Sawdust can be used as growing medium or as spawn for other substrates such as logs, outdoor mushroom beds and straw. It is common practice to grow a variety of species on “blocks” made from enriched sawdust or a mix of sawdust and woodchip. By adding a nitrogen rich amendment to the hydrated substrae before being sterilised the yield of fruit bodies can be increased significantly. A common additive used to enrich sawdust is wheat bran. Sterilisation of the substrate rather than pasteurisation is necessary after adding a nitorgen rich ammendment.
- 1kg hardwood sawdust (1 – 5mm shavings)
- 10 – 20g gypsum (calcium sulphate); 1 – 2% dry weight of substrate
- 10 – 20g Limestone flour (calcium carbonate); 1 – 2% dry weight of substrate (optional)
- 100g – 200g wheat bran ( 10 – 20 % of dry weight)
- large mixing container
- measuring jug
- polypropylene filter patch mycobags
- pressure cooker
- kilner screw bands or other suitable “spacers”
The first step in processing sawdust for use as a mushroom growing substrate is to filter out the finer dust particles and, when necessary, the larger shavings to ensure you have the right consistency of substrate. This prevents compaction within the substrate which could create anaerobic environments. This can be most easily achived by throug the soaking of the sawdust in water thereby separating the different sized shavings, allowing the finer particles to sink to the bottom of the container,
The most important factor for increasing yields is the moisture content of the substrate. However, excess moisture can create anaerobic environments, thereby suffocating the fungus and creating the possbility of contaminants. Fresh sawdust is hydrophobic meaning that it resists water absorption so the longer you can soak the sawdust the better. I now soak sawdust in water for at least for at least week, changing the water once or twice during that period. However, 24hr is sufficeint for basic hydration.
To avoid any excessive moisture drain the sawdust thorougly so it is not dripping wet. After some practice you will eventually get a feel for the maximum holding capacity o fthe material you are working with. Until then err on the side of caution and drain thoroughly. You are now ready to add the gypsum and wheat bran. The gypsum is a mineral ammendment adding calcium and sulphur for increased fungal growth and development. It will not alter the PH of the substrate, this is the role of Limestone flour (calcium carbonate). Adding in larger ammount it will rais ethe Ph of the substrate. It is added in smaller ammount to act a s aPh “buffer”.
Gypsum is best aded by sieving onto the hydrated sawdust as you are mixing it in.
Mix thoroughly and then fill the polypopylene mycobags to about two thirds full. Try to ensure that the top of the bag is free from the mixture this helps with sealing the bag and limits a potential vector of contamination. If it does get round the inside top of the bag wipe it off with a paper towel. Now fold the top of bags down so there’s no excess air in the bag, ensure that the bags don’t unfold during the sterilisation, you may have to use some tape to secure them. Now place into the pressure cooker making sure the bags aren’t touching the bottom or the sides of the pressure cooker. Bring it up to 15psi and and maintain this pressure for 2hrs. Leave to cool for 6 to 8 hours before opening in a sterile environment for inoculation. Do not move the pressure cooker until it has returned to 0psi.
- sieve out finer dust and large shavings
- soak for at least 24hrs, longer if possible
- thoroughly drain, optimise moisture content; avoid excess moisture
- thoroughly mix sawdust with gypsum and wheat bran
- put into polypropylene bags
- ensure the bags are not touching the bottom or sides of the pressure cooker and there is space between the bags; use plastic spacers
- sterilise at 15psi for 2hrs