John Miller
Spent mushroom compost is a by-product of mushroom growing. It is a blend of natural products such as wheat straw, horse manure, hay, corn cobs, poultry manure, and gypsum. Mushroom growers pasteurize the compost to kill any pests or pathogens. Spent mushroom compost is high in organic matter and nutrients, making it a desirable soil amendment or conditioner. It improves soil structure, increases pH levels, and enhances moisture retention. However, due to its high alkalinity and salt content, it may not be suitable for all plants, especially acid-loving ones. Spent mushroom compost is available commercially and can be purchased in bulk for use in gardens and landscaping.
| Characteristics | Values |
|---|---|
| Composition | Horse manure, poultry manure, wheat straw, hay, corn cobs, cottonseed hulls, cottonseed meal, cocoa bean hulls, gypsum, ground soybeans, seed meal supplements, peat moss, limestone, and other natural products |
| pH | 6.5-7 or 6.6 |
| Alkalinity | High |
| Nutrients | Nitrogen, phosphorus, potassium, calcium, magnesium, sulphur |
| Organic matter | High |
| Soluble salts | High |
| Pesticides | Low residue |
| Pasteurization | Steam |
| Texture | Loose and lightweight |
| Use cases | Soil amendment, soil conditioner, mulch, growing vegetables and ornamental plants |
| Benefits | Improved soil structure, weed reduction, moisture retention, nutrient provision, enhanced plant growth, increased soil pH |
| Drawbacks | High salt content, potential toxicity for ericaceous plants, unsuitable for shallow-rooted plants and potting mixes |
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What You'll Learn
Composition
Spent mushroom compost is a by-product of the mushroom growing process. It is made from the spent mushroom substrate used in commercial mushroom production. The substrate consists of organic materials, such as straw, hay, corn cobs, cocoa bean hulls, poultry and horse manure, and other agricultural by-products. These materials are combined and pasteurized to eliminate weed seeds and harmful pathogens, creating an ideal mushroom-growing medium.
The pasteurization process also helps to break down the organic materials, further enriching the substrate with nutrients and beneficial microorganisms that support mushroom growth. Once the mushrooms have been harvested, the remaining substrate is no longer suitable for further mushroom production. However, it has become a valuable resource for gardeners and landscapers looking to improve the quality of their soil.
The composition of spent mushroom compost will vary depending on the grower. Common ingredients include 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 also add ground soybeans or seed meal supplements during the production cycle. On top of the substrate, farmers apply a "casing" layer, which is typically a mixture of peat moss and ground limestone. This casing layer provides support for the growing mushrooms.
Spent mushroom compost is a good source of nutrients, including nitrogen, phosphorus, potassium, calcium, magnesium, trace minerals, and organic matter. It can help improve soil structure, increase nutrient availability, enhance water retention, and support overall plant growth. However, it is important to note that spent mushroom compost has a high salt content and alkalinity, which can be harmful to certain types of plants, such as acid-loving plants and shallow-rooted plants. Therefore, it should be mixed with two parts soil to one part mushroom compost when applied to garden beds.
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Uses
Spent mushroom compost is a versatile soil amendment that can be used for a variety of purposes in gardening and landscaping. Here are some of the common uses for spent mushroom compost:
Soil Amendment and Conditioner
Spent mushroom compost is often used as a soil amendment or conditioner to improve soil health and structure. Its high organic matter content enhances all types of soils, from heavy clay to light, free-draining sand. By mixing spent mushroom compost with soil, you can improve aeration, moisture retention, and drainage capabilities. It is also rich in nutrients like nitrogen, phosphorus, potassium, magnesium, calcium, and sulphur, which are essential for plant growth and health.
Vegetable Gardening
Spent mushroom compost can be beneficial for growing a wide range of vegetables. It is particularly useful for brassicas (vegetables in the cabbage family) as its higher pH helps defend against club root. Tomatoes also benefit from the higher calcium levels, which can prevent blossom end rot. Root vegetables, leafy greens, and fruit vegetables can thrive in the loosened soil provided by spent mushroom compost. However, it is important to mix it with soil, especially for acid-loving plants, as the high alkalinity of spent mushroom compost may create an unfavourable environment for certain plant types.
Mulch
Spent mushroom compost can be spread as a thick mulch over flower or vegetable beds. This no-dig approach allows the compost to be gradually drawn into the soil by soil organisms, improving the soil structure and nutrient availability over time. As a mulch, spent mushroom compost can also suppress weeds and improve moisture retention in the soil.
Landscaping
Spent mushroom compost is valued by landscapers for its ability to improve soil quality and enhance plant growth. Its light and loose texture make it easy to work with, and its high nutrient content can support the growth of various plants, fruits, and vegetables. Landscapers often use it in combination with soil to improve the overall health and appearance of landscapes.
It is important to note that spent mushroom compost has a high salt and chalk content, which can affect soil alkalinity and nutrient availability. Therefore, it should be used in moderation and mixed with soil to avoid any negative impacts on plant health.
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Benefits
Spent mushroom compost is a by-product of mushroom farming, which can be used as a fertilizer and organic soil amendment for many plants. It is a great option for gardeners who want to improve their soil quality and enhance plant growth.
Improves Soil Structure
Spent mushroom compost helps create a good soil structure, aiding water infiltration, soil aeration, and nutrient availability. It contains organic matter and high nitrogen content, which help to break up heavy soil and loosen compact clay particles.
