The Q&A Archives: Sterilizing Soil

Question: I understand the basics of soil sterilizing - 180 degrees for 30 minutes, using a sterile, soilless starting mix, etc. but IF I wish to use a mixture of garden soil/compost does it hurt to overheat it? I have used electricity to heat my mixture to nearly400 degrees and have used a microwave oven to heat it near that with no apparent problems but wonder if this may have destroyed beneficial organisms that would have escaped harm by more conventional methods.

Answer: You're absolutely correct - overheating soil will kill beneficial organisims. What you're producing is really far more sterile than necessary, and will be lacking the 'good guys' that help break down organic matter to make the nutrients available to plant roots. I'm also curious why you mention sterilizing a "sterile, soilless starting mix". <br><br>Compost contains many beneficial organisms and has been shown to have disease-suppressing qualities; some people use a compost "tea" as a pesticide spray. Are you using this mix to start seeds? You may have good luck starting the seeds in straight compost--if it is well "cooked" it should need no further sterilization. If you do decide to go ahead and sterilize yourgarden soil, I'd suggest heating just long enough to kill disease pathogens - 30 minutes at 180 degrees.

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