Many organisms pathogenic to plants reside in the soil and can contaminate even "soilless" mixes used to propagate plants in nurseries. Using pathogen-free potting media is an essential starting point for producing nursery container stock that is free of soil-borne plant diseases. Diseased plant material that is outplanted can introduce pathogens from the container into the new planting site. If conditions are suitable, these pathogens can affect not only the transplanted stock but may spread beyond the planting sites into the environment where thay can affect other vegetation.Heat treament is often referred to as sterilization, but temperatures routinely used to heat soil will not result in completely sterile soil / potting media. The goal is to heat the potting mix to a point that kills the plant pathogens of concern.
Heat treatment of soil or potting mix involves reaching a target temperature for a specific minimum period of time. In general, lower temperatures require a longer heating time to achieve the same degree of kill of plant pathogens. Moist heat is also more effective than dry heat at any given temperature.
The entire mass of soil or potting mix needs to reach the desired temperature for the minimum treatment time. Begin timing when the coolest area of the treated mix reaches the target temperature. Uniformity of heating can be affected by a variety of factors, including variation in compaction, moisture, clods, bin shape, and method of heating. The coolest part of the heated mix may be at the center or near the edge, depending on these factors.
Fumigation is the best method which is used widely. Fumigants are applied before crops are planted; there are no fumigant residues in the crops grown in the fumigated soil.