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I have built a raised bed (about 60cm high) and want to fill it so I can start growing stuff. Unfortunately the cost of vege mix/garden mix is quite high - so I'm wanting to add something as a filler.

I have a considerable amount of wood chips which I managed to get for free when the neighbour cut down trees (wood chips has been sitting for a little over a month). I'm seriously considering filling the bottom half to 2 thirds of the bed with this mulch - but I'm very concerned about their absorption of nitrogen - and then covering it with vege mix for the top 1/3rd to half, and growing stuff.

The only thing I've been able to find online - which says its OK - is https://backyardgardengeek.com/bark-or-mulch-in-bottom-of-raised-beds/ - but I'm not convinced

Is it a mistake to do this, and are there common veges that are better/I should steer away from for the first year if I do proceed? (FYI, its spring here. I'm in the equivalent of US hardiness zone 9b I believe)

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    When I filled my island beds (only raised by about 10cm/4", but I had block paving and hardcore to replace so probably 30 cm in total), a lot of the bulk came from composted bark, which was cheap in sacks. I also buried a lot of oak leaves - and still do if they come down faster than I can otherwise dispose of them, as well as rotting them down in sacks.
    – Chris H
    Oct 2, 2023 at 13:08

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I think it would work OK, it would certainly drain well. You would be more or less making a Hügelkultur. This study found adding a layer of wood chips under the soil did not reduce Nitrogen availability, but they also had twice as much topsoil as you're intending, and used equal parts leaf mould and woodchips. I would try to get my hands on a load of fresh manure or something to mix in if I could. It would be a big improvement.

Diagram of 16 inches of top soil over 8 inches of leaf mould, 8 inches of wood chips, a thin layer of brush and corn stover, and 8 inches of loosended soil4

You will also want to think about what plants you will be planting there, and how deep their roots can reach. I think you want at least 30 cm of soil for the roots to go down into, and maybe much more. I'm not sure how well the wood chips would serve as a growth medium by themselves, but I've worked very large amounts of wood chips into the under-layer of garden beds I have double dug with great success. I hope your more severe version is successful too :-).

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    You could also mix in grass clippings - they're often available free and are quite rich in N
    – Chris H
    Oct 2, 2023 at 9:37
  • Note that without a filtering layer above the chips--plastic onion or potato bags?-- everything above will slowly seep in to eliminate the expected drainage. Oct 2, 2023 at 22:28
  • A bit hyperbolic, @YosefBaskin - If the soil above was that impermeable, water would never get through it to carry it down. Also nothing would grow. Over time the woodchips break down and become part of the soil, and it drains depending what it's made of, like any other 60cm deep bed does.
    – Ecnerwal
    Oct 4, 2023 at 14:16
  • @Ecnerwal Sorry, a misunderstanding here. When people pot plants with coarse material like broken crockery below and soil directly above, the soil quickly falls through to fill all the desirable air pockets below. Same with the layering here. I was not referring to anything being impermeable. Oct 4, 2023 at 14:34

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