Spent grains from brewing are likely to be an excellent material for
growing some sort of mushroom, I would be surprised if adding soil would
be useful in shiitake cultivation as they are wood decomposers. the best
source of information on this type of research is the journal Mushroom
Science, it is the proceedings of the international conference on mushroom
cultivation which occurs every two to three years. i have seen articles on
using coffee grounds, cane waste etc. for mushroom cultivation there. you
should be able to get the journal through a good research library.
todell at u.washington.edu
Dr. Thomas E. O'Dell, Ph.D.
Dept. of Botany, Box 355325
University of Washington
Seattle, WA, 98195-5325