Mushroom compost is technically known as Spent Mushroom Substrate, or SMS, which is the material remaining after a crop of mushrooms has been harvested. This organic byproduct is highly valued as a soil amendment because it significantly improves soil structure, particularly in heavy clay soils. The substrate is a rich blend of materials like straw, hay, gypsum, and various manures, making it a source of organic matter and slow-releasing nutrients. Gardeners seek out this material to enhance aeration, boost water retention, and provide a gentle nutrient supply for their plants. The logistics of acquisition depend entirely on the volume required, with purchasing options ranging from small, convenient bags to large, cost-effective bulk loads.
Purchasing Bagged Mushroom Compost
For small-scale projects, container gardening, or topping off existing garden beds, purchasing bagged mushroom compost offers maximum convenience and accessibility. These smaller volumes are readily available at a wide variety of retail locations, making them the simplest option for most home gardeners. National home improvement chains are the most common source, stocking the material seasonally, often beginning in early spring. Local, independent garden nurseries and farm supply stores also reliably carry bagged SMS. The bagged product is typically sold in volumes measured in cubic feet, such as 1.5 or 2.0 cubic feet bags, ideal for amending smaller areas without the need for specialized delivery. While the per-unit cost is higher than bulk, the ease of transport and storage makes it the preferred choice for minor amendments.
Sourcing Mushroom Compost in Bulk
When undertaking large landscaping projects, establishing new raised beds, or amending an entire vegetable garden, sourcing mushroom compost in bulk becomes the most cost-efficient method. This approach requires different suppliers and logistical planning compared to buying bags. The most direct source is often a commercial mushroom farm, as they are the producers of the SMS. Mushroom farms frequently sell the spent substrate directly to the public, usually by the cubic yard. A cubic yard of compost can cover approximately 100 square feet at a depth of three inches, suitable for substantial needs. Bulk SMS is also available through local landscape supply yards and independent soil or mulch companies, which typically offer delivery services. Many bulk suppliers offer discounted pricing for self-loading or self-pickup options for those with a pickup truck or trailer.
Essential Quality Checks Before Buying
Regardless of whether the SMS is purchased bagged or in bulk, a few quality checks are necessary to ensure the material is safe and beneficial for plant use.
Curing and Ammonia Content
Confirming the material is fully cured, often referred to as “weathered” SMS, is essential. Freshly spent substrate, which has not been aged for several months, can contain chemical characteristics that are detrimental to plants. A strong, pungent ammonia smell is a clear sign that the composting process is incomplete, making the material too “hot” due to unstable nitrogen compounds. Properly aged mushroom compost should instead have a rich, earthy aroma, similar to healthy soil. The supplier should confirm that the substrate has been aged, ideally for six months or longer, to stabilize the organic compounds and nutrients.
Salinity and pH
Another vital consideration is the potential for high residual salinity, or soluble salts, which are byproducts of the mushroom growing process. These salts can burn the roots of sensitive plants, such as azaleas or young seedlings, if the compost is applied too heavily. While the pH of SMS is typically near neutral, ranging from 6.6 to 7.75, it is advisable to inquire about the material’s soluble salt content. This is crucial before applying it to salt-sensitive plants.
Weed Contamination
While the initial steam pasteurization at the mushroom farm kills most weed seeds and pathogens, bulk piles sitting outdoors for an extended period should be visually inspected. Look for the presence of new, visible weed contamination before purchasing.