One of the best ways to ensure your seedlings and potted plants remain healthy is by planting in sterile soils. Sterile means the soil is clean and less likely to spread diseases or pests to your plants. Purchased soil, soil from your garden or soil reused from past plants must all undergo sterilisation to ensure your plants remain healthy.
Sterile potting soils have no active weed seeds, insects or disease organisms, which allows you to grow healthier plants. The sterilisation process, called pasteurisation, uses heat to kill or neutralise any pests living in the soil. Chemical sterilisation is also used for some commercial soils. Seeds started in sterilised soil are less prone to damping-off disease and other fungi that kill the young plants.
Commercial potting mixes usually combine sterilised potting soils with naturally sterile mediums, such as a peat moss or vermiculite. Not all commercial mixes have undergone sterilisation, so it's vital to check that the soil mix is labelled sterile or pasteurised. If you prefer purchasing compost over traditional potting soil mixtures, it must also undergo sterilisation procedures and should be labelled as such. Using a nonsterile soil, in either pots or garden beds, can introduce weeds seeds and pathogens to your garden.
Unless you assemble your homemade potting mixes with sterile purchased components, you must heat treat any soil or compost before planting. Peat moss, vermiculite, perlite and other mineral potting soil amendments are already sterile and shouldn't be heat treated. Baking moistened compost and garden soil at 82.2 degrees Celsius for 30 minutes kills most weed seeds and disease organisms, making these components sterile for use in your homemade potting recipes.
Keeping Soil Sterile
Both purchased and homemade sterile soils must remain sterile until they are planted. Storing the soil in sealed bags or plastic tubs prevents pathogens and insects from compromising the safety of the soil. Reused pots and garden tools may also transfer unwanted pests to sterilised soil. Cleaning these items with a weak bleach solution sterilises them so they do not infect otherwise healthy potting soils. Heat treatment sterilises used potting soils, but should not be used on soils containing vermiculite or perlite as these minerals respond poorly to heat.