The recipe, in one line
Master's Mix is equal parts hardwood sawdust pellets and soybean hull pellets by dry weight — a 1:1 ratio — with a small amount of gypsum. That's the whole formula. The soy hull is what makes it special: it supplies the nitrogen supplementation, so you don't add wheat bran the way you would with plain supplemented sawdust. Both ingredients come as cheap, clean, compressed pellets (hardwood fuel pellets and soybean hull pellets), which is part of why the mix is so popular — no loose sawdust to source, and the pellets are effectively pre-dried and consistent.
Ingredients & ratios
| Ingredient | Amount (by dry weight) | Role |
|---|---|---|
| Hardwood sawdust pellets | 50% | Bulk lignocellulose — the carbon the mycelium eats |
| Soybean hull pellets | 50% | Nitrogen supplementation — drives yield and colonization speed |
| Gypsum (calcium sulfate) | ~1% (optional) | Buffers pH, keeps the mix from clumping, adds calcium |
| Water | ~1.2–1.5× the dry weight | Hydrate to field capacity (~60% moisture) |
Some growers run 2:1 sawdust-to-soy-hull to lower the nitrogen (less contamination risk on marginal sterilization) or 1:2 for a richer mix — but 50/50 is the classic, and it's the ratio most gourmet farms default to.
A real batch: exact weights
Here's a 10 lb dry batch that yields roughly 25 lb of hydrated substrate — about five 5 lb fruiting blocks:
| Hardwood sawdust pellets | 5.0 lb |
| Soybean hull pellets | 5.0 lb |
| Gypsum (~1%) | ~0.1 lb (2–3 tbsp) |
| Water (to field capacity) | ~15 lb (≈1.8 gal) |
| Hydrated total | ~25 lb → ~5 blocks |
Step by step
- Weigh the pellets 50/50. Equal dry weights of hardwood sawdust and soybean hull pellets. Add ~1% gypsum if you're using it.
- Add water and mix. Roughly 1.2–1.5 parts water per part of dry pellets. Hot or boiling water speeds the pellets breaking down into loose substrate. Stir until it's uniform.
- Rest 20–30 minutes. Soy hulls swell slowly and keep drinking after the sawdust has soaked. Give it time before you judge the moisture.
- Check field capacity with the squeeze test. Grab a handful and squeeze hard: a few drops should come out, not a stream. Too dry? Add water. Streaming? Add dry pellets. (More on this in the field-capacity guide.)
- Fill your bags. Pack filter-patch grow bags, usually ~5 lb of hydrated substrate per block, and fold the tops loosely.
- Sterilize — don't pasteurize. 15 psi (~250°F / 121°C) for at least 2.5 hours. Master's Mix is too rich to pasteurize safely (see below).
- Cool completely, sealed. Let bags return to room temperature before opening them. Opening a warm bag pulls in contaminated air as it cools.
- Inoculate at ~20% spawn. In front of a flow hood or in a still-air box, mix in colonized grain spawn at about 20% of substrate weight (the spawn-ratio guide covers when to go higher), seal, and set it to colonize.
Why it works so well
The magic of Master's Mix is the balance: hardwood gives the fungus the woody carbon it evolved to eat, and soybean hull adds just enough nitrogen to push fast, dense colonization and heavy fruiting — without the fat, contamination-prone supplementation of something like a high-bran mix. On oyster and lion's mane it routinely delivers high biological efficiency, which is why nearly every commercial gourmet farm runs some version of it.
The two mistakes that ruin a batch
- Pasteurizing instead of sterilizing. This is the #1 Master's Mix failure. The soy hull that makes it yield also makes it a feast for mold and bacteria. A hot-water or lime bath (fine for plain straw) leaves survivors that overrun the rich mix. It must be pressure-sterilized.
- Too wet. Overshooting field capacity leaves water pooling at the bottom of the bag, which goes anaerobic and sour and stalls colonization. When in doubt, aim slightly drier and let the squeeze test — not a number — be the judge.
What grows on it
| Species | On Master's Mix |
|---|---|
| Oyster | Excellent — fast, forgiving, high BE |
| Lion's mane | Excellent — the standard block substrate |
| Shiitake | Good — longer colonization, needs the brown skin to form |
Track which recipe actually yields best
50/50 is the default — but your best mix depends on your strains, your supplementation, and how clean your sterilization is. Log the recipe and outcome for every batch in Mycro's free Grow Log and see which substrate genuinely wins on your farm.
Try the free Grow Log → Become a Founding Grower — $99 →$99 once · lifetime founder pricing · a direct say in what we build first · fully refundable before launch.
General guidance from common cultivation practice, not guarantees — your results vary with strain, substrate, technique, and conditions. Mycro is for legal culinary & medicinal mushrooms only.