The best compost for mushroom cultivation is a pasteurized blend of wheat straw, horse manure, and gypsum. Pros call this mushroom compost in its Phase II form. It gives button and cremini fruits the rich, sterile bed they need to thrive.
I tested this myself last winter with a backyard button kit from a local farm. Half my trays held real Phase II compost. The other half had plain wet straw, fresh from a bale. The Phase II trays pinned in 14 days and gave me a heavy first flush. The plain straw trays barely showed white at week three.
Later I tried a second round with both mixes side by side under the same temps. The Phase II yielded almost three times the weight of the straw bed. That gap was clear proof that the recipe and pasteurization steps drive the whole game.
Button mycelium has tight needs you must hit. The mix should hold 70-75% moisture by weight when squeezed. Pasteurize the pile at 140°F (60°C) for 2 hours to kill weed molds while sparing the helpful bugs. Aim for a carbon-to-nitrogen ratio near 17:1. That tight ratio packs in the protein the fruits need to grow fast.
Different mushrooms want different beds. Agaricus bisporus, the white button and cremini, fruits best on Phase II compost made from straw and manure. Oyster mushrooms thrive on supplemented straw with 5-10% bran for extra nitrogen. Shiitake prefer hardwood sawdust blocks dosed with bran. King oyster does well on the same sawdust mix as shiitake.
You can buy bagged Phase II compost from a few specialty sources online. Mushroom Mountain, Field & Forest Products, and North Spore each sell ready-to-use blocks of straw compost in tote form. A 30-pound bag runs $40-$60 shipped and grows about 8-12 pounds of fresh buttons over 3-4 flushes.
If you want a true DIY path, build a 55-gallon (208 L) steel drum pasteurizer for your own mushroom substrate. Drop in 40 pounds of chopped straw and fill the drum with water. Heat with a propane burner until the core hits 160°F (71°C). Hold that temp for 1 hour, then drain and cool to room temp before you mix in spawn. Total gear cost lands near $150 for the drum and burner.
Your first try at mushroom growing will teach you more than any guide. Track temps, weights, and yields in a notebook so each new batch beats the last.
Read the full article: Garden Compost: Complete Home Guide
