Hi Frel. I have half a dozen cuttings I took Last year from a boxus and they have all rooted well. The two I have in pots outside the door seem to thrive well with a feed of Chempax (not sure if I spell it right) it can be bought in grades 1,2,& 3 I think. I use it for everything.
I think Epsom Salts is generally used for conifers if they are going a bit yellowish. I'm sure you'll get more advise though, as I'm not well up on fertilizers having only ever used the above.

I just add bone meal when planting and the odd liquid fertiliser when it looks discoloured.