sorry there is also a plum tree just noticed spelling error I meant compost

You will need large containers, nonetheless, Osprey - they are still trees, even if they do only reach 6 or 7 feet. I'd use John Innes No. 3, as long as heaviness of the pots isn't an issue - if it is, mix John Innes No. 3 with multipurpose compost, half and half. If your trees are very small plants currently, choose John Innes No. 2 (unless you're mixing half and half as mentioned).

rember the roots need at least the with of the branches, so either verry big containers or out in the garden if grown in containers you will get six to nine fruit in the garden count that in LB's and at least double it after the first year it is entirly up to you but I know what my choise is.