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What Is Mushroom Compost?

We generally know compost as the end product that results from composting varied organic waste under controlled moisture and temperatures. It might, therefore, seem odd when compost is defined by a single product as in the case of mushroom compost.

So, what is mushroom compost? Mushroom compost refers to two different products. The first is the mushroom substrate that is commercially manufactured to grow mushrooms. The second refers to the spent mushroom substrate that has been used to grow mushrooms and is recycled to create the mushroom compost (mushroom soil) that you buy in bags from a garden center.

If this explanation of mushroom compost sounds complex, this article will break it down for you. Read on to find the simplified details on the nature of mushroom compost, the uses, when mushroom compost should not be used, the DIY process of making mushroom compost, alternative forms of mushroom compost and the benefits of using mushroom compost.

Mushroom Compost: What Exactly Is It?

Mushroom growers produce mushrooms in a substrate specifically made for this purpose. The mushroom substrate is commonly created by mixing wheat straw, gypsum, and chicken or horse manure.

When mushroom compost is manufactured for commercial use in large quantities, wheat straw is first submerged in water before it is chipped into thin pieces. These are then mixed with the horse or chicken manure and gypsum and placed in large hot compost heaps. The piles are watered and turned daily for a fortnight to facilitate composting.

After the initial two weeks, the mushroom compost piles are converted into long rows and left to mature for an additional two weeks. By this time the mushroom compost has acquired a dark-brown color and an earthly smell.

What follows is the pasteurization and sterilization process. The heat used in this process kills any bacteria or wheat seeds that may have survived the composting period. The resulting ‘clean’ compost is now ready to receive the mushroom spores and produce mushroom for consumption.

While mushrooms will consume a lot of the nitrogen and other minerals in the substrate, the compost is still packed with nutrients by the time the crop is harvested. It is also true that fungal inoculation causes further breakdown to the compost which improves its quality and makes it more like soil.

It is the left-over compost that has been used to grow mushrooms which is recycled and packed in bags for sale as mushroom compost. This explains why it is described as Spent Mushroom Substrate (SMS) or Spent Mushroom Compost (SMC). SMS is our main focus for the rest of the article.

The Uses of Spent Mushroom Substrate

While spent mushroom substrate may be low on nutrients considering that it has already been used to grow mushrooms, the straw used to make the compost continues to break down. This makes it perfect for improving the quality of soil in your garden.

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In crop production, mushroom compost can be used to enrich the soil and improve its structure. The positive effects of using mushroom compost have been scientifically proven for the following.

1. Greenhouse farming

Studies have shown that mushroom compost has positive effects on the production of greenhouse crops including fruits, vegetables, and flowers.

A study in 2019 reported that mixing mushroom compost with other organic compost ameliorated the production of greenhouse vegetable plants (lettuce, tomato, cucumber) except when the mushroom substrate had not been pasteurized. In such cases, the growth of mushrooms alongside the vegetable crop hindered the augmentation of the crop yield, different from the improved production when a pasteurized option was used.

2. Structural and organic amendment of soil

The application of mushroom compost to physically degraded and poor-structured soil has recorded improved soil structure and physiochemical properties. Two properties of mushroom compost are especially key to the structural and organic amendment of soil: the high amount of straw and the high organic salt content. 

Owing to the high amount of straw, the mushroom substrate is great in improving the texture of clay soils. It is also perfect for mulching to promote water retention and control weeds. This means that it can be used to save irrigation water or control extreme water evaporation in the hot seasons.  As a mulch, mushroom compost is also optimal under shrub plants where it is difficult to remove weeds.

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While mushroom compost may not favor the growth of plants that do not like salty soil, it is an ideal organic amendment option for acidic soils. Acidic soils have low levels of organic matter and mushroom compost increases their fertility. Besides, mushroom compost is rich in a good number of minerals including calcium, potassium, iron, manganese, zinc, and copper among others.

3. Vegetable gardening

The soluble salts in mushroom compost restore alkalinity levels in acidic soils. Since most vegetables grow well in alkaline soils, mushroom compost is a perfect choice for your vegetable garden.

Using mushroom compost with cauliflower, Brussels sprouts, kale, cabbage and broccoli among other vegetables protects them from clubroot disease. SMS is rich in calcium and has a high capacity for conserving water. Both characteristics are excellent in protecting tomatoes from Blossom end rot.

