Akadama or dark-coloured lava
I can't think of any natural rock that would be harmful to the soil. If there was such thing then noting would grow in the area where it occurs so we would see large areas devoid of plant life.Mostly i was wondering if some type of rocks can harm the bottom soil.
And yes, this top layer is meant for the developing trees. For the trees that are already in pots and are 'better' ones i use sphagnum moss / regular moss mixture.
Some of my trees are at my parents place where i keep the trees in development, collected ones etc.. But when i start to work on the tree i take it to my apartment. And because all of my trees are on my balcony i try to make them look as nice as i can (also keeps the GF happy.), event if they are still in training pots .
Top dressing is completely unnecessary and can be detrimental as it can wind up "channeling" water, instead of allowing it to drain directly into the soil. It's not a huge issue, but it can crop up. uniform soil mix from top to bottom in the pot is ideal.
I have used aquarium gravel (because I had it)
It is a bigger issue for trees in training than established trees, particularly if those trees in training have been recently root-pruned. those trees need evenly supplied moisture in the soil. Overuse of top dressing can create drier spots in the soil and prevent moisture from draining through properly. Newly developing roots require Oxygen exchange as much as water. Water draining evenly through a uniform soil pulls in O2 more effectively...I can understand that but it's a training pot (large one) so i think that water shouldn't be an issue.
Would assume that if it doesn't harm the fishes it also shouldn't harm the tree.
Get some fishThanks all. Have already decided against
Now i just have to figure it out what to do with large bag of aquarium basalt grit