Sawdust can be used as growing medium either directly as a substrate, or, as spawn for other substrates such as logs, outdoor mushroom beds and straw. Depending on the species of fungus you intend to cultivate considerations must be made with regard to the type wood from which the sawdust was sourced, the size of sawdust particles, the age and how the sawdust has been stored.
It is common practice to grow a variety of species on ” fruiting blocks” made from sterilised sawdust or a mix of sawdust and woodchip that is then “enriched” with a source of nitrogen. By adding a nitrogen rich amendment to the hydrated substrate 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 nitrogen rich amendment.
The following describes how to make an enriched sawdust block using 1kg of hardwood sawdust.
- 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 mycobag
- 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. Removing the finer dust particles prevents compaction within the substrate which could create anaerobic environments. The shavings are too larger there will be too much space in the substrate increasing colonisation time and compromising the structure of the block. When processing smaller amounts, a sieve can be used (but use a respirator!), if processing larger amounts it is easier to soak the sawdust in water thereby separating the different sized shavings, allowing the finer particles to sink to the bottom of the container. If you are using a sieve, be sure to wear a respirator dust mask.
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 possibility 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 sufficient for basic hydration.
To avoid any excessive moisture, drain the sawdust thoroughly so it is not dripping wet. After some practice you will eventually get a feel for the maximum holding capacity of the 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 amendment 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 i.e., calcium carbonate (CaCO3), which, in larger amounts will raise the Ph of the substrate. It is added in smaller amounts to act as a Ph “buffer”.
Gypsum is best added by sieving onto the hydrated sawdust as you are mixing it in.
Mix thoroughly and then fill the polypropylene filterbags 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 bag down so there’s no excess air in the bag, ensure that the bag doesn’t unfold during the sterilisation, you may have to use some tape to secure it. Now place into the pressure cooker making sure the bags aren’t touching the bottom or the sides of the pressure cooker if it doesn’t have an inner container. Bring it up to 15psi and maintain this pressure for 2hrs. Leave to cool for 6 to 8 hours before opening in a sterile environment for inoculation. You can inoculate at ratio of 10-20% the dry weight of substrate, so, for this example of processing 1kg of sawdust you would need 100-200g of grain spawn. 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, ideally, there is space between the bags
- sterilise at 15psi for 2hrs