There will be very little nutrition in there. In the ground, they need virtually nothing in the way of extra food as long as the soil is in decent condition. A slow release food like Blood Fish and Bone once a year in spring is more than enough, and it's the addition of organic matter that's the best method of keeping them healthy. In a container, they're totally reliant on you for everything. A seaweed feed occasionally is good, as that suits foliage plants, but a soil based medium is necessary to keep them happy, and a bit of organic matter added each year will keep that soil in good condition. A mulch of bark is good for retaining moisture, and also sets them off nicely. Any shrub potted long term needs soil, because compost alone isn't enough to sustain them. You can buy John Innes composts which will suit - it's really just a formula, and you can choose the most suitable one for any plants you have in pots. You can also mix some ericaceous compost with that, but most Acers are perfectly happy in neutral to acidic soil, so it isn't always necessary
It's a place where beautiful isn't enough of a word....
I live in west central Scotland - not where that photo is...
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In a container, they're totally reliant on you for everything. A seaweed feed occasionally is good, as that suits foliage plants, but a soil based medium is necessary to keep them happy, and a bit of organic matter added each year will keep that soil in good condition.
A mulch of bark is good for retaining moisture, and also sets them off nicely.
Any shrub potted long term needs soil, because compost alone isn't enough to sustain them. You can buy John Innes composts which will suit - it's really just a formula, and you can choose the most suitable one for any plants you have in pots. You can also mix some ericaceous compost with that, but most Acers are perfectly happy in neutral to acidic soil, so it isn't always necessary
I live in west central Scotland - not where that photo is...