acidicpoor

Acidic soil

A chemical nature (pH below 7), common on granite, sandstone or sand. Ideal for ericaceous plants, trickier for many vegetables.

Simple definition

'Acidic' describes the chemistry of the soil, not its texture: its pH is below 7. It is common on siliceous rocks (granite, sandstone), on leached sand and under a rainy climate. The same acidic soil can be sandy, silty or clayey, texture and pH are two distinct things. In very acidic soils, aluminium and manganese can reach levels toxic to roots (Kochian et al., 2004).

Visual and tactile appearance

Acidity cannot be seen with the eye: it is measured (strips, analysis) or guessed from the wild flora, heather, gorse, foxglove, sheep's sorrel and bracken often signal an acidic soil. The vinegar test stays silent (no fizzing, unlike chalky soil).

Main properties

pH below 7, sometimes well below.
Iron and manganese stay well available: no iron chlorosis.
When very acidic, it can hold enough aluminium and manganese to hinder some plants.
Phosphorus, calcium and magnesium are less available; these soils are often poor in calcium.

Advantages in the garden

A dream terrain for ericaceous plants: blueberry, rhododendron, azalea, camellia, heather.
Also suits chestnut and potato (less prone to scab in acidic soil).
No iron chlorosis: iron stays available.

Limits & common problems

Many vegetables prefer a pH close to neutral.
Frequent calcium and magnesium deficiencies; phosphorus partly locked up.
Encourages clubroot (a Brassica disease) in acidic, moist soil.
Very acidic, aluminium toxicity can slow growth.

Recommended improvements

For a classic vegetable garden: gently raise the pH by liming (lime, maerl or a little wood ash), based on an analysis.
Add mature compost: it buffers the pH and improves structure whatever the texture.
Do not over-lime: aim for neutral through small repeated inputs rather than one large dose.
Rather than fighting everywhere, devote the truly acidic areas to soft fruit and ericaceous plants.
Avoid what acidifies further (excess ammonium fertiliser).

Suitable plants

Blueberry
It needs an acid soil, ideally pH 4.5 to 5.5.
Chestnut
It prefers a slightly acid soil, even tolerates very acid, poor sands, but dislikes chalk.
Potato
Common scab is worse in alkaline soil: in acid ground, its tubers suffer less from it.
Raspberry
It prefers a slightly acid soil, pH 6 to 6.7.

Plants to avoid or watch

Lavender
It prefers a neutral to chalky soil and succeeds almost anywhere as long as the ground drains well and is not too acid.
Cabbage
Clubroot, a disease of its roots, thrives in acid soil: lime the ground to raise the pH above 6, as the RHS advises.
Sage
It prefers a chalky soil and likes neither heavy nor acid ground.
Sainfoin
It needs a neutral to alkaline, well-drained soil; it even succeeds on thin soils over chalk.

These reasons rely mostly on the RHS and on Plants For A Future, both in the United Kingdom: they describe an oceanic climate. In another climate, each plant's page gives its dates and advice for your climate zone.

Watering tips

Acidity imposes no particular watering rule, texture is what commands. But watering with rainwater rather than tap water (often chalky) helps keep the soil acidic where you want it.

Link with crop rotation

A classic vegetable rotation first requires raising the pH towards neutral (liming, compost); the areas left acidic come out of the rotation. On soil that stays acidic, avoid following on with Brassicas, which are prone to clubroot.

Heavy metals: caution

Higher-risk behaviour: an acidic pH increases the bioavailability of several heavy metals (cadmium, zinc, lead, manganese), more readily taken up by roots. In an urban or old industrial context, raise the pH and have the soil analysed. See the Heavy metals concept and the Choosing and testing your ground guide.

Definition: Heavy metals →Guide: Choosing and testing your ground →

Key takeaways

“Once I know my soil, I know what to plant, what to pair, what to improve and which guild to build.”

Sources

RHS (Royal Horticultural Society) : Lime and liming (rhs.org.uk/soil-composts-mulches/lime-liming)
RHS (Royal Horticultural Society) : Club root (rhs.org.uk/disease/club-root)
RHS (Royal Horticultural Society) : How to grow blueberries (rhs.org.uk/fruit/blueberries/grow-your-own)
RHS (Royal Horticultural Society) : How to grow raspberries (rhs.org.uk/fruit/raspberries/grow-your-own)
RHS (Royal Horticultural Society) : Potato scabs (rhs.org.uk/disease/potato-scabs)
RHS (Royal Horticultural Society) : How to grow cabbages (rhs.org.uk/vegetables/cabbages/grow-your-own)
Plants For A Future, « Castanea sativa » (pfaf.org/user/Plant.aspx?LatinName=Castanea+sativa)
Plants For A Future, « Lavandula angustifolia » (pfaf.org/user/Plant.aspx?LatinName=Lavandula+angustifolia)
Plants For A Future, « Salvia officinalis » (pfaf.org/user/Plant.aspx?LatinName=Salvia+officinalis)
Plants For A Future, « Onobrychis viciifolia » (pfaf.org/user/Plant.aspx?LatinName=Onobrychis+viciifolia)
Kochian, L.V., Hoekenga, O.A. & Piñeros, M.A. (2004). How do crop plants tolerate acid soils? Mechanisms of aluminum tolerance and phosphorous efficiency. Annual Review of Plant Biology, 55, 459-493.
Baize, D. (2000). Guide des analyses en pédologie (2e éd.). Paris : INRA Éditions.
Kabata-Pendias, A. (2011). Trace Elements in Soils and Plants (4e éd.). Boca Raton : CRC Press.

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