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I am not sure whether I should use potting soil or raised bed soil. This would have a bottom so the roots would not be going down to the actual soil (going to be putting it on concrete anyways).

  • composted wood chips make good soil if you have time to let them compost – black thumb Aug 5 at 22:42
  • Unfortunately I am not patient enough for that – Andrew Aug 6 at 1:36
  • A moist organic soil good for growing plants should corrode through galvanized sheet steel in a few years,depending on thickness. – blacksmith37 Sep 6 at 16:17
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This page from the Royal Horticultural Society provides the formulations of a range of compost specifications used in the United Kingdom; from this source:

[John Innes No 2] contains double the amount of nutrient in John Innes No 1 to suit established plants. It is suitable for most houseplants and vegetable plants in containers.

John Innes No 2 is formulated as:

  • 7 parts sterilised loam (good quality topsoil - raised bed soil is probably approximately this)
  • 3 parts peat or peat substitute
  • 2 parts sand

Plus the addition of a variety of concentrated fertilisers and minerals described as "John Innes Base" which include:

  • Hoof and horn meal (bonemeal or fish, blood and bone is not equivalent; the NPK ratios are different)
  • Superphosphate
  • Sulphate of potash

You may not find products labelled as such in your market, but if you look for potting compost with a similar formulation, you shouldn't go far wrong. You could also make your own to the above ratio with topsoil, well-rotted organic matter, sand, and a balanced slow-release fertiliser applied as per the pack directions.

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