Mushroom manure, more accurately termed Spent Mushroom Substrate (SMS), is a valuable organic byproduct of commercial mushroom cultivation. This material is the growing medium left over after the edible fungi have been harvested. SMS has gained popularity among gardeners and agricultural producers as an excellent, resource-rich soil amendment that significantly improves soil health.
How Spent Mushroom Substrate is Produced
Spent Mushroom Substrate is the residual material from growing mushrooms, most commonly Agaricus bisporus (the white button mushroom). Production begins with blending and composting raw ingredients to create the initial growing medium. This substrate typically includes materials like wheat straw, hay, corn cobs, and supplements such as gypsum, poultry litter, or horse manure.
The blended materials undergo an initial composting phase, often lasting two to three weeks, where microbial activity heats the pile to high temperatures. This thermophilic process breaks down complex organic matter, preparing the substrate for the mushroom mycelium. Following the harvest, the entire growing bed is subjected to steam pasteurization, heating the substrate to approximately 140 degrees Fahrenheit. This final step ensures the SMS product is free of weed seeds, insect pests, and pathogens, making it a clean garden additive.
Nutritional Value and Soil Improvement
The primary benefit of Spent Mushroom Substrate is its high organic matter content, functioning as a slow-releasing soil conditioner. This organic material improves soil structure by promoting better aggregation, which enhances both water retention and aeration. The porous substrate helps to loosen dense clay soils and provides moisture retention in sandy soils.
SMS contains a stable, relatively low nutrient profile that is released gradually as the material decomposes. Analyses typically show low concentrations of nitrogen (N), phosphorus (P), and potassium (K). The material is a good source of micronutrients, including calcium, often due to the initial addition of gypsum (calcium sulfate). SMS sustains soil fertility and physical properties over many growing seasons.
Best Practices for Garden Use
Gardeners should treat fresh Spent Mushroom Substrate with caution due to two characteristics: a naturally high pH and a high soluble salt content. The inclusion of limestone and gypsum gives SMS an alkaline nature, often with a pH range between 7.28 and 7.75, which is unsuitable for acid-loving plants. High levels of soluble salts, byproducts of the composting process, can also damage young plants and seedlings if applied directly.
The most effective way to mitigate these issues is to age or cure the substrate before application. Weathering the SMS in an outdoor heap for six months or longer allows rainfall and microbial activity to leach out excess salts and stabilize the material. When using SMS, it should be blended thoroughly with existing garden soil or other composts, rather than used as a straight planting medium.
A common recommendation is to mix SMS into the top few inches of soil at a ratio of no more than 25% SMS to 75% native soil. This practice ensures the benefits of organic matter and aeration are gained without risking harm to salt-sensitive plants like azaleas, rhododendrons, or young vegetable starts.