Whether you're starting vegetable seedlings for your garden or growing potted flowers inside, sterile soil is a must for keeping harmful micro-organisms away from your plants. Many potting mixes are made with sterile or sterilized materials that you can substitute for sterile soil, or you can sterilize garden soil yourself right in the oven.
1. Potting Soils
Potting soils are made from a combination of materials that have been treated to provide a sterile growing medium for new plants. Sterile materials in potting soil that your average garden soil cannot provide include bark, perlite, vermiculite, compost, soil, peat and coarse sand. Soilless potting mixes made entirely from nonorganic materials are naturally sterile. Potting mixes that contain organic materials such as bark, soil, compost or peat are often heat-treated to kill any unwanted seeds or organisms in the mixture. You can use potting soils made from these materials to start your garden plants early or to grow indoors with relatively little risk of infection from soil-borne diseases.
2. Sterilizing Soil
You can use your kitchen oven to kill seeds, fungi and bacteria in soil before you use it in your flower pots or garden seedling starter trays. To fully sterilize soil, you need to heat it to 180 degrees Fahrenheit for roughly 30 minutes. Use a spare oven thermometer to monitor the temperature. Make sure the temperature stays below 200 degrees F, because many soils will emit a foul odor and release chemicals that are harmful to plants at higher temperatures. The most effective method is to preheat your oven and then place your soil in the oven in a baking pan covered with aluminum foil. Spread the soil out so it's less than 4 inches thick to make sure it gets sterilized thoroughly.
A potted plant brightening up your living room or the starter trays supporting the vegetables for your spring garden are the best places to use sterile soil. Sterile soil also helps reduce rot in cuttings from your favorite potted flowers. Sterilize soil that has shown signs of harboring harmful micro-organisms before you plant in it. Using sterile soil on plants you are going to put outside in pots can harm them, however, because sterile soil provides a fertile breeding ground for the bacteria and fungi that abound outdoors.
Sterilizing soil for new houseplants has little benefit if you put it in used pots already harboring harmful microorganisms. But you can get your containers clean and ready to use with dish soap and water from the kitchen and a dash of disinfecting isopropyl alcohol or hydrogen peroxide. Make sure you wipe out all of the dirt from each container's previous occupants, and then wipe it down with disinfectant and allow it to air dry.
- Colorado State University Extension: Start Seed and Transplants in Sterilized Soil
- Pennsylvania State University: How to Pasteurize Medium and Sterilize Containers and Tools
- Saddleback College: Propagation Soil Preparation
- Colorado State University Extension: Yes Houseplants Experience Stress Too
- University of Connecticut: Packaged Potting Media
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