Potting media should be heat treated using steam, aerated steam, or other moist heat applications. If using a dry heat source, media should be moistened to near field capacity. Heat potting media until the temperature of the coolest portion of the treated soil has maintained a temperature of at least 140 F (60 C) for at least 30 minutes. This heat treatment regime is lethal to most plant pathogenic fungi and oomycetes such as Phytophthora but does not do not kill all soil microooganisms and will not result in “sterile” soil.
Excessive heating at high temperatures (generally above 180-212° F [82-100°C]) can increase the potential for phytotoxicity. Potting media, especially those containing readily-decomposed organic matter, can develop levels of ammonium, manganese, or other compounds that are phytotoxic to some plants. Phytotoxicity is usually temporary and is reduced over time or with leaching.
Solarization. Heat-treating nursery potting media via solarization requires sufficient direct sunlight and relatively warm ambient temperatures. Solarization is most practical for treating nursery potting media in situations where the coolest portions of the treated soil mass can sustain a minimum temperature of 113°F (45°C).
In a static solarization system, potting media should be piled no more than 6-10 inches (15-25 cm) deep to facilitate heating to the bottom of the pile. Media should be moistened to near field capacity before solarization. Solarization should continue until the coolest portion of the potting media has been heated to a temperature of 113°F (45°C) for at least 15 hours or a temperature of at least 122°F (50°C) for at least 2 hours.
It may take from two days to more than a week to attain these time/temperature thresholds depending on the weather and your solarization setup. If you want to use plastic sheeting, 6 mil clear thermal anti-condensate greenhouse film is preferable. This material has efficient thermal qualities and a long service life. In cooler areas, using a double layer of plastic film separate by an air gap reduces heat loss. Using a layer of insulation (e.g., a foam insulation panel) beneath the media will also reduce heat loss. Alternatively, soil can be heated more efficiently in an insulated solar oven. Heating will be more efficient and uniform if hot air can circulate beneath and around soil container(s) within the solar oven.
As noted in the BMPs, care must be taken to avoid contamination of potting media after heat treatment. Heat treated material should only be transferred into sanitized containers using sanitized tools by workers with clean gloves following phytosanitary working practices. Read more about heat treating soil at Using heat to eradicate soil-borne plant pathogens from nursery potting media ("soil sterilization")