Do you have a local mushroom farmer or gardening store selling (or giving away) spent mushroom compost? Are you wondering will mushrooms grow from spent mushroom compost? In this article we’ll answer that question with videos and how to instructions.
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What Is Mushroom Compost?
Mushroom compost is composed of broken down organic matter. It’s used most commonly to grow button mushrooms and portobello mushrooms. It’s made of a mixture of hay, straw, and other dry organic matter like corn cobs combined with manure, usually from cows, horses, or chickens.
Depending on the mushroom grower it can also include cottonseed meal (the leftover materials from squeezing cottonseed oil), crushed grapes from pressing grape juice, potash, lime, and even urea.
Spent mushroom compost refers to used compost that has grown one batches of mushrooms. One batch of mushrooms will usually fruit two to three times. After that to prevent contamination from other mushrooms, the spent compost is disposed of.
If there’s a mushroom grower nearby you should have access to literally tons of spent mushroom compost. The question is, can you reuse it for your own mushrooms?
To answer that we have to dive deeper into how mushrooms compost is made.
How Is Mushroom Compost Made and Used to Grow Mushrooms?
Mushroom compost is made like any other hot compost. A large pile of dried plant materials (browns) and higher nitrogen materials like fresh manure or green plants (greens) are mixed in a ratio that varies between 2 parts browns to 1 part green all the way to a 1:1 ratio of browns to greens.
The mixture sits for 30 days and bacterial action heats up the pile. This kills any weed seeds, pathogenic bacteria or viruses from the manure, and any harmful fungus in the mix. Ideal thermophilic temperatures can reach up to 160 degrees Fahrenheit.
Once the hot compost is finished it’s moved to growing beds and the top few inches are pasteurized at 140 degrees Fahrenheit to kill any remaining bacteria or fungus lying on the surface.
The beds are then innoculated with button mushroom spawn. This is a form of baby mycelium that’s been grown from spores onto specialized media that prevents contamination from other mushrooms.
By first inoculating spores on media to make spawn then moving the spawn to the pasteurized beds you limit the amount of contamination and you increase time to fruiting. It gives your mushroom beds a fighting chance compared to all the other floating spores and bacteria that’d love to colonize the fresh mushroom compost bed.
Watch the video below to see an example of large scale mushroom compost production using giant rows and machinery.
What Is Spent Mushroom Compost Used For?
Once the mushrooms are grown on fresh compost, the question becomes, what do you do with the spent compost? Can it be reused?
Spent mushroom compost is still very high in organic matter. It makes an excellent soil addition to gardens or farms.
Most mushroom farmers don’t reuse the spent compost to make more mushrooms. It’s more economical to create fresh mushroom compost and start over. The economics factors in the cost of the new compost plus the loss of revenue and time from the higher rates of contamination on spent mushroom compost beds.
Will Mushrooms Grow From Spent Mushroom Compost?
Yes, it’s possible but it’s better to use it to make new mushroom compost. To do that do the following:
- Use the spent compost as the browns
- Add in new manure (greens)
- Shoot for a ratio of 1 to 2 part browns to 1 part greens
- Build a big enough pile to get it up to 160 degrees Fahrenheit – You’ll want a bin that’s at least 3 feet on each side as a cube. 4 pallets makes a quick perfect sized bin.
- Wait 30 days. You can speed up the process by adding a perforated PVC pipe under your bin and using an air blower to push forced air through the bin.
- Lay the new compost into bins that are 4 to 6 inches deep. Pasteurize the top using steam or cover the bins with saran wrap and let it sit in the hot sun for 4 hours. Doing so will kill off any contaminating spores or diseases that could hurt your mushroom crop.
- Inoculate the bins with button mushroom spawn or portobello spawn.
Is Salt In Mushroom Compost Harmful To Your Garden?
No, the salt levels in mushrooms are not high enough to hurt your garden. A study of 30 mushroom growing facilities in Pennsylvania. The study found that mushroom compost didn’t contain enough salt to harm soils or gardens (1).