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How to improve clay soil

Val Bourne / 24 January 2013 ( 30 August 2017 )

Read gardening expert Val Bourne on the best ways of improving clay soil in your garden.

Digging clay soil in garden
The first thing to do is to find out whether your soil is actually real clay or just moisture-laden

Do you have clay soil?

The first thing to do is to find out whether your soil is actually real clay or just moisture-laden. Clay soil is smooth yet sticky and, if you take a small handful, you can actually roll it into a tight ball rather like plasticine.

If the soil sticks together and you can press out water this indicates a lot of clay. If it stays shiny after squeezing, that’s a further indicator of heavy clay content.

Don’t just assume you are on clay, however, because gardens on the same street, or even within just yards of each other, do vary. That applies to your garden, too. Clay may not necessarily cover your whole plot.

Find out the best plants for clay soil

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Planting in clay soil

When you’ve ascertained whether you’re on clay, don’t panic. Some of the finest gardens, including the late Christopher Lloyd’s Great Dixter in East Sussex and RHS Rosemoor in Devon, thrive on heavy clay soil because clay is a fertile growing medium.

Clay soil holds nutrients really well and woody plants (ie trees, roses and shrubs) do really well – as do certain perennials. Avoid drought-tolerant plants with either aromatic, silver or finely divided foliage, however. If you must grow them dig in lots of coarse grit and create raised beds that are higher than the paths so that the water can drain away.

Find out the top trees and shrubs for clay soil

How to improve clay soil

Dig over your soil in autumn

Generally the soil is not as damp in early September, so it’s easier to turn over with a fork. Don’t break up the lumps though. Allow the frosts and wintry weather to do this instead: it will give you a finer tilth.

Don’t walk on your soil

As clay become compacted if under pressure, it’s important not to walk on it. Get into the habit of standing on a plank when digging and only work on your soil in dryish conditions.

Improve the drainage

Be prepared to work at improving your soil by digging in coarse grit which will last for several years. You can also make it more airy by adding garden compost, although after two or three years this rots away to nothing.

Read our guide to improving your soil

Plant in spring once the weather has warmed up

Clay remains wet in winter and the soil stays cold, so autumn planting can lead to high losses. Try to plant in the spring, once the buds begin to break, as this means the ground is warming up. Once planting is complete, water well in droughts. Never ever plant in really, really wet and cold conditions.

Dig larger holes when planting

When planting, always dig a hole about twice the size of the rootball so that the roots find it easier to grow away. Break up the bottom of the hole with a fork to improve drainage. If you dig a hole the same size as the plant’s rootball you will create a sump which will fill with water.

Plant on a mound

Raising up an area of soil when planting trees and shrubs, before you plant, improves drainage.

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When clay dries out it shrinks and cracks appear so it’s a good idea to prevent this by adding a mulch in late spring on  moist soil that has warmed up.

A good quality fine bark will do the job, but be aware that as the bark decomposes it will rob nitrogen from the soil. Sprinkle on a nitrogen-rich fertiliser (like pelleted chicken manure or powdered 6X) before mulching to compensate. In herbaceous borders the foliage of the plants usually acts as a barrier if you make sure your plants mingle together. Avoid having bare soil between each one. 

Read our guide to mulching

Add plenty of paths and stepping stones

Link up the key areas of your garden by laying either stepping stones or hard paths so that you’re not walking on the grass or soil. This will prevent lawns etc from becoming distressed and muddy.

Aerate and scarify your lawns

Worm casts on lawns can be a severe problem on clay soil and so is moss. Wire rake the thatch away from the lawn in autumn and then use a scarifier (or the tines of a fork) to produce drainage holes. This helps greatly.

Consider raised beds

If you're planning a vegetable patch in your clay soil you could consider a raised bed. It will allow the soil to drain more easily and warm up more quickly in spring.

Read our guide to using raised beds

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The opinions expressed are those of the author and are not held by Saga unless specifically stated. The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.