Spent Mushroom Substrate (2024)

Sometimes this material is called spent mushroom compost. This article briefly explains mushroom growing, so that the reader knows what is in the prepared substrate, and then describes the characteristics and possible uses of the material.

Mushroom Growing

Substrate prepared specifically for growing mushrooms is a blend of natural products. Common ingredients are wheat straw bedding containing horse manure, hay, corn cobs, cottonseed hulls, poultry manure, brewer's grain, cottonseed meal, cocoa bean hulls and gypsum. Growers may add ground soybeans or seed meal supplements later in the production cycle. On top of the substrate, farmers apply a "casing" layer, which is a mixture of peat moss and ground limestone. The casing material provides support for the growing mushrooms.

Spent mushroom substrate still has some nutrients available for the mushroom; however, it is more economical to replace the substrate and start a new crop. Before removing the spent substrate from the mushroom house, the grower "pasteurizes" it with steam to kill any pests or pathogens that may be present in the substrate and casing. This final pasteurization kills weed seeds, insects, and organisms that may cause mushroom diseases. Users may consider spent substrate clean of weed seeds and insects.

Mushroom growers sometimes apply a registered pesticide during the crop cycle. The local garden center sells most of the same pesticides a mushroom farmer uses. Even if pesticides have been applied, they are generally hard to find for two reasons. Organic matter in the substrate effectively binds pesticides. Also, these compounds decompose rapidly at the high temperatures used for pasteurizing the completed crop. It is safe to assume that the pesticide residue on spent substrate is low. Some farms are strictly "organic" and will not use chemical pesticides. These farms can be identified by contacting your Extension office.

Characteristics of Spent Mushroom Substrate

The typical composition of spent mushroom substrate fresh from a mushroom house will vary slightly. Since raw materials and other cultural practices change, each load of fresh spent substrate has a slightly different element and mineral analysis. Therefore the characteristics shown in Table 1 indicate a range of values for each component. Sometimes, fresh substrate is placed in fields for at least one winter season and then marketed as "weathered" mushroom soil. This aged material has slightly different characteristics because the microbial activity in the field will change the composition and texture. The salt content may change during the aging period. If you have any specific questions concerning characteristics of either fresh or aged spent substrate, please contact your local Extension agent.

Appropriate Uses of Spent Substrate

There are many appropriate uses for spent mushroom substrate. Spent mushroom substrate is excellent to spread on top of newly seeded lawns. The material provides cover against birds eating the seeds and will hold the water in the soil while the seeds germinate. Since some plants and garden vegetables are sensitive to high salt content in soils, avoid using fresh spent substrate around those plants. You may use spent substrate weathered for 6 months or longer in all gardens and with most plants. Obtaining spent substrate in the fall and winter, allowing it to weather, will make it ready to use in a garden the following spring. Spring and summer are the best time to use weathered material as a mulch.

As a soil amendment, spent substrate adds organic matter and structure to the soil. Spent substrate primarily improves soil structure and it does provide a few nutrients. Spent substrate is the choice ingredient by those companies making the potting mixtures sold in supermarkets or garden centers. These companies use spent substrate when they need a material to enhance the structure of a soil.

Table 1. Average Analysis of Spent Mushroom Substrate
ContentsUnitsAvg. FreshWeathered 16 mos.
Sodium, Na% Dry Wt.0.21 - 0.330.06
Potassium, K% Dry Wt.1.93 - 2.580.43
Magnesium, Mg% Dry Wt.0.45 - 0.820.88
Calcium, Ca% Dry Wt.3.63 - 5.156.27
Aluminum, Al% Dry Wt.0.17 -0.280.58
Iron, Fe% Dry Wt.0.18 - 0.340.58
Phosphorus, P% Dry Wt.0.45 - 0.690.84
Ammonia-N,NH4% Dry Wt.0.06 -0.240.00
Organic Nitrogen% Dry Wt.1.25 - 2.152.72
Total Nitrogen% Dry Wt.1.42 - 2.052.72
Solids% Dry Wt.33.07 - 40.2653.47
Volatile Solids% Dry Wt.52.49 - 72.4254.24
pHStandard Units5.8 - 7.77.1
N-P-K ratioPPM Dry Wt.1.8 - 0.6 - 2.22.7 - 0.8 - 0.47

*% x 10,000 = PPM
Prepared by David M. Beyer, Penn State

Spent Mushroom Substrate (2024)


What do you do with spent mushroom substrate? ›

Spent mushroom substrate is the soil-like material remaining after a crop of mushrooms. Spent substrate is high in organic matter making it desirable for use as a soil amendment or soil conditioner. Sometimes this material is called spent mushroom compost.

