Lime is usually added in winter for annual crops, such as vegetables, just prior to digging, as the lime can take effect over the winter months and will not damage young growth. 

Before planting

Perennials are plants that live for multiple years. They come in all shapes and sizes and fill our gardens with colourful flowers and ornamental foliage. Many are hardy and can survive outdoors all year round, while less hardy types need protection over winter. The term herbaceous perennial is used to describe long-lived plants without a permanent woody structure (they die back to ground level each autumn), distinguishing them from trees, shrubs and sub-shrubs.

Lime applied to the surface, around

Established plants have been in their current location for two or three years and so have well-developed root systems able to support strong growth with healthy foliage and flowers.

This describes how acid or alkaline soil is, and is expressed as a number between 0 and 14. Soil with a pH of 7 is neutral, an acid soil has a lower value, and an alkaline soil has a higher one. Most plants grow best in soil that’s within a specific pH range, so it’s useful to know your soil’s pH. Testing kits are easy to use and widely available in garden centres and online.

pH

A soil pH test measures the acidity or alkalinity of the soil. A pH 7.0 is considered neutral. Above pH 7.0, the soil is alkaline and below pH 7.0, the soil is acid. See our page on soil pH testing for more detail.

It is especially worth checking soil pH and making any adjustments before designing or planting a new garden, making vegetable plots, planting fruit or when growth is disappointing.

Testing can be done at any time, but if carried out within three months after adding lime, fertiliser or organic matter, the test may give misleading results.

If your soil pH test comes back at 7.0 or higher, you have alkaline soil, and liming is not necessary. If the pH is above 6.5, or if you wish to grow acid-loving ericaceous plants, you may wish to reduce the pH by adding an acidifying material.

Liming materials

Lime raises pH and is usually added as ground limestone, commonly called ‘garden lime’. The active ingredient is calcium carbonate. Ground limestone is easy to spread, widely offered in garden centres, and is the recommended liming material for gardeners. Calcified seaweed and ground chalk are other forms of calcium carbonate offered to gardeners.

Ground magnesian limestone, often called ‘Dolomite lime’ is a ground limestone rich is magnesium carbonate as well as calcium carbonate and is used to lime soils that lack magnesium (see our page on plant nutrition for more information on plant nutrients).

Hydrated lime (calcium hydroxide), sold for use by builders, can also be used. It is a fine powder, quick acting, but can irritate skin and eyes if not handled carefully.

The effectiveness (called the ‘Neutralising Value’ (NV)) is expressed as a percentage of the neutralising value of pure calcium oxide (which is caustic so cannot be used). The NV varies between materials – typical examples include:

• Ground Limestone (garden lime)  – NV 50-55
• Ground Chalk – NV 50-55
• Ground Magnesian Limestone (Dolodust) – NV 56
• Calcified Seaweed – NV 44
• Hydrated Lime – NV 70

Therefore, if you use hydrated lime, you will need much less than products containing calcium carbonate.

Liming materials are very finely ground so that they work quickly. Avoid any products with lumps, as they will take years to have any effect.

Method of application

When applying more than 0.5kg per sq m (14¾oz per sq yd), it is best to dig half into the soil and sprinkle the rest on the surface after digging.

When applying less than 0.5kg per sq m (14¾oz per sq yd), dig the entire amount in, but you can sprinkle it on the surface if digging is not practical.

Quantities to apply

The clay in soils resists changes in pH (called buffering capacity) so that much more lime is needed to change the pH in soils with high clay content than in soils with little clay, such as coarse sandy soil. See our page on soil types for more information on assessing your garden soil type.

The recommendations below are for the finely ground garden lime (calcium carbonate) sold in garden centres in kilograms (kg) per square metre or ounces per square yard. They are based on the Department for Environment, Food and Rural Affairs (DEFRA) recommendations for incorporation into the top 20cm (8in) of soil and are enough to raise the soil pH to pH6.5. This is considered the best all-round pH for the majority of garden plants.

Take care if using builders’ lime (hydrated lime), as it can be irritating to the skin and eyes. Wear gloves and consider safety goggles if adversely affected. 

Some plant diseases, such as clubroot of brassicas, are associated with insufficient lime levels in the soil.

Some plant nutrient deficiencies can be associated with acid or alkaline soil conditions.