What is substrate?

The substrate sets which species you can grow, how fast it colonizes, how much it yields and how much prep it needs. Cornell's Small Farms Program defines substrate as "any material that is a food source for mushroom mycelium". Low-tech growing uses low-nitrogen, high-carbon materials, like oyster on straw or shiitake on logs. Higher-tech growing adds a high-nitrogen supplement to sawdust or wood pellets for faster colonization and higher yields, and has to sterilize it (Cornell).

The common mushroom substrates are straw, supplemented sawdust, logs and wood chips. Each needs its own prep.

Substrate	Grows	Prep
Straw	Oyster, the best species for it (Cornell)	Pasteurize, soak in lime, or ferment
Supplemented sawdust	What most specialty growers fruit on (Cornell)	Steam-sterilize, inoculate clean
Logs, stumps, wood chips	Shiitake on logs; outdoor beds	None if the tree was healthy
Coffee grounds	Oyster (UF/IFAS)	Inoculate the day they're brewed, mixed with 50% sawdust

The straw, sawdust, log and coffee-ground rows come from Cornell unless another source is named (Cornell). Master's Mix, half hardwood sawdust and half soy hulls, is the best-known supplemented sawdust. Grain is too expensive and too rich for bulk fruiting, Cornell says; it's used as grain spawn instead.

Which substrates you pasteurize and which you sterilize comes down to how nutritious they are: see sterilization vs pasteurization.

A mushroom substrate should hold about 50–70% water, depending on the material, according to GroCycle, a company that sells growing kits and courses (GroCycle). A UC Cooperative Extension handout gives Stamets' enriched sawdust recipe at 60–65% water, and expects straw to hold about 65% once it's drained well (UC Cooperative Extension). In practice, growers wet it to field capacity and check with a squeeze test.

• Softwood. GroCycle says to avoid softwood sawdust and pellets (GroCycle). UC notes that cedar and redwood resist most cultivated mushrooms, and only a few colonize pine (UC Cooperative Extension).
• Too much supplement. Nitrogen raises yields, but UC says adding it "always increases the risk of contamination" (UC Cooperative Extension). GroCycle suggests starting supplements at 5% and working up (GroCycle).
• Waiting too long with logs. Sources split here. GroCycle says to store fresh-cut logs for a few months first (GroCycle). Cornell says there's no harm in inoculating right after cutting, calls the idea that logs must sit first a misconception, and says to inoculate within a month (Cornell).
• Too wet. GroCycle names overhydrated substrate as the most common cause of slow colonization (GroCycle).

Hypha keeps your substrate recipes and supplies in its inventory, and on Pro and Farm works out cost per fresh pound by block and strain, so you can see which substrate earns its keep.