Several students have asked why I prefer pumice to lava. They are interchangeable for the most part. In all, lava is a pretty nice particle. Drains well, holds a good amount of moisture, and roots grow well in it. If you’ve a lot of it on hand, or it’s cheap where you live, sure, go ahead and use it. But I have a beef with this particle on several counts, and mostly I’m comparing it unfavorably to pumice.
1. Lava is heavy. If you have larger trees, those pots and boxes get ponderous very quickly.
2. The gas exchange appears higher in pumice than in lava. Pumice gets much lighter when it’s dry. From my experience, this promotes better root growth.
3. The most egregious problem with lava is how hard it is. If you repot many trees in the spring, and are doing detail work on rootballs, you will likely discover that lava destroys your tools. Tweezers wear down quickly. Chopsticks become nubs. Root scythes lose their teeth fast. Your formerly sharp root scissors now has dings in its edges. Scissors get dull fast with lava in the mix; pumice, on the other hand, is soft and the scissors will cut right through it. All volcanic materials are abrasive, but lava is the worst.
For drainage we often choose whatever is most easily found where we live. I know many in the Midwest who cannot easily find pumice. But when I have the option, I always choose pumice over lava for the interior of the soil mass. If the looks of your soil are an issue, use some small lava and akadama as a top dressing in the last 3/4″. That is best use of lava.