In mushroom growing, the term substrate refers to the organic material you're growing the mushrooms on, that is, what the mycelium is feeding off.
Inside a mushroom grow kit is a block of substrate in a special mushroom grow bag which has been sterilised to get rid of any biological competition to the mushrooms. Once it's cleared of competitors the mushroom culture, or mycelium, is introduced to the substrate so it can start feeding off the organic material.
There's many organic materials you can use as mushroom growing substrate such as straw, wood chip, sawdust, and paper. The types of mushrooms in our grow kits are wood decomposers so we use a wood-based substrate.
We make our substrate blocks in Brisbane at our farm, using clean Australian hardwood sawdust with some additional nutrients. The sawdust is recycled from sawmills around Australia. This sawdust would be a waste product, but instead it's growing food!