Alkaline soil is soil with a pH above 7.0, and it is common in dry regions, on limestone or chalk bedrock, and in gardens built on rubble or new-build sites where concrete and mortar have leached lime into the ground. At pH 7.5 and above, iron, manganese, zinc and phosphorus become less available to plants even when they are present in the soil, so plants can show yellow leaves with green veins — iron chlorosis — while sitting in perfectly fertile ground. Alkaline soil is harder to change permanently than acidic soil, because the soil keeps pushing pH back up, so the practical approach is a mix of gentle lowering, heavy composting, and choosing plants that suit the conditions.
That last point matters most. Many excellent garden plants prefer alkaline ground, and a chalk garden can be beautiful and productive once you stop fighting it. This guide explains how to test for alkalinity, how to lower pH safely where it is worth doing, how compost helps, and which plants to grow.
How to test for alkaline soil
Test before you do anything. Home pH kits and meters give a rough reading, but a laboratory test is more accurate and reports free lime (calcium carbonate) content, which decides whether lowering pH is realistic at all. Soils with high free lime resist acidification — you can add sulfur and the lime simply neutralises it — so knowing that number saves you wasted effort. Your local extension service can usually test soil for a small fee and advise on realistic targets; use that service if it is available.
As a rough guide, pH 7.0–7.5 is mildly alkaline and manageable for most plants with compost and care. pH 7.5–8.0 is where iron chlorosis starts to appear on sensitive plants, and pH above 8.0 is strongly alkaline and needs a deliberate strategy. Also do a fizz test: put a spoonful of dry soil in a dish and add a few drops of vinegar. Vigorous fizzing means free lime is present and the soil is strongly buffered — lowering pH will be an uphill job. No fizz at pH 7.5 suggests the alkalinity may be easier to shift. See our soil pH testing guide for the full method, and our loamy soil guide for the broader soil picture.
What causes alkaline soil
The main cause is geology: limestone, chalk and dolomite bedrock produce alkaline soils, and in dry climates there is not enough rainfall to wash the calcium away, so it accumulates. Human causes matter too: concrete, mortar, brick rubble and ash are all alkaline, and gardens on new-build sites often have alkaline rubble mixed into the topsoil. Over-liming is another common cause — gardeners who lime every year out of habit can push pH well above 7.5. Hard tap water adds to the problem in containers and in beds watered heavily with alkaline water. Finally, low organic matter makes alkalinity worse, because organic matter buffers pH towards neutral; a bare, pale, chalky bed with no compost is the worst case.
How to lower soil pH
Elemental sulfur is the standard treatment. Soil bacteria convert sulfur to sulfuric acid over several weeks, which lowers pH gradually. Typical rates are roughly 100–150 g per square metre to lower pH by about half a unit in loam, less on sand and more on clay, but these are broad ranges and a laboratory recommendation should always win. Apply sulfur in spring or early summer when the soil is warm and bacteria are active, spread it evenly, and water it in. Retest after three to four months — sulfur works slowly and over-application can damage soil life, so lower pH in stages. Never apply sulfur and lime together, and do not expect a single application to hold pH down permanently in a chalk soil: the lime in the ground will keep pushing it back up, so annual re-testing and repeat applications are part of the routine.
Other acidifying materials help at the margins. Acidic organic matter such as composted pine bark, leaf mould and peat-free ericaceous compost nudges pH down a little and, more importantly, improves structure and nutrient availability. Ammonium-based fertilisers such as ammonium sulfate acidify soil as bacteria convert them, which is useful on alkaline beds but should be used in moderation. Iron sulfate (roughly 50–100 g per square metre) can lower pH slightly and supplies iron directly, which helps chlorotic plants quickly, but its effect is short-lived. Aluminium sulfate works faster but can build aluminium to harmful levels and is best avoided in food beds. If your soil has high free lime, accept that large-scale pH lowering is impractical and focus on compost, mulching and plant choice instead.
Gardening in alkaline soil with compost
Compost is the single most useful tool on alkaline ground, because it does two things sulfur cannot: it buffers pH towards neutral, and it feeds the soil life that makes iron and other nutrients available even when pH is high. Add 5–8 cm of compost each year and mulch thickly. Compost also improves the structure of chalky soils, which are often thin and stony, and it holds moisture that chalk drains away. For acid-loving plants such as blueberries, the practical answer on alkaline soil is almost always a container or a raised bed filled with ericaceous (acidic) compost — trying to acidify a whole chalk bed for one blueberry bush is a losing battle. Our growing blueberries guide covers container growing. For general beds, see our raised bed soil mix guide and work out volumes with the raised bed soil calculator.
