Soil pH and Lime

The pH or Acidity of the soil is one of the most important factors of your garden or vegetable plot.

The pH of the soil is often neglected by gardeners, who then wonder why plants will not grow. It has a very major effect on what plants will grow in your soil. The Acidity or Alkalinity of your soil is determined by its Lime content.

The pH is measured in units from 1 to 14. A neutral soil has a pH reading of 7. Any reading below 7 is regarded as being Acid, while above 7 is Alkaline. Testing the pH of your soil is very easy to do and can be done using one of the cheap proprietary test kits available from garden centres.

Once you know the actual acidity of your soil you can do two things.

Only grow plants or vegetables that are suitable to that particular acidity.


Change the soil acidity to suit the plants that YOU WANT to grow.

In ornamental gardens it is probably best to grow plants suitable to your soil condition rather than trying to change it, as there are a very wide range of plants to suit all soil conditions.

In the vegetable garden though it is a different matter. Most plants thrive and do best in a fairly neutral pH (around 6.5) so if your soil varies from this you will need to change it.

Make the Soil more Alkaline

It is easy to make an acid soil less acid by adding Lime but will not be done overnight. Adding too much Lime at one time can scorch plant's roots. Small amounts at a time should be added and the pH regularly checked until it is the pH that you require. Bear in mind that like nutrients, Lime can be washed through the soil by rain and the addition of manure can also change the acidity back again.

Applying Lime has other advantages; for example, adding Lime to a heavy clay soil will help to bind particles together.

Adding too much lime can be detrimental and lock up vital nutrients, particularly the trace elements that plants need.

Types of Lime

Lime is available in different forms. The more expensive types like ground limestone or calcified seaweed usually last a little longer in the soil.

Slaked Lime

This is the most common form to be used.

Hydrated Lime (builders Lime)

Usually sold for the addition to cement, does not last as long as slaked lime and has to be used annually.

Ground Limestone

Will last in the soil for several years and also contains Magnesium.

Calcified Seaweed

This is in fact a ground coral and is very useful as it contains Magnesium and other trace elements. It lasts a lot longer than other forms of lime and will stay in the ground for 2-3 years.

Application rates for Hydrated lime

Sandy Soil Loam Clay or Peat Soil
Very Acid 1lb per Sq. Yd. 1½lb per Sq. Yd. 2lb per Sq. Yd.
Acid ½lb per Sq. Yd. 1lb per Sq. Yd. 1½lb per Sq. Yd.
Neutral ¼lb per Sq. Yd. ½lb per Sq. Yd. 1lb per Sq. Yd.
Alkaline Do Not Lime Do Not Lime Do Not Lime
These figures should only be used as a rough guide. A soil pH test kit should be used to check the actual pH.

The Best Time

The best time to lime is just after digging in the autumn. If Manure has been applied to the ground, postpone applying the lime until the February. Never manure and lime at the same time always leave the liming for at least two months after.

Manures, composts, fertilisers and seeds can safely be added to the soil one month after liming.

Make the Soil more Acid

Very few gardens are likely to be so alkaline as not to be able to grow vegetables and usually the addition of compost or manure is sufficient to change the pH enough to enable this.

However in very chalky soils, there may be problems associated with the excess lime locking up the nutrients.

In this case it would be advisable to grow on a raised bed system, whereby the bed is raised enough to prevent any alkaline water from draining into it, then by regularly applying manure or compost as a mulch, the acidity will be raised.

© copyright 1999, P. A. Owen

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