Smelly Potting Soil in Oven Disputed

Q. Earlier in March you talked about fungus gnat problems in potting soils. You recommended baking the potting soil in the oven. But I’ve heard this stinks up the whole house! I might try it in a gas grill outside, but not inside.

Potting soils are lightweight soil mixes that usually contain peat moss and perlite as part of the soil component to minimize weight and maximize porosity.

A. I used to think the same thing about baking potting soil in the oven. The common response by gardeners is not to do it for that very reason; it stinks up everything! But we tested it recently and it worked just fine as long as the temperature of the oven was below 180F. I think the smell problem occurs when the temperature of the oven is set too high.

            I prefer “solarizing” potting soil in sealed, clear plastic bags for a couple of days to kill fungus gnats. The temperature of the soil does not have to be as high as it does to kill everything in the soil, a.k.a soil sterilization. If the temperature of the potting soil reaches 140 – 150 F for 30 minutes, through and through, it’s enough to kill insects.

            To sterilize potting soil, completely ridding it of all insects as well as harmful microorganisms, the temperature must reach 180° F for 30 minutes throughout the soil. Attaining this temperature by solarizing is doable if there is plenty of sunshine, the soil is moistened, placed in a clear plastic bag and left in the sun long enough.

            To be on the safe side, I solarize the soil with one side facing the sun the first day. Then I turn the bag over and solarize the other side on the second day. I check the temperature with a temperature probe through the plastic bag to make sure the temperature was high enough.

            I would not recommend putting potting soil in a gas grill and firing the temperature up high. Low temperatures in gas grills are harder to control. Excessively high temperatures will “burn up” the organics in the potting soil. That’s when it gets stinky.

            This cool, overcast spring presented some real problems. Cool temperatures and overcast skies did not allow solarization to reach the temperatures needed inside the bag. I enlisted the aid of a brave volunteer with an oven that had accurate temperature controls. She placed a half bag of potting soil on cookie sheets and into a domestic oven.

            The lowest thermostat setting for the oven was 160° F. The temperature inside the oven stayed around this temperature for 30 minutes and the soil allowed to cool inside of it. Voilà. No fungus gnats and no off smell.