Spent casing, Sphagnum moss, grass fibers, and green compost as peat alternatives in casing soils for Agaricus bisporus cultivation.

Agaricus bisporus, the button mushroom, is commonly grown in compost, which is overlaid with casing soil to induce fructification of the crop. Casing currently consists mainly of peat, but peat use is under pressure due to environmental concerns. We previously identified multiple materials that can be used to partially replace peat in casing soil without having a substantial negative effect on mushroom yield or disease suppressiveness. Here, we investigated what percentage of peat can be replaced by each of these materials (spent casing, Sphagnum moss and a processed grass component) as well as green compost, and if combinations of materials perform better than individual constituents. We found that peat can be replaced by up to 75% of alternative casing materials without a significant effect on yield or ginger blotch incidence, with the exception of an unsteamed grass component. Across all casing soils, moisture content at low suction pressure was the best predictor of yield. Physico-chemical properties did not predict blotch incidence, which was instead associated with sourcing of the material and could be mitigated by steaming. Several alternative materials contained increased levels of heavy metals and fungicides, but in all cases, these were still well below the legal limits.