Partial substitution of peat in mushroom casing with fine particle coal tailings

Casing prepared from different peat sources and sugar beet lime (SBL) was substituted by 25% (v/v) with different materials: composted bark fines, coconut fibre (coir), or fine particle, dewatered coal tailings (CT). Casing substituted with 25% (v/v) bark or coir had similar water retention near saturation (matric potential −0.7 kPa) to casings prepared from three different peat sources with SBL: brown surface and deep-dug peats and black deep-dug peat. However, casing partially substituted with bark or coir retained less water under applied suction (matric potential −15 kPa), and resulted in a lower mushroom yield than peat + SBL casing. By contrast, casing substituted with 25% (v/v) CT retained less water than non-amended peat + SBL near saturation, but the effect of partial substitution on water retention under applied suction was either small or not significant. Partial substitution with CT did not affect mushroom yield. These results indicate that water retention near saturation and under applied suction, i.e. desorption curve characteristics, are important in determining the suitability of a casing for mushroom cropping. Partial substitution of casings based on black peat and brown surface peat with CT resulted in an increase in mushroom dry matter content and the proportion of marketable mushrooms respectively. Mushroom cleanness was not affected by any of the treatments. The heavy metal contents of the mushrooms were not affected by the inclusion of CT in the casing. The partial substitution of peat and SBL in casing with CT will have both environmental and economic benefits.