hello Anders ,
nice to have a reply from you.
this is the "mixture" from a "cactus shop" here in Germany,
and maybe it is especialla the Zeolith, which help to prevent the tubers from rotting.
Körnung 0,5-2 mm, spezieller Mix aus Lava, Bims, Zeolith und Schwarzsand. Hervorragend geeignet als Topferde für sehr kleine Töpfe, zur Bewurzelung von Stecklingen und zur Aussaat.
hope you can find an equivalent, good luck.
lMK.
cheers
dieter