How to improve the drainage of your clay soil

Improving the drainage within your garden makes a huge difference to the overall performance of your plants. It also ensures that during heavy rain, the water will run away from your property and won’t gather in as easily in puddles either. One particular type of soil needs our help more than the others in these circumstances and that is clay.

What is clay soil?

Clay soil also known as ‘Heavy Soils’ is earth that contains a greater amount of fine clay particles. Fine clay particles are tiny, microscopic pieces of rock and minerals that occur through physical weathering. The inclusion of the particles means that clay soils will swell when wet and shrink when dry. This action means that the soil cultivates itself.

It has the potential to be very fertile, but it is harder to work with, particularly when it is wet, as it becomes very heavy. You’ll also find or know if you have clay soil, that the earth is cooler due to the moisture retention of clay, this means it takes longer for the ground to warm up in spring. This delay often means your crops or plants won’t appear until later in the season.

How do I know if I have clay soil?

The soil in your garden will be dense and feel a little sticky. In fact, you’ll be able to a lump of clay soil into a ball just be rolling the clump in your hands and then again into a sausage. If the roll does not crack, then you have ‘heavy clay’.

Other telltale signs that you have clay include:

• The ground is difficult to work.
• It will drain slowly after rain.
• The water retention in the soil is very good.
• How can I improve clay soil?

There are several methods in horticulture that have been shown to improve clay soil. These include:

• Use raised beds, so you don’t walk on the surface and trample the soil structure causing compaction.
• Use a no-dig approach.
• Dig in sharp sand – avoid other types.
• Dig in organic matter, like manure or composted bark.
• The application of mulches to retain moisture in warmer weather and prevent soil cracking.

I’m going to focus on one method today and that is whether you should add Lime to the soil or Gypsum. I’m mentioning both Lime and Gypsum as you apply Lime to soils that are acidic, and Gypsum to soils that are alkaline.

This is really complicated – how will I know whether I have acid or alkaline soil?

Speak to your local garden centre and ask for a pH testing kit. Once you have done the test, you’ll know what type of soil you have. The kits are simple pieces of testing paper that you dip into muddy water from your garden, or devices that you pop into the ground, and they give you a reading.

Just like using a thermometer to test whether your roast chicken has been cooked through.

What are the benefits of using Lime or Gypsum?

Firstly, they improve the structure of the soil. The addition of calcium will bind the clay particles into soil crumbs. It’s a plant food. Plants need calcium like humans, they just benefit from a moderate amount. The addition of Lime or Gypsum will unlock nutrients that are trapped within the soil that will help the plant thrive. Lime in the soil can reduce the number of pests in the soil. You’re less likely to see slugs, wireworms, and club root. As very few plants like to grow in acidic soil, Lime can help neutralize its impact, making the soil more habitable for bacteria, and earthworms.

What’s the difference between Lime and Gypsum?

Let’s look at Lime first.

Lime (either calcium hydroxide or calcium carbonate) reduces the acidity of the soil by neutralizing the acid reactions in the soil. There are also different types of Lime that react with the soil at different speeds and have different Neutralising Values or NV. The number that is expressed alongside the Lime is a percentage of the pure calcium oxide.

• Chalk and Ground Limestone are slow acting and have an NV of 50-55.
• Ground Magnesian Limestone also known as Dolomite Limestone has an NV of 56.
• Calcified Seaweed is a long-lasting Lime and has an NV of 44.
• And Hydrated Lime is the strongest and works the fastest with an NV of 70.

If you need to use Hydrated Lime, always use gloves, and wear safety goggles, as this product is an irritant to the skin.

How much Lime do I need to use?

Before you start make sure you know:

• Your soil pH level.
• Your garden square meterage.

This is important as you need to know the amount you need to use and whether you need to dig into the soil or whether you can sprinkle on the surface.

The general rule is less than 0.5kg per square metre you can dig in, but if it’s too difficult then sprinkling on the surface is fine. If it is greater than 0.5kg per square metre, then dig half of the Lime into the soil and the rest onto the surface.

Always read the packaging carefully before you buy or apply.

What about Gypsum?

Gypsum (calcium sulphate) will add calcium to the soil without neutralising the soil acidity. So, this is ideal if you are looking to grow acid-loving plants like camellias, heathers, rhododendrons, or blueberries, in your clay soil.

How much Gypsum do I need to use?

Before you start make sure you know:

• Your soil pH level
• Your garden square meterage

Always read the packaging carefully before you buy or apply.

You need to break the surface of the soil so that the gypsum can be absorbed. Follow the instructions on the packet, so for example, apply 2kg of gypsum per square metre and then mix it in. If the soil is very dry, you’ll need to water the gypsum into the soil for it to be absorbed.

For a range of plants that will thrive check out my blog on the best plants for clay soil.