How Do I Kill Algae in the Potting Soil?

The dark green color that develops in potting soil consists of microscopic growths called algae, which eventually cover the top of the soil. Algae form when spores contaminate the soil before the bagging process. When the potting soil absorbs water from the air, such as when you open the bag, the spores begin to grow. Heating the potting soil to high temperatures kills the existing growths and the spores.

Roasting bag
Water
Twist tie
Baking sheet
Scissors
Oven
Meat thermometer
Air-tight container

Instructions

- 1
  Open an oven roasting bag and pour in approximately 1 to 2 pounds of the infected potting soil. Add one-quarter to one-half cup of water to the bag. The soil may appear damp, but it should not be soggy or muddy.
- 2
  Gather the open end of the bag together tightly and wrap a twist tie around it to hold it closed.
- 3
  Set the bag on top of a baking sheet or other flat pan. Use your hands to spread the bag out as much as possible. Then cut a small, one-quarter-inch slit in the top of the bag using scissors.
- 4
  Place the pan inside the oven and set the oven to 200 degrees Fahrenheit. Allow the soil to cook for approximately 30 minutes.
- 5
  Remove the pan from the oven. Insert a meat thermometer into the slit in the top of the bag. When the thermometer reads between 160 and 170 F, the soil is sterile. If the soil is not at this temperature, put the pan with the bag of soil back into the oven and check the temperature periodically.
- 6
  Open the top of the bag carefully and let the soil sit until it cools to room temperature.
- 7
  Pour the sterilized soil into a large plastic tub or other container with a tight-fitting lid until you are ready to use it.

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