Watering matters too. If your tap water is hard and alkaline, it slowly raises the pH of containers and beds. Rainwater is better where you can collect it, and it is essential for acid-loving plants in pots. Mulching with composted bark also helps, and keeping the soil covered reduces the drying and crusting that chalk soils are prone to.
Plants that love alkaline soil
Many plants prefer alkaline ground, so lean into them. Lavender, rosemary, thyme and most Mediterranean herbs thrive at pH 7.0–8.0 — our growing lavender guide covers one of the best. Brassicas such as cabbage and broccoli prefer slightly alkaline soil and suffer clubroot less there. Clematis, honeysuckle, lilac, buddleia and many roses do well, as do beets, asparagus, peas and beans. Among vegetables, lettuce, spinach and onions tolerate pH up to about 7.5. Avoid acid-lovers — blueberries, rhododendrons, azaleas, camellias and heathers — unless you grow them in containers with ericaceous compost.
Treating iron chlorosis
Yellow leaves with green veins on young growth, while older leaves stay green, is the classic sign of iron chlorosis on alkaline soil. The iron is in the soil; the plant cannot take it up at high pH. Short-term fixes include watering with a chelated iron feed or spraying leaves with a chelated iron solution, which greens plants up within a week or two but does not fix the cause. Long-term fixes are compost, sulfur, and mulching with acidic organic matter. Do not simply add more general fertiliser — excess phosphorus makes iron uptake worse, and over-feeding will not help. If chlorosis is severe and the plant is an acid-lover in the wrong soil, moving it to a container is the honest answer.
Common mistakes with alkaline soil
The biggest mistake is trying to acidify a chalk soil permanently with heroic doses of sulfur. It does not hold, and large sulfur doses can harm soil life and plant roots. Aim for gentle, staged lowering and accept a pH around 7.0–7.5 for most beds. The second mistake is liming alkaline soil out of habit — never lime without a test. The third is overusing phosphorus fertiliser, which is already less available at high pH and can build up to levels that block iron and zinc. The fourth is planting acid-lovers in the open ground and then being surprised when they fail. And the fifth is leaving chalk soil bare: it dries, crusts and erodes, while mulch and cover crops protect it and add the organic matter that buffers pH.
Safety notes
Wear gloves and a dust mask when handling sulfur or iron sulfate powders, which irritate eyes, skin and lungs. Keep sulfur away from open flames — it is flammable as a dust. Do not apply sulfur at the same time as lime, and do not use aluminium sulfate in food beds. If your pH is above 8.0, or if plants are failing despite compost and care, check your local extension service for a soil test and a written plan rather than guessing.
Frequently asked questions
What pH is too alkaline for vegetables?
Most vegetables grow best at pH 6.0–7.0 and tolerate up to about 7.5. Above pH 7.5, iron chlorosis and nutrient lock-up become common, especially for beans, tomatoes and acid-sensitive crops. Brassicas, beets and asparagus tolerate higher pH better than most.
How much sulfur do I need to lower soil pH?
As a broad range, roughly 100–150 g of elemental sulfur per square metre lowers pH by about half a unit in loam, less on sand and more on clay. But the right dose depends on starting pH, texture and free lime content, so a laboratory recommendation should always take priority, and retest after three to four months.
Can compost fix alkaline soil?
Compost does not permanently change the pH of a chalk soil, but it buffers pH, improves structure, and makes nutrients more available at high pH — which often matters more than the pH number itself. Annual compost at 5–8 cm plus mulch is the most reliable routine on alkaline ground.
Why are my plant leaves yellow with green veins?
That pattern — yellow young leaves with green veins — is usually iron chlorosis, common on alkaline soil. Treat it with a chelated iron feed for quick relief and compost plus sulfur for the longer term, and check the pH before adding more fertiliser.
Should I grow blueberries in alkaline soil?
Not in the open ground. Blueberries need pH 4.5–5.5 and will fail in alkaline soil. Grow them in containers or raised beds filled with ericaceous compost, watered with rainwater. It is far more reliable than trying to acidify a chalk bed.