Sterilizing potting mixture

Gastil Gastil-Buhl
Wed, 03 Apr 2013 13:12:50 PDT
For sterilizing dirt or potting mix, in the summer I use my solar sterilizer, which is just a wire bookshelf wrapped in a huge plastic bag. Works great. 2 hours near noon and the dirt can reach 135 F two inches deep. The air temperature was much higher.…

But for winter, or cloudy weather, I needed an alternative for a small batch of potting soil in February. 
The soil is already in the plastic pots so that I do not have to handle it afterwards.
I sterilized them as if they were jars of jelly, that is, cooked in a water bath. 
Here is a photo of the setup:…

Because I have seen graphs of how slowly heat penetrates soil dry and wet, I knew I should saturate the soil to enable the heat to penetrate. Even wet, it takes a long time to heat soil. 
I put two cheap kitchen meat thermometers into the soil and watched.
The water bath was boiling initially when first poured into the dish.
One hour in the oven at 170 F brought the soil to around 130 to 140 F.
So do not assume the temperature of the oven is the interior temperature of the soil.
And do not assume pouring boiling water onto soil heats it to near a boil.

There is some pumice in this mix, a much smaller fraction than for my SA bulbs. This is a peat-based mix. 

These pots were for a very specific purpose where I knew they would be enclosed, not open to fresh air. 
That is not the usual case; normally all my seed pots are outdoors with lots of fresh air.
It has been 5 weeks now and not a single sign of any mildew so I think it worked.

- Gastil

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