Soil pH Guide
Look up the optimal soil pH for specific plants and learn how to adjust soil pH using lime (to raise) or sulfur (to lower). Soil pH affects nutrient availability and plant health.
pH scale for gardening
The pH scale runs from 0 (highly acidic) to 14 (highly alkaline), with 7 being neutral. Most plants thrive in slightly acidic to neutral soil (6.0-7.0), where most nutrients are soluble and available.
Optimal pH by plant type
| Plant | Optimal pH |
|---|---|
| Blueberries, azaleas, rhododendrons | 4.5-5.5 (acidic) |
| Potatoes, sweet potatoes | 5.0-6.0 |
| Tomatoes, peppers, beans | 6.0-6.8 |
| Carrots, lettuce, spinach | 6.0-7.0 |
| Brassicas, asparagus | 6.5-7.5 |
| Lavender, sage (Mediterranean herbs) | 6.5-7.5 |
Adjusting soil pH
- To raise pH (more alkaline): add garden lime (calcium carbonate). Work into soil 2-3 months before planting.
- To lower pH (more acidic): add elemental sulfur, pine needle mulch, or acidic compost. Sulfur acts slowly (weeks to months).
Test soil pH with a home test kit before and after amendments. Changes take time, large pH shifts should be done gradually over a season.
Typical UK soils and what they mean for your garden
UK soils vary sharply by region. Knowing your starting point saves money on amendments:
- Chalk and limestone downlands (SE England, Yorkshire Dales): pH 7.5-8.5, naturally alkaline. Lavender, clematis and most brassicas thrive; blueberries and rhododendrons will struggle without ericaceous beds.
- Acidic moorlands and uplands (Scotland, Wales, Peak District): pH 4.5-5.5. Heathers, bilberries and conifers are natural. Adding lime is needed for most vegetables.
- Heavy clay (Midlands, Thames Valley, Kent): pH 6.0-7.0, often closer to neutral. Generally good for vegetables but drainage is the main issue, not pH.
- Sandy soils (Norfolk Broads, Lincolnshire): pH 5.5-6.5, can be acidic and drain quickly. Regular organic matter addition is more important than pH correction alone.
The RHS recommends a soil test every 3-5 years. Basic kits (£5-10) are available at garden centres. For a detailed report, the RHS and DEFRA-accredited labs offer full analysis for around £30. Ericaceous compost and sequestered iron feed are widely available at UK garden centres for growing acid-lovers in containers.