There are two kinds of media that are generally used for greenhouse potting mixes and seedling flats: soilless media, compost or a mixture of a small percentage of field soil and soilless media. Soilless media is generally purchased in bags or custom mixed on-site and contains no field soil. Soilless media purchased in bags does not have to be treated before use. Preventative applications of one or several synthetic fungicides or biological fungicides may be necessary with some crops that are prone to damping off.

Field soil, whether to be used by itself or as an amendment in a soilless medium, must be treated to eliminate soilborne plant pathogens, insects, and weed seeds. Once the soil has been treated, take care to avoid reinfestation.