Roberto said:

and the other one has to do with the Epsom
salt, is it good for some others trees too? I usually fertilize my
trees every 15 days with different fertilizers, depending on the tree.
For
example I use a 20-20-20 mix for general purposes (trees without
flowers or fruits) and another one for Fruits or Flowers. So, do you
recommend me to combine the Epsom salt with my actual fertilizers?
Once again,
let me thank you for your advice.

I say:
As far as I know, Epson salts are used to make foliage "bluer" by some
unknown mechanism. All it is is Magnesium chloride. Magnesium is a
necessary micronutrient in plants, but not the only one. Because we
commonly grow bonsai in non-soil mixes, they can be deficient in
micronutrients and regular NPK fertilizers like 20-20-20 will not
supply them. Such fertilizers are fine when applied to regular soil,
because most soils have reasonable levels of the micronutrients plants
need. But I grow my bonsai in a turface/granite/bark mix, and I need
to add micronutrients.

However, full-spectrum fertilizers are easy to find. For conifers, I
use MirAcid and get magnesium as well as all other necessary
micronutrients, without having to worry about dose. That's what I
would recommend.

Nina