What is garden lime?

Ground limestone, commonly known as garden lime, is sometimes added to garden soils that are acidic, or low in lime. The active ingredient of garden lime is calcium carbonate. Do not confuse garden lime (calcium carbonate) with the industrial product quicklime (calcium oxide). Quicklime is not suitable for adding to garden soil because it's stronger and quicker-acting than garden lime, and can cause eye, skin and respiratory injury if not handled properly.

Why apply garden lime?

Garden lime improves growth and productivity of lime-loving vegetable crops if the soil is naturally acidic. Garden lime can also be applied to improve the structure of clay and compacted soils, encouraging the tiny particles to combine in larger crumbs.

Assessing your soil before applying lime

Finding out the natural level of soil acidity or alkalinity in your garden is essential before adding lime. This level is known as pH, and you can find out the pH of your soil using cheap and simple test kits, which are widely available in garden centres and online. A pH level higher than 7 shows that the soil is alkaline or lime-rich and does not need extra lime. Neutral soil has a level of 7. Below that level indicates an acid soil.

Which crop plants need limy soil

Brassicas such as cabbage, Brussels sprouts, swede, kale and broccoli grow best on slightly alkaline soil and can be more susceptible to the disease clubroot in neutral to acidic soils.

For ornamental plants, it's far better to select plants that thrive in your natural soil conditions, rather than trying to alter it.

When to apply garden lime

While garden lime can be applied at any time of year, it's best to apply it in the autumn, well in advance of spring sowing and planting – otherwise it will not have had time to take effect on the soil and will not be of greatest benefit to the growing plants.

How much garden lime to use

The amount of lime to apply depends not only on the natural pH of the soil before treatment, but also the type of soil. Clay soils need the most lime, sandy soils the least, and loam soils are in the middle. For example, to raise the soil pH from 5.5 to 6.0 takes 700g of garden lime on sand, 800g on loam and 1kg on clay.

Bear in mind that you can add too much lime to your soil. Too much lime will raise the pH of your soil so much that many plants won't grow well and will start to show signs of nutrient deficiencies. Too much lime will also lead to high levels of calcium in your soil, which will prevent plants from absorbing magnesium (an essential part of chlorophyll). This is why it's essential that you test the soil before adding lime, or simply grow plants that are better suited to your soil.

How to use garden lime

As a very rough guide, one handful of lime weighs around 35g, although obviously this will vary from person to person. Using a container to weigh out the required amount of lime per square metre is strongly recommended. Apply lime on a dry, windless day when the soil is damp or fairly dry, but not actually wet. Take care not to inhale dust and wear protective clothing including gloves and a dust mask.

Ideally, incorporate lime thoroughly into the top 20cm of soil by digging over the ground after application.

Applying lime and manure

Do not apply lime and manure at the same time because manure is rich in nitrogen, a major nutrient required by plants, and this reacts with the lime to release the nitrogen in the form of ammonia. This means nitrogen won’t be available to the plants and ammonia may ‘scorch’ and damage roots.