Physicochemical characterisation of casings in relation to mushroom (Agaricus bisporus) cropping performance

Peat-based casings have been used for button mushroom (Agaricus bisporus) cultivation for decades but there is environmental pressure to find sustainable alternatives. This work aimed to characterise the physicochemical properties of peat and peat-substituted casings and to determine their influence on mushroom cropping to enable alternatives to be identified. British milled peat and German wet-dug peat casings produced smaller mushrooms than Irish wet-dug peat casing although yield was unaffected. Substitution of milled or wet-dug peat casings with 25% v/v bark, green waste compost or spent mushroom casing, except Irish wet-dug peat casing with spent peat mushroom casing, caused reductions in mushroom yield and/or size. These poorer results of casings compared with Irish wet-dug peat casing corresponded with lower water retention volumes at matric potential (Ψ_m) -15 kPa but not after drainage from saturation or at -1 kPa. Air-filled porosity (17-22% v/v), compacted bulk density after drainage (670-800 g L^-1) and electrical conductivity (0.30-0.54 mS cm^-1) of casings were unrelated to their mushroom cropping performance. In-situ casing measurements with electronic tensiometers confirmed laboratory casing physical analysis: at the same casing Ψ_m, Irish wet-dug peat casing had a higher water content than German wet-dug peat casing and produced larger mushrooms for the same yield. Solid-state foam-based tensiometers were more robust than water-filled tensiometers but they did not detect the full decrease in casing Ψ_m during a flush of mushrooms. The results indicate that if sustainable materials are to replace wet-dug peat casing with the same mushroom yield and size quality performance, they should have equivalent water retention volumes at Ψ_m -15 kPa. Measurement of casing Ψ_m with electronic tensiometers to control mushroom crop irrigation should assist in this transition.