Abstract: A process and apparatus are provided for the sterilization of horticultural material, which may be substrate, fertilizer, peat or especially mushroom casing. The sterilization may be complete or may be partial, removing only harmful organisms while leaving beneficial organisms unharmed. The horticultural material is passed through a tunnel and is exposed to radiofrequency electromagnetic waves for a period sufficient for sterilization. In a preferred process and apparatus the horticultural material is heated to a suitable temperature by the waves and is then maintained in a region not exposed to the waves for a period long enough to sterilize the material, before allowing it to cool to ambient temperature.