Boosts Soil Drainage
The organic matter in spent mushroom compost creates pore spaces in the soil structure, allowing excess water to drain more easily. This improves the water-holding capacity of the soil, enhancing its moisture retention and drainage capabilities.
Reduces Weed Growth
When applied as a layer of mulch, spent mushroom compost suppresses weed growth by blocking sunlight from entering. Its high nitrogen content also promotes the production of flowers and fruit, as high nitrogen can stimulate leaf production.
Amends Clay-like Soils
Spent mushroom compost is a great soil amendment for denser soil. The straw content will easily break up clay-like soils and improve soil quality. It can be used as a planting mix or raised bed medium, as the straw provides great organic material.
Environmentally Friendly
By using spent mushroom compost, gardeners can repurpose waste materials that would otherwise be discarded, reducing their environmental footprint while improving their garden's health and productivity. It also supports a circular economy by reducing waste in the mushroom industry.
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Drawbacks
Spent mushroom compost (SMC) is a popular choice for gardeners and landscapers due to its many benefits, such as being lightweight, easy to work with, full of nutrients, and reasonably priced. However, there are several drawbacks to using SMC that should be considered.
One major concern is the presence of peat. SMC often contains between 10% and 15% peat, which is used as a casing during the mushroom growing process. While peat helps maintain a moist environment essential for mushroom growth, it is not environmentally friendly. Peat bogs are a critical carbon sink, and their destruction for peat extraction contributes to climate change. Additionally, peat extraction damages natural habitats and threatens the survival of rare plant species.
Another drawback of SMC is its potential impact on certain types of plants. SMC is alkaline due to the presence of chalk, and this can affect the pH level of the soil. While it is beneficial for plants that prefer alkaline soil, such as tomatoes, it can be detrimental to acid-loving plants like blueberries. The high salt content in SMC can also harm salt-sensitive plants, including azaleas, rhododendrons, camellias, and other members of the Ericaceae family.
Furthermore, SMC may not be suitable for all gardening purposes. While it is excellent for improving soil structure and water retention, it is not a suitable replacement for soil when growing plants other than mushrooms. It should be mixed with soil, with a ratio of 25% SMC to 75% soil being a good starting point for container use.
Additionally, there are potential issues with the quality and sourcing of SMC. Commercially available SMC may not always be truly "spent," as it is sold when it is no longer producing commercially viable yields of mushrooms. If the compost was stored outside, it may contain insects or other contaminants, and chemicals may have been used to treat the straw or sterilize the compost. Therefore, gardeners, especially those practicing organic gardening, must be cautious about the sourcing of SMC and may need to test samples for contamination.
Lastly, the nutrient content of SMC can vary. While some sources claim that SMC is high in nutrients and contains various minerals and trace elements, others state that it is low in nutrients, especially nitrogen, which is depleted by mushroom growth. This discrepancy may be due to the different methods of producing SMC.
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Buying advice
Spent mushroom compost is a by-product of the edible mushroom industry. Mushroom farmers cultivate mushrooms on a blend of composted straw and well-rotted animal manure. After two or three crops of mushrooms, the compost is sterilised at high temperatures to remove pests, diseases, and weeds. It is then sold as a soil conditioner and mulch for ornamental and vegetable gardens.
Spent mushroom compost is available from retailers in large bags, bulk bags, or loose loads. It is a useful soil conditioner and mulch but may contain peat, which is no longer recommended for horticultural use due to the damage to peatland ecosystems. Spent mushroom compost often contains chalk, which makes it alkaline in nature. This needs to be considered when choosing what plants and soil to use it with. It is best used on acid soils that are low in organic matter, where the liming effect of the chalk is an added benefit to soil fertility.
Before buying, it is important to note that spent mushroom compost is not a suitable replacement for soil. It is a slow-release fertilizer and is low in heavy metals but can be higher in soluble salts than other fertilizers. It has a pH level of around 6.6, which is in the neutral range. It is best to start with a mix of 25% spent mushroom compost to 75% soil for container use and adjust from there.
Spent mushroom compost is light, easy to work with, and improves soil structure and water retention. It is reasonably priced, with a one-tonne bulk bag costing around £80-100. However, it is important to be cautious about the sourcing of mushroom compost, as it may contain chemicals used to treat the straw and sterilize the compost. Samples can be analysed for contamination, and in the UK, the Department for Environment, Food and Rural Affairs can advise on this issue.
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Frequently asked questions
Spent mushroom compost is the soil-like material remaining after a crop of mushrooms. It is a by-product of the mushroom-growing process and cannot be used again for growing mushrooms.
The typical composition of spent mushroom compost includes wheat straw bedding with horse manure, hay, corn cobs, cottonseed hulls, and poultry manure.
Spent mushroom compost is used as a soil amendment or conditioner to improve soil fertility, structure, and water retention. It can be mixed with soil and used to grow a wide range of plants, fruits, and vegetables.
Spent mushroom compost is available from various retailers, including local garden centres, soil and manure suppliers, and mushroom farms. It is often sold in large bags, bulk bags, or as loose loads.
To use spent mushroom compost, mix two parts soil to one part mushroom compost. Apply it to the top of your soil to a thickness of around 5cm to 50mm and work it into the soil.
