How to Improve Heavy Clay Soil

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 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 that you are likely to have poor drainage. This ability to hold onto water makes it swell up during periods of heavy rainfall and then, in summer, it shrinks, leaving your soil looking cracked and parched. On top of this, clay soil likes to hold onto weeds. It seems unusually attached to them. So much so that it won't let you dig out a weed without taking half your soil with it.

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 the problem?

You don't need a sophisticated soil-testing kit to discover the extent of your problem. All you need is a jam jar, your soil and some water.

How easy or difficult your soil is to manage is all down to the size of the particles. Sandy soils have large particles, silt has medium sized particles and clay soil has small particles. The small particles in clay soil bind together more easily, squeezing out the air and making it more difficult for water to drain away. This lack of air in the soil is also damaging to plants, worms and tiny organisms in the soil because it literally suffocates them.

It is unlikely that your soil will be pure clay. There will be particles of sand and silt and hopefully organic matter in there too. Ideal soil (loam), has roughly equal particles of each. 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 sandy particles will settle first, then the smaller silty particles and, last of all, the clay. 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.

The solution

We 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.

Adding organic matter pushes apart the small particles and opens out the spaces in between. This will not only improve your drainage but also make the soil more workable over time. As an added benefit, it also 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 also helps to open out the soil and increase the amount of air.

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 good, big, messy 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 giving away your precious organic waste to the council, be a bit more selfish and keep it to yourself. In fact, most local councils these days positively encourage this practice and many will even sell you a composting bin at a reduced price. Check out the wrap website for information on compost and composting bins.

Why not simply add sand or grit?

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 you would have to be careful you don't make the problem worse by using soft sand which will tend to bind to the clay giving you something resembling concrete.

Adding grit 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.

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?

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 soil by simply mulching it. Mulching is the term we gardeners use 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 will 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 will 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 will take 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.


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