Leatherback, thank you so much for your message, I really didn't think at all about procuring my own soil from nature! I mainly thought that the tree is coming from Japan or China and the soils must be different from what I have in Romania. On the other hand, someone has told me my bonsai is a Siberian Buddhist Pine but I am not exactly sure what he meant, my bonsai skills haven't yet reached that level.
Alain Leon, thank you a lot for the useful links you provided me with, they were really eye opening.
In the end I think I have found the recipe for my Buddhist Pine's soil:
60% Akadama
10% Organic compost
30% Grit
Please let me know your opinion about this mix.
I have also found the products in the Netherlands:
www.bonsaishop.nl/grond/grondsoorten/voo...potgrond-mix-4-liter
www.bonsaishop.nl/grond/grondsoorten/aka...bonsai-aarde-2-liter
Thank you in advance!