Coir is a commercially available mix of ground up coconut husk and shells. Coir is one of the best bulk mediums to grow mushrooms in. It’s naturally resistant to mold and bacteria and mushrooms absolutely love it. When wet it expands to over 2.5 cubic feet.
The product of powdering brown rice is a popular substrate for growing mushrooms. Its use as a substrate was made popular by the use of cake spawns in the PF Tek method of growing. It is most often powdered into a flour and combined with vermiculite.