@Meomye Yes, I mean a liquid ericaceous feed (although you can buy it in slow release granule form too). They probably don't need feeding if they are in the ground, but I would always feed if they are in pots. They quite fussy plants, so we prefer to give them every bit of help we can. This is only for our smaller ones that have been moved a few times over the years.
We have a very mature specimen, which we don't feed, but we put fresh ericaceous compost around the roots every Spring.
I too have acers in pots. I try to replace a thin layer of ericacious compost annually and give them some Osmacote which is slow release fertiliser. The main thing I do is water them. One of my green ones which I think is Emerald Lace, bought from Tesco, suffered a black fly infestation last year, but now I have a flock of sparrows, encouraged by regular feeding and they keep everything clear of both black and greenfly. I wouldn't have believed it had I not seen them in my small trees, pecking away!
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We have a very mature specimen, which we don't feed, but we put fresh ericaceous compost around the roots every Spring.