What is considered a bulk substrate?

A bulk substrate is a substrate of greater volume that is used for mushroom cultivation on a large scale.

Examples of different types of bulk substrates used for cultivation of Ps. cubensis or Pan. cyanescens may be straw, dung, or coco coir supplemented with additives such as worm castings.

Bulk substrates are  pasteurized and are  inoculated using spawn made with whole grains such as rye berries or wild bird seed; as opposed to grain or flour based substrates, which are sterilized.