- Very chalky soils have a whitish colouring and contain lumps of chalk (white stones) and limestone soils contain pieces of limestone
- Chalky soils contain calcium carbonate and are alkaline (a pH greater than 7.1) in nature
- Soil froths when placed in a jar containing vinegar (due to the release of carbon dioxide)
- Large flints may also be present
Chalky soils are derived from chalk or limestone and, as a result, are alkaline in nature. Suitable plants need to be able to tolerate these alkaline conditions.
What to look for
Do you have a chalky soil?
Chalky soils are derived from chalk or limestone and, as a result, are alkaline in nature (pH of 7.1 and higher). Suitable plants need to be able to tolerate these alkaline conditions.
Chalky soils can be very variable – from solid, pure chalk or limestone, to gravelly to good, deep, alkaline clay soils. They may contain large numbers of flints and large stones – and even good chalky soils can contain lumps of chalk or limestone of varying sizes.
The actual topsoil depth may be shallow, and soils without clay may be very free draining and lack nutrients, resulting in a dry, impoverished soil.
Before planting, it is important to establish exactly how deep the topsoil is. Even trees and shrubs that tolerate alkaline/chalky soils may struggle on very shallow topsoil over chalk. The topsoil may only be a few inches deep before hitting solid chalk.
You can test the pH of the soil with a simple pH or soil testing kit.
What’s good about chalky soils?
Deep chalky soils can be moisture-retentive and don’t dry out in summer. Where there is a good percentage of clay, the soil can have, and retain, good levels of organic matter and plant nutrients. There is a huge range of plants that thrive in such alkaline soils.
Chalky soils rarely flood or become waterlogged. They warm up quickly in spring, so plants start into growth early and seed sowing can start in a timely manner.
What’s bad about chalky soils?
Chalky soils with a shallow topsoil are very free draining and have low fertility levels.
Solid chalky soils are difficult to dig and cultivate and to make planting holes. This is accentuated in soils containing a lot of flints and/or large stones.
In very chalky/alkaline soils, plant growth may be poor and plants suffer from yellowing leaves (lime-induced chlorosis), due to their inability to absorb iron and manganese from the soil.
Very chalky, alkaline soils cannot be made acidic, so are unsuitable for lime-hating/acid-loving ericaceous plants. As a result, it is better to choose plants that will thrive in alkaline conditions – of which there are many.
The following are particularly suitable for alkaline, chalky soils.
Acer campestre, A. negundo, A. platanoides, A. pseudoplatanus
Sorbus aria, S. intermedia
Pinus mugo, P. nigra
Soil testing kit