Mushroom compost is an organic byproduct of the commercial mushroom-growing industry, consisting of the material left over after the fungi have been harvested. This material, technically known as Spent Mushroom Substrate (SMS), is highly valued by gardeners as a soil amendment to improve soil structure and gradually release nutrients. Its composition typically includes a blend of agricultural wastes like straw, hay, animal manure, and other organic supplements.
The Direct Answer: pH Range of Spent Mushroom Substrate
Mushroom compost is generally not acidic; it is typically neutral to slightly alkaline. The pH of this spent substrate usually falls within a range of 6.5 to 7.5, which is considered neutral to mildly alkaline for soil standards. While the average pH is often around 6.6, it can occasionally reach pH 8 or higher. This range means that mushroom compost can slightly raise the pH of acidic garden soil when incorporated.
Ingredients That Determine the pH
The alkaline nature of mushroom compost results directly from the materials added during cultivation. Commercial growers use a base of organic materials, such as straw, hay, and well-rotted animal manures, like horse or poultry litter. These ingredients are supplemented with liming agents to create an optimal environment for the fungi. Materials like ground limestone or chalk (calcium carbonate) are intentionally added to buffer the substrate. This addition maintains the substrate’s pH at a level that suppresses competing molds and bacteria, allowing the mushroom mycelium to thrive. Gypsum (calcium sulfate) is also a common additive that helps condition the substrate’s structure and contributes to the final mineral content.
Practical Application: Using Mushroom Compost in the Garden
The slightly alkaline nature of the spent mushroom substrate dictates its best uses in the garden. Plants that prefer a neutral to slightly alkaline environment, such as most vegetables, brassicas, and many ornamental flowers, benefit greatly. The higher pH can help suppress club root disease in cabbage-family plants, and the calcium content helps prevent blossom end rot in tomatoes. It is also effective as a soil conditioner on lawns and improves the structure of heavy clay soils by increasing aeration and water retention.
Gardeners must exercise caution when using mushroom compost around plants that require acidic soil conditions. Ericaceous plants, including blueberries, rhododendrons, and azaleas, should not receive large quantities of this compost. The addition of an alkaline amendment can interfere with the ability of these acid-loving plants to absorb necessary micronutrients from the soil.
Another consideration is the soluble salt content, which is often elevated in fresh mushroom compost due to the nutrients present in the original manure and supplements. High concentrations of these salts can harm tender seedlings or cause leaf burn on established plants. To mitigate this risk, use older, well-aged compost or apply fresh material sparingly and mix it thoroughly into the soil. Leaching the compost with heavy watering or rainfall before planting will help dilute the salt concentration.