. Careful: it stains paving and clothes with rust, and in excess it can acidify a spot too much.
• Acidic organic matter: compost, sphagnum peat, pine needles, oak or chestnut leaf mould. A gentle, gradual effect with no risk of overdoing it. It's the option for acidifying bit by bit and naturally, especially in already-planted beds.
• Practical solution for difficult cases: if your soil is very chalky (with high active lime), acidifying is a constant battle against the soil itself and against the irrigation water, and probably a losing one. It's much more sensible to plant the acid-lovers in pots or raised beds with specific acidic growing medium, isolated from the soil with a liner, and water them with rainwater.

If your irrigation water is hard (lots of lime), it will gradually raise the soil pH and undo your acidification work. For acid-loving plants, collect rainwater: it's slightly acidic and adds no carbonates. A tank connected to the downpipe solves the problem; you can also acidify the irrigation water with a few drops of vinegar or citric acid to pH 6.

How to raise pH (correct an acidic soil)

Less common in Spain, typical of the humid north and of siliceous or granite soils.

• Garden lime (ground calcium carbonate): the standard option. Spread it in autumn and work it in lightly with the rake. It takes weeks or months to act. The dose depends on the analysis and the texture (more lime the more clay the soil).
• Dolomitic lime (dolomite): calcium and magnesium carbonate. Use it if the analysis shows the soil is also short of magnesium, something common in acidic and sandy soils.
• Wood ash: raises the pH quickly and supplies potassium, but very sparingly (a handful per m² at most, occasionally): in excess it salinises and cakes. Never use coal ash, chipboard ash or treated or painted wood ash.

Raise the pH gradually: no more than 1 point per season. Sudden changes block other nutrients (if you raise the pH all at once you'll cause chlorosis) and stress soil life.

Common pH mistakes

• Adding lime "just in case" without measuring. If the soil was already neutral, you cause chlorosis in every sensitive plant in the garden. Never change the pH without having measured it first.
• Expecting results in a week. pH amendments work over months. Measure first, apply in autumn, and measure again in spring before deciding whether to apply more.
• Confusing pH and active lime. Two soils with the same pH 8 can behave very differently depending on their active lime content: one acidifies relatively easily, the other is almost impossible because it has an enormous "buffering power". A laboratory analysis clears this up.
• Ignoring that pH varies by zone. Measure next to the walls and in the middle of the beds separately, and plant acid-lovers away from the cement.
• Fighting the soil instead of choosing plants adapted to what you already have. It's nearly always the smarter, cheaper and more sustainable decision.
• Using vinegar or lemon juice as a permanent acidification method. The effect is minimal, lasts hours and can harm soil life. It's useful for acidifying pot irrigation water on a one-off basis, not for changing the pH of a bed.

Symptoms of nutrient lock-up by pH (not real deficiency)

Many gardeners fertilise a yellowing plant over and over with no result, because the problem isn't that the nutrient is missing from the soil, but that the pH stops the root absorbing it. Learn to tell the difference:

The pH of the irrigation water counts too

It's not only the soil pH that matters: the water you irrigate with, if it has a high pH and a lot of lime (hard water), gradually deposits carbonates on the soil surface and in pots, raising the pH bit by bit. In pot growing and hydroponics, the water pH is as important as that of the growing medium.

• Check the pH and hardness of your water: many municipal companies publish it, or you measure it with strips.
• Hard water (> 7.5 and lots of lime): collect rainwater for acid-lovers and for pots; for the rest of the garden it's usually not a problem if the soil has good organic matter.
• Acidifying the irrigation water to pH 6-6.5 with citric acid, a few drops of diluted nitric acid or a commercial product is the practical solution for watering pots of camellias, azaleas, blueberries or gardenias.

How long each amendment takes to act

Frequently asked questions

My hydrangea was blue and now it comes out pink, what happened?

The colour of the hydrangea (of the varieties that change) depends on the pH and the available aluminium: blue in acidic soil (low pH, soluble aluminium that the plant absorbs), pink in neutral-to-alkaline soil (aluminium locked up). The soil or the irrigation water has probably raised the pH. To get the blue back: acidify with aluminium or iron sulphate and water with rainwater. White hydrangeas don't change colour with pH.

Does the pH of bagged growing medium also change over time?

Yes. With watering (especially if the water is hard) and with the natural breakdown of the organic matter, the pH of potting mix tends to rise. That's why it's worth refreshing the growing medium of acid-lover pots every 1-2 years and watering them with rainwater or acidified water.

How much sulphur do I need to lower the pH by one point?

It depends a lot on the texture and the lime in the soil: as a guide, between 50 and 150 g/m² to lower one point, more in clay soil than in sandy. Split it into two applications a few months apart and measure again before a third. Never add a large dose all at once: you can create a spot that is too acidic and "scorch" the root.

Do coffee grounds acidify the soil?

Very little and temporarily (spent grounds are almost neutral). Their real value is in the organic matter and nitrogen they add, mainly through the compost. To acidify properly and lastingly, sulphur or iron sulphate.

Can I trust a cheap €10 probe meter?

For a rough reading (acidic / neutral / alkaline), yes, if you push it into moist soil and clean it between readings. For fine adjustments — for example, blueberries that need pH 4.5-5.5 — you need a reagent kit or a laboratory analysis, because a tenth of a pH point changes iron availability a lot.

Which plants can I put in if my soil is very chalky and I don't want to fight the pH?

The whole Mediterranean palette thrives in chalky soil: lavender, rosemary, sage, rock rose, oleander, olive, lilac, mock orange, pomegranate, fig, almond, bougainvillea, Teucrium, Phlomis, ornamental grasses, strawberry tree, cotoneaster. It is, in fact, their natural habitat. See drought-resistant plants.

Does the lawn have an ideal pH?

Yes, it prefers 6.0-7.0. In very acidic soil it grows weak and moss appears; in very alkaline soil it yellows and clover comes up. An analysis and a gentle adjustment (lime or sulphur as appropriate, plus good feeding) greatly improve a lawn that just won't get going.

How do I acidify only the irrigation water for my acid-lover pots?

Add a few drops of vinegar or citric acid (or a commercial "irrigation water acidifier") until the water is at pH 5.5-6, checking it with strips. It's a useful workaround for pots; for a large bed it's not practical.

Is peat the best way to acidify?

It acidifies well and improves water retention, but its extraction has a high environmental impact (it destroys peat bogs, which are huge carbon sinks). To acidify, sulphur is better; for acidic potting mix, look for blends with coir and composted bark instead of peat.

How long does it take for the pH to rise again after acidifying?

In a soil with high active lime and hard irrigation water, it can start rising in 1-2 years and return almost to the starting point in 3-4. That's why, in very chalky soils, acid-lovers in the ground are constant maintenance and the pot is usually the realistic solution.

Does pH also affect soil life?

Yes. Most worms and beneficial bacteria prefer a pH between 6 and 7.5. Very acidic soils favour fungi over bacteria and slow down the breakdown of organic matter; very alkaline ones limit the availability of micronutrients for the whole soil network.

For more context, look at the soil types and how to amend them, and how to make compost, which is the best way to improve any soil without touching the pH abruptly.