How much topsoil to add on top of clay?

paulacton123489326 · 15 February

I have recently moved into a new house with a very large garden which was completely covered in concrete flags which have further compacted already thick clay. I am planning to build borders using sleepers to raise the height so I can add compost etc on top of the clay. The cost is an issue though, in that I can only raise it so high without it costing the earth. What is the minimum amount of soil you would recommend adding on top of the clay to grow a good selection of perennials without them dying in wet winters which we tend to get here? Thanks in advance!

Fairygirl · 15 February

The short answer is - I would never add soil to sticky clay

Clay is a wonderful growing medium, but it needs some work. The way to sort it is to add loads of organic matter which helps the soil structure, improving drainage in wet parts of the country, and in hot, dry areas, it prevents the soil cracking by improving moisture retention.
The quickest way to get planting is to create those raised beds, and then add a good mix of soil and your compost, but manure is also ideal, for the reasons given. Your location and climate has a bearing, as well as the type of plants you want to grow

punkdoc · 15 February

What she says.🙂

Dovefromabove · 15 February

Dovefromabove · 15 February

Just wondering … is the question about how deep the raised beds need to be?

Dovefromabove · 15 February

Sorry … don’t know how I posted the same thing three times … whoops! 🙄

Loxley · 15 February

20-30cm should do it.

It's not a bad idea at all to be honest, the roots will get established in the imported soil but deeper roots will also be able to access the clay which helps during drought.

You should, in fact, use sharp sand rather than soil, if you are planning to grow plants which prefer dry conditions. This is basically the no-dig sand-bed method which is becoming quite popular with landscape designers.

For woodland planting, the layer of sand is substituted with green waste/compost.

You also don't necessarily need to use sleepers to form a raised edge, you can slope the bed down at the edge to meet existing grade.

Tom Stuart-Smith describes the technique here, in case you though I was going mad. (Scroll to the 57 minute mark).

https://youtu.be/A8o_NogMYO8?t=3418
It does seem counterintuitive, but I think it's a tried and tested technique by now, and increasingly popular. Sand promotes excellent root systems, and the lack of nutrients promote tough resilient plants.

That said, clay is NOT an inherently bad soil, and it would be absolutely valid to ditch the raised beds and work plenty of organic matter into the existing soil, breaking up any compaction. And choose perennials which prefer clay rather than sharply drained soil.

barry island · 15 February

My allotment plot has about six inches of soil before subsoil is reached and we can grow just about anything so I would say six inches would be ok.

Fairygirl · 15 February

It's fine adding a layer of soil if the underlying soil is decent. Not the case for what the OP has

I think he's adding compost in the raised beds @Dovefromabove. That would still need a bit of extra organic matter if the sleepers are on top of sticky clay IMO. It does depend on what's going in them though.

paulacton123489326 · 15 February

Thanks for the answers everyone. I will be initially working in plenty of manure to help improve the clay so from the answers it sounds like so long as I do a good enough job of that I shouldn't need too much extra soil on top of that to have something that most plants can work with rather than just ones that are suitable for clay. It doesn't help my favourite plants aren't hugely fond of wet feet over winter. (agastache, salvia, echinacea, lavender) I think I will work with the clay and improve that the best I can and add approx 20cm of compost on top, hopefully that should do the trick.

punkdoc · 15 February

You might have to change your plant palette.

How much topsoil to add on top of clay?

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