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Spawn & Substrates
Agricultural Lime (Calcium Carbonate)
Finely milled calcium carbonate. A gentle, slow pH buffer that holds your casing and bulk mixes steady and adds calcium, without the caustic bite of hydrated lime.
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Food-grade calcium carbonate. Not classified as a hazardous chemical.
Holds pH steady around pH 8, will not overshoot like hydrated lime
Supports firmer fruit bodies and conditions peat or coco casings
1 kg, stored dry and sealed it keeps indefinitely
The short version
Agricultural lime is finely milled calcium carbonate (ground limestone or chalk). In mushroom growing it does one main job: it acts as a slow, mild pH buffer. It holds a casing layer or bulk substrate steady as the mycelium grows and naturally turns the mix more acidic.
It is not the same as hydrated lime, and it is not gypsum. Calcium carbonate is barely soluble in water and settles the mix to a little above neutral, around pH 8, rather than spiking it. That makes it very hard to overdose. Typical use is roughly 2 to 3 percent by wet weight in a casing mix and around 1 to 2 percent of dry weight in a bulk substrate.
What it is
What agricultural lime actually is
Agricultural lime is calcium carbonate (CaCO3), the same compound as chalk, limestone, marble and eggshell. Ours is calcitic lime, milled to a fine powder so it mixes evenly through peat, coco and bulk substrates. It is not dolomitic lime, which also carries magnesium and acts more slowly.
The important point is that calcium carbonate is a buffer, not a fast pH raiser. It is barely soluble in water, so it dissolves only as acids build up and reacts with them as it goes. That holds the pH steady rather than pushing it up in one jump, and it settles a wet mix to a little above neutral, around pH 8. That is why it is hard to overdo, and why it keeps working over the life of a mix, unlike hydrated lime which acts fast and then fades as it reverts to carbonate.
How to use it
Using it in casing and bulk mixes
Add the lime as a dry powder and mix it through thoroughly. For a bulk substrate, weigh by the dry weight of the material. For a casing layer, the usual figure is given by the wet weight of the finished casing.
Weigh your material
For a bulk substrate, work from the dry weight of your peat, coco or substrate. For a casing layer, work from the wet weight of the made-up casing. This keeps the percentage accurate.
Add the lime
For a casing layer, use about 2 to 3 percent by wet weight of the casing, which is roughly 20 to 30 g per kg of made-up casing. For a bulk substrate, around 1 to 2 percent of dry weight is typical.
Mix it in, then hydrate
Blend the powder evenly through the material first, then bring it to field capacity with water and pasteurise or prepare as normal.
Because calcium carbonate buffers rather than spikes the pH, there is a wide safe margin. If you are unsure, start at the lower end of the range. A little extra will not burn mycelium the way hydrated lime can.
What it is not for
What this is not
This is not hydrated lime (calcium hydroxide). Hydrated lime takes a wet mix to around pH 12.4 and is used to swing pH up fast or to knock back mould on a casing or substrate. It is caustic and easy to overdose. Calcium carbonate will not reach those numbers, so do not reach for it when you want a high pH to suppress contaminants. It is the gentle, long term buffer instead.
It is also not gypsum (calcium sulphate). Gypsum adds calcium and sulphur and helps stop grain spawn clumping, but it is pH neutral and does not buffer. The two are often confused. If you want pH buffering, use this lime. If you want to keep grain loose, use gypsum. Some growers use both, for different reasons.
Storage and safety
Storage and handling
Calcium carbonate is not classified as a hazardous substance and is non-toxic in normal handling. The only real caution is dust. Fine powder can mildly irritate the eyes and airways, so mix it in a ventilated space and avoid breathing the dust. A simple dust mask is sensible if you are working with a lot at once.
Keep the pouch sealed in a cool, dry place. Kept dry it does not go off. Keep it away from strong acids, which react with it and release carbon dioxide.
Common questions
Frequently asked
No. Hydrated lime (calcium hydroxide) is caustic and takes a mix to around pH 12.4. This is calcium carbonate, a gentle slow buffer that settles a mix near neutral and is hard to overdose.
No. Gypsum (calcium sulphate) adds calcium and sulphur and keeps grain spawn loose, but it is pH neutral and does not buffer. Use lime for pH, gypsum for grain.
Roughly 2 to 3 percent by wet weight in a casing mix, or about 1 to 2 percent of dry weight in a bulk substrate. For the casing, weigh by the wet weight of the made-up casing and mix the powder in before topping up the water.
It is very forgiving. Because it is barely soluble and buffers rather than spikes the pH, a little extra will not burn mycelium. Start low if unsure.
No. It does not reach a high enough pH to knock back contaminants. Use hydrated lime if you want a high starting pH to suppress mould.
As mycelium grows it tends to acidify the mix. The carbonate neutralises that slowly, holding the pH steady over the whole grow.
Sealed, cool and dry. Kept dry it keeps indefinitely. Keep it away from strong acids.
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Questions and answers
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Calcium carbonate is not classified as a hazardous chemical. Avoid breathing the dust and keep it away from strong acids. Sold for legal gourmet mushroom cultivation and substrate preparation.
We work hard to keep this information accurate and to cite reputable sources, but the occasional mistake can still slip through. Always check the product label and a current reference before relying on any figure for something important.