Soil/Media Suggestions for Containerized Layering of M26

Greetings,

I'm about to try an experiment - see attached image from p549 of Hartmann and Kester's Plant Propagation Priciples and Practices: Containerized Layering.

I am getting an M26 apple rootstock and want to try this method as a simple way of producing a small amount of root stock for personal/non-commerical purposes.

Can anyone suggest a soil/media recipe for planting my M26 bare root plant - not rooting media for the top pot w/layers.

Thanks,
Michael