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Hydrated Lime (Calcium Hydroxide) 1kg

This is hydrated lime, also called calcium hydroxide or slaked lime. Mushroom growers use it for cold water lime pasteurisation. You stir it into the soak water for straw or similar bulk substrate, which pushes the pH up high enough to knock back competing moulds and bacteria without boiling or steaming.

It is a fine white powder and it is an irritant. Keep it off your skin and out of your eyes and lungs. Wear nitrile gloves, fitting goggles and a dust mask when you scoop and stir it.

£5.00

ChemicalCalcium hydroxide, Ca(OH)2

CAS number1305-62-0

Typical soak strengthAbout 1.75 to 2 g per litre of water

Target pHRoughly 11 to 13

Soak timeAbout 12 to 24 hours

Pack1kg resealable pouch

What it is

What hydrated lime does

Hydrated lime is calcium hydroxide, Ca(OH)2. When it dissolves in water it makes the water strongly alkaline. A saturated solution sits around pH 12.4 at room temperature. That high pH is what does the work. Most of the moulds and bacteria that would compete with your mushroom mycelium cannot survive in it, so a long soak in lime water leaves the substrate clean enough to inoculate.

You are not pasteurising with temperature, you are pasteurising with pH. That makes it cheap and simple for bulk substrates like straw, where heating a large volume would be a lot of effort. It also stays a cold method by nature. Calcium hydroxide is unusual in that it dissolves less in hot water than in cold, so warming the bath does not help.

Use hydrated lime, not garden lime or dolomite lime. Those are mostly calcium carbonate, do not raise the pH anywhere near far enough, and often carry magnesium that can hold back mycelium. This product is low in magnesium and is the right type for the job.

How to use it

Cold water lime pasteurisation

This method suits chopped straw and other lower nutrient bulk substrates such as cardboard, plain hardwood sawdust or pellets, and sugarcane bagasse. It pairs well with fast, tolerant mushrooms like oyster. It is not the right method for rich substrates or for slower, fussier species.

Mix the lime water

Use about 2 grams of lime per litre of water. Some growers use a little less, around 1.75 grams per litre. That gives a strongly alkaline bath, roughly pH 11 to 13. More lime than this does not pasteurise better, so there is no reason to overdose. Put your gloves, goggles and mask on, then stir the lime fully into the water in a clean container or drum.

Submerge the substrate

Push the straw or other substrate fully under the lime water. Weigh it down so it stays submerged and does not float. The whole batch needs contact with the high pH water.

Soak

Leave it to soak for roughly 12 to 24 hours. Sawdust and finer materials usually need less time than coarse straw.

Drain and inoculate

Lift the substrate out and let it drain to field capacity, so a hard squeeze gives a few drops but no stream. There is no need to rinse the lime off. Then mix in your spawn and pack your grow container. Discard the used lime water, do not reuse it batch after batch.

Lime pasteurisation lowers contamination, it does not sterilise. Work cleanly and inoculate generously with healthy spawn for the best result.

Handling and storage

Storage and safety

Calcium hydroxide is classed as an irritant under GB and EU rules, with the signal word Danger. It can irritate the skin and the respiratory system and it can cause serious eye damage. The risk is the high pH, not poisoning.

Wear nitrile gloves, fitting goggles and a dust mask, and avoid raising dust. If it gets in your eyes, rinse with plenty of water for several minutes and seek medical advice. If it gets on skin, brush off the dry powder then wash with plenty of water. Keep it away from children.

Chemical	Calcium hydroxide, Ca(OH)2
CAS number	1305-62-0
EC number	215-137-3
Appearance	White to off white fine powder, odourless
pH	About 12.4 in saturated solution at 20C
Classification	Skin Irrit. 2 (H315), Eye Dam. 1 (H318), STOT SE 3 (H335, inhalation). Signal word Danger
Pack	500g resealable pouch

Store the pouch closed in a dry place. Lime slowly reacts with moisture and with carbon dioxide in the air, which turns it back to calcium carbonate and weakens it over time, so press the air out and reseal between uses. Keep it away from acids.

Good to know

What this is not for

This is not a long term pH buffer for grain spawn or fruiting mixes. Hydrated lime swings pH hard and high, which is exactly what you want for a pasteurising soak but not what you want sitting in a substrate. For gentle buffering, growers use calcium carbonate or gypsum instead.

It is also not a sterilant for jars or agar work, and it is not a feed or supplement for the mushrooms. Its one job here is the cold water soak.

Sources

Hydrated Lime (Calcium Hydroxide) 1kg