How to Make Your Own Potting Soil
(Page 3 of 4)
December 2008/January 2009
By Barbara Pleasant
In summer, a solar cooker made from a cardboard box (See Solar Cooking for Free) will heat a 3-gallon black plastic pot filled with soil and enclosed in a plastic bag within a few hours (search for solar cooker and you’ll find three articles that describe easy-to-build models).
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The rest of the year, use your oven and a big heat-retaining Pyrex baking dish (mine came from a thrift store) to pasteurize compost or soil. When done correctly, you will smell an earthy fragrance as the process unfolds. (Reports of foul odors from oven pasteurization are wildly exaggerated.) Here’s the basic method.
- Preheat the oven to 200 degrees. Place 3 to 4 quarts of screened, mature compost (or screened rich garden soil) in a bucket or pail, and mix in enough water to make it lightly moist. Spread the moistened material in a large metal or glass pan, but don’t pack it down because steam needs to be able to circulate through the crevices. Cover tightly with aluminum foil. Poke a meat or oven thermometer into the center of the cover at a diagonal angle.
- Place the pan in the oven, and check the temperature at 15 minute intervals. Turn the oven off when the thermometer shows 150 degrees. Compost heats up quickly, while denser soil can take up to 30 minutes to hit 150 degrees. After the oven is turned off, the temperature should rise to 170 degrees; vent the oven if it goes to 180 degrees. Sharp odors indicate that things in the oven have gotten too hot. Allow the pan to sit in the warm oven for at least 30 minutes. To free up the oven for dinner, remove the pan from the oven and wrap it snugly in dry towels. This is an optional step, but it will slow cooling, which makes for more thorough pasteurization.
- When the pan is cool, dump the pasteurized compost or soil into a clean container with a lid. Let it rest until you need it. As long as you have your pan and foil handy, consider doing a second batch.
Germinating seeds, young seedlings and plants being rooted from stem cuttings benefit greatly from a pasteurized mix, as do long-lived houseplants and trees that are seldom repotted. But you need not heat-treat compost-soil mixtures used for potting up plants that are almost ready to grow outdoors. In fact, I think switching to an unpasteurized, bio-active soil mix during the last “potting up” before transplanting reduces transplant shock. When plants spend a week or two in a potting mix made from the same stuff they will encounter in the garden, there is no break in the growing action as they settle into their permanent home.
