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I want to grow vegetables in my garden in beds because the soil is clay and I feel that’s the best option.

Wooden beds seems the best way to go as recycled plastic seems awfully expensive.

There seem to be a lot of debate online regarding tanalised beds with the chemicals eventually sipping through the soil and getting in your veg.

If That’s the case, what wood is best to use? What alternatives do I have? I live in the UK

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Tanalised wood will leach inorganic arsenic into the soil but the crucial question is whether it's a significant amount. The short answer is that plants closest to the wood, just a couple of inches, and especially root vegetables are most likely to be affected. And they'll get sick before you do.

More concerning is you, and your children, touching the exposed wood and transferring it to your mouth.

Some people recommend painting the wood or covering it with black plastic to reduce the hazards.

Best wood otherwise to use? Any hardwood should do. Or use any other material, or just use a sloped edge to the beds.

https://www.tuin.co.uk/blog/tanalised-pressure-treated-garden-timber/

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I do the vegetable garden in clay soil, without problems. Fertilizers and manure will acidify the soil, enough for garden. Some vegetables prefer clay, most are neutral (if there are enough nutrients). If you can, add some sand and good soil on top. One problem of clay soil is that it tend to dry quickly (but less than sand soil), and it could be hard (so forget carrots).

Tannin are taken from trees, so it is natural. I doesn't think we (as human) we get problems, but it is also a good herbicide, so not so nice for vegetables. The choice of soil is much more important (heavy metals).

In general I recommend raw wood. You need to replace from time to time, but this is good: it gives you the possibility to improve the bed (construction), and rot wood will give back some nutrients.

EDIT:

I confused tannin with tanalised procedure. Tannin were used (and still used) to preserve wood and leather. Tanalised wood has much more chemicals. I doesn't think enough to get health problems (still oligo-elements, so essential for us, and it should not give so much to vegetables, and then we eat just a part (fruits or some leaves).

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Alternatives? Dirt. Make your raised beds by simply defining where the paths are and tossing that soil up onto the remaining bed area, and work with a rake. Pile on any amendments to the bed areas only. The same as "hilling" only on a bed-sized area (in farming, often a "wide row") - in all cases, the big deal is not walking on the beds, so don't make them too big, or at least too wide (I've shrunk mine 3 times over the years.)

Rocks, bricks, or other masonry, but usually that's only economically viable if you have a free or very low-cost source of recycled material.

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