Advice for soil for Japanese black pine

I have 3 very young (about 6month old) japanese black pines which I am growing from seeds. They come in a kit and about 4months ago I put each one in its own pot. However I now think that the soil might not be the most appropriate. I used the Westland Bonsai Tree Potting Mix but I understand now that a mix of 33% Akadama 33% Pumica and 33% Lava is a good soil especially for Japanese black pines. What I would like to ask you guys is the following

• Shall I change the soil now to this Akadama/pumice/lava mix. Do you have any soil mixes to recommend for Japanese black pines?
• Where can I purchase such a mix please. I live in London UK and I havent found anything onlline with this composition (ie Akadama/pumice/lava)

Many thanks

gmiliotis wrote: (about 6month old) japanese black pines

Shall I change the soil now to this Akadama/pumice/lava mix.

There's no need for that.

The recommended mix is for trees that are bonsai.