According to Better Homes and Gardens, mushrooms come from spores rather than seeds like most vegetables. Instead of plant nutrient or fertilizer, mushrooms prefer nourishment called "spawn," composed of wet, wood-based components.
Mushrooms will get adequate nourishment from a mix of sawdust, grain waste, water, wood chips, straw and water. Nearly any wood by-product effectively nourishes mushrooms. This mixture, or spawn, goes on top of the substrate or ground where the mushrooms are being grown. The spores continually have exposure to the spawn, allowing them to take nourishment as often as they need to. Spawn should be reapplied as often as necessary to ensure that the mushrooms "eat" as much as they can.