Calcium Carbonate (Chalk) Mushroom Supplement

Calcium Carbonate — commonly known as chalk — is one of the most reliable and widely used supplements in mushroom cultivation. Its primary role is as a pH buffer: as mycelium colonises a substrate, it produces acids that can gradually push the pH into a range that inhibits growth or invites contamination. A small addition of calcium carbonate keeps the environment stable, giving your mycelium the consistent conditions it needs to colonise fully and fruit well.

What it does in your substrate

Buffers pH: counteracts the natural acidification that occurs during colonisation, maintaining a more stable and mycelium-friendly environment throughout the grow.

Adds calcium: calcium is an essential mineral for fungal cell wall development and helps mycelium grow stronger, denser, and more resilient.

Reduces contamination risk: a stable, slightly alkaline substrate is a less welcoming environment for many competing moulds and bacteria, helping your mycelium get established before contaminants can take hold.

Where to use it

Calcium carbonate is suitable for sterilised and pasteurised bulk substrates, casing mixes, and grain spawn. It's particularly useful in supplemented hardwood substrates (Masters Mix and similar) where higher nitrogen content can accelerate acidification. Recommended at 0.5–1% by dry weight of your substrate mix.

It's also excellent in garden applications — a gentle, slow-release source of calcium for vegetable beds, raised planters, and pots where pH drift is a concern.

Available sizes

Available in 100g, 250g, 500g, and 1KG bags — scaled for hobbyist to small commercial grows. Sourced and packed in Australia.