That is a good question. I do not like to keep trees in containers, prefer to let them grow and search out their own nutrition by spreading their roots. I did note that a manure stash, when we got to the bottom had roots coming up into it from my nearby Bramley Apple tree. Good results from the tree too, most apples weighed over a pound each.
Some one I know puts a shovel full of manure in the bottom of his water butt every year and gives it a good stir with a broom handle weekly. He uses it to water his container plants, shrubs and trees, none of which are fruiting varieties though.
I put a new tree in a huge container this year because the area I wanted to actually plant it was not ready and I put some tomato feed in the watering can once a week. It has certainly shot up, the instructions which came with the tree were to take off the fruit the first year to give it a chance to build up strength.
My assumption is that the tree should be fed whilst it is making fruit and if you are not going to make your own liquid manure then use the dilution or distribution ratio recommended with your preferred brand of manufactured feed.
Top up the container with a decent compost mix too, if you are not re-potting.
Lets hope we all get some plums this coming year, it was a poor harvest this last season, due to the low temperatures during blossom time.
