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).
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)
- 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.