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Too dry and mycelium stalls; too wet and bacteria win. Most contamination in bulk substrate starts with guessing the water. This calculator works out the exact amount of water to reach your target moisture, accounting for the moisture already in the dry ingredients, and how much grain spawn to mix in.
The formula
Moisture content is expressed on a wet basis — the percentage of the final, hydrated substrate that is water. With dry weight D, starting moisture i and target t:
- Dry solids = D × (1 − i)
- Final wet weight = solids ÷ (1 − t)
- Water to add = final wet weight − D
Example: 5 lb of Master’s Mix at 9% moisture, targeting 60%, needs 5 × 0.91 ÷ 0.40 − 5 = 6.4 lb of water — about 3.1 quarts (2.9 L).
Target moisture by substrate
| Substrate | Target moisture | Typical spawn rate | Good for |
|---|---|---|---|
| Master’s Mix (50% hardwood pellets, 50% soy hulls) | ~60% | 10–20% | Oysters, king oyster, lion’s mane, chestnut, pioppino |
| Hardwood sawdust/pellets + 10–20% bran | ~60% | 10–20% | Shiitake, lion’s mane, reishi |
| Pasteurized straw | ~65–70% | 5–15% | Oysters |
| Coffee grounds + straw | ~60–65% | 15–20% | Oysters (use grounds within 24 h) |
Checking by hand: the squeeze test
Take a handful of hydrated substrate and squeeze hard. At field capacity, a few drops appear between your fingers. A steady stream means it’s too wet — drain or add dry material. Nothing at all means it’s too dry.
FAQ
What spawn rate should I use?
10–20% of the wet substrate weight is standard for supplemented sawdust. Higher rates colonize faster and outcompete contaminants but cost more spawn; lower rates work on pasteurized straw, which is less nutrient-rich.
Why does Master’s Mix need sterilizing, not pasteurizing?
Soy hulls make the mix very nutritious, which contaminants love. Supplemented substrates need sterilization — typically 2.5 hours or more at 15 psi in a pressure cooker or autoclave, depending on load size. Straw can be pasteurized because it’s low in nutrients.
Does the water need to be weighed?
Weighing is most accurate: 1 liter of water weighs 1 kg, and 1 US quart weighs about 2.09 lb.
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