Why Soil pH is the Invisible Factor Ruling Your Plant List
- Ian Green
- Jun 3
- 5 min read
By Ian Green, Pre-Registered Member of the Society of Garden & Landscape Designers
One of the most common conversations I have with clients goes something like this:
"We've tried three different plants there and none of them seem happy." The light looks right, the drainage seems fine, the soil appears healthy - and yet something is still wrong. The leaves are yellowing, the flowers are sparse, and the plant just looks unhappy.
But beneath the surface, there's often a hidden factor shaping every planting decision in the garden: soil pH. If your soil pH is working against your plants, nothing else matters. Here's everything you need to know — and exactly what to grow.
In my earlier journal post Right Plant, Right Place, I outlined pH as one of the four non-negotiables before any plant goes in the ground. In this post, I want to go deeper - because once you truly understand it, it changes everything.
What Exactly Is Soil pH?
pH is a measure of how acidic or alkaline your soil is, expressed on a scale of 0 to 14. As the RHS explains, a reading of 7 is neutral — anything below 7 is acid, and anything above 7 is alkaline. Most garden plants prefer a slightly acidic to neutral range, roughly 6 to 7, because that's where the widest range of nutrients are freely available in the soil.
What Goes Wrong When pH is Off
The RHS notes that in very acid soils, most phosphates become locked up and unavailable to plants below pH 5.1, and bacteria cannot rot organic matter properly below pH 4.7 - meaning fewer nutrients reach the plant. At the other extreme, on alkaline and chalky soils, iron and manganese become locked up, leading to a condition called chlorosis - that frustrating yellowing of leaves with green veins that makes a rhododendron on chalk look permanently ill.
The better solution is not to fight your soil's chemistry indefinitely. It is to understand it, then plant accordingly.
Understanding your soil pH isn't just for serious gardeners - it's for anyone who's ever
wondered why their rhododendron looks yellow, or why their hydrangeas are stubbornly
pink despite everyone else's being blue.
How to Test Your Soil pH
Testing is far simpler than most people expect. There are two approaches: a simple DIY test kit from any garden centre (typically a few pounds, taking around ten minutes) or sending a sample to a laboratory for a more detailed analysis.
Practical Tip Test multiple spots across your garden - pH can vary considerably between areas. Take samples from around 15cm depth for the most representative reading. And if vinegar dropped on your soil sample fizzes, you have free calcium carbonate present - confirming alkaline chalk conditions. |
Acid soil, pH 4 . 5 – 6.5, Ericaceous & Acid-Loving Plants
Rhododendrons, camellias, heathers, and blueberries are classic examples of plants preferring acidic conditions. On neutral or alkaline soils, they will show chlorosis and decline.
Rhododendron | pH 4 . 5 – 6 .0 | Spectacular spring flowering but will struggle above pH 6.5. Iron lock-up causes characteristic yellow leaves on chalk. |
Camellia | pH 5 . 0 – 6 . 5 | Gorgeous winter-to-spring colour in acid soils. Excellent for woodland gardens. Cannot cope with lime-rich soils. |
Pieris | pH 5 . 0 – 6 . 0 | Evergreen shrub with striking red new growth and lily-of the-valley like flowers. Superb for sheltered acid borders or containers. |
Hydrangea | pH 5 . 0 – 5 . 5 for Blue | Flower colour is directly controlled by pH. In acidic soil, aluminium availability produces blue flowers. On alkaline soils, flowers turn pink or red. |
Alkaline soil, pH 7.0– 8.0, Chalk & Lime Tolerant Plants
Chalky and limestone soils are widely distributed across Britain - across the Chilterns, South and North Downs, Yorkshire Wolds, and much of Lincolnshire. While these soils present challenges (free-draining, often shallow), they support a wide and often spectacular range of plants.
Clematis | PH 7 . 0 – 8 . 0 | A confirmed lime-lover. Listed prominently on the RHS chalky soil plant list. Wild clematis growing in your hedge is a strong indicator of alkaline conditions |
Lilac (Syringa vulgaris) | PH 7 . 0 – 8 . 0 | A quintessential cottage garden shrub that positively thrives on chalk. Stunning, fragrant spring blossom. Named on the RHS chalk soil list. |
Salvia nemorosa & cultivars | PH 6 . 5 – 8 . 0 | Superb perennial for alkaline borders — featured in Ian's Right Plant, Right Place pH table. Long-flowering, drought tolerant, excellent for pollinators. |
Quick-Reference pH & Planting Table
pH Range | Condition | Best Plants |
4.5 - 5.5 | Strongly acid | Rhododendrons, blueberries, heathers (Calluna vulgaris), Pieris |
5.5 - 6.5 | Mildly acid | Camellias, magnolias, blue hydrangeas, most conifers, Pieris, heathers, ferns |
6.5 - 7.0 | Neutral (optimal) | Roses, most vegetables, most perennials - the broadest plant palette available |
7.0 - 8.0 | Alkaline / chalky | Clematis, Lilac (Syringa), Beech (Fagus), Buddleia, Lavender, Salvia, Viburnum, Dianthus, Weigela |
Soil pH is just one of four factors I consider before any plant goes in the ground. In my journal post Right Plant, Right Place, I walk through all four - light levels, soil conditions, pH, and final plant size - and explain how getting all four right is what separates a garden that struggles from one that looks effortless.
Working With Your Soil, Not Against It
Alkaline chalk gardens can be extraordinary -think of classic, romantic English borders
consisting of clematis, lilac, dianthus, and lavender. Acid gardens are equally spectacular - think of rhododendrons, Pieris and camellias, as well blue hydrangeas in late summer.
Neither is a problem. Both are an opportunity - but only if you choose the right plant for the right place and understand your garden environment.
Test your soil. Know your number. Then plant with confidence. |
Frequently asked questions
What is the ideal soil pH for most garden plants?
Most garden plants perform best between pH 6 and 7, where nutrients are most readily available.
How do I know if my soil is alkaline?
Chalky soils are usually alkaline. A simple soil test kit will confirm this.
Does Lincolnshire have alkaline soil?
Is it better to change my soil or choose plants that suit it?
In most cases, choosing plants that naturally suit your soil conditions is the better long-term approach. While it is possible to make small adjustments to soil pH, permanently changing the chemistry of an entire garden can be difficult, expensive, and often short-lived. Gardens tend to perform best when planting works with the existing conditions rather than against them.
Can I grow rhododendrons in alkaline soil?
Generally no. While containers can help, long-term success usually comes from choosing plants suited to your existing soil.Can I grow rhododendrons in alkaline soil?
About the author Ian Green is a garden and landscape designer with over 25 years’ experience, working with clients across the East Midlands and beyond. He believes that the best gardens start with listening, to the space, and to the people who’ll live in it.
To learn more visit iangreengardendesign.co.uk
About the author Ian Green is a garden and landscape designer with over 25 years’ experience, working with clients across the East Midlands and beyond. He believes that the best gardens start with listening to the space, and to the people who’ll live in it.
To learn more visit iangreengardendesign.co.uk
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