What should be done with the growing medium after the crop is harvested? 

After several growing cycles, the substrate used for mushroom production becomes less productive. Rather than treating it as waste, it can be repurposed as "champost" (mushroom substrate compost), thereby contributing to a circular economy approach: "nothing is lost, everything is transformed."

When is a substrate considered "finished"?

A substrate is generally considered depleted when it exhibits the following signs:

• Significant drop in production: mushroom yields decrease markedly. Growing space is generally limited, so after 1, 2, or 3 harvests, the mushroom beds are usually discarded. This depends largely on the varieties of mushrooms being grown, growing conditions, and the space the grower has available. It is sometimes possible to continue with a fourth, fifth, or even sixth harvest, but it is generally not worth the effort.
• Contamination: the growth of mold or bacteria, which can then spread more easily to other bales and reduce productivity.
• Insect infestation: the presence of gnats or other pests. Nobody likes finding fly larvae in their mushrooms, do they?

These indicators suggest that it is time to replace the substrate and consider how to reuse it.

Why is used substrate so valuable?

Unlike other organic materials, the mycelial substrate has been partially broken down by fungal enzymes. Mycelium is highly effective at breaking down complex materials such as cellulose, lignin, and wood fibers, making nutrients more accessible to plants.

This transformation process improves:

• soil structure, by binding soil particles and increasing water retention;
• microbial biodiversity, by promoting bacterial and fungal life;
• plant protection, as certain fungal species produce natural antifungal compounds that inhibit soil-borne pathogens.

A recent study conducted by Khalil et al. (2024) showed that mushroom compost improves vegetable growth and significantly reduces the incidence of root diseases. 

How can we recycle used growing media?

Used substrate from the cultivation of wood-decaying mushrooms (shiitake, oyster mushrooms, lion’s mane, etc.) is a resource rich in organic matter that has been partially broken down by fungal enzymes. Far from being mere waste, it can be given a useful second life in many contexts, thereby contributing to a circular economy.

🌱 1. Organic soil amendment and fertilization

Substrates made from straw, sawdust, wheat bran, or wood chips have already been pre-digested by the mycelium, making them easier for soil microorganisms to break down.

• Improves soil structure: better water retention, aeration, and reduced compaction.
• Stimulates microbial activity: residual mycelium + organic matter = living soil.
• Usage: Use directly as mulch or work into the soil after 2–3 months of curing.

⚠️ Warning: Using fresh compost too soon can lead to nitrogen competition. It is recommended that you let it mature or partially compost it before use.

🌾 2. Vegetable farming and permaculture

In systems rich in organic matter, such as permaculture raised beds, the fungal substrate is a valuable ally:

• Use in lasagna, raised beds, or as mulch around vegetables (e.g., tomatoes, squash).
• A base for sowing or planting, after additional composting.
• Boosts soil resilience thanks to the microbial diversity it provides.

🍄 3. Re-inoculation for secondary culture

A mycelium substrate that is considered “exhausted” can still produce fruiting bodies, especially if it is rehydrated and placed in the right conditions:

• Oyster mushrooms: often capable of fruiting one or two more times after fruiting controlled environments.
• Cultivation of less demanding secondary species (Stropharia, Coprinellus, etc.).

This practice is common in low-cost systems or urban farms

🐄 4. Bedding or animal feed (experimental)

Some farmers use the substrate as dry bedding for poultry, goats, or cattle. It absorbs moisture and improves comfort.

Research (particularly in Asia) has also explored its use as enriched feed, but this requires strict controls (digestibility, toxicity, spore residues).

🔥 5. Incineration or anaerobic digestion

Rich in cellulose and lignin, the substrate can be used for energy production:

• Anaerobic digestion following pretreatment, with biogas production.
• Heating pellets or briquettes, after drying and compression (performance varies depending on moisture content).

