My friends in the great snowy north tell me that they still have lots of winter weather. But here in the sunny south, the daffodils are popping up and the witch hazel is in full bloom. Although it is still several weeks until the last yearly frost date, I start thinking of my spring container garden now. This is the time to clean containers and, if you plan to recycle your soil, to sterilize it.
Why Sterilize Soil?
The longer that a plant grows in the soil, the more that diseases, fungus and pests like parasite nematodes build up in soil. This is one reason why farmers practice crop rotation: to give the soil pathogens a chance to die out before planting the same plant in the soil again.
Although gardeners can replace containers and soil yearly, it is less expensive to sterilize them than to replace potting soil and containers. Especially if you use dirt in large quantities.
Sterilizing potting soil will also kill any plant seeds that were left behind in the soil from last year.
How to Sterilize Soil.
Heat treating soil is one of the best means for sterilizing it. Heating soil above a certain temperature kills everything in soil from seeds to fungus spores and bacteria. This is how greenhouse owners prepare their soil between harvest and planting: by injecting superheated steam that was generated by huge machines into the soil.
While you may not have a large steam generator, you can still use heat to treat your used potting soil.
What to do:
Fill a pan with slightly damp potting soil. The pan should be one that you can cover to keep the soil moist, such as a roasting pan. If you don’t have a roasting pan, you can use almost any oven safe pan and cover it with tin foil.
Pinch a hole in the foil and insert a meat or candy thermometer into the center of the soil. Place the pan inside your oven and heat the oven to between 200 and 250 degrees Fahrenheit. Keep the soil in the oven until the temperature of the soil reaches between 160 and 180 degrees. Turn the oven off and let the soil cool before removing the pan.
A Word of Warning:
The baked soil will stink! If you (or the rest of your family) do not want to smell the odor of baked potting soil, make sure to open a window before sticking it in the oven. If you have a place outside where you can bake the soil, such as a gas grill, pizza oven or a patio kitchen with an oven, your family will thank you if you use that instead.
The sterile soil will not only be free of disease and weeds, it will also be pretty devoid of nutrients as well. Before using the soil, you should mix a good potting soil fertilizer into the soil. A granulated, time release fertilizer such as Osmocote is a good choice. Always mix a fertilizer such as this with your potting soil according to package directions. Granulated fertilizers are available in various time release increments. These should be mixed at different rates.
Don’t Do This.
Now that your potting soil is sterile, you can contaminate it again by putting it in dirty containers or by using dirty tools. Now that you’ve worked so hard to clean your soil, don’t contaminate it again using careless techniques.