What is field capacity for mushroom substrate?

Field capacity is the ideal moisture level — the point at which substrate holds maximum water without dripping. Test by squeezing a handful: only a few drops should fall. Most substrates reach field capacity at 60–65% moisture content by wet weight. Too wet causes anaerobic conditions and contamination; too dry slows colonization.

What is the best substrate ratio for oyster mushrooms?

Straw substrate: pasteurize 1 lb dry straw with 2–3 lbs water to reach field capacity. Hardwood substrate: 5 parts hardwood sawdust + 1 part wheat bran (weight), hydrated to field capacity. Master's Mix: 1:1 hardwood sawdust to soy hulls by dry weight, hydrated to field capacity — highest yield but needs full sterilization.

How much spawn do I need per block?

Grain spawn inoculation rate: 10–20% of dry substrate weight. Higher spawn rates (15–20%) colonize faster and resist contamination better. For a 5 lb hardwood block (wet weight), use 8–16 oz of grain spawn. Sawdust spawn can be used at 10–15%.

How do I calculate substrate for a grow bag?

Standard mushroom grow bags (Unicorn 14B or similar) hold 3–5 lbs of dry substrate when hydrated to field capacity. A 5 lb colonized block typically produces 1–2 lbs of mushrooms over 2–3 flushes. Use our calculator below to compute exact amounts based on your container size.

Can I reuse substrate after harvesting?

Spent substrate can be buried in garden beds to produce a bonus outdoor harvest and improves soil structure. It can also be composted. However, re-inoculating spent substrate for another indoor grow cycle is generally not worth the contamination risk — start fresh for best results.

Mushroom Substrate Calculator

Calculate exactly how much substrate, water, and spawn you need for any container size. Select your substrate type to get component ratios, hydration amounts, and expected yield ranges.

🧮 Substrate Calculator

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Substrate Ratios by Species

Species	Best Substrate	Dry Ratio	Sterilize or Pasteurize	Typical Yield
Blue Oyster	Pasteurized straw or HWSD+bran	5:1 HWSD:bran or pure straw	Pasteurize	25–35% biological efficiency
King Oyster	HWSD + wheat bran	5:1 HWSD:bran	Sterilize preferred	20–30% BE
Lion's Mane	HWSD + wheat bran	5:1 HWSD:bran	Sterilize	15–25% BE
Shiitake	HWSD + wheat/rice bran	4:1 HWSD:bran	Sterilize	15–25% BE
Reishi	Hardwood sawdust (no supplement)	100% HWSD	Sterilize	10–15% BE
Button Mushroom	Composted manure + casing	Manure bulk + 1" casing layer	Phase II composting	20–30% BE
Enoki	HWSD + rice bran	4:1 HWSD:rice bran	Sterilize + cold fruiting	20–30% BE
Wine Cap (outdoor)	Wood chips (garden bed)	N/A — bulk garden bed	None — outdoor bed	Variable by season

Understanding Biological Efficiency (BE)

Biological efficiency is the ratio of fresh mushroom weight to dry substrate weight, expressed as a percentage. A 25% BE means 1 lb of dry substrate produces 0.25 lbs of mushrooms over the entire run (all flushes).

Master's Mix typically achieves 50–100%+ BE under optimal conditions because the soy hull supplement provides additional nitrogen. Standard hardwood/bran substrates typically run 20–30% BE. Straw typically runs 25–40% BE for oyster mushrooms.

How to Check Field Capacity

Field capacity is the ideal moisture level for mushroom substrate colonization. Too wet → anaerobic conditions → contamination. Too dry → slow colonization → contamination.

The squeeze test: Take a handful of your hydrated substrate and squeeze it as hard as you can. You should see:

A few drops to a thin stream: Good — this is field capacity
No water coming out: Too dry — add water in small increments and mix
Steady stream of water: Too wet — allow to dry or add more dry substrate