🧪 6. Extraction of molecules or enzymes

Fungal residues still contain lignolytic enzymes, such as laccase and peroxidase. These molecules have potential for:

• Bioremediation (degradation of pollutants).
• The production of industrial enzymes (for the paper, textile, and food industries, etc.).

🦗 7. Cultivating decomposer insects (earthworms, soldier flies, beetles, etc.)

Used substrate from wood-decaying fungi provides an excellent base for raising decomposer insects. These insects convert organic matter into high-quality protein, fertilizer, or humus, while contributing to a natural cycle of biological recycling.

🐛Earthworms ( Eisenia fetida, Eisenia andrei)

• Ideal for partially composted or mature substrates.

• Their slow but thorough action produces stable, nutrient-rich vermicompost.

• Ideal for gardeners, communities, or small-scale composting systems.

🐛Black soldier fly larvae (Hermetia illucens)

• Highly effective at rapidly digesting moist substrates rich in decomposed plant matter or mycelium.

• Produce protein-rich biomass that can be used as animal feed (poultry, fish).

• The digestion residue (frass) is an excellent natural fertilizer.

• Recommended for agricultural settings or semi-industrial insect farms.

🐛 Saproxylic beetles (Tenebrio molitor, Dermestes spp.)

• They prefer dry, cellulose-rich substrates (sawdust, bran, straw).

• The mealworm (Tenebrio) is often raised for the production of edible protein (for human or animal consumption).

• Less susceptible to fungal infection than other species.

📊 Comparison by substrate type

Composting and the circular economy

As sustainable resource management becomes a priority, the reuse of spent mushroom substrate is fully in line with the principles of the circular economy. This approach aims to reduce waste, extend the life of materials, and create value at every stage of the production cycle.

src: https://www.mdpi.com/2313-4321/9/4/58

♻️ Giving a byproduct a second life

Rather than treating used substrate as bulky waste, it is repurposed as a useful resource. Transformed into organic soil amendment, mulch, or compost, it returns to the earth and continues to nourish living organisms.

This principle oforganic upcycling closes the loop between production, processing, and regeneration.

🌍 Reduce waste and emissions

• Avoidingthe landfilling orincineration of growing media helps reduce CO₂ and methane emissions.
• Reusing this organic material locally reduces transportation and disposal costs.
• Organic fertilization avoids the useof chemical fertilizers, which are energy-intensive to produce and contribute to soil pollution.

🌾 Support local and sustainable agriculture

By distributing the used growing medium to local market gardeners, farmers, and gardeners, we support a positive local economic cycle:

• Soils naturally accumulate organic matter.
• Agricultural yields can be improved without synthetic inputs.
• Collaboration among producers (mushroom growers, vegetable growers, composters, etc.) helps build the resilience of local supply chains.

🌱 An educational and civic opportunity

Mushroom compost is also an educational tool for raising awareness about the challenges of the ecological transition. It provides a concrete example of how agricultural "waste" can be transformed into fertile resources, and howenvironmental sustainability and productivity can go hand in hand.

Local governments, schools, or organizations can take the initiative to launchurban farming, permaculture, or community composting projects.

🔗 Toward a local recycling loop

Establishing partnerships between mushroom growers, farmers, composting facilities, livestock farmers, and local governments helps build a truly circular supply chain:

• The used substrate becomes a product in its own right.
• The resource remains within the region, creating local jobs and reducing imports of soil amendments.
• Every stakeholder benefits from an optimized and sustainable supply chain.

In short: incorporating used substrate into a circular economy transforms a challenge into an opportunity—for the soil, for people, and for the environment. It turns fungi into key players in the soil… and in driving change.

Where can I get used potting mix?

At La Mycosphère, we offer used substrates from our own crops.

These substrates are available to gardeners, farmers, or anyone else who wants to use them in an environmentally friendly way.

About the author

Quentin Gobert
Founder of La Mycosphère

He has been working for several years on mycelium cultivation, organic mushrooms, and functional extracts. Through his articles, he shares practical expertise gained from the production, experimentation, and processing of mushrooms.

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