Preparation of mushroom (Agaricus bisporus) composts in controlled environments: Factors influencing compost bulk density and productivity

Mushroom composts were prepared under controlled temperature and aeration in bulk tunnels following a 4-day wetting and blending process. The temperature (51–68°C), O2 concentration (<1–9% v/v) and duration (6–42 h) of an initial pasteurization phase were varied before all the substrates were given a standard aerated conditioning period at 47–49°C. An aerated (7–9% O2 v/v) pasteurization regime of 58–61°C resulted in composts with the highest mushroom yield and, with the exception of ammonia, low levels of odour. No benefit was found from extending the pasteurization period beyond 6 h. A low O2 concentration during pasteurization resulted in reduced moisture loss and increased compost bulk density which influenced the filling weight and mushroom yield per unit of cropping area. Substrate decomposition (based on ash content) and mushroom yield were reduced and strong odours were produced during the pasteurization. Increasing the compost nitrogen content from 1•6 to 3•1% of the compost dry matter (by increasing the quantity of deep litter poultry manure added to straw) resulted in a greater yield of mushrooms. Further increases in nitrogen content resulted in incomplete clearance of ammonia from the substrate and subsequently low or no mushroom yield. Recycling a proportion of processed substrate (18% of dry matter) in the compost ingredients reduced the processing time required to clear ammonia and increased compost bulk density with no significant effect on yield.