Short answer: Mushroom compost is the spent growing substrate left after commercial mushrooms have been harvested. It starts as composted straw and gypsum (sometimes with a small amount of manure) that’s pasteurised before mushrooms are grown — not afterwards. When sold, it’s no longer rich in nutrients but can still improve soil texture and moisture balance.

What It Contains

• Straw and other fibrous crop residues
• Gypsum (adds calcium and sulphur, moderates pH)
• Occasionally small amounts of manure or synthetic nitrogen
• Lignin-rich, partially humified straw after several mushroom crops

How It Differs from Multipurpose Compost

Best Uses

• Improving heavy clay or acidic soils
• Conditioning vegetable beds in autumn or winter
• Mulching established borders (avoid acid-lovers)

When to Avoid

• For acid-loving plants such as rhododendrons or blueberries
• In pots or seed trays — too coarse and unbalanced
• On chalky soils — may increase alkalinity further

Good to Know

• It is not sterilised before sale — it remains biologically active
• Think of it as a structural soil improver, not a nutrient feed
• Its slow-decaying carbon fraction makes it long-lasting but low-energy
• Often classed closer to a stabilised compost than to active compost