Vermicomposting Spent Mushroom Substrate Blocks
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Time to read 5 min
What is Spent Mushroom Substrate (SMS)?
SMS is the "waste" product of mushroom cultivation. Fungi colonize the substrate and turn many of the nutrients within it into mushrooms. The spent blocks are typically either a mix of oak sawdust and soybean hulls (commonly referred to as Master's Mix) or a manure/compost mix depending on what mushrooms are being grown. Your mushroom provider will be able to tell you what they use. You will want to know what kind of substrate it is before using it!
A block of Lion's Mane on a Worm Bin
Why Use Spent Mushroom Substrate?
1. Worms love spent mushroom blocks!
2. It is a dual purpose material in that it serves as a bedding and a food source for the worms.
3. SMS can create higher quality worm castings.
4. If you are near a mushroom farm you can get large amounts of it readily and may never need to look anywhere else for your worm's nutritional needs!
5. It is a consistent product. Mushroom growers follow pretty consistent procedures so each batch should be quite similar.*
6. There should be no insect pests or other hitchhikers with the food (unlike manure for example) so if you need your vermicomposting operation clean of any insect or other macroscopic life SMS can be a good pick.*
*Note that it is common practice for mushroom growers to compost the blocks, once it has started to compost, or if the blocks have been left sitting for a while, some variability is introduced and outdoor organisms might find their way into the compost.
How to Use Spent Mushroom Substrate in a Vermicomposting System
In my personal experience I have found there are some hurdles to clear to make sure it works well. Here are the main considerations with using Spent Mushroom Substrate in a vermicomposting system;
Mycelium vs. Worms
When you add the SMS to your worm bin it is not just some inert material, you are introducing a new life form to the bin. The mycelium is alive and does not want to be eaten by worms. If it is given the opportunity to "recover," the mycelium will bind up the substrate and solidify into a solid mass that red wigglers may have a hard time penetrating. The more you pulverize/blend up the SMS the easier the worms can eat it before the mycelium can recover.
These two videos show how I prepare my SMS to feed to worms, and what happens if the mycelium does recover. I used a paint blender for this, but other people use attachments with sharp blades to break the SMS even better.
Hydration
The fungi inside the substrate put a lot of water into the actual mushroom once it begins growing, making the substrate dry out. When added to your vermicomposting system, it will absorb water from its surroundings fairly aggressively so unless your bin needs to be dried out you should consider pre-moistening the substrate before adding it.
Inoculate the Spent Substrate
Though you may feel as though the substrate comes inoculated with microbial life already, its really just inoculated with one single species of fungi. It does not have the diverse microbiome found in a healthy worm bin that the worms rely on. Without that microbiome, foods you add may take longer to decompose, pH swings may be more dramatic, and organic nitrogen might remain in more harmful forms for longer. Mixing the SMS with healthy vermicompost or extract will circumvent this potential threat.
Thermal Composting
Though the blocks are considered "spent," there is still nutrition in them. Depending on how many "flushes" your mushroom grower got out of the block, there may be more or less nutrition, but either way there will still be some. If you add too much SMS at a time it can cause hot composting, so start small and pay attention to your temperatures!
Salt Levels
If your mushroom grower was using a manure or compost mix to grow the mushrooms, then their salt levels could be too high for worms. Test out a small amount and see how the worms react. To remove salt from the substrate you can wash it through water.
Will Spent Mushroom Substrate Improve the Microbiome in a Worm Bin?
Mushroom farms grow their fungi on sterile conditions. This fungi has likely never had to compete for a meal its entire life. When it gets placed into a worm bin, its going to get introduced to all sorts of competition, so it's not very likely that it will colonize through the worm bin or survive long term.
Kind of like if a person was grown without exposure to any germs, not developing any antibodies, and then was dropped off in ground zero for most every mild sickness known to man... aka a daycare.
If you know you know!
However, if the blocks have some mushrooms on them, or grow new mushrooms, the spores these mushrooms drop have a better chance at growing in your system as the spores, which are dropped by the millions, have different genetics and other factors that give them a bit of a better chance of establishing.
You will get better chances of the fungi establishing if your bin is full of fungal foods, most materials referred to as "woody" will serve that purpose.
In this picture, you can see oyster mushrooms fruiting from the SMS added in a worm bin. The spores this mushroom produces will fall into the bin below and potentially begin to grow.
Additionally, the spent blocks are fungal foods, and even if the fungus that was on the block doesn't colonize, it can help facilitate the growth of fungal species that were already in your bin.
If you want to get spores, keep the block intact and moist. You could even keep it out of the bin for a while, keeping it misted with water to encourage mushrooms to grow. If you can get mushrooms, let them finish growing over your bin so that any spores will drop into the vermicompost like in the picture below. A single mushroom can drop billions of spores.
Do Oyster Mushrooms Reduce Nematode Populations?
Some people fear that Oyster mushrooms (which are technically carnivorous!) may harm the nematode population in their worm bin, but based on my own use of oyster substrate on large scale I have not found that to be the case.
Will the SMS create more mushrooms?
It definitely can. If it recovers well enough it will likely try to fruit again.
Can I Grow Worms in Mushroom Susbtrate without Mushrooms having grown in it first?
Though you can, it is really not cost effective. Let the mushroom growers use it and get the value out of it first, then you can just use the "waste" portion of it.
Is it Harmful to Breathe Spores?
If your worm bin has a bunch of spores yes you should probably wear a mask, or try to not breathe in all the spores that may be present. It is not likely a risk to your health short term, but over time can become problematic.
Get Worms for Vermicomposting Your Spent Substrate
If you're a mushroom farmer and need a good way to process your SMS, consider getting some red wigglers to turn your waste into worm castings to sell alongside your mushrooms!
Scott Kent
Owner of Hungry Worms. Worm farmer, soil microbiologist, and educator since 2014. Read more about me here.