Spent mushroom substrate (SMS) is waste every mushroom farm produces. It is a fully colonized substrate, in our case a rich mix of oak or birch hardwood sawdust and soya hull, that has given us 2 flushes of gourmet mushrooms and is no longer economically viable to keep in the fruiting room. However, these are mycelium-rich and still have enough nutrients to produce a couple more harvest/flushes. Just soak them overnight and leave them in natural light, on a table top, away from direct sunlight or direct radiators.
Spent substrate makes great fungal-dominated compost and many home growers use these in their organic raised bed, no-dig gardening, be it in their backyard gardening or allotments.
1) Put the spent substrate straight on the soil. Don’t put cardboard or fabric under it as this reduces the breathability between the substrate and the soil below.
2) Make sure the spent substrate is laid down at least 5 cm thick. The thicker the more mushrooms you get.
3) Make sure the spent substrate is in a shady place or add a thin layer of leaves, bark, wood chips or landscape fabric on top of the bed. This ensures more mushrooms will not dry out during the growing phase.
4) Water regularly.