Premixed Calcium Supplement for Mushroom Substrate

Stop fighting clumping. Start growing better bags.

Premixed Calcium Supplement is a precision-blended, ready-to-use calcium supplement for mushroom bulk substrate bags. Combining organic calcium sulphate (gypsum/CaSO4) and calcium carbonate (chalk/CACO3) in a single premix, it solves two of the most common substrate headaches at once — clumping and pH drift — so your bags colonise cleaner and faster.

Bulk discount up to 20% off on larger orders!

Key Benefits:

• Reduces clumping — calcium sulphate keeps substrate particles separated and workable, improving mycelium penetration throughout the bag
• Helps maintain balanced pH — calcium carbonate (chalk) acts as a natural buffer, supporting a stable environment for healthy colonisation
• Double calcium boost — both calcium sulphate and calcium carbonate contribute available calcium, a key mineral for strong mycelial growth
• Premixed and ready to use — no guesswork, no separate measuring of two powders, just scoop and add
• Compatible with all common bulk substrates — hardwood, straw, coco coir blends and Masters Mix
• Simple, consistent application — clear dosing rates for small and large bags

Why Calcium Sulphate and Calcium Carbonate Work Together:

Calcium sulphate (gypsum) is the workhorse of substrate conditioning. It improves particle separation, reducing the dense clumps that block gas exchange and slow mycelium spread. It also adds calcium without significantly shifting pH — making it safe to use alongside other additives.

Calcium carbonate (chalk) complements gypsum perfectly. It acts as a gentle pH buffer, helping to counteract the slow acidification that can occur during sterilisation or pasteurisation and throughout colonisation. It also contributes an additional source of calcium, giving your mycelium the minerals it needs to thrive.

Together, they create a more forgiving, better-structured substrate that colonises evenly and supports healthier, more productive grows.

Application Rates:

• Large bags (2.2 – 2.5 KG substrate): 2 tablespoons / 40g per bag
• Small bags (1 – 1.6 KG substrate): 1 tablespoon / 20g per bag

Mix thoroughly into dry substrate before hydrating and sterilising or pasteurising as normal.

How Much Will One Bag Treat?

• At 40g per bag → approximately 25 large substrate bags per 1KG of Gypsum + Chalk
• At 20g per bag → approximately 50 small substrate bags per 1KG of Gypsum + Chalk

Frequently Asked Questions:

What mushrooms is Gypsum + Chalk suitable for?
It's suitable for the full range of commonly cultivated species — oyster mushrooms (all varieties), lion's mane, shiitake, chestnut mushrooms, and most other wood-loving species grown on bulk substrate bags.

Is it safe to use with sterilised and pasteurised substrates?
Yes. Gypsum + Chalk is added to your dry substrate before hydrating, then sterilised or pasteurised along with everything else. Both calcium sulphate and calcium carbonate are heat stable and will not degrade during the sterilisation or pasteurisation process.

Can I use too much? Will it affect my pH badly?
At the recommended rates, Gypsum + Chalk is well-tolerated and won't push pH to harmful levels. Calcium carbonate is a mild buffer rather than a strong alkali, so it raises pH gently. Stick to the recommended dosing and you'll be in a safe range for most species.

Do I need to add any other pH adjusters if I use Gypsum + Chalk?
For most standard bulk substrate recipes, Gypsum + Chalk at the recommended rate provides sufficient buffering without needing additional pH adjusters. If you're using an unusually acidic substrate or supplementing heavily with high-nitrogen additives, you may want to test your substrate pH after hydrating.

Is this food-grade?
No, both calcium sulphate and calcium carbonate used in this blend are not food-grade minerals.

How should I store it?
Store in a cool, dry place away from moisture. Keep the bag sealed between uses. Properly stored, it has an indefinite shelf life.

Will it clump itself during storage?
The calcium sulphate content helps resist clumping in the product itself. If stored in a dry environment, it should remain free-flowing and easy to scoop.

Does it affect the smell or taste of mushrooms?
No. At these application rates, Gypsum + Chalk has no detectable impact on the flavour or aroma of your mushrooms.