Spent mushroom substrate (SMS) is an organic manure that can be used with advantage in agriculture. Under European Union (EU) (Good Agricultural Practice for Protection of Waters) Regulations, SMS cannot be applied to land over the winter months and must be stored on concrete surfaces, either covered or uncovered, to prevent nutrient-rich runoff seeping into groundwater. Spent mushroom substrate at four storage facilities, two covered and two uncovered, was analysed for physical and chemical characteristics after storage for up to 12 mo. Significant differences (P<0.05) were identified for all parameters across the four sites, except for pH, but there were no consistent differences that correlated with uncovered or covered storage conditions. The content of nitrogen (N) and manganese (Mn) was significantly lower in uncovered SMS, while the content of iron (Fe) and copper (Cu) was significantly higher. The chemical nitrogen-phospous-potassium (NPK) fertiliser equivalent value of SMS, when applied at a rate of 10 t/ha, was between €105 and €191 per hectare. Nitrogen-phospous-potassium concentrations per kg wet weight were all higher in SMS that was stored under cover, meaning higher chemical fertiliser savings are possible. The high pH of stored SMS (7.8–8.1) means it could be used with good effect on acid soils instead of ground limestone. The low bulk density of SMS (0.545–0.593 g/cm3) makes it an ideal amendment to soils to improve soil structure and quality. There is some variability in the nutrient content of SMS from different sources, so it is advisable to get the material analysed when including in nutrient management plans.