The mushroom growing substrate referred to as ‘compost’ is quite different from garden compost or soil. The ideal substrate for humus-inhabiting mushrooms is bedding cleared out of horse stables (horse manure mixed with straw), which has been left to rot for around three weeks. If this type of substrate is not readily available, then a mushroom compost can be prepared by combining fresh horse manure with straw; the decomposition process can take up to three weeks or more (see Resources: Growing Button Mushrooms). At this stage, the compost is partially decomposed by bacteria; the next step is pasteurisation – the compost is heated to a temperature that neutralises harmful species, without letting it get so hot that it kills beneficial microbes. When the compost has cooled to ambient temperature, gypsum (calcium sulphate) and the mushroom spawn are added.
In order to facilitate ‘spawn run’, (where the mycelium grows through the substrate), the mushroom compost must be covered by or enclosed in polythene. When the spawn run is completed, it is necessary to increase air circulation by removal of the polythene and a garden compost layer called ‘casing’ is applied on top of the mushroom compost. The casing provides a reservoir of moisture and it creates a low-nutrient zone (compared to the compost), signalling the mycelium to start forming the fruiting bodies. When the tiny mushroom buttons begin to form, light watering is required as the mushrooms enlarge.
Thanks to Clifford Davy of Forest Foragers.