Our supplemented hardwood substrate mushroom grow blocks are the perfect fruiting medium. We use 100% organic oak sawdust perfectly hydrated for growing mycelium. Our hardwood substrate blocks are supplemented with grain flour, sulfur and calcium for optimal results. We use the thickest bags available, to ensure the sterility of the block. Each block is 5 lbs in an extra large bag, so they can be easily mixed after inoculation.