Post-Crop Pasteurization and Spent Mushroom Substrate (SMS)

When a house becomes unproductive, the crop is usually terminated. Before removing the spent substrate from the mushroom house, the grower «pasteurizes» it with steam to kill any diseases, pests or other biological activity that could interfere with a neighboring house or subsequent crop. The steaming-off procedure is accomplished by maintaining a compost temperature of 140–150ºF (60–70ºC) for anywhere from 8 to 24 hours. The spent compost should be removed from the farm to reduce the chances of contaminating the subsequent mushroom crops at the farm (Figure 19).

Figure 19. SMS being emptied from a mushroom farm.

Spent mushroom substrate (SMS) is the soil-like material remaining after a crop of mushrooms has been harvested. Spent substrate is high in organic matter, making it desirable for use as a soil amendment or soil conditioner. Sometimes this material is called spent mushroom compost. SMS still has some nutrients available for the mushroom. However, replacing the substrate and starting a new crop is more economical. Users should consider spent substrate clean of weed seeds and insects.

The typical composition of SMS fresh from a mushroom house will vary slightly. Since raw materials and other cultural practices change, each load of fresh spent substrate has a slightly different element and mineral analysis. Sometimes fresh substrate is placed in fields for at least one winter season and then marketed as »weathered» mushroom soil. This aged material has slightly different characteristics because the microbial activity in the field will change the composition and texture. The salt content rapidly decreases during the weathering or composting.

Spent mushroom substrate has many appropriate uses. SMS is excellent to spread on top of newly seeded lawns because it will provide cover against birds eating the seeds and will hold water in the soil while the seeds germinate. Since some plants and garden vegetables are sensitive to high salt content in soils, avoid using fresh spent substrate around those plants. You may use spent substrate weathered for 6 months or longer in all gardens and with most plants. Obtaining spent substrate in the fall or winter and allowing it to weather will make it ready for use in a garden the following spring. SMS can be applied as mulch in small amounts on turf all year-round. Spent substrate is a choice ingredient for companies that make potting mixtures sold in supermarkets or garden centers. These companies use spent substrate when they need a material to enhance the structure of a soil.

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