Potting Soil

How to Sterilize Potting Soil

Q. I want to sterilize my potting soil which I buy in bags. It seems that tiny seeds must be inside since there are green stems growing after I pot and water. How and can I do this?

A. Yes you certainly can use the microwave oven to sterilize soil and kill off some seeds and organisms. The result is fine. I pour a little bit of water to the soil in a plastic cup with lip unlocked for 1 or 2 minutes. Let the soil sterilized by the steam. Generally I repeat once more. 

The old method was to heat it in a conventional oven to 180 F and keep it at that temperature for 30 minutes. The problem is heating soil creates a strong stench that will permeate the entire home.

Now that most homes have microwaves I've had inquiries as to whether they can be used to sterilize soil. The answer is "yes." Here's the procedure.

Do about four to five pounds of soil at a time. It should be moist and crumbly but not oozing water. If it's dry enough to plant in and work it is dry enough to microwave.

Put the soil in a plastic bag (polypropylene bags used for baking are less likely to burst than more common polyethylene bags). Twist the top of the bag slightly but do not seal; it might explode as steam builds up. Alternatively you could use a loosely covered microwave-safe mixing bowl.

With oven at full power heating for seven minutes will kill most diseases. If you are seriously concerned with damping-off disease or have had problems with other soil-borne disease extend or even double the heating time.

High water content and myriad pore spaces in compost or leaf mold will considerably slow the heating process if they're included in the mix. After sterilization let the soil stand uncovered until cool and store in sealed containers. It's advisable to clean the oven thoroughly particularly the door seal.