I tend to be frugal when gardening and tending houseplants. I don’t buy pricey garden or potting soils, but I learned the hard way that you shouldn’t bring outside soil into your home. It is loaded with bugs, microbes and weed seeds. I did some research into the processes that commercially sold soil goes through. It goes through a sterilization process similar to the pasteurization commonly used for food products such as milk. I found a way to do this in my home with a common kitchen appliance: the microwave oven.
A microwave oven cooks things by super-heating the water molecules present in food or other items. Because of that, it is important that the soil you are sterilizing is not too dry. Use a spray bottle to moisten the soil if it is dry, then mix it up to evenly distribute the water.
Place the moist soil into a microwave-safe casserole dish. Place the soil into your microwave oven and heat it at 50% power and watch for steam to appear. Continue cooking the soil for 30 seconds after the appearance of steam. How long this takes will depend on several factors such as the amount of soil, the initial temperature of the soil, the level of moisture in the soil and how powerful your microwave oven is. Generally, this should take between one to five minutes. Let the soil cool slightly before removing it from the microwave oven.
Once cooled, the sterilized soil is now ready for use. I use this clean soil for both my houseplants and for garden plants that I start inside. Some gardeners feel that food plants need to be planted in natural soil loaded with microbes, but I disagree. Once you transplant the seedling to your outdoor garden, the soil there will spread its microbes to the newly planted plant and the sterilized soil that surrounds it. I have never had troubles producing healthy plants with this method. Quite the contrary, I have less problems with blight and other plant diseases when I start my plants in sterilized soil.
The Best Microwave Oven for Sterilizing Soil
You will need a microwave that is capable of even heating, or you will risk having soil that is not completely sterilized. One small area with pathogenic bacteria can repopulate the rest of the soil with stuff that you don’t want. Look for one with a turntable; that will contribute to even heating.