Does Sterilizing Soil Kill the Microorganisms?

Pasteurizing Vs. Sterilizing

• Technically, pasteurizing and sterilizing soil are two different processes. Adhering to the strict definitions of these terms, sterilizing soil will rid it of all organisms, including pests, weed seeds, diseases and beneficial microorganisms. Pasteurizing soil will kill only harmful microorganisms, not those that are beneficial to soil.

Sterilized Soil Basics

• Sterilized soil, such as that you would find in home and garden stores, often contains fertilizers and other beneficial ingredients but will never contain beneficial microorganisms that could help your plants grow. While this type of soil is healthy for houseplants and will not introduce disease, pasteurized soil is healthier.

Harmful Microorganisms

• Pasteurizing or sterilizing soil will rid it of harmful microorganisms such as pythium, rhizoctonia and fusarium, which can cause damping off disease. This disease causes plants to brown and die and can quickly spread through many plants. Pasteurizing your potting soil or using store-bought pasteurized soil will ensure that these and other microorganisms do not harm your plants. Eliminating harmful insects from the soil is also beneficial as they can carry disease or eat plants' roots.

Beneficial Microorganisms

• Beneficial microorganisms include bacteria and fungi that help plants grow. Two common microorganisms are rhizobia and mycorrhizae. Rhizobia attach to the roots of plants like beans and peas and give them nitrogen while the plant feeds the bacteria with nutrients and sugars. Mycorrhizae are fungi that also adhere to roots and behave as extensions of the root system, enabling roots to take in more water and nutrients. The roots also supply fungi with necessary sugars and nutrients. Mycorrhizae help plants persevere during droughts, cold and transplants and improve soil structure. Because sterilizing soil would rid it of these helpers, pasteurization is desirable.

Pasteurizing at Home

• If you plan to reuse soil from last year's garden, you can pasteurize it yourself at home. This method, if followed carefully, will kill undesirable microorganisms while retaining beneficial types. Keep in mind that this can create odors in your oven or home. Preheat your oven to 180 degrees Fahrenheit. Place 4 inches of moist soil in a pan, cover it with aluminum foil and place a meat thermometer in the center. Once the thermometer reading reaches 180 degrees, leave soil in the oven for 30 more minutes, allow it to cool and begin sowing seeds or potting.

Considerations

• If you pasteurize soil but fail to sterilize gardening tools and potting containers used for planting or garden work, plants will still be susceptible to harmful microorganisms. To make sure these are disease-free, soak items in a mixture of 1 part bleach to 9 parts water for 30 minutes. Rinse tools and pots thoroughly and allow them to dry before use.