• Garden Maintenance

How to Improve Heavy Clay Soil

  • By Julie Kilpatrick
Clay soil is both a blessing and a curse. If you have it, you may not be able to think of it as a blessing at all but you may feel better to know that clay soil is capable of holding onto more of the nutrients your plants need than any other type of soil.

The trouble with clay soil is that it doesn't just hold onto nutrients. It holds onto water too meaning, if you have clay soil, you are likely to have poor drainage. This ability to hold onto water makes clay soil swell up during periods of heavy rainfall and then, in summer, it shrinks, leaving your soil cracked and parched. On top of this, clay soil likes to hold onto weeds. It seems unusually attached to them. So much so, it won't let you dig out a weed without taking half your soil with it.Air spaces in clay soil are limited, meaning plant roots have less access to oxygen.

Clay soil is slow to warm up in spring, difficult to work when wet and even harder to dig when its completely dry. However, clay soil's ability to hold onto nutrients is a real benefit, if only it was easier to work with.

If you have clay soil, the good news is that it is possible to make it more gardener-friendly and the method is, mercifully simple.

How bad is your clay soil problem?

You don't need a sophisticated soil-testing kit to find out what type of soil you have and how much of it is clay. All you need is a jam jar, your soil and some water.

It is unlikely your soil will be pure clay. There will be particles of sand and silt and hopefully organic matter in there too. To find out how much clay is present in your soil, simply put a sample of your soil in a jam jar, add some water and shake it vigourously.

Leave the soil in the jar to settle. The bigger sandy particles will settle first, then the smaller silty particles and, last of all, the tiny clay particles. Eventually, you should get three distinct bands, with the clay soil band at the top. If you have a high percentage of clay, this band will be wider than the rest. Any organic matter in the soil will float near the top.

jam jar test for soil type

The solution to clay soil issues

I did tell you the method of improving clay soil was mercifully simple and it is. All you have to do is to add organic matter.

Organic matter is the decomposing remains of once living organisms - plant or animal. When organic matter is fully decomposed, we call this humus. You can get organic matter from various sources: well-rotted manure, home compost, leafmould, used compost from tubs and hanging baskets and composted bark. You can buy in compost or peat from your garden centre but this is costly to both you and the environment and should only be used as a last resort.

Clay soil particles are very small and, because of this, there is little room for air spaces in between these tiny particles. This means there is also little room for water to drain. Organic matter binds the tiny soil particles together, causing them to crumb and these crumbs allow for more air space. Think of it like rubbing butter into flour. At the end of the process, you get bigger crumbs.

butter and flour

Adding organic matter to clay soil will not only improve your drainage, but also make the soil more workable over time. As an added benefit, it improves fertility, providing essential carbon and other nutrients. Another huge benefit is that earthworms and other soil organisms have a taste for organic matter and the tunelling effects of these tiny creatures opens out the soil and increases the volume of air spaces.

By far the best source of organic matter is your own compost bin. Gardeners of old wouldn't have considered themselves to be true gardeners unless they had a compost heap, happily rotting and steaming away at the bottom of the garden. These days, we're a little more high-tech. Composting areas don't have to look like a mini version of the local landfill site. You can buy nice plastic composting bins that will sit neatly in a hidden corner of the garden or beside your normal bins and you may be surprised to learn too that it's not just garden waste you can compost. Spent kitchen roll holders, toilet roll holders, newspapers, cardboard, vegetable peelings and many other items of household waste can all be put into use to create compost.

So, instead of throwing away your precious organic waste, be a bit more selfish - keep it to yourself and make some compost.

Why not simply add sand or grit to your clay soil?

You could add sharp sand or grit to improve the drainage but this is only really a short term solution. You would need to use plenty of it and be careful you don't make the problem worse by using soft sand which will bind to the clay giving you something resembling concrete. To change your soil type from predominately clay to one which is more sandy, you would need an awful lot of sand. Think about the jam jar test and how the sand in soils sinks to the bottom of the jar. The same would occur over time if you added sand to clay soil.

Adding grit or sand to your soil won't give you improved fertility, it won't feed your beneficial soil-dwelling creatures and it won't give you the lovely, satisfying dark brown humus-rich soil you've been dreaming of. Only organic matter can do all of these things and improve your clay soil in the process.

Unless you happen to have a quarry at the foot of your garden, you will also have to shell out good money for enough sand to make a difference and why would you want to do that when you can make your own compost for free?

person holding dark, crumbly soil

Applying the solution

The best method for adding organic matter is to dig it well in to your soil. As you dig you not only incorporate the organic matter but you add air to the soil too. Of course, digging the soil over like this is only practical if your border is not full to the brim with plants and is a good method for improving the soil in vegetable plots.

Most of us however, have established borders and, if you're feeling really energetic, you can, of course, temporarily move your plants out of the way while you give the border a good going over with the spade. If you can't do this, you can still improve your clay soil by simply mulching it. Mulching is the term used for adding material to the surface of the soil.

When you mulch with organic matter such as home-made compost, you encourage earthworms to visit this layer and, over time, they pull the organic matter down into your soil, acting like your own private army of very mini diggers. It may take some time and repeated applications of compost before you see a benefit by using this method but, in the meantime, your mulch of compost will help suppress weeds and anything that does that has to be a bonus.

You might find that, if your soil is really heavy, you don't have many earthworms. Given time, they will appear but you can speed up this process by borrowing some from a friend who has an excess. Earthworms breed rapidly if the conditions are right so you only need a handful or two to get things going.

Improving your clay soil takes time and patience, whatever method you choose for applying the organic matter. In case you run out of patience, here's a thought to keep you going. There will come a time when it will take you no more than half an hour to weed your garden with that hoe you've never been able to use instead of hours and hours on your knees with a trowel.

Julie is a lecturer in horticulture, editor of Gardenzine and author of The Plant Listener.