Bake Soil to Pasteurize Before Germinating Seeds

January 31, 2011, by Ken Jorgustin


For best results to successfully germinate seeds, soil from the garden should not be used unless it is first pasteurized to kill weed seeds and fungi. It is also safe practice to pasteurize potting soil bought from the store prior to planting seeds, although theoretically you can purchase ‘sterile’ potting mixes for this purpose.

Why should I pasteurize the soil before planting or germinating seeds? To eliminate some of the organisms that could harm your seedlings, and to eliminate their spores. The process usually kills most diseases, weeds and insects in the soil.

The procedure is pretty simple if you follow just a few rules.

How to pasteurize Soil or Dirt prior to planting and germinating seeds

Add soil or dirt to a baking pan
Cover the pan with aluminum foil
Bake at 180 degrees F (82 C)
When internal soil temperature reaches 180 F, Bake for 30 more minutes

Be sure that the soil is moist (not dry, not soaking wet). If dry, add a bit of water and mix it up. If wet, then let it evaporate out first. The soil should clump together somewhat, when squeezed in your palm.

Use a Jelly thermometer or Fry thermometer. Poke a hole in the foil and insert thermometer until it is about in the middle of the soil depth. Be sure to use a thermometer that is oven safe.

You can also use a non-oven-safe thermometer by sliding out the oven tray and occasionally check the internal temperature until it is 180 degrees F. Once at 180, then start a 30 minute timer.

My experience is that baking soil with 9×13 baking dishes (about 2 inches deep) takes about 45 minutes to get internal soil temperature to 180 F (with oven set to 190 F). So the entire process takes about one hour and a half.

It is important not to let the temperature of the soil rise above 200 degrees F, since plant toxins are reportedly produced above that temperature (I’ve read this, but am not absolutely sure if the ‘toxin’ part is accurate). If you have digital controls or a digital gauge on your oven, then it’s easy – just set the oven to 180. Otherwise, you will need to keep an eye on it.

Be prepared for an unpleasant odor (My experience actually has not been that bad… it’s not like cooking brownies though.

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