What are the benefits of spent mushroom substrate? ›

The addition of spent mushroom substrate in the nutrient poor soil leads to an improvement in soil texture, water holding capacity and nutrient status.

How to sterilize spent mushroom substrate? ›

3. Pasteurization. An effective pasteurization will eradicate harmful bacteria, nematodes, insects and fungi. In general a compost substrate temperature of 140° F for 4 hours is adequate for a complete pasteurization.

What is the composition of spent mushroom substrate? ›

Spent mushroom compost (SMC) is the substrate remaining after mushroom production, with approximately 5 kg of SMC produced for each kg of mushrooms. Mushroom compost is a mixture of 60 to 70% straw, 28 to 34% poultry litter, and 2 to 4.5% gypsum.

How do you know if mushroom substrate is bad? ›

Spend time getting to know what your mycelium is supposed to look like so you can better identify unhealthy or contaminated patches. While mycelium is typically white and filamentous, contamination generally takes the form of green, blue, gray, or black patches or discolorations in your substrate.

Can you use spent mushroom substrate as mulch? ›

Use spent mushroom substrate (SMS) as mulch to suppress weeds + retain moisture or as soil amendment to add organic matter + structure. Cellar Mushrooms SMS is free from pesticides, fungicides, chemicals. This block was the food source for our culinary mushrooms grown in Vancouver, WA.

What happens if you don't sterilize mushroom substrate? ›

You don't always need to sterilise or pasteurize the mushroom substrate. This simply minimises the risk of contamination-related growth issues or sickness. Some mushrooms cannot be cultivated on sterile media and must instead be grown outdoors or in nutrient-rich environments that do not allow for sterilisation.

What plants do not like mushroom compost? ›

Mushroom compost is also high in salt, which can be problematic for some plants such as blueberries, camellias, rhododendrons and azaleas. These soluble salts along with other nutrients in fresh mushroom compost are too concentrated to germinate seeds or plant young seedlings.

Should you pasteurize or sterilize mushroom substrate? ›

Studies have shown sterilized substrates often outperform pasteurized substrates, in terms of total yield, yield per flush, and mushroom quality.

Can chickens eat spent mushroom substrate? ›

The results showed that incorporating the spent substrate into the diet of broiler chickens positively influenced their performance, suggesting that SMS could be a valuable component for optimizing broiler growth and development.

What is the most nutritious mushroom substrate? ›

Manure-based compost is a good substrate for growing mushrooms because it is high in nutrients and provides a good environment for mushroom growth. However, it is important to use manure that has been properly composted to kill any harmful bacteria or mold.

Can you reuse mushroom substrate? ›

Yes, you can most definitely reuse mushroom grow kits! Mushroom grow kits are generally designed to give you more than one harvest depending on the species of mushroom inside your growing kit. There is enough water nutrition available in the substrate to give you multiple harvests over a period of 2-10 weeks.

How to dispose of mushroom substrate? ›

Composting is the main disposal and reuse method for spent mushroom substrate, as well as landfilling. Incineration is a main disposal method for agricultural and domestic waste but is problematic for this type of waste.

What can I do with leftover mycelium? ›

Spent substrate as an inoculant for a fresh substrate gives the reused mycelium a nutrient boost. When preparing a new substrate, crumble and mix the old one in with it. Incubate and fruit again.

How long can you keep mushroom substrate? ›

Store your 50/50 substrate in a cool and dry location. 50/50 substrate can keep for up to 4 months in these conditions. There will be no need to rehydrated or sterilize the products a second time. If you need to store the product longer it can be refrigerated for up to 6 months and frozen for up to 1 year.

What is the use of spent mushroom substrate as animal feed? ›

Due to the enzymatic conversion processes that occur during mushroom cultivation, ruminants can easily digest spent mushroom substrate. Consequently, spent mushroom straw may be an appropriate feed additive for sheep and Holstein steers.


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