Optimal moisture levels in incubated compost depending on the degree of decomposition and the structure. The various varieties of mushrooms all have their own preference. Hybrids for example prefer a slightly moister nutrient medium and intermediate hybrids still perform well on drier compost. If the moisture content is too high, there will be too little gas exchange between the compost and the room air, causing the compost to become inactive. On the other hand, a too low moisture content will mean the mycelium extracts fewer nutrients from the compost.
Composters aim to achieve an optimal moisture content. However, practical experience shows that extra measures is often needed. The final moisture content of compost is largely determined during the fermentation process. When the pasteurisation tunnels are filled, additional water can be added to compost that is too dry. It is highly inadvisable to apply water to inoculated compost; this can have serious consequences for quality! Colonised compost can take watering either at the tunnel company or at the growers. The amount of water depends on the moisture content structure and how well grown the mycelium is through the compost. Never use a standard amount, but assess each batch separately before deciding whether to give water and how much.
Con Hermans, AdVisie ‘The mushroom cultivation advisors’