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Two gardeners plant the same blueberry bushes. One harvests bowls of fruit; the other gets yellowing leaves and a few sour berries. The difference often isn’t water, sunlight or fertiliser. It’s soil pH.
Soil pH is one of the most important and least visible factors in plant growth. It doesn’t feed plants directly. Instead, it controls the chemistry of every nutrient in the soil, deciding whether each one is dissolved and available to roots or locked away in forms plants can’t use.
The soil pH scale
Most soils fall between pH 4 and pH 8.5:
| Soil pH | Description | Typical situation |
|---|---|---|
| below 5.0 | strongly acidic | peat bogs, heathland, pine forests, heavily leached soils |
| 5.0–6.0 | moderately acidic | many rainy-region soils, woodland |
| 6.0–7.0 | slightly acidic to neutral | most productive farm and garden soils |
| 7.0–8.0 | slightly alkaline | soils over chalk or limestone |
| above 8.0 | strongly alkaline | dry regions, soils rich in sodium carbonate |
Most garden plants and crops grow best between pH 6.0 and 7.0.
Why pH matters: nutrient availability
Plants absorb nutrients as dissolved ions through their roots. Whether an ion stays dissolved depends heavily on pH.
| Nutrient | Best available at | What goes wrong |
|---|---|---|
| Nitrogen (as NO₃⁻, NH₄⁺) | 6.0–8.0 | microbes that release it slow down in acidic soil |
| Phosphorus (as H₂PO₄⁻, HPO₄²⁻) | 6.0–7.0 | locked up with iron and aluminium in acid soils, with calcium in alkaline soils |
| Potassium, calcium, magnesium | 6.0–8.5 | washed out (leached) of acidic soils |
| Iron, manganese, zinc, copper | below 6.5 | form insoluble hydroxides and oxides in alkaline soils |
| Molybdenum | above 6.0 | less available in acidic soils |
| Aluminium | toxic below about 5.0–5.5 | dissolves as Al³⁺, damaging roots |
The sweet spot around pH 6.5 is where the most nutrients are available at once. Move too far either way and something goes wrong.
Phosphorus: the trickiest nutrient
Phosphate is a good example of how pH controls availability. In acidic soil, dissolved iron and aluminium ions react with phosphate to form insoluble iron and aluminium phosphates. In alkaline soil, calcium reacts with phosphate to form insoluble calcium phosphates. Only in the narrow window of roughly pH 6–7 does a useful amount stay dissolved.
Iron: why alkaline soils cause yellow leaves
In alkaline soil, iron exists mainly as insoluble iron(III) hydroxide:
Fe³⁺ + 3OH⁻ → Fe(OH)₃
Even if the soil contains plenty of iron, roots can’t absorb it. The plant can’t make enough chlorophyll, and new leaves turn yellow between green veins, a condition called lime-induced chlorosis. Acid-loving plants such as rhododendrons, azaleas and blueberries are especially sensitive.
Aluminium: the danger in very acidic soil
Aluminium is the most abundant metal in the Earth’s crust, but in soils above about pH 5.5 it’s locked in insoluble minerals. Below that, Al³⁺ ions dissolve. They damage root tips, stunt growth and interfere with calcium and phosphate uptake. Aluminium toxicity is one of the main limits on crop yields in acidic soils worldwide.
Why soils become acidic
- Rainfall. Rainwater is naturally slightly acidic (about pH 5.6) because carbon dioxide dissolves to form carbonic acid. Over years, rain leaches basic ions like calcium and magnesium out of the soil.
- Organic matter. Decomposing leaves and roots release organic acids and CO₂.
- Fertilisers. Ammonium-based fertilisers acidify soil when soil bacteria convert ammonium to nitrate, releasing H⁺: NH₄⁺ + 2O₂ → NO₃⁻ + 2H⁺ + H₂O
- Plant uptake. Roots release H⁺ ions when they take up positive ions such as K⁺ and NH₄⁺.
- Acid rain from sulfur and nitrogen oxides, though this has decreased in many countries.
Why soils become alkaline
- Parent rock. Soils over chalk or limestone contain calcium carbonate, which neutralises acids and buffers pH near 7.5–8.3.
- Dry climates. With little rain, basic salts aren’t washed out and can accumulate at the surface.
- Irrigation with hard water adds calcium and hydrogencarbonate.
Plants and their preferred pH
| Preferred pH | Examples |
|---|---|
| 4.5–5.5 (acid-loving, “ericaceous”) | blueberries, cranberries, rhododendrons, azaleas, camellias, heathers |
| 5.5–6.5 | potatoes, strawberries, raspberries, sweet potatoes |
| 6.0–7.0 (most plants) | tomatoes, beans, lettuce, carrots, roses, most lawns |
| 6.5–7.5 | cabbage, broccoli, asparagus, clematis, lavender |
| 7.0–8.0 (lime-tolerant) | many Mediterranean herbs, lilac, some grasses |
How to test soil pH
- Take samples from several spots, about 10–15 cm deep, avoiding surface debris. Mix them.
- Use a soil test kit: shake soil with distilled water and an indicator solution, let it settle, and compare the colour with a chart.
- Or use a pH meter designed for soil, following its instructions.
- For accurate farm decisions, send samples to a laboratory, which also measures nutrient levels.
The same indicator principles used in school labs are at work here. See universal indicator.
How to change soil pH
Raising pH (making soil less acidic): liming
Ground limestone (calcium carbonate) is the most common material. It reacts slowly with soil acidity:
CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂
Hydrated lime (calcium hydroxide) works faster but is caustic and easier to overdo. Dolomitic limestone also supplies magnesium. Wood ash has a mild liming effect.
Liming takes months to act fully and should be based on a soil test, because the amount needed depends on the soil’s buffering capacity. Clay and organic-rich soils resist pH change much more than sandy soils, for the same reason that concentrated buffers resist change more than dilute ones (see buffer capacity).
Lowering pH (making soil more acidic)
- Elemental sulfur: soil bacteria slowly oxidise it to sulfuric acid. It takes weeks to months.
- Aluminium sulfate or iron sulfate: faster, because the hydrated metal ions are acidic (see salt hydrolysis), but easier to overuse.
- Acidic organic matter such as pine needles or peat-free ericaceous compost.
In practice, it’s much easier to raise pH than to lower it, especially in soils over chalk, where the calcium carbonate keeps neutralising the added acid. Gardeners on chalk often grow acid-loving plants in pots of ericaceous compost instead.
The hydrangea colour trick
Bigleaf hydrangeas (Hydrangea macrophylla) act as living pH indicators, but not in the way you might expect. Their blue colour comes from a complex between an anthocyanin pigment and aluminium ions in the flower cells.
- In acidic soil (roughly below pH 5.5–6), aluminium is soluble. The plant absorbs it, and the flowers turn blue.
- In alkaline soil, aluminium is locked up as insoluble compounds. Without it, the flowers stay pink or red.
Gardeners who want blue flowers add aluminium sulfate and keep the soil acidic. It’s a vivid example of how pH controls metal availability.
Key takeaways
- Most plants grow best at soil pH 6.0–7.0, where the most nutrients are available.
- Acidic soils release toxic aluminium and lock up phosphate; alkaline soils lock up iron, manganese and zinc.
- Rain, organic matter and ammonium fertilisers acidify soil; limestone bedrock and dry climates make it alkaline.
- Lime raises pH; sulfur and aluminium sulfate lower it; buffering capacity decides how much is needed.
- Hydrangea flower colour depends on aluminium availability, which depends on pH.
- For more on the pH scale, see the pH scale explained.
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