4. Cultivation of other mushrooms

It is estimated that by the time of harvest, mushrooms use 70% of the nutrients in the mushroom substrate. The spent mushroom compost can be used to grow other mushrooms by adding a suitable amount of fresh mushroom substrate. Besides, other forms of manure can be added to the spent mushroom compost to re-ferment it and replenish it with adequate nutrients for another crop.

5. Alternative pesticide

A liquid extract of spent mushroom substrate is used as an alternative pesticide. The compost tea is made by mixing one part of SMS with four parts of water and using it on the soil to prevent the growth of pests. Also, this compost tea contains healthy microorganisms (fungi, bacteria, nematodes, and protozoa).

Whichever context mushroom compost is used in, it is important to note that SMS cannot be used as a substitute for soil as is the case with other organic-waste compost. Mushroom compost is better mixed with soil before being used. This should be done at the ratio of 3:1 or 75% soil and 25% mushroom compost.

Although mushroom compost is an excellent choice for the above uses and with most plants, it may not be so for some plants and should be avoided altogether.

When Should Spent Mushroom Substrate be Avoided?

Spent mushroom substrate has a high content of soluble salts which may affect the growth of plants that don’t like high levels of salt. These include camellias, azaleas, magnolia, rhododendrons, heathers, and blueberries. You will notice lime-induced chlorosis (yellowing of the foliage) and slowed growth or reduced fruits and flower production if your plants are absorbing too much of soluble salts from mushroom compost.

The level of soluble salts in SMS is also unsuitable for germinating seed and young seedlings. It is advisable not to use it at all in these cases.

Once mushrooms are harvested, the entire growing space and the mushroom substrate are heat-treated to avoid the spread of pathogens. This sterilization process also kills valuable microorganisms which means that exclusive use of SMS can predispose your soil and plants to pests and disease. A remedial approach is mixing mushroom compost with other organic compost before using it.

The high amount of straw in mushroom compost that helps in water retention can become a problem if mushroom compost is used for plants like succulents that do not thrive with a lot of water. Mushroom compost should be avoided with such plants as well as with potted plants that have slow or no drainage.

Can You Make Mushroom Compost at Home?

Yes. While mushroom compost is widely obtained from industrial mushroom growing due to the largescale production, you can choose to make your mushroom compost at home. The same compounds as those used in large-scale production are applicable: wheat straw, chalk, and horse or chicken manure.

Follow these steps for the DIY mushroom compost production.

  1. Shred the wheat straw into thin pieces. You can use a woodcutter or any appropriate tool that would produce the desired pieces.
  2. Dunk the straw shreds in water. This serves both to clean the straw of any bad microorganisms and to moisturize it.
  3. Remove the straw from the water and rinse it with clean water.
  4. Submerge the straw in a container of boiling water to pasteurize it.
  5. Reduce the heat to around 800 F and let the straw sit in the water for an hour or two.
  6. Use a forked spade to remove the straw and drain it on a clean surface. Make sure you do not re-contaminate it.
  7. Mix the straw with chicken or horse manure and gypsum. The ratio should be 1: 35: 1 (a bale of straw with 35kg of manure and 1kg of gypsum.
  8. Pile the mixture in a compost heap and leave it for composting for a couple of weeks. Turning should be done every 2 or 3 days to air the compost and ensure each part of the compost gets to the high temperatures at the core of the heap. You should target temperatures of around 1600 F to ensure that any pathogens and wheat seeds are dead. Watering should also be done regularly.
  9. After the few weeks of hot-composting, transfer the compost to a secondary pile for curing. This is the stage where the compost is left to age for another couple of weeks. No turning should be done during this time, but the compost should be kept moist to facilitate the breakdown process.
  10. You will know that your mushroom compost is mature when there is no more heating and the compost has acquired a dark brown color.
  11. Depending on the purpose of your mushroom compost, you can introduce the mushroom spores to the compost when it is mature or mix the compost with the soil in your garden. Remember that the ratio of mixing is 3 quantities of soil for a quantity of mushroom compost.

When DIY mushroom compost is meant for mushroom growing, some people will prefer doing the pasteurization and sterilization when the compost is mature. In this case, the compost is placed in small bales and submerged in boiling water for 1-2 hours.

