Q: For his science fair project, my son is researching how soil with and without microorganisms affects the growth of plants. How long and at what oven temperature does soil need to “cook” to be sure the microorganisms are destroyed?

A: Here is one way to sterilize small amounts of soil: stir in enough water to make it quite moist but not soupy. Place in a shallow pan in a 180 degree oven for 30 minutes. The soil should be sterile after this length of cooking. Remember that while it cooks, the soil will smell dreadfully bad. Another option is to place small amounts of soil in a kitchen colander, having lined the interior with cheesecloth. Pour boiling water through the soil a couple of times. Yet a third option is to heat moist soil in a microwave until it “steams” for five minutes.