Bulk substrate is the term used for any type of medium used to grow mushrooms. There are a huge range of different materials that can be used to grow different mushrooms. The most common being supplemented hardwood sawdust/shavings, coco coir, straw, cardboard, coffee grounds, vermiculite, peat and many other materials. Alternatively you can use one of our pre-made mushroom grow bags with grain and bulk substrate and nutrients all in one bag.