Extreme Clay Soil

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You've heard about clay soil and you maybe know what it can and can't be used to grow. The stuff that you've got in your garden however isn't quite clay soil. The word soil in that phrase doesn't seem to belong at all, you've just got clay - not soil

Here are a couple of questions received via the (now removed) any questions facility on this site and the answers. I am very grateful to George Shiels of McMillan-Shiels Associates for his input into this section. Contact details

Q. I have inherited a clay hillock for a back garden, with about an inch at best of clayish top soil, very clayish sub soil, and total clay beneath that. Is there an easy answer to growing a lawn with out resorting to the obvious. (importing tons of worms/topsoil)

A. I keep hearing about the clay you have in West Sussex, I've had several questions about it!

The good news is that grass will grow on the most unpromising of soils, though could always do with a little help. If you're going to lay turf, then a ton or two of topsoil will help tremendously, expect to pay about £25-£30 a ton, not a lot more to the cost if some-one is going to do the job for you.

You could just lay the turf directly on what you have (it comes rooted in about an inch of soil anyway) and it will more likely than not be ok (how is the grass in neighbouring gardens doing?). If I were starting from scratch though as it sounds that you are, then your "obvious" answer is the one to go for. If you're going to do it yourself, a ton or so of topsoil is not so bad to shift in reality as it is in prospect.

As for an easy answer to turning tonnes of clay into nice fertile soil, I wish there were I'd be making a fortune from it!

Q. I have a Garden 10 meters by 3 meters. The topsoil seems very thin, then there is clay for at least another 60 cm or so. Underneath this is a chalk/flint layer (so my next door neighbour tells me). When it rains, and for some time after the ground is waterlogged and I can't even step into the area without loosing my shoes! It is fairly depressing. Unfortunately the only access to the garden is through the house and I have no outside drains in my garden (It is a terraced house). How can solve this problem??

One of my neighbours dug out all of the clay and replaced it with topsoil and put in drainage to the main drains (but they are at the end of the terrace with easy access and a back gate. The other dug a big hole (2'*3'*2') and filled it with pea shingle. I can't help thinking that the reason mine is so bad is because the sump hole is next to my fence! What can I do?

(I live in Burgess Hill, this is on the south downs and used to be well known for its pottery works! Hence the heavy clay soil)

A. Not an easy problem to solve. If you want to deal with it properly then you aught to call in a drainage expert who could assess the site at first hand and suggest a solution, probably involving pipes set beneath the soil leading to a soak away at the end of your garden. If you want a cheaper answer however it's going to entail a lot of work. It's difficult to advise not seeing the garden directly. My first approach would be in penetrating the clay layer to the chalk and flints underneath, a soak away like your neighbours may help or a series of mini-soak aways. Dig a couple of holes about 12" in diameter (about 6 ft apart) down to the chalk layer and fill the hole with shingle apart from the top few inches where you can replace topsoil. See if this has an effect and then extend if it appears to work.

You'll end up with a lot of clay to dispose of down the tip, but there's no real alternative other than call in the professionals, do it a bit at a time and you'll get there.

It will also help to dig organic matter, and pea shingle into the soil in as large quantities as possible, ultimately however it is the removal of the clay that will help unless you want your garden to be elevated by a foot or so thanks to the added material.

A. from George Shiels

With soils and drainage it is always better to see the site and look at the profile to see what is really there and what is causing the problem It is not always a case of the obvious cause being the actual cause. Topsoil can be a clay topsoil and chalk might be a hard or a soft type. the soft types do not always drain well and may not be good for a soakaway. The problem may be more due to compaction of the upper layer than to soil type problems, in which case compaction relief would be the best advice.

Generally the advice is reasonable but I would not advise adding sharp sand or pea gravel to a clay soil because the clay, if very wet, will simply absorb the sand and you then have gritty clay which in the long term will be no better drained. The idea of linking through to the chalk is the most obvious one for a gardener.

On a larger scale we use a post hole borer but a pit excavated down to the chalk may be all that is required. Several pits across the area of the garden would be effort well invested if the chalk is suitable. In Burgess Hill it should be suitable. The pits can be spade width in such a small site. As you say, backfill any pit with pea shingle and blind with a grit, cover with a good 150-200mm depth of sandy loam topsoil or the existing topsoil mixed with a lot of medium/coarse sand. The mix must remain permeable and will be around 9:1 sand:soil if it is to work long term. A larger pit with drains running into it would be another option. The drains should be 45cm deep minimum and backfilled with pea gravel and then covered with a sandy loam topsoil to 200-300mm depth to ensure adequate growth as well as drainage.

Plants for extreme clay

McMillan-Shiels Associates. 21 York Road, Woking, GU22 7XH tel: 01483 726997 fax: 01483 771442 mob: 07774 986331 Email George R Shiels

Turf, landscape and soils consultancy by agronomist of 27 years experience including turf preparation for major golf tournaments in Europe and many construction projects. Drainage and technical specifications. Trouble shooting and litigation specialist.

Please note George may be contacted in his professional capacity only.
He is not available to answer emailed questions

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