Alternative Mushroom Compost

For varied reasons, wheat straw may be replaced by other materials especially when mushroom compost is made for home use. Wheat straw may not be readily available or the alternative material may be cheaper and easier to use. These alternative substrates make alternative mushroom compost. They include:

  • Coffee grounds (from home coffee usage or chain coffee shops)
  • Hardwood dust
  • Coconut coir
  • Soybean meal
  • Rice straw
  • Cottonseed hulls,
  • Peat moss

Whichever material is used, the horse or chicken manure and gypsum are added to the main material. Other vegetarian manure may be used as an option. In all cases, the pasteurization process must be done.

The Benefits of Using Mushroom Compost

We’ve already alluded to the benefits of spent mushroom compost by discussing where it is used and when it is avoided.

To be more specific, the following are some of the benefits of using spent mushroom compost in your garden or farm.

  • Spent mushroom compost continues to breakdown even after being used for growing mushroom because of the high quantity of straw. This makes it a perfect choice for enriching your garden soil with nutrients.
  • Because of the increased mushroom production, spent mushroom compost is produced in large quantities, making it less costly to purchase.
  • The high capacity of mushroom compost to retain water makes it appropriate for the transmission of water through the soil, helping it stay moist. This makes it good as mulch for plants that consume high amounts of water.
  • Because of its capacity to maintain moisture in the soil, mushroom compost is an economically viable product; it helps gardeners who need to save on water and can be a good option for farmers using irrigation.
  • Mushroom compost is environmentally friendly considering that it is naturally created for growing mushroom and then recycled for use in moistening and enriching the soil without additional synthetic chemicals.
  • Spent mushroom compost is good for improving the structure and drainage of poorly aerated soils such as clay. This renders the soil more suitable for farming and gardening.
  • Unlike synthetic fertilizers that are nutrient-dense and readily support the growth of weeds alongside the crop, mushroom compost releases nutrients to the soil gradually through the progressive breakdown of the substrate. This supports a natural growth process for both the plants and weeds. The farmer or gardener will not be overwhelmed by the need to control the spread of weeds.
  • Even though the sterilization process kills the good microorganism in mushroom compost, it also preempts the spread of disease-bearing pathogens from the mushroom growing industry to the soil where the spent mushroom substrate is used.

Irrespective of these benefits, organic farmers should be keen when purchasing spent mushroom substrate. Often, mushroom growers use synthetic pesticides such as organochlorides to control fungus gnat. This means that the spent mushroom substrate may have traces of these pesticides. Also, synthetic chemicals may be used to treat straw and sterilize mushroom compost.

All these may work against the standards of organic farming, implying that organic farmers should be careful about the source of their compost. To be on the safe side, have your spent mushroom compost analyzed before applying it to the soil.

Concluding Summary: FAQ

What is mushroom compost?

In the industrial context of mushroom production, mushroom compost refers to the substrate for growing mushrooms that is made from wheat straw, gypsum, and horse or chicken manure.

In the crop growing context, mushroom compost is the spent mushroom compost that is recycled from the compost used to grow mushroom. You will readily find this substrate for sale in bags at garden centers.

Can mushroom compost be used to grow all kinds of crops?

Generally yes. Mushroom compost is rich in many nutrients including calcium, copper, iron, zinc, and manganese among others. But mushroom compost is also has a high content of soluble salts and should be avoided with plants that don’t like high levels of salt. Using SMS with these plants will cause foliage discoloration and stunted plant growth.

Can I make mushroom compost at home?

Yes. You can use the same material used in the industrial production of mushroom compost to make it at home. These materials include wheat straw, gypsum, and horse or chicken manure. Where wheat straw is not available, alternatives such as coffee grounds, hardwood dust, coconut coir, soybean meal, rice straw, cottonseed hulls, and peat moss can be used.

Are there benefits in using spent mushroom compost?

There are several benefits associated with the use of spent mushroom compost. These benefits include its capacity to enrich the soil and maintain moisture and its availability for relatively low prices. Besides, mushroom compost is made naturally which works in favor of the environment.

ReadyToDIY is the owner of this article. This post was published on 2020-03-24.

Organic food farmers opting for spent mushroom compost should ensure that no synthetic pesticides have been used in the initial preparation of the mushroom substrate. This will save them any problems with the food control agencies when it is time to sell their crop.