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Substrate hydration and field capacity
Mushroom mycelium needs moisture to colonize a substrate, but there's a narrow band that works. Too dry and the mycelium stalls; too wet and oxygen is pushed out, inviting bacteria and mold. The target is field capacity — the substrate holds all the water it can against gravity, with no free water pooling.
The squeeze test
The classic check, used across the cultivation literature: grab a handful of hydrated substrate and squeeze it hard.
| What you see | Verdict |
|---|---|
| A steady stream runs out | Too wet — drain and dry back |
| A few drops are forced out | About right — field capacity |
| No water, substrate springs apart | Too dry — add water |
Target moisture by substrate
Different substrates hold water differently, but most fruiting substrates land around 60–65% moisture content by weight:
| Substrate | Typical target |
|---|---|
| Hardwood sawdust / pellets | ~60% |
| Straw | ~70% |
| Supplemented masters mix | ~60–62% |
Moisture content is the weight of water as a fraction of the total wet weight. If you weigh substrate before and after fully drying a sample, you can calculate it precisely rather than guessing.
Free water in the bottom of a bag or tub goes anaerobic and breeds bacteria that outcompete the mycelium. When in doubt, err slightly drier; you can mist a colonized block, but you can't easily un-soak a contaminated one.
Hydrate before pasteurizing/sterilizing, then check again. Heat treatment and cooling can change moisture. Confirm the squeeze test after the substrate has cooled, not just when you mixed it.
Flush calculates your water-to-substrate
Enter your substrate, dry weight, and target moisture and Flush returns exactly how much water to add to hit field capacity — alongside spawn-rate, agar, and biological-efficiency tools for 11 species. Free to download.
Sources
- Stamets, Growing Gourmet and Medicinal Mushrooms; Cotter, Organic Mushroom Farming and Mycoremediation
- Cornell Small Farms mushroom best-management practices; Field & Forest grower guides
Targets are typical ranges; tune to your species, substrate, and container.