If you don't have one, make an IG account and follow Kazuo Onuma so that you can witness and verify, day after day, week after week, year after year, that lava is not a death sentence or even really a problem at all for a wide range of species including deciduous broadleaf species (there is AFAIK nothing quite like IG for following Japanese growers progress day-by-day and seeing how they actually do stuff in development stages, so it really is worthwhile). He's got bazillions of trees in pure lava including all sorts of broadleaf deciduous stuff like maples.
Seriously, spend 30 minutes flipping through every picture on this account and you will be convinced that lava isn't a problem, because you will see a lot of non-pine things in
pure lava:
5,230 Followers, 37 Following, 233 Posts - See Instagram photos and videos from 大沼 和男@盆じぃ、山岳盆栽を目指す！ (@bonsai.kazuo.oonuma)
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If lava was inherently a problem, Onuma didn't get the memo, and neither did his trees.
It is true, there may be impacts on root structure, and it doesn't hold much water, and it's heavy, and it eats our tools for breakfast, but lava isn't automatically poison in a horticultural sense -- at least not all lava. I would say your sparing amount of lava isn't necessarily a problem.
I've grown everything from pine to maple to black cottonwood in varying amounts of lava and have never had a horticultural issue I could plausibly blame on the lava (unless it was related to shorter water retention, but for some pine projects that is a plus).
I still heed Andrew Robson's advice about not using it in later stages because eventually I need (or want) finer root systems and density, but for development, and for health, lava isn't something to fear IMO. And in low amounts I think your elm will be OK.
