Elms will grow on a wide variety of substrates, but if they stay on the wetter side, they form those fat,white water roots. These fleshy roots are more susceptible to rot and decay when cut.
Therefore, I use a drier mix of maybe 20 percent pine bark, 20% sand, 30% lava, 30% pumice. This minimizes fleshy roots and maximizes fine feeder roots.
Elms also seem to be able to take up water through their leaves. It's probably more to do with reduced transpirational loss, but watering elm leaves seems effective.