Lime and its uses


Strictly speaking, true lime is calcium oxide, which is slightly toxic and is rarely used by gardeners.

Other forms of lime are; calcium hydroxide, calcium carbonate and less commonly calcium sulphate.

All of these can added as soil sweeteners, to liberate other chemicals, and to improve the texture of heavy ground.

Some act quite quickly, and others have better moisture-holding qualities.


Lime helps to regulate the acidity / alkalinity level of soil.

The level is measured using the pH scale, which ranges from 1 (highly acidic) to 14 (highly alkaline).

An increase in pH from 6 to 7 results in a tenfold increase in alkalinity.

A pH of 7 is neutral, with conditions neither acid nor alkaline.

The optimum pH for most garden plants and vegetables is 6.5 (slightly acid).

If the pH varies significantly from this value, plants may exhibit signs of nutrient deficiency.

High pH levels cause a reduction in phosphate and trace element availability, while low pH levels reduce the availability of nitrogen, potassium and magnesium, and discourages beneficial earthworm activity.

It is important to carry out pH tests to determine whether liming is necessary.

If your soil already has a pH of 6.5 or higher, liming will be of little benefit and may in fact be detrimental.


Autumn is the best season for liming soils, as the lime can take effect over the winter months and will not damage young growth.

Lime should not be applied at the same time as organic matter or fertilisers, it may cause the release of ammonia, which wastes nitrogen and may damage tender growth.

In the event that you incorporate organic prior to the onset of winter leave the task of liming until the spring by which time the organic matter wil have rotted down to virtually eliminate this problem.

Over-liming is difficult to correct, therefore, add less than the recommended amounts at first, and readjust the following season after further tests.

Change is not permanent, as lime is used by plants and leached out in solution, especially on sandy ground, so you may need to repeat the treatment every three to four years.

This attribute can work in favour of those people who operate a three to four year crop rotaion programme, as it can mean that lime applications can fit in with the needs of the crops in the planting out plan.

For example: Liming kitchen garden soil does not benefit all crops - most thrive at around pH 6.2 - but brassicas prefer neutral (pH 7) to slightly alkaline conditions (> pH 7) which also discourages diseases such as clubroot.

(If clubroot is known to be present, raise the pH to 7.5 as an effective and inexpensive safeguard).

Application rates:

These vary with soil type and the degree of pH change required. (see below)

If a large change in pH is required, it is better to add the lime in small amounts over a period of time, rather than all at once.


Chalk: (calcium carbonate)

This type is the most widely available and it is usually advertised as garden lime, and / or carbonate of lime, ground limestone or ground chalk.

It is slow in acting, although this depends very much upon how finely it has been ground down.

It is especially suitable for light soil, as it holds moisture.

Spread at a rate of up to 900 gm (2lb) per sq metre depending upon pH.

As a rule of thumb, a loamy soil will require an application of 200g (6oz) per sq m to raise the pH by half a unit.

A clay soil will require 420g (12oz) per sq m and a sandy soil 140g (4oz) per sq m.

Ground Limestone:

This is another form of calcium carbonate, and is even slower acting, but again much depends on fineness.

Spread at a rate not greater than 900gm (2lb) per sq metre depending upon pH.

To alter pH by 1 unit apply 300gm (10oz) per sq metre.

Gypsum: (calcium sulphate)

Commonly used for improving soil structure and particularly for heavy clay soils.

Spread at a rate not greater than 225 gm (8oz) per sq metre on heavy soils, depending upon pH.

Hydrated Lime: (Calcium hydroxide or air-slaked lime)

Is soluble in water and has a stronger liming action than calcium carbonate, but is less pleasant to handle and easier to over apply.

In powder form, it is the quickest acting of the limes, and can be used with safety around plants.

Spread at a rate not greater than 450gm (1lb) per sq metre depending upon pH.

Quicklime: (calcium oxide)

This type kills insects etc. in the soil, and is quick acting, but caustic, and is only suitable for use on vacant land.

Spread at a rate not greater than 450gm (1lb) per sq metre depending upon pH.

Choose a windless day to spread any of the above, and always wear protective goggles to avoid damaging your eyes.

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