Building raised beds on clay soil

[catgirl]

Hi guys,

I finally got an allotment a couple of months ago and have been busy working it since then. Since it's a bit late to be sowing most fruits and vegetables I'm mostly concentrating on getting the plot ready for growing everything in the spring. The problem I have is that the plot is pretty much pure clay - the really heavy kind that you can roll into any shape and is impossible to break up unless it's bone dry. The previous allotment holder said half the plot tends to flood in the winter as drainage is so poor. My question is, am I okay to build raised beds straight onto the soil without trying to break up the clay underneath them, or will this just result in waterlogged beds because the rain can't drain into the soil underneath? I'm a bit short on money as well so can't afford to bring in tons and tons of new soil, so the beds will only be about 15cm high.

I have dug over the soil in one of the beds, throwing out the really unworkable stuff and digging new compost into it. But it took me two days to do a 4x1 meter section because it's so hard going. I'm now scared that it's going to take me months to get the whole plot done, what with only having a couple of free days every week and other obligations as well! Does anybody have any other solutions?

[ButternutSquash]

I have heavy clay soil, and the neighbours complain about it, but it is wonderful. I dug mine, when not too wet and not dry and hard. I then spread compost and over winter the frost broke it up nicely. I avoid walking on it, to retain drainage. I dug some more beds this year, after the frosts, and although not ideal, the soil is usable. It will be better next year due to more compost going in this winter, once it is cooked. The beds in their second year are very fertile, and loose, and carrots, courgettes, tomatoes, chillis etc are thriving. I think you should be grateful for clay soil.

google no dig gardening and Charles Dowding. He has shown that digging is not needed, even on heavy clay. In the latter case he spread a thick layer of compost in autumn, and planted in spring. The first year was not great. He spread more compost, and the next year was good. He did not dig. The compost goes down into the soil, and worms gradually drag it down lower, improving the soil.

I intend to adopt the no dig approach. I will only dig to pull out bindweed roots, which are still a problem, but I won't dig otherwise.

[ButternutSquash]

I forgot to add that I do not use raised beds. It is heavy clay, water did puddle on the surface, but now it is fine, it drains nicely. I suspect a big problem with clay soil is compaction, which long term composting can cure, along with not walking on it.

[Lazy Gardener]

Mines not totally clay, But gets waterlogged in the winter/wet weather. I dug down about a spade deep and fille it with manure, grass clippings and leaves and left it open during the winter to rot
down.

Start of the season i put the earth taken from that spot back ontop and made a nice little mound to grow my onions. Best ive had so far.

I intended on repeating it, But it seemed to be waterlooged all the way through the winter.

A lot of waste matter makes a small amount of compost though, I have noticed that. My green dalek bin in the garden often overflows with grass clippings and kitchen scraps (teabags, veg peelings and offcuts only).

Yet a few days after i say its full there is more room again.

[taff]

Pure clay as in grey colour clay?
Or pure clay as in much blacker clay?
If it's the former, you're probably not going to get it anywhere without a shed loan of compost stuff.

If the latter, it's great stuff, very rich, full of nutrients but don't walk on it, and also feed it lots of compost/mulch/grit


It doesn't matter how long you take just keep doing a bit at a time. The only people who can do an allotment in no time are retired or have lots of money.

Everyone else takes years

ANd compost everything you can, weeds/grass etc too. You can ust chuck it on the surface and cover it with mulch if you don't want to dig it in, worms will find it wherever it is.

[teakdesk]

Hmmm... a few comments come to mind!

1/ a clay soil with good additions is an excellent growing medium especially for allotments where watering can be sporadic!

2/ If you have, as I do in parts of my plot, a thin layer of soil on top of solid brown clay then you should avoid digging into the clay as you will just make a "pond" under the bed. I would remove all the soil from the path around your beds and add that to raise the bed. The paths will then be compacted clay and water will then run off the raised bed and into the sunken path and drain away (you may need to lower the path to coax the water away in the direction you want).

3/ I wouldn't leave a bed you have just dug without anything in it! There are plenty of things you can plant at the moment and even if you decide not to grow a crop then I would sow a green manure to cover the bed and hold nutrients. An empty bed will just grow weeds and compact itself again so you will need to re-dig next year.

Some people will cover a dug-over bed with carpet or plastic but covering with a crop is much more preferable (add a mulch around the crop as well).

[catgirl]

Thanks for your comments everyone, you've been really helpful.

It is the grey/brown clay rather than the nice kind, and because the weather has been so nice lately it has all baked into something which is really rather difficult to dig over! I have found in the last few days that it will break up if you try hard enough though, so that's what I've been doing. There are lots of deep-rooted weeds still on the plot so I'm going to have to dig them out before I start thinking about no-dig methods.

I think the main problem is that the person who had the plot before me only had three beds and had covered the rest of the plot with weed control fabric. Great for suppressing weeds, but it does mean the soil underneath it is really compacted. Even the monster rotavator they have on the allotment had trouble getting through it! But hopefully if I dig it over once I can then just add compost to it and not have to worry about drainage. And I will definitely use green manure on the beds that haven't already got Brussels sprouts in them.

[Srh.sloan]

Hi cat girl,
Congrats on getting a plot! Don't panic about the clay- ours is really heavy clay and was covered in grass/brambles when we got it 3 years ago. It bakes hard in the sun and cracks, and there are parts of it like a swamp at times when it rains (which is often in South Wales!). We have gradually dug it all over- no dig doesn't seem like it would work for us as we have 2 large beds rather than small ones so need to walk on parts of it for weeding etc. one year's paths get dug over as next year's beds.

The best thing I can suggest is to get hold of as much manure as you can and add plenty of it to everywhere you dig- this has made a huge difference and already we are seeing a difference in the soil structure and drainage has improved massively.

We took ours on in the late spring 3 years ago and like you mainly concentrated on readying it for the following year- since then we have had fantastic harvests of nearly everything we grow- as mentioned above clay soil is great for holding onto nutrients so things do grow well if it's not too damp. We have learned to grow different things in different areas- the swampy area is no good for root crops but squashes and brassicas love it.

Good luck with your plot! Don't feel like you have to do it all at once either- we started with about a quarter of it and are up to maybe 7/8ths of what we want to cultivate now- I cleared brambles from the remainder only this week. It takes time, but is so worth it!
Sarah

[catgirl]

Thanks Sarah, that's really encouraging!

[bario1]

Hi Catgirl,

If your raised beds are going to be permanent, it might be worth digging your paths a spade's depth down and filling them with woodchip or something like that, that's what I did. Not only does it provide natural drainage channels for your beds, it also gives you loads of free soil to fill them so you can make them deeper - then add your organic matter to the beds over time to improve